m

macerating pump A pump that reduces solid waste material to small pieces or animal waste to a slurry.

machine A mechanism that transmits power in the performance of a useful task. More than one machine in one place, either connected or performing separate functions, is termed machinery.

machine bolt See bolt.

machine design See mechanical engineering.

machine element Any individual part used in the design of a machine, particularly a standardized component such as a bearing, fastener, gear, seal, or spring.

machine screw A relatively small screw, usually less than 20 mm in diameter, with the thread running along the whole length up to the head, intended to be screwed into threaded holes. If inserted through plain holes in assembled parts and held together by a nut, an undesirable contact surface of threads bearing against the surface of a hole results. See also fitted bolt.

machine tool A powered machine, such as a borer, grinder, lathe, milling machine or planer, used for cutting and shaping metal, plastics, composites, etc. (machining). A machining centre is a CNC machine tool working about several axes, having a stock of tools and automatic tool changing ability, which is capable of diverse machining operations under automated control. See also computer numerical control.

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• Workshop and manufacturing terms

machining indication See surface-texture indication.

Mach number (M) 1. At a point in a steady flow, the ratio of the fluid velocity V to the local speed of sound c, i.e. M = V/c. 2. For an object moving through a fluid, the ratio of its speed (relative to the fluid) to the speed of sound. A Machmeter is an instrument, typically based on a pitot tube, that determines the Mach number of an aircraft. See also Bairstow number; Cauchy number; subsonic flow; supersonic flow; transonic.

Mach wave (Mach line) An infinitesimally-weak wavefront of expansion or compression in a supersonic flow, caused by an infinitesimal disturbance, usually on a surface. The Mach angle (μ), with unit rad or °, is the angle that a Mach wave makes with the direction of a supersonic flow. If M is the Mach number, μ = sin⁻¹(1/M). A Mach cone is the cone within which are confined the disturbances from an infinitesimal source moving at supersonic speed relative to a fluid. The cone angle is equal to the Mach angle. The two-dimensional equivalent is a Mach wedge, where the disturbances are produced by a line. Mach’s construction is the geometric construction of a mach cone or wedge in which the distance travelled by a source in time t, Vt, is combined with that travelled by the disturbance, ct, V being the source speed, and c is the speed of sound. See also Prandtl–Meyer expansion; shock wave.

Mach–Zehnder interferometer An apparatus in which two parallel beams of light from the same source pass through a gas flow, the reference beam in undisturbed flow, the other through a disturbed region of non-uniform density. The two beams are focussed on a screen to produce interference fringes that can be analysed for information about the density variations. See also interferometry.

Macpherson strut A type of front suspension for motor vehicles comprising a long strut, with an internal shock absorber, surrounded by a coil spring attached to the body and the strut.

macro A prefix denoting a scale size very much larger than that of atoms or molecules. See also meso; micro; nano.

macromechanics The mechanics of materials at the continuum level, i.e. where the different behaviour of microstructural components is averaged out so that the material can be considered to have homogeneous (but not necessarily isotropic) properties.

macroporous material See microporous material.

macroscopic property Any thermodynamic or material property that can be defined at a scale where the continuum hypothesis remains valid.

macrosonics The application of high-intensity sound or ultrasound to such industrial processes as atomization, degassing, mixing, and particle agglomeration.

maglev (magnetic levitation) A system of transportation, usually a train, in which vehicles are supported by contactless magnets and propelled by linear electric motors. See also linear generator.

mag meter See electromagnetic flow meter.

magnetic bearing A lubricant-free, low-friction bearing in which a load is supported by forces generated by electromagnets.

magnetic brake A form of non-contact brake in which a metal fin (typically copper or copper/aluminium alloy) passes between rows of neodymium magnets. Eddy currents are generated in the fin together with a magnetic field that opposes its motion. Commonly used on roller coasters.

magnetic clutch (electromagnetic clutch) A clutch in which an electromagnet is used to magnetize a rotor that attracts one or more friction discs, either directly or through an armature. In an alternative design, torque is transmitted via teeth on the armature and rotor.

magnetic diffusivity (λ) (Unit m²/s) A quantity analogous to kinematic viscosity or thermal diffusivity that accounts for the diffusion of a magnetic field.

magnetic-disc coupling A non-contacting coupling in which power is transmitted across the air gap between two discs, carried by co-axial shafts, each of which incorporates powerful rare-earth magnets.

magnetic domain See magnetic materials.

magnetic drag dynamometer (eddy-current dynamometer) A dynamometer in which torque is applied to a rotor attached to the input shaft. The torque arises from eddy currents induced in the spinning rotor by the magnetic field created by an electromagnet.

magnetic flow meter See electromagnetic flow meter.

magnetic levitation See maglev.

magnetic materials Materials in which the magnetic moment of adjacent atoms can adopt either parallel or antiparallel alignment. Ferromagnetism occurs when the magnetic moments adopt parallel alignment. When adjacent magnetic moments cancel due to antiparallel alignment, the material exhibits antiferromagnetism and has no overall magnetic moment. Examples of the latter include ferrous oxide and manganese oxide. The phenomenon of ferrimagnetism occurs when adjacent magnetic moments adopt antiparallel alignment but have unequal magnitude. The region of ferromagnetic or ferrimagnetic material in which the magnetic moments are aligned is known as a magnetic domain and adjacent regions are separated by domain walls. Diamagnetic and paramagnetic materials exhibit weak forms of magnetism that results from independent (uncoupled) alignment of magnetic atomic dipoles induced by an applied magnetic field. They do not retain magnetization in the absence of an applied field and are classed as non-magnetic.

Ferromagnetic and ferrimagnetic materials can acquire high degrees of magnetization in relatively weak magnetic fields. They have different hysteresis characteristics and are classified as either hard magnets or soft magnets. Hard magnets retain large magnetization in the absence of an applied magnetic field and are used as permanent magnets. Conventional materials include magnet steels alloyed with tungsten or cobalt. Rare-earth magnets made from alloys of rare-earth elements produce significantly stronger magnetic fields. The most common of these are samarium-cobalt and neodymium-iron-boron permanent magnets. Their strength allows them to be small and used in numerous applications, including cordless tools, disk drives, headphones, toys, and motors for automotive components such as wipers, fans, washers, and windows. Soft magnets may be magnetized and demagnetized relatively easily. When subjected to an alternating magnetic field the energy loss due to hysteresis is small. Common materials include iron-silicon alloys (known as electrical steels), nickel-iron alloys (permalloy), soft ferrites, and amorphous nanocrystalline alloys, the last of which can be produced in the form of tape by melt-spinning. Soft magnets are used to enhance the magnetic flux produced by an electric current. Applications include transformer cores, magnets for lifting scrap metal, and magnetic shielding. See also hysteresis.

magnetic moment (magnetic dipole moment) (Unit N.m) A vector quantity, m, that relates the torque, T, experienced by a magnet dipole to align it with the field vector in a magnetic field with flux density . The direction of m points from the south to the north pole of the magnet, and the magnitude is proportional to the magnetic field of the dipole.

For a single atom, the magnetic moment arises from its individual electrons, each of which produces a moment due to orbital motion about the nucleus and another due to spin. See also dipole; magnetic materials.

magnetic powder clutch See powder clutch.

magneto An electrical generator that produces alternating current for the ignition systems of petrol engines where there is no other electrical supply, such as for lawn mowers, bicycles, and some aircraft engines.

magnetohydrodynamic generator A power-generation device in which a flow of hot ionized gas through a channel is subjected to a magnetic field, thereby generating an electric field. Electrical power can be supplied to an external circuit by electrodes built into the channel walls.

magnetohydrodynamics (MHD) The study of the flow of an electrically-conducting fluid subjected to a magnetic field. Counterparts of the Reynolds and Prandtl numbers which arise are the magnetic Reynolds number, defined by VL/λ, where V is a characteristic flow speed, L is a characteristic length, and λ is the magnetic diffusivity of the fluid, and the magnetic Prandtl number, defined by ν/λ where ν is the kinematic viscosity of the fluid.

magnetorheological fluid (MR fluid) A suspension of fine soft magnetic particles in a liquid such as silicone oil. The particles align in an applied magnetic field and the rheological properties change from Newtonian to viscoelastic. The change in the shear yield stress of the viscoelastic solid under high magnetic field is reversible and can occur in milliseconds. Used, for example, in shock absorption and vibration control, such as in engine mounts, dampers in prosthetic legs, and earthquake-resistant structures. See also non-Newtonian fluid.

magnetostriction The expansion or contraction of a ferromagnetic material in a magnetic field, e.g. to produce ultrasonic vibration.

magnet steels See magnetic materials.

Magnus effect The phenomenon whereby a lift force arises on a spinning object in a transverse viscous fluid flow. The force acts in a direction perpendicular to the spin axis and that of the flow. In the diagram, the fluid pressure is higher on the surface marked H than on the surface marked L.

Magnus effect

main bearings In a reciprocating machine, the bearings that locate and support the crankshaft.

main rotor(s) The rotor(s) of a helicopter or other rotorcraft that provide lift and thrust.

major diameter See screw.

make up (Unit kg or L) The water added to boiler feedwater to compensate for that lost through blowdown, exhaust, leakage, etc.

male fitting (male coupling) The inner part of a connection in which one part fits into another. See also female fitting; screw.

male thread See screw.

malleable iron See cast iron.

Maltese-Cross mechanism See Geneva mechanism.

mandrel 1. A circular bar, either cylindrical or tapered, that acts as a form on which soft or flexible material can set or be wound or on which partially machined work is supported for further machining. 2. The driving or headstock spindle of a lathe.

Mangler transformation A method for transforming the axisymmetric boundary-layer equations to the plane (i.e. two-dimensional) boundary-layer equations.

manifold See exhaust manifold; fuel system; intake manifold.

manifold pressure (Unit Pa) The absolute pressure in the intake manifold of a normally-aspirated piston engine. See also boost pressure.

manipulator See robot.

manometer An instrument used in the measurement of a pressure difference in a fluid (manometry). See also differential pressure gauge; inclined-tube manometer; U-tube manometer; well-type manometer.

manostat A device for controlling and regulating pressure.

manual control unit A hand-held control panel used to program a robot. See also teach.

manufacturing engineering See production engineering.

maraging steels Very high-strength iron alloys named after ‘martensitic’ and ‘aging’. They contain nickel, chromium, cobalt, and molybdenum, and form martensite on air cooling with a strength of about 1 GPa. When reheated to about 500°C and aged for some hours, a room-temperature strength of some 2.4 GPa is reached.

Marangoni convection (Bénard–Marangoni convection) A form of natural convection arising as a consequence of thermocapillary instability, in which vertical hexagonal cells occur in a thin horizontal layer of a liquid heated from below. The motion is a consequence of surface-tension gradients in the liquid surface. The flows are characterized by the non-dimensional Marangoni number (Ma), defined by where Δσ is the difference in surface tension due to a temperature difference across a fluid layer of depth H, μ is the dynamic viscosity of the fluid, and α is its thermal diffusivity. See also Bénard convection.

marginal stability (neutral stability) A system shows marginal stability if, when given a non-periodic input (for example, an impulse), it shows oscillation that is bounded (i.e. finite) and continues indefinitely.

margin of safety See factor of safety.

Margoulis number See Stanton number.

marine-screw propeller See propeller.

Marlborough wheel An extra-wide gear wheel that can mesh with two or more standard-width gear wheels side-by-side.

martensite See steel microstructures.

mass (m) (Unit kg) The quantity of matter in a body or system. See also inertial mass.

mass balance weight A mass attached to an aircraft control surface to prevent flutter.

mass-conservation equation See conservation equations.

mass density See density.

mass-diffusivity coefficient See Fick’s law.

mass effect In heat treatment, the rate of cooling of a component during quenching is progressively slower from the outside to the centre. Thus, in the case of steels, martensitic microstructures may be produced on the surfaces of components, but equilibrium microstructures in the centre. See also hardenability.

mass flow meter An instrument, such as a Coriolis flow meter, which measures the mass flow rate of a fluid flowing through a pipe or other duct, rather than its volume flow rate.

mass flow rate (ṁ) (Unit kg/s) The mass of a material, usually a fluid or powder, that flows across a surface or through a pipe or other duct per unit time. The corresponding mass flux (ṁ″), with unit kg/s.m², is the mass flow across a real surface or through a duct, divided by the surface or cross-sectional area A, i.e. ṁ″ = ṁ/A or ṁ″ = ρV where ρ is the material density and V is its velocity normal to the surface.

mass fraction The ratio, in a mixture of substances, of the mass of an individual component to the mass of the mixture. See also volume fraction.

mass moment of inertia See moment of inertia.

mass salinity See salinity.

mass transfer The flow of fluid through a porous surface, to or from another fluid, involving diffusion due to concentration gradients within the fluid and movement of the fluid. The diffusive phenomena occur within the boundary layer. The mass-transfer coefficient (coefficient of mass transfer, convective mass-transfer coefficient, mass-transfer conductance, g), with unit kg/s.m², is the proportionality factor in the mass-transfer equation ṁ″ = gB where ṁ″ is the mass flux and B is the mass-transfer driving force. For mass transfer of component j of a fluid across a porous surface into another fluid, where m_j is the mass concentration of j. The subscript ∞ indicates a value far from the surface in the surrounding fluid, S a value at the surface, and T a value far from the surface in the reservoir of component j.

mass transport In fluid flow, the movement of a substance by advection or diffusion.

master A reference gauge or instrument against which others are compared or calibrated.

master cylinder A piston-cylinder pair attached to a reservoir of hydraulic fluid in a hydraulic circuit, such as in the braking or clutch systems of a motor vehicle, used to transfer fluid to the brake cylinders, clutch, etc.

master/slave manipulator A robot which follows the movement of a manually moved master arm, but often with greater force or range of movement. Used for remote operations in hazardous areas, for example nuclear-plant decommissioning. See also teleoperator.

material derivative See substantial derivative.

material particle See particle.

materials science The study of the properties, behaviour, and application of solid substances such as metals, ceramics, glasses, polymers, composites, biomaterials, and semiconductors, at all scales from the atomic to the macroscopic. The topic has its origins in metallurgy.

materials selection The process of selecting the most suitable material for a particular application. In the case of solid materials, it involves assessment of candidate materials in terms of their physical properties (density, elastic moduli, fracture toughness, yield strength, hardness, fatigue resistance, coefficient of thermal expansion, thermal conductivity, resistance to thermal shock, creep resistance, corrosion resistance, radiation resistance, etc), and such factors as the function of the final product, its shape, method of manufacture, required tolerances, number to be made, ease of inspection and quality control, cost, and design methodology employed.

matrix The continuous phase in any reinforced material, which may be polymeric, metal, ceramic, or complex biomaterial.

Matthew effect Applied to such phenomena as crystal or drop growth, whereby large crystals or drops develop at the expense of small ones.

Maupertius principle See principle of least action.

MAV See micro-air vehicle.

maximum allowable operating pressure (Unit Pa) The highest pressure at which any pressure system may be operated, usually 10 to 20% below the maximum allowable working pressure.

maximum allowable working pressure (Unit Pa) The pressure on which the design of a pressure system is based and the highest pressure at which relief valves should be set. The lowest-rated component in the system typically has a design safety factor of 4.

maximum-and-minimum thermometer (minimum-maximum thermometer, Six’s thermometer) A liquid-in-glass instrument designed to register the maximum and minimum temperatures experienced over a given time period.

maximum continuous load (maximum continuous rating) (Unit kg/s) The maximum rate of steam output that a boiler can supply for a specified period, usually 24 hours.

maximum material condition (maximum metal condition) The situation where the volume of a manufactured component corresponds to the upper limit for all toleranced external dimensions, and to the lower limit for all internal dimensions. See also minimum material condition.

Maxwell model (Maxwell material) A mechanical model for a solid or liquid viscoelastic material comprising a dashpot (i.e. a viscous damper with viscosity μ) in series with a linear-elastic element (modulus of elasticity G) such that both carry the same load that gives rise to a common stress τ. The elastic displacement rate is /G, being the rate of change of shear stress with respect to time, the viscous displacement rate (or shear rate) being τ/μ, and the overall displacement rate τ/μ + /G. The shear stress τ is given by τ + λ = μ where λ = μ/G is the relaxation time and is the overall strain rate. See also Kelvin–Voigt model; non-Newtonian fluid.

Maxwell relations (Maxwell equations) A set of equations which thermodynamic properties must satisfy for any thermodynamic system in a state of equilibrium:

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where T is absolute temperature, p is pressure, v is specific volume, and s is specific entropy.

Maxwell’s lemma A relation from which the minimum weight of a structure to support a given set of loads may be obtained, given by , where V_T is the total volume of members in tension and V_C that of those in compression, σ_T and σ_C are the failure stresses in tension and compression, respectively, and F is the planar vector force applied at the ith node of the truss situated at vector distance r from an assigned origin. The design loading on the structure is given by and is known. For a given framework, the geometry gives the length of each member (in both tension and in compression) and thus V_T and V_C for specified cross-sectional areas. Multiplying by σ_T and σ_C permits to be evaluated and compared with . For members of the same material and hence same density (the usual case), the minimum weight depends on the total volume, V_T + V_C. The minimum weight is arrived at by adjusting the cross-sectional areas.

Maxwell’s theorem See reciprocal theorem.

Mayer’s formula For an ideal gas, C_P − C_V = R where C_P and C_V are the specific heats at constant pressure and constant volume respectively, and R is the specific gas constant.

McCabe wave pump A wave-energy device consisting of three hinged pontoons pointed parallel to the wave direction, which flex in response to the waves.

M contour On the Nyquist diagram, the locus of the real and imaginary parts of the closed-loop transfer function with unity feedback plotted as a function of frequency.

mean See mean value.

mean diameter (Unit m) 1. The average of the inside and outside diameters for a helical spring or hollow circular cylinder or sphere. 2. See Sauter diameter.

mean-effective pressure See brake mean-effective pressure; indicated power.

mean film temperature See film temperature.

mean-free path (Λ) (Unit nm) In a gas, the average distance that a gas molecule travels before collision with another gas molecule.

mean stress (Unit Pa) The average of the maximum and minimum stresses for a material subjected to a stress cycle, as in a fatigue test.

mean temperature See bulk temperature.

mean temperature difference (ΔT_M) (Unit K or °C) In a heat exchanger, ΔT_M is defined by where U is the overall heat-transfer coefficient, A_S is the surface area associated with U, and is the heat-transfer rate. ΔT_M can be obtained from the log-mean temperature difference (LMTD) for a counterflow heat exchanger and a correction factor F for the specific type of heat exchanger concerned according to ΔT_M = F.LMTD.

mean value (mean) Where a material property is dependent upon either the thermodynamic state (i.e. temperature or pressure) or a variable such as strain (in a solid) or shear rate (in a fluid), an average over the range of the state or variable. For example, the mean stress for a work-hardening solid deformed over a range of strain from 0 to ε would be

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measured relieving capacity (Unit m³/s or kg/s) The actual flow rate through a pressure-relief device measured at its design pressure. The rated relieving capacity is that portion of the measured relieving capacity used as the basis for the application of a pressure-relief device, determined from the applicable code or regulation.

measuring tank (dump tank, metering tank) A calibrated tank used to measure the mass of a volume of liquid that flows into it in a measured period of time. Used to calibrate flow meters.

mechanical admittance (mobility) (Unit m/(N.s)) The inverse of mechanical impedance.

mechanical advantage (force ratio, leverage) The ratio, for a machine, of the output force (load or resistance) to the input force (applied effort).

mechanical alloying Production of alloys from powders by consolidation, e.g. hot-pressing or extrusion, followed by hot and/or cold working and final annealing. Materials such as dispersion-strengthened alloys, high-temperature aluminium alloys, and amorphous alloys can be made in this way.

mechanical comparator A contact device, such as a dial gauge, in which movements of the indicator are magnified by levers and gear trains. See also optical comparator.

mechanical damping The damping caused by dry friction in a vibrating system.

mechanical draught See draught.

mechanical-draught cooling tower A cooling tower in which fans force or draw air through the tower. See also forced-draught cooling tower; induced-draught cooling tower.

mechanical efficiency (η) 1. In general for a machine, the ratio of output work to input work. 2. For a compressor, the ratio of indicated power to shaft power; for a reciprocating engine or an expander, the ratio of shaft power to indicated power.

mechanical energy (Unit J) The sum of kinetic energy and potential energy for an object or a mechanical system, including the energy stored in springs, etc.

mechanical-energy integral equation An equation relating the variation with streamwise distance x of the fluid kinetic energy within a two-dimensional boundary layer to the work done by the shear stress τ. The equation is valid for laminar and turbulent flow. For a flow with constant density ρ, it can be written as

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where U_∞ is the free-stream velocity and ū is the mean velocity a distance y from the surface. The quantity on the right-hand side is termed the dissipation integral.

mechanical engineering That branch of engineering concerned with energy conversion, stress analysis, vibration, dynamics, and kinematics, especially applied to design (machine design, mechanical-engineering design).

mechanical equivalent of heat (Joule’s equivalent, 𝓙.) According to the first law of thermodynamics, for a closed system taken through a cycle, the sum of the net work done on the system (or delivered to the surroundings), ΣδW, plus the net heat delivered to (or taken from) the surroundings, ΣδQ, is equal to zero. If the units of and are the same, then . If the unit of is joule and that of is calorie (an obsolete non-SI unit), then 𝓙 = 4.186 8 J/cal. See also calorie.

mechanical hysteresis loop

adiabatic and isothermal elasticity

thermoelastic hysteresis

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• Manufacturing design

mechanical hysteresis A phenomenon exhibited by materials when stress recovery lags behind strain recovery on unloading. A simple example of the former is illustrated in the figure for a strain cycle between the limits , such as occurs in bending between fixed displacements. The initial loading curve to is O–A (elastic) and A–B (plastic). On reversing the strain to , yielding in compression occurs at a lower stress C (Bauschinger effect). On reloading, a stress–strain loop develops. The area of this mechanical hysteresis loop is the energy dissipated in the cycle per unit volume of material. Since plastic deformation is not completely reversible, further cycles for the same strain amplitude may exhibit a stress amplitude which is higher (cyclic hardening) or lower (cyclic softening) or unchanged from the original value of . Similar cyclic curves can arise when the limits of strain (or stress) are not of the same magnitude.

Mechanical hysteresis from cycles in the elastic region can occur due to the thermoelastic effect. A metal specimen loaded to stress σ_c isothermally, i.e. sufficiently slowly to equilibrate with its surroundings, exhibits the stress–strain line ABC in the figure, with gradient equal to the isothermal elastic modulus. However, if loaded sufficiently rapidly that there is no heat exchange with the surroundings, i.e. adiabatically, the internal temperature of the specimen reduces and the adiabatic elastic modulus line AC′ with higher slope results. If held at stress σ_c and allowed to return to ambient temperature, the specimen undergoes thermal expansion C′C. On unloading slowly, the isothermal line CBA is followed, but if unloading is rapid and again adiabatic, the temperature of the material rises and line C″C is followed. Strain falls to zero by thermal contraction when the specimen cools to ambient temperature. The hysteresis loop AC′CC″A is an idealized form. In practice there will be heat exchange in the cycle and the parallelogram becomes a loop, as shown in the figure. For metals, the difference between the isothermal and adiabatic moduli is generally small (for steel it is about 0.3%) and the loss of energy per cycle is also small. However, in vibration when many cycles occur in succession the losses become important and produce one of the internal friction mechanisms responsible for damping vibration. Rubber shows pronounced elastic hysteresis in its stress–strain curve that converts large amounts of mechanical energy to heat, hence its use for shock absorption. Its high hysteresis is also important for skid-resistant tyres. See also cyclic stress–strain curve; damping; fatigue; hysteresis; shakedown; thermoelastic effect.

mechanical impedance (Z) (Unit N.s/m) A measure of the resistance to motion. If a force F, varying with angular frequency ω, applied to a mechanical system leads to a velocity V(ω) at the point of application, the mechanical impedance Z(ω) = F(ω)/V(ω). The mechanical reactance is the imaginary part of Z and the mechanical resistance is the real part. The mechanical ohm, equal to 1 N.s/m, is a non-SI unit of mechanical impedance. See also angular impulse.

mechanical irreversibility See irreversibility.

mechanical linkage A mechanism consisting of rigid bars connected at pivot points which can transmit force but not torque.

mechanical pressure (p, average mechanical pressure) (Unit Pa) In a moving fluid, the negative of the average value of the three normal components of stress over the surface of an infinitesimally small sphere. Generally taken to be the same as the static pressure. For an incompressible fluid and for a perfect monatomic gas, the mechanical pressure at a point is identically equal to the thermodynamic pressure p_TH at that point. More generally it can be shown that where λ is the second coefficient of viscosity and Θ is the divergence of the velocity. In practice it is usually assumed that the mechanical pressure is equal to the thermodynamic pressure. See also pressure; thermodynamic pressure.

mechanical properties of solid materials. The strength and stiffness properties of solid materials such as fracture toughness, the moduli of elasticity, percent elongation, Poisson’s ratio, proof stress, tensile strength, ultimate stress, and yield stress. See also intensive property.

mechanical scales A weighing device consisting of one or more pivoted beams along which are moved counterweights until balance is achieved. See also Roberval balance.

mechanical seal A device that prevents the leakage of fluid from a high-pressure to a low-pressure region. See also O-ring; packing.

mechanical separation The partial or total removal of solid particles from a fluid, or liquid droplets from a gas, by cyclone separators, filtration, or gravitational settling.

mechanical twinning See deformation twinning.

mechanical units The units of physical quantities, the dimensions of which include mass, length, and time. See also International System of Units.

mechanical vibration The motion of a particle or body which oscillates about a position of equilibrium.

mechanics See applied mechanics.

mechanism A kinematic chain in which one element or link is fixed so that motion can be transmitted or transformed.

mechanize To convert a hand-operated device into a powered machine.

mechatronics The integration of mechanical engineering, electrical engineering, electronic engineering, and software engineering. See also engineering science.

medium-enthalpy resource See geothermal energy.

medium-head hydroelectric plant See high-head hydroelectric plant.

medium-technology robot A robot having four to six joints and not incorporating a vision system or other advanced sensors. Such robots are typically employed for pick-and-place tasks.

medium vacuum See vacuum.

meehanite iron See cast iron.

mega (M) An SI unit prefix indicating a multiplier of 10⁶; thus megawatt (MW) is a unit of power equal to one million watts.

melamine resins (melamine-formaldehyde resins) Thermosetting plastics based upon the polymerization of melamine and formaldehyde. Physical properties include relative density 1.5, Young’s modulus 7 GPa, and ultimate tensile strength 30 MPa. The resins are hard and strong, and have good electrical-insulation properties and surface hardness, and good resistance to water, weak acids, and alkalis.

Melde’s experiment An apparatus in which standing waves are excited on a fine thread under tension by vibrating one end either longitudinally or transversely.

melt fracture Instabilities in flow during polymer extrusion, resulting in rough surfaces and variations in dimensions of the extrudate. In extremis, the extrudate fractures.

melting point (Unit K or °C) The temperature at which a solid material undergoes the phase change to a liquid at a specified pressure, usually 1 atm. Pure metals and eutectics have single-valued melting points, while alloys with other compositions melt over a range of temperature such that there is a well-defined start and end to the melting process, but there are states in between where solid and liquid are both present. See also latent heat; liquidus line; solidus line.

melt spinning A method of manufacturing fibres in which molten polymer is pumped through a die having numerous holes about the size of the fibre diameter (a spinneret). The fibres are then reeled, during which stretching occurs, giving the fibres orientation.

member A component of a structure such as a beam, column, plate, strut, or tie rod.

membrane 1. A thin sheet of material, usually flexible, but with negligible resistance to bending such that the only stresses are in-plane and tensile. 2. A thin sheet of semi-permeable material, often polymeric, used in separation processes such as water purification, reverse osmosis, ultrafiltration, and dehydrogenation of natural gas.

membrane analogy (soap-film analogy) An analogy based upon the fact that the differential equation governing the elastic-stress distribution in a cylindrical bar of arbitrary cross section under torsion is the same as that for the surface slope of a membrane with an outline corresponding to the bar and subjected to a differential pressure. See also sand-heap analogy.

membrane pump See diaphragm compressor.

membrane stresses (Unit Pa) The in-plane stresses in shell structures such as thin-walled pressure vessels. There are circumferential (hoop) stresses, orthogonal to which are meridional stresses that, in the case of a cylinder, are the axial stresses. Membrane analysis (membrane theory, thin cylinder theory) is the analysis of thin shells assuming only in-plane compressive or tensile stresses, with zero bending stresses.

membrane valve See diaphragm valve.

MEMS See micro-electromechanical systems.

MEMS actuator See microactuator.

MEMS thruster (microthruster) A miniature propulsion device with applications to, for example, nanosatellites. The devices typically have radial dimensions up to 500 μm, are fabricated from silicon, use hydrogen peroxide or hydrazine as propellant, and develop thrust up to about 500 μN.

meniscus The curved portion of a liquid surface where it comes into contact with a solid surface. If the contact angle (measured from the solid surface) is acute (i.e. < 90°), as for water, the solid surface is wetted and the meniscus is concave. The opposite is true when the angle is obtuse, as for mercury. See also interface; wettability.

mep See brake mean-effective pressure.

mer The group of atoms that constitute a structural repeat unit in a polymer chain molecule. For example, in the carbon–carbon chain of polyethylene, the mer CH₂ consists of two hydrogen atoms attached to a carbon atom.

mercury (Hg) A heavy (relative density 13.546) metallic chemical element, liquid in the range −38.8°C to 356.7°C and widely used in bulb-thermometers, barometers, and manometers.

mercury barometer A barometer in which the working liquid is mercury. See also Fortin barometer.

mercury manometer A manometer, such as an inclined or U-tube manometer, in which the working liquid is mercury.

mercury thermometer A bulb thermometer in which the working liquid is mercury.

mercury vapour cycle That part of a binary vapour cycle in which the high-temperature fluid is mercury.

meridional stress See membrane stresses.

Mersenne’s law The fundamental frequency f of a stretched string of length L, mass per unit length m′, and tension T is given by .

mesh 1. A pair of rotating gears in contact is said to mesh. 2. The arrangement of elements in finite-element, finite-difference and finite-volume simulations. 3. See screen mesh; sieve.

mesh size See sieve.

meso A prefix meaning ‘in between’ other scale sizes; its use depends upon the application.

mesomorphic state See liquid crystal.

mesopause See standard atmosphere.

mesosphere See standard atmosphere.

mesoporous material See microporous material.

metacentre For a floating object in the neutral position, a point M vertically above the centre of buoyancy B lying on a line through the centre of gravity G. If M is above G, the situation is stable to a small tilt Δθ but unstable if M is below G. In the diagram, W is the weight of the object and F_B is the vertically upward buoyancy force. The metacentric height is the distance between G and M. It is an important parameter for the stability of any floating object, such as a ship.

metacentre

metal inert gas welding See welding.

metallic bonding See interatomic bonding.

metallic glasses See amorphous metals.

metallize A non-metallic object, such as a glass mirror, is metallized by coating with a metal, typically by vacuum deposition or thermal spraying.

metallography The structure of metals and alloys revealed using microscopy to view polished, and usually etched, surfaces enabling microconstituents, grain sizes, and so on to be identified along with any processing defects.

metalloid See periodic table.

metallurgy The study of the atomic, crystallographic, microstructural, mechanical, and physical properties of metallic elements and alloys and their applications. Now part of materials science.

metal-matrix composite A composite material in which a reinforcing material, such as carbon fibre, alumina or silicon carbide, is embedded into a metal matrix. See also cermets.

metal spraying The creation of a metal coating using arc, flame, or plasma spraying.

metarheology The rheology of spatially-heterogeneous materials, such as fibre suspensions, that display elements of both particulate and continuous systems.

metastable A state of matter that drops to a lower energy state due to a slight disturbance; for example, the condensation of a supercooled vapour.

meter factor 1. The ratio of the true flow rate passing through a liquid flow meter to the flow rate indicated by the meter. The true flow rate is determined using a meter prover whereby an inflatable spheroid is forced through a calibrated section of piping to displace a precise volume of liquid. 2. The calibration factor for a flow meter, i.e. the true flow rate divided by the meter output.

metering pump A pump that supplies a specified volume of fluid in a specified time period. Types include bellows, diaphragm, piston, and travelling-cylinder pumps. See also proportioning pump.

metering rod A type of needle valve consisting of a thin tapering rod used in a carburettor to vary the size of the jet opening and so control the fuel flow rate.

metering tank See measuring tank.

method of characteristics A numerical procedure for solving hyperbolic partial differential equations, commonly used for problems in supersonic flow and rigid plasticity.

method of joints A procedure for determining the forces in the individual members of a statically determinate truss by considering the equilibrium of forces at each pinned joint.

method of sections A procedure for determining the internal forces in the individual members of a statically-determinate structure by making an imaginary cut through the framework and considering the equilibrium of forces on each of the two parts.

metre (m) The base unit of length in the SI system. It is equal to the distance travelled by light in vacuum in 1/299 792 458 seconds.

metric ton (tonne, t) An SI-accepted, but non-SI, unit of mass defined as 1 t = 10³ kg. See also long ton; short ton.

metrology The science and technology of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology.

Meyer hardness (Unit Pa) A material hardness similar to the Brinell hardness but based upon the projected area of the impression rather than the surface area.

MGU See motor generator unit.

MGU-heat (MGU-H) See motor generator unit.

MGU-kinetic (MGU-K) See motor generator unit.

MHD See magnetohydrodynamics.

Michell turbine See Banki turbine.

Michell–Kingsbury bearing A thrust bearing containing a number of sector-shaped pads, arranged in a circle around a rotating shaft. They are free to pivot, thus creating wedge-shaped films of oil inside the bearing between the pads and a rotating disc.

micro 1. (μ) An SI unit prefix indicating a submultiplier of 10⁻⁶; thus micrometre (μm) is the unit of length equal to one millionth of a metre, formerly called the micron. 2. A prefix usually used to indicate a device or an object with dimensions in the range 1 μm to about 1 mm although now frequently used to indicate a much larger but still small-scale device, such as a micro-air vehicle or micro-CHP. See also macro; meso.

microactuator (MEMS actuator) An actuator with dimensions typically in the range 100 to 500 μm and generating a force on the order of tens of μN. Basic principles include constrained bent beams that move due to Joule heating and forces generated electrostatically.

micro-air vehicle (MAV) An unmanned aircraft that may be fixed, flapping, or rotary wing, typically with a scale in the range 50 mm to 200 mm.

microballoon A pneumatic actuator used for turbulence control, in which a silicone-rubber membrane about 120 μm thick is attached to an etched silicon wafer.

microbubbles Bubbles of air or oxygen less than 0.5 μm in diameter. Applications include surface cleaning and drag reduction.

microcalorimeter 1. An X-ray detector used in spacecraft and consisting of an X-ray absorber, a thermistor, and a heat sink. 2. A calorimeter, fabricated on a silicon wafer and incorporating a heater and a thermometer, used to study the thermal properties of small (typically a few μg) samples.

microcasting See soft lithography.

microchannel (microfluidic channel) A channel etched in glass or silicon having depth and width typically in the range 1 to 500 μm. Typical applications include microfluidics, micro heat exchangers, and microreactors.

microchannel reactor See microreactor.

micro-CHP A combined heat and power system, on the scale of a family house or small office building, based upon the use of a Stirling engine.

microcombustor (microburner) A combustor micromachined from silicon, designed to burn a gaseous fuel such as hydrogen and supply hot gas to a microturbine. Such devices are about 10 mm in diameter and generate power on the order of 100 W.

microconstituent In materials science, an element of a material’s microstructure with clearly identifiable appearance when seen under an optical microscope. Microconstituents may be either phases or mixtures of phases. For example, the microconstituent pearlite in steels is a eutectoid with fixed proportions of iron (ferrite) and Fe₃C (cementite) combined in a characteristic pattern. A microstructure diagram is a phase diagram labelled in terms of microconstituents rather than phases. In the case of the silver-copper diagram shown under the entry for phase diagram, the central (α+β) region would be labelled (α+ε) to the left of the 71.9% Ag eutectic composition and (ε+β) to the right, where ε means eutectic. The components of an alloy system are the pure elements or compounds from which the alloy is formed, e.g. iron and Fe₃C, and copper and silver in the two examples above. See also steel microstructures.

microcooler 1. A solid-state device, typically using the Peltier effect, for cooling semi-conductor chips. 2. A MEMS device for cooling electronics, typically using a micro-capillary pumped loop incorporating an evaporator and a condenser.

microdispenser A device used to distribute liquid volumes in the range 1 μL to 1 mL.

micro-electromechanical systems (MEMS, microsystems) Small-scale devices manufactured from silicon, polymer, or glass, including microchannels, micro heat exchangers, microreactors, and micropumps.

microengine (micro gas turbine, microjet) A miniature gas-turbine engine based upon a microcombustor. Applications include micro-air vehicles, experimental and model aircraft.

microexchanger See micro heat exchanger.

microfabrication The fabrication of components from silicon, elastomers, and plastics on scales from fractions of 1 μm to 1 mm. Techniques include photolithography, etching, deposition, moulding, casting, injection, and laser ablation.

microfilter A filter that uses a microporous membrane with pore sizes in the range 0.1 to 10 μm to remove contaminants from a fluid. Applications include drinking-water treatment.

microfluidic channel See microchannel.

microfluidic mixer See micromixer.

microfluidics The study of flow in artificial microsystems, including miniaturized fluidic devices. See also microchannel.

microfuel cell A fuel cell with dimensions typically a few mm, used for low-power electronic devices. Methanol and hydrogen are commonly used fuels.

micro gas turbine See microengine.

microgeneration See micropower generation.

micrograph An image taken through a microscope or similar optical or electronic device to show a magnified image, for example of metallurgical microstructure.

microgravity (micro-g environment, μg) (Unit μm/s²) An environment in which the effective acceleration due to gravity has been reduced to a negligible fraction (typically one-millionth) of its normal value on Earth (9.81 m/s²).

microgripper A device designed to handle objects ranging from 1 to 100 μm in size.

microhardness (Unit kgf/m² or Pa) The hardness of a material determined under light loads to give shallow, small-volume indentations, thus enabling the hardness of microconstituents in alloys, thin layers, and brittle materials to be measured. Microhardness machines employ Berkovich, Knoop, or Vickers indenters and usually incorporate a microscope.

micro heat engine A heat engine in which thermocapillary pumping within a microchannel is used to convert heat to power.

micro heat exchanger (microexchanger) A heat exchanger with fluid flowing through microchannels about 100 μm deep, used, for example, to cool electronic components and also in microreactor systems.

micro heat pipe A heat pipe with sufficiently small dimensions (typically < 1mm) that capillary action rather than a wick can be used to return the condensate to the evaporator.

micro-hydroelectricity (micro-hydro power) An imprecise term usually taken to mean a hydroelectric power plant generating less than 100 kW.

microjet See microengine; microsynthetic jet.

microlithography See photolithography.

micromachining A process used to produce components for microsystems with critical dimensions on the order of a few μm. In bulk micromachining, a silicon wafer is selectively etched to produce microstructures. In surface micromachining the microstructures are built up by etching of several layers deposited on the substrate. Owing to the smoothness of the surfaces, large adhesion forces (resulting from capillary action, hydrogen bridging, electrostatic forces, and van der Waals forces) arise between fabricated structures and the substrate, and can be greater than elastic recovery forces. This leads to stiction in microdevices. In wet etching, capillary forces occur after rinsing but special drying processes can minimize friction.

micromanipulator A machine used to position an end effector within the view of a microscope, typically to make contact with a semiconductor device during manufacture.

micromanometer See Betz manometer.

micromechanism Any of various crystallographic or microstructural mechanisms, such as dislocation movement, void-initiation, growth, and coalescence, by which plastic flow, fracture, etc. occur.

micrometer (micrometer gauge) A mechanical-contact device for the accurate measurement of the length, width, diameter, etc. of an object, the depth of a hole, the height of a step, etc. The usual arrangement is a spindle that is moved by rotation of a thimble, the distance then being read off a vernier scale.

micromixer (microfluidic mixer) A microchannel-scale mixing device in which mixing occurs either due to molecular diffusion and chaotic advection (passive micromixers) or to externally-imposed effects such as pressure variation, thermal, acoustics, electrohydrodynamics, and MHD (active micromixers).

micromotor A device, on the scale of micrometres, immersed in a liquid and containing material which reacts with the chemicals in the liquid to produce jets of gas bubbles which cause it to move. Possible applications include the delivery of drugs to parts of the human body. See also nanomotor.

micromoulding High-precision moulding of miniature plastic components, typically with a mass of a few μg.

micron The former, non-SI, name for 1 micrometre (μm). See also micro.

micronozzle A nozzle with a diameter in the range of a few μm to 1mm machined in a thin metal sheet. Applications include ink-jet printing and micropropulsion.

micron rating The ability of a fluid filter to remove particles of a given size. See also beta ratio.

microplasticity Plasticity in the microstrain range (strain ≈10⁻⁶–10⁻⁵) that occurs below the yield point of a crystalline material. At the 0.2% proof stress commonly used to define the yield strength of materials, the offset plastic strain of 0.002 results from the movement of many dislocations over distances comparable with the grain size, i.e. ≫1 μm. Microplasticity occurs due to reversible movement of a few dislocations over distances <1 μm. In polycrystalline material, only grains most favourably oriented for dislocation glide contribute to microplasticity. See also dislocation; proof strength.

microporous material (nanoporous material) Porous material with pore diameters less than 2 nm. Pore sizes for micro-, meso- (2–50 nm) and macroporous (greater than 50 nm) materials follow IUPAC guidelines.

micropower generation (microgeneration) The stand-alone, small-scale generation of low-carbon heat and/or electricity typically using solar photovoltaics, solar thermal water heating, micro CHP, wind turbines, fuel cells, micro-hydro systems, biomass boilers, air-, ground-and water-source heat pumps, and passive flue-gas recovery devices.

micropropulsion Small-scale propulsion, typically for use in outer space (e.g. for satellite positioning), using pulsed-plasma electric propulsion devices.

micropump A pump with internal dimensions in the μm range, typically powered by piezoelectric actuators and incorporating passive check valves. Applications include cooling of electronic components, ink-jet printing, and drug delivery.

microreactor (microchannel rector, microstructured reactor) A device in which flow processes involving chemical reactions take place in microchannels. Especially suited to strongly exothermic and dangerous reactions.

microsensor A miniature sensor, typically using capacitive and piezoresistive effects and manufactured by wet or dry etching of silicon.

microstereolithography A rapid-prototyping process in which an object is formed through the addition of successive layers of a liquid photopolymer that solidifies when exposed to ultraviolet light. See also additive-layer manufacturing.

microstrain The measure of strain obtained from a strain gauge and equal to 10⁶ × actual strain.

microstructure See microconstituent.

microstructured reactor See microreactor.

microsynthetic jet (microjet) A device consisting of a cavity, typically 15 μm deep and 1 to 5 mm long, with a flexible actuating membrane on one side and an orifice, typically 50 to 800 μm wide, on the other, usually machined into a silicon wafer. Movement of the membrane towards the orifice pumps fluid out of the orifice to create a jet. Movement away from the orifice draws fluid into the cavity.

microsystems See micro-electromechanical systems.

microtechnology The technology involved in the design and manufacture of microsystems. See also microfabrication.

microthruster See MEMS thruster.

microturbine See microengine.

microvalve A MEMS device used to regulate or seal fluid flow in a microfluidic system. Actuation effects include electromagnetic, electrostatic, piezoelectric, bimetallic, thermopneumatic, and shape memory.

mig welding See welding.

mild steel See steel microstructures.

mile See nautical mile; statute mile.

milli (m) An SI unit prefix indicating a submultiplier of 10⁻³; thus millimetre (mm) is a unit of length equal to one thousandth of a metre.

millimetre of mercury A non-SI unit of pressure equal to the pressure exerted by a vertical column of mercury 1 mm high. Approximately equal to 133.3 Pa or 1 torr.

millimetre of water A non-SI unit of pressure equal to the pressure exerted by a vertical column of water 1 mm high. Approximately equal to 9.81 Pa.

milling A machining process, typically for metals and plastics, in which a multi-tooth rotary cutter removes material to produce flat or profiled surfaces, slots, grooves, etc.

mimic panel A control panel which shows diagrams (mimics) of each part of the plant being controlled, with sensor readings and actuator positions displayed alongside each diagram.

MIMO system See multiple input, multiple output system.

Miner’s rule See fatigue.

mini-hydroelectricity (mini-hydro power) An imprecise term, usually taken to mean a hydroelectric power plant generating less than 5MW.

minimal realization The smallest number of states of a system that properly represent the transfer function and allow both observability and controllability.

minimum fluidization velocity See fluidization.

minimum material condition (minimum metal condition) The situation where the volume of a manufactured component corresponds to the lower limit of all toleranced external dimensions and to the upper limit for all internal dimensions. See also maximum material condition.

minimum-phase system A linear system where the poles and zeros have zero or negative real parts. Such systems are thus stable due to the zero or negative real parts of the poles.

minimum potential energy (theorem of minimum potential energy) (Unit JK) A rigid body or structure will locate itself so as to have minimum potential energy when in a position of equilibrium. In a gravitational field, a deformable body or structure will take up the shape and position giving minimum potential energy.

minimum quantity lubrication A term relating to machining with the least possible quantity of coolant/lubricant, as most traditional coolants are ecologically unfriendly, yet dry machining is not acceptable.

minor diameter See screw.

minor loop control The placing of an additional control loop around the plant so as to effectively modify the plant transfer function to simplify the design of the main control loop.

minute (min) A non-SI unit of time. The conversion to SI is 1 min = 60 s.

misfiring The behaviour of a piston engine when the fuel/air mixture in one or more cylinders fails to ignite. Causes include a non-combustible fuel:air ratio or, in a petrol engine, the absence of spark.

mist See spray nozzle.

mist eliminator See demister pad.

mist extractor See demister pad.

mist flow See boiling.

mistuning Small, random, blade-to-blade differences in gas-turbine rotors that lead to increased levels of blade vibration and stress.

Michell–Bánki turbine See Banki turbine.

mitre gears See toothed gearing.

mixed cycle See Sabathé cycle.

mixed-flow heat exchanger See shell-and-tube heat exchanger.

mixed-flow impeller A pump (mixed-flow pump), compressor, or turbine (mixed-flow turbine) impeller, which discharges the working fluid with both an axial and a radial component of velocity.

mixed-mode fracture Fracture resulting from a combination of tensile, in-plane or out-of-plane shear, or twisting loading, i.e. a combination of modes I, II, and III of fracture mechanics. See also mode of fracture.

mixed-phase flow See multiphase flow.

mixed-pressure turbine A steam turbine in which steam is admitted from two or more independent sources that may be at different pressures.

mixing The process by which two or more fluids become distributed within each other. Mixing performance is a term for how well mixed materials are after a period of time in a mixer. It is generally used in a qualitative sense, but can be quantified in terms of the statistical properties of the concentration distribution of each constituent. Complete mixing means that the mixture properties are uniform throughout.

mixing chamber A chamber within which mixing occurs, typically of fuel and oxidant prior to injection into a combustion chamber.

mixing layer The laminar or turbulent shear layer that develops when there is a tangential velocity difference between two miscible viscous fluids.

mixing length (l) (Unit m) The basis of an elementary turbulence model in which it is assumed that the turbulent shear stress τ_T can be calculated from the equation

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where ρ is the fluid density, ū is the mean velocity a distance y from a reference plane (usually a solid surface), and is the gradient of mean velocity with respect to y. See also van Driest damping function.

mixing valve A valve in which two or more miscible fluid streams, which may be different in temperature or composition, are mixed together to produce a fluid stream of desired temperature or composition.

mixtures See gas mixtures; solution.

mixture strength See stoichiometric mixture.

MKS The obsolete system of units based upon the metre, kilogram, and second that preceded the SI system.

MMC See metal-matrix composite.

mmscfm An abbreviation for million standard cubic feet per minute; a non-SI unit for volumetric gas flow rate.

mobile robot A robot where the base frame is mounted on a vehicle.

mobility See mechanical admittance.

modal analysis 1. The experimental study, using transducers such as accelerometers and load cells, of the dynamic response to excitation of structures, objects, systems, etc. 2. Methods of calculating the modal shapes and frequencies at which vibration occurs for given inputs.

mode (mode of oscillation, mode of vibration) A pattern of vibration of a component or structure corresponding to a natural frequency.

model-following problem The problem of making the behaviour of a real plant accurately match that of a model reference system; that is, the problem whose solution is attempted by model reference adaptive control.

modelling 1. The analysis, usually approximate and empirical, of complex physical phenomena, such as turbulence or viscoelasticity, based upon simpler systems or processes for which the behaviour is well understood. See also Kelvin–Voigt material; Maxwell model. 2. Sometimes used to mean the process of simulation of results in FEM.

model reduction The process of simplifying the mathematical model of a real system so as to give a model reference system.

model reference system A mathematical model of an idealized plant which shows desirable behaviour. For example, a real robot may experience vibration when a joint is moved, while the model reference system for the joint would show the same basic dynamics without the vibration. A model reference adaptive-control system would then attempt to make the behaviour of the real system (that is, the robot) match that of the model and thus avoid the vibration.

model testing The use of either small- or large-scale models to determine the behaviour of geometrically-similar full-scale structures and systems using the principles of dimensional analysis and physical similarity.

mode of failure The various ways in which a material can fail when subjected to loading, surface damage, chemical attack, heating, cooling, or radiation, in either isolation or combination. The failure modes include fracture, fatigue, creep, corrosion, erosion, and embrittlement.

mode of fracture The different types of crack propagation, viz: tensile opening (mode I); in-plane shear (mode II); out-of-plane shear (twisting, mode III). See also fracture mechanics.

mode of oscillation See mode.

mode of vibration See mode.

moderator A medium used in a nuclear reactor to slow fast neutrons so that a chain reaction with uranium²³⁵ is possible. Water, heavy water, and graphite are the most commonly used moderators. See also nuclear fission.

modern control A control method that represents system behaviour by states and the resulting low-order time-domain differential equations, rather than through Laplace transforms. The controller is then designed to achieve optimal control.

modification A change in the design of a component or machine, to correct a fault or deficiency, improve performance, or simplify manufacture.

modified Goodman diagram See fatigue.

modulation The systematic time variation of the amplitude, frequency, or phase of a periodic signal.

module See toothed gearing.

modulus of compression See bulk modulus.

modulus of decay (Δ) (Unit s) The time taken for the amplitude of a damped oscillation to decay to 1/e of its initial value, where e is Euler’s number, 2.718 281 83….

modulus of elasticity (Unit Pa) A term that usually refers to Young’s modulus of an isotropic solid, although there are also the shear modulus and the bulk modulus.

modulus of elasticity in shear See shear modulus.

modulus of resilience (u_R) (Unit J/m³ or Pa) The strain-energy density in a material stressed to the proportional limit σ_PL and given by where E is Young’s modulus.

modulus of rigidity See shear modulus.

modulus of rupture (flexural strength, fracture strength in bending) (Unit Pa) A measure of the breaking strength of a brittle material, typically determined from a three-point bending test in which the test specimen is loaded to fracture. It is not an absolute quantity, as it depends upon fracture toughness and flaw size.

modulus of torsion See shear modulus.

Mohr–Coulomb fracture criterion A fracture criterion, primarily for brittle materials, according to which failure occurs when the stress at a point in a material falls outside the envelope created by the Mohr’s circles for uniaxial tensile strength and uniaxial compressive strength.

Mohr–Coulomb yield criterion A pressure-dependent yield criterion, according to which yielding occurs when the stress at a point in a material falls on the envelope created by the Mohr’s stress circles at yielding for various tests such as tension, shear, and compression that have different components of hydrostatic stress.

Mohr’s strain circle For a body strained along orthogonal x,y axes by normal strains ε_x and ε_y, and shear strains γ_xy, the normal strain ε_θ and shear strain γ_θ on a plane inclined at angle θ to the x-axis are given by and

In terms of principal strains ε₁ and ε₂,

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and . These relations plot as a circle (Mohr’s strain circle) on axes of ε,(γ/2) using the tensor definition of shear strain, i.e. one-half of the engineering shear strain γ. When used for plastic strain increments, simultaneously with the corresponding stress circle, the origin of the strain circle coincides with the value of the hydrostatic stress.

Mohr’s stress circle (Mohr’s circle) For a body loaded along orthogonal x,y axes by normal stresses σ_x and σ_y, and shear stresses τ_xy, the normal stress σ_θ and shear stress τ_θ on a plane inclined at angle θ to the x-axis are given by

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and

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Elimination of θ gives

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that, on σ,τ axes, is the equation of a circle having radius

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centred on the σ-axis at , called Mohr’s stress circle. In terms of principal stresses σ₁ and σ₂, and , which plots as a circle centred at (σ₁ + σ₂)/2,0 having radius (σ₁ − σ₂)/2. For loading in three dimensions, there are three Mohr’s circles having diameters (σ₁ − σ₂), (σ₂ − σ₃) and (σ₁ − σ₃) where σ₁, σ₂, and σ₃ are the principal stresses.

Mohr’s stress circle

Mohr’s theorem See reciprocal theorem.

Mohs scale A scale of scratch hardness originally developed for minerals. See also hardness test.

moisture content See specific humidity.

moisture separator See steam separator.

moisture separator reheater See reheater.

mol See mole.

molar analysis The number of moles of each component in a mixture of substances. See also gravimetric analysis; mole fraction.

molar gas constant See universal gas constant.

molar heat capacity (molecular heat capacity, C̃) (Unit J/kmol.K or J/kmol.°C) The energy required to raise the temperature of one kmol of a substance by one degree kelvin without change of phase. For practical purposes, a liquid or a solid is generally regarded as having a single specific heat, whereas for a gas the energy required depends upon how the energy-transfer process is executed and two values are defined: the molar heat capacity at constant volume (C̃_V) and the molar heat capacity at constant pressure (C̃_P). The thermodynamic definitions are and where the subscript V denotes a reversible non-flow process at constant volume and P denotes a reversible non-flow process at constant pressure, ũ is the molar internal energy, h̃ is the molar enthalpy, and T is the absolute temperature. Molar heat capacity is an intensive thermodynamic property dependent on temperature and pressure. See also specific heat.

molar property A form of intensive property obtained by dividing the extensive value of a property by the number of kilomoles present. Molar mass (𝓜) with unit kg/kmol, is the mass in kg of one kmol of any substance. Molar volume, with unit m³/kmol, is the corresponding volume, at a given temperature and pressure, and equivalent to the molar mass divided by the substance density. See also atom.

mole (mol) The base unit of the amount of substance in the SI system. It was previously defined as the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kg of carbon-12. From 20 May 2019 it has been defined as the amount of substance containing exactly 6.022 140 76 × 10²³ elementary entities. This number is the fixed numerical value of the Avogadro constant N_A when expressed in the unit mol⁻¹. See also atom; Avogadro constant.

molecular drag pump (molecular pump) A high-vacuum (down to about 10⁻⁴ Pa) pump in which momentum is transferred to the gas molecules by a rapidly rotating solid surface. See also turbo/drag pump; turbomolecular pump.

molecular gauge See friction vacuum gauge.

molecular heat capacity See molar heat capacity.

molecular motor See nanomotor.

molecular weight See atom.

mole fraction In a mixture of substances, the ratio of the mole number of an individual component to the mole number of the mixture. See also molar analysis.

mole number (N) The number of moles in a given mass m of a substance, equal to m divided by the molar mass 𝓜 of the substance.

Mollier diagram (Mollier chart, enthalpy-entropy chart) A plot of specific enthalpy vs specific entropy, for a substance such as steam, on which are shown isobars, isotherms, lines of constant dryness fraction or quality, and sometimes also density. See also temperature–entropy diagram.

Moll thermopile A heat-flux sensor used to measure thermal radiation (see heat transfer). It consists of a blackened disc to which are connected thermocouples connected in series with a reference junction maintained at constant temperature. See also thermopile.

molten The state of a substance, such as a metal, which is normally solid at STP, when caused to become liquid (usually by heating). See also frozen.

moment diagram See bending-moment diagram.

moment of force (moment (M) (Unit N.m) The tendency of a force to rotate an object to which it is applied. If the force vector is F and the displacement vector is r, r being the length of the lever arm from the putative axis or point of rotation to the point at which the vector is applied, M = r × F. If the magnitude of the force vector is F and the angle between F and r is θ, then the magnitude of M is M = rFsinθ. See also bending moment; couple; torque.

moment of inertia (mass moment of inertia, I) (Unit kg.m²) For a particle of mass m rotating about an axis, I = mr² where r is the perpendicular distance of the particle from the axis. For rigid a body regarded as a system of particles rotating about an axis, where particle i has mass m_i and is a distance r_i from the axis. More generally for a rigid body

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Similarly for rigid bodies rotating about a point. See also second moment of area.

moment of momentum See angular momentum.

moment sensor A sensor used in robotics that allows the determination of the load carried by the end effector from the moment produced in a link of the robot.

momentum (linear momentum, M) (Unit kg.m/s) For a solid body of mass m moving with vector velocity V, M = mV. It is a vector quantity which satisfies the linear-momentum conservation equation. In a fluid flow, the rate of flow of momentum per unit area (momentum flux), with unit kg/ms², is ρV² where the fluid density is ρ. The rate of flow of momentum across a given area A in a given direction (momentum flow rate, Ṁ), with unit kg.m/s², is then

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momentum integral equation (integral momentum equation, Kármán integral equation, von Kármán momentum integral equation) An equation that relates the wall shear stress for a boundary layer to the variation of integrals of the mass and momentum flow within the boundary layer, the pressure gradient, and the surface mass-transfer rate ṁ″. It is conveniently written in terms of the skin-friction coefficient c_f and the displacement and momentum thicknesses, δ* and θ as

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where ρ is the fluid density, U_∞ is the free-stream velocity, t is time, and x is the distance along the surface.

momentum source A concept in fluid mechanics in which there is a flow of momentum initially in the absence of mass flow. See also entrainment; jet.

momentum theorem (impulse theorem) A form of Newton’s second law of motion as used in fluid mechanics, which can be stated as follows: the sum of all the forces F acting on a fixed control volume 𝓥 is equal to the time rate of change of momentum M contained in the volume plus the net flow rate of momentum through its surface . As an equation, the momentum theorem can be written as

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where t is time, ρ is the fluid density, V is the vector velocity, n is the outward-pointing unit vector normal to d𝓢, and

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See also rocket equation.

momentum thickness (momentum-deficit thickness, θ) (Unit m) A thickness that arises in the analysis of a boundary layer based upon the momentum-integral equation. It is related to wall shear stress. From conservation of momentum within the boundary layer,

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where u(y) is the distribution of streamwise velocity within the boundary layer, y is the distance from the surface, and U_∞ is the free-stream velocity. See also displacement thickness.

momentum thickness

momentum thrust (Unit N) In a jet or rocket engine, if the exhaust mass flow rate is ṁ and the exhaust-gas velocity is V, the product ṁV is termed the momentum thrust.

momentum transfer The transfer of momentum from one body to another upon impact or from one fluid stream to another upon mixing. In both instances, momentum is conserved. See also conservation equations.

MON See octane number.

monatomic gas A gas for which each molecule consists of a single atom. Examples are helium, neon, argon, krypton, xenon, and radon. The specific-heat ratio γ for most monatomic gases at room temperature is 5/3. See also diatomic gas.

monitor To observe, and often record, the output of one or more instruments measuring process or system variables, structural integrity, etc., over a period of time.

monobloc A single-piece piston-engine block incorporating all cylinders and possibly also the crankcase.

monochromatic absorptance (monochromatic absorptivity, α_λ) The ratio of the absorptance of a surface at a given wavelength and temperature to the absorptance of a blackbody at the same wavelength and temperature. For emittance, the corresponding quantity is the monochromatic emittance (monochromatic emissivity, ε_λ).

monochromatic absorption coefficient (α_λ) A measure of the decrease in thermal-radiation intensity at a given wavelength λ due to absorption by a gas. According to Beer’s law, where is the intensity after absorption by a gas layer of thickness x and is the initial intensity.

monocoque structure A structure such as an aeroplane fuselage consisting of a shell that is torsionally very stiff. A unitized motor-vehicle body has a monocoque structure.

MONO pump See progressive-cavity pump.

Moody chart (Moody diagram) A set of curves of Darcy friction factor vs Reynolds number for fully-developed turbulent flow in rough pipes, based upon Colebrook’s equation.

Moody friction factor See Darcy friction factor.

morse taper A standardized series of tapers employed on the shanks of large-size drills, etc. to mate with internal tapers on machine tool spindles.

Morton number (Mo) A non-dimensional parameter that arises in the study of bubbles and drops rising or falling in another fluid and is defined by where Δρ is the density difference and σ is the surface tension at the interface between the two fluids, g is the acceleration due to gravity, ρ is the density of the continuous phase, and μ is its dynamic viscosity. See also Bond number; Eötvös number.

motif unit See crystal structure.

motion Any progressive change in the position of a solid object, a solid particle, or a fluid particle.

motive fluid A high-pressure fluid used to produce flow of another fluid, as in an ejector.

motive power That which imparts motion to a machine, for example steam, a head of water, or wind.

motor See engine; prime mover.

motor generator unit (MGU) A device that is used as both an electric motor and an electrical generator. When attached to a turbocharged internal-combustion engine, an MGU can generate electrical power from the turbocharger exhaust turbine (MGU-heat) and provide additional drive to the turbocharger compressor under acceleration to increase intake pressure more rapidly, optimizing throttle response and initial power delivery, or be connected directly to the engine’s crankshaft (MGU-kinetic), either to recover power under retardation or provide additional power (from the battery or the MGU-heat) under acceleration. See also energy recovery system.

motor octane number (MON) See octane number.

mould A hollow form filled with liquid, pasty, soft, or molten material that is allowed to harden and adopt the shape of the mould. A major application is to the process of casting.

moulding pressure (Unit Pa) The pressure required to force material into a mould, particularly highly viscous material.

moulding shrinkage (Unit m) The reduction in the dimensions of a moulded component as it hardens.