SAE Numbers	Minimum	Maximum
10	90	less than 120
20	120	less than 185
30	185	less than 255

Viscosity Problems

If the viscosity of the hydraulic fluid is too high (fluid does not flow as freely as desired), the following undesirable actions may result:

• Internal resistance, or fluid friction, is high, which means a high resistance to flow through the valves and pumps. • Power consumption is high, because fluid friction is high.

• Fluid temperature is high, because friction is high.

• Pressure drop through the system may be higher than desired, which means that less useful pressure is available for doing useful work.

• The motion and operation of the various parts may be slow and sluggish. This is a result of the high fluid resistance.

If the viscosity of the hydraulic fluid is too low (fluid flows more freely than desired), the following undesirable actions may result:

• More leakage may occur in the clearance space than is desired.

• A lower pressure than is desired may occur in the system.

• An increase in wear may occur because of the lack of a strong fluid film between mechanical parts that move in relation to each other.

• Pump leakage may increase, resulting in reduced pump delivery and efficiency.

• A loss of control may occur because fluid film strength is reduced.

With respect to Saybolt readings, the viscosimeter readings of oils in service should not exceed 4000 seconds, and they should not read less than 45 seconds.

Viscosity Index

Ideally, the dynamic viscosity of any oil should change only slightly as the temperature changes. In the automobile engine, the oil in the crankcase is operated over a wide range of temperatures. On a very cold winter morning, until the car has been operated for some length of time, the temperature of the oil may be very low, and the dynamic viscosity of the oil may be very high. If the dynamic viscosity of the oil is excessively high, large forces and large amounts of power may be required to shear the oil films. Also, after the engine has been operated for a period of time on a hot summer day, the temperature of the oil may be very high, and the dynamic viscosity of the oil may be too low. Therefore, the oil may not form a suitable lubricating film between the sliding surfaces. A breakdown of the oil film may result in excessive wear of the metal surfaces and a loss of power in the engine.

The term viscosity index is an arbitrarily defined ratio. It indicates the relative change in Saybolt Universal reading, with respect to temperature. The most desirable oils are those that have a high viscosity index (that is, the change in Saybolt reading is relatively small as the temperature changes). Oils having a small viscosity index register a relatively large change in Saybolt reading as the temperature changes.

Lubricating Value

The terms oiliness and lubricity are used to refer to the lubricating value of any oil. These terms are most often used when the moving surfaces are relatively close and may make metal-to-metal contact. At the same pressure and temperature, oil A may be a better lubricant than another oil B . Therefore, oil A possesses more oiliness or lubricity than oil B. The lubricating value of a fluid depends on its chemical structure and its reaction with various metal surfaces when the metal surfaces are relatively close to each other. Thus, oiliness and lubricity are extremely important in the performance of the oil.

Pour Point

The pour point of a fluid is defined as the lowest temperature at which the fluid flows when it is chilled under given conditions. The pour point is important when the hydraulic system is exposed to low temperatures. As a general rule, the most desirable pour point should be approximately 20°F below the lowest temperature to that the fluid will be exposed.

Oxidation and Contamination

Oxidation is a chemical reaction in which oxygen combines with another element. Because air contains oxygen, the oxygen that is involved in fluid oxidation comes from exposing or mixing the fluid with air. The oxidation reaction increases with increased exposure of the oil to the air.

Undesirable quantities of air in hydraulic systems can be caused by mechanical factors, such as air leakage into the oil suction line, low fluid level in the oil reservoir, and leakage around the packing. Air leakage may result in the erratic motion of mechanical parts, and it may cause the fluid to oxidize more rapidly. All oils contain some air in solution that may not cause any trouble. If the air is not in solution, a foaming action may result. If trapped in a cylinder, air that is not in solution is highly compressible. However, oil is not as highly compressible as air. Irregular action of a cylinder, for example, may result if a significant quantity of air becomes undissolved.

Ferrous metals are destroyed by rust. Rust can develop in a hydraulic system if moisture is present. This moisture may be the result of condensation from air that enters through leaks on the intake (low pressure) side of a pump.

The oxidation stability of any oil refers to the inherent ability of oil to resist oxidation. Oxidation increases with increases in temperature, pressure, and agitation. Oxidation also increases as the oil becomes contaminated with such substances as grease, dirt, moisture, paint, and joint compound. Various metals also promote oil oxidation, and the various fluids have different oxidation characteristics. Table 13-2 lists the essential properties of the commercially available hydraulic fluids.

Table 13-2 Properties of Available Hydraulic Fluids

Petroleum-Base Fluids
Viscosity range, Saybolt Universal reading, in seconds, at 100°F	40 to 5000
Operating temperature, in °F	-75 to 500
Minimum viscosity index	76 to 225
Fire-Resistant Fluids (water-oil emulsions, water glycol, phosphate-ester, chlorinated hydrocarbon, sili cate ester, silicon)
Viscosity range, Saybolt Universal reading, in seconds, at 100°F	20 to 5000
Operating temperature, in °F	-100 to 600

Hydraulic Filters

Hydraulic filters are needed to aid in eliminating many of the potential causes of hydraulic system failures. Proper filters and proper filter maintenance are important in obtaining satisfactory results in a hydraulic system.

Although most hydraulic systems are considered closed, they are not free from contaminants. Following are four sources of contaminants common in hydraulic systems:

• Wear—As the sliding members of components move, small particles of metal and seals enter the fluid. A typical example of this action is the movement of a cast-iron piston within a steel cylinder tube. Wear begins as soon as the cylinder is placed in operation, although it may not be visible to the eye for a long period.

• Formation of sludge and acids due to fluid breakdown—When extreme heat and pressure are encountered, chemical reaction within the fluid causes the formation of sludge and acids that are harmful to the precision parts of the components. For example, resinous coatings may cause a valve spool to freeze within the valve body by forming on moving parts, or small orifices may become clogged. Acids cause pitting and corrosive conditions.

• Built-in contaminants in the manufacture of components—In castings with intricate cored passages, core sand is difficult to remove, and small quantities of sand can enter the system as the fluid flows through the cored passages under high pressure. Lint and small metal chips are also encountered.

• Contaminants from outside the system—Lint may enter the system if the filter cap on the oil reservoir is not replaced. Dirt that clings to the piston rod of a cylinder or to the stem of a valve may enter the system. Water or coolant may enter a system.

Factors that should be considered in selecting a hydraulic filter are flow rate, pressure drop, degree of filtration, capacity, ease of servicing, compatibility with the fluid in the system, and pressure to that the filter is subjected. The filter in a hydraulic system can be located in a number of places, including the following:

• In the sump or oil reservoir—This is a sump-type filter (see Figure 13-3). The filter should have a capacity twice that of the hydraulic pump in keep pressure drop at a minimum and to eliminate the possibility of cavitation in the pump.

Figure 13-3 A sump-type filter equipped with magnetic rods for collecting minute particles of steel and iron. (Courtesy Mar vel Engineering Co.)

• In the discharge line from the relief valve—Since, in most systems, a considerable quantity of fluid passes through the discharge of the relief valve, a low-pressure filter with fine filtration is recommended. The filter should be equipped with a low-pressure bypass valve to avoid filter or system failure. The capacity of the filter should be large enough to handle the full flow of the pump without imposing a backpressure on the relief valve. Backpressure on the exhaust of the relief valve can cause malfunctions with the system.

• In the bypass line from the pump—In bypass line filtration, a small percentage (approximately 10 percent) of the flow from the pump passes through the bypass filter, and returns to the reservoir as clean oil. A pressure-compensated flow control should be installed between the pump and the filter to maintain constant flow at minimum pressure through the filter. An internal bypass valve in the filter is recommended to avoid filter failure if the filter becomes clogged completely.

• In the pressure line between the pump and the directional control valve—To protect the components of the system that are located beyond the pump, a high-pressure filter that can handle full pump pressure and flow is often employed. Although a 25-micron filtration is an often-used standard in industry, a 5-micron, or less, filter is sometimes desirable if close-fitting parts are to be protected. The flow capacity through the filter should be as high as possible (four to five times pump capacity). A built-in bypass relief valve should be incorporated within the filter, because the filter may become overloaded with contaminants. Figure 13-4 shows a high-pressure filter equipped with a warning switch that provides a signal when the filter requires cleaning.

Figure 13-4 A high-pressure filter equipped with a warning switch that provides a signal when the filter requires cleaning. (Courtesy Marvel Engineering Co.)

• In the intake line between the sump and the pump—The intake line filter is similar to the sump-type filter, except that it is encased and is mounted outside the reservoir. A 100-mesh filter is commonly used for hydraulic oils, and a 60-mesh filter is usually specified for aqueous-base fluids. The filter elements can be replaced without disturbing the piping (see Figure 13-5).

• In the exhaust line between the four-way directional control valve and the reservoir—The exhaust line filter is helpful when the bulk of the fluid is returned to the reservoir through the four-way directional control valve. However, if most of the fluid is returned through the relief valve or bypass valve, this type of filter is of little value. When the return-line filter is used, it should have more capacity than the maximum flow of the return line, to reduce backpressure to a minimum on the exhaust of the control valve. Sudden surges and shock can have a detrimental effect on the element in this type of filter.

Figure 13-5 Intake-line filter placed between the sump and the pump. (Courtesy Marvel Engineering Co.)

Mobile-type Hydraulic Filter Units

Figure 13-6 shows a mobile-type filter unit. Such a unit has many uses in industry, including the following:

• Cleaning old hydraulic systems.

• Filtering make-up oil (ideal for adding oil to servo-controlled systems and other systems where clean oil is absolutely essential).

• Cleaning new, in-plant systems before startup and systems on OEM machines and equipment before shipment to customer.

• Filtering out hydraulic system contamination resulting from failure of a component in the system.

• Recirculating and filtering power unit oil without having to shut down the power unit.

The unit in Figure 13-6 is equipped with a gear-type hydraulic pump driven by an electric motor; a 10-micron nominal paper element final filter; two hoses, a suction hose and a pressure hose; a utility box for storing spare element and small tools; and a drip tray that catches oil spills from filters and hoses. Figure 13-7 is similar to Figure 13-6, except that the hydraulic pump is driven by an air motor that receives its air through a filter, regulator, and lubricator unit. This unit is also equipped with a stored barrel caddy. In operation, the barrel caddy wheels rest on the floor and the oil drum is placed on the caddy.

Figure 13-6 Mobile-type hydraulic filter unit. (Courtesy Marvel Engineering Co.)