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Figure 7.18 Two-pump circuit with unloading valve. C. Unloading Valves In some circuits, two pumps may be used to meet substantially variable flow requirements. In the case, shown in Figure 7.18, flow from the larger volume pump is used to ensure the rapid advance of the machine tool to a certain point but then, as the work activity is performed, the volume of the flow decreases markedly. During the rapid advance cycle, the discharge volume of both pumps is required but while actual work is being performed, only the small pump volume is required, causing a rise in system pressure. As the pilot pressure rises (pilot pressure set below relief valve pressure setting), the unloading valve opens, allowing the flow volume from the larger pump to be discharged to the reservoir at low pressure. If one constant volume pump were used, most of the oil pumped during the work cycle would be discharged at full system pressure through the relief valve. Its energy would be wasted and excessive heating could occur. Use of a variable volume pump could be an alternative to the method featuring two pumps and an unloading valve. D. Sequence Control Valve In some machines, two or more movements may need to be hydraulically operated in sequence. When one movement must not begin until another has ended, a pressure-operated sequence valve may be used. Referring to the circuit shown in Figure 7.19, when oil flow stops at the end of the clamping-cylinder stroke, pressure rises sufficiently to open valve A. Full line pressure is then available to activate the feed cylinder. Sequence valves can also be activated by pressure-sensing pilot valves or electronically, by position or other pressure-sensing devices. Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. Figure 7.21 Flow control valve: the orifice shown in the meter-in circuit in Figure 7.20 usually will not maintain constant speed throughout the work stroke. The workload will usually vary, causing the pressure drop across the orifice and flow to vary. To maintain constant pressure drop and flow, a pressure compensator such as shown may be used. the right. Since the two pressures act on equal areas, the pressure at A will be greater than the pressure at B by a constant amount determined by the spring tension. Any imbalance will cause the spool to move in a way that opens or throttles the inlet, at C, and restores the balance. With constant pressure drop across the orifice, the flow remains constant. F. Accumulators Although not control valves, accumulators can act as flow control mechanisms. As we briefly discussed in connection with Figure 7.3, accumulators can be used to store energy of an incompressible fluid. Some presses and other machines require large volumes of oil under pressure for short duration cycles with relatively long periods of time between cycles. A pump and motor large enough to generate the necessary flow and pressure for such an application could be very costly. Instead, accumulators (Figure 7.22), in conjunc- tion with small pumps and motors, can often be used as shown in Figure 7.23. Energy is stored in the accumulator by the pump during the long periods between high energy requirements. This is done by pumping hydraulic fluid into the accumulator and raising a weight, compressing a spring, or compressing a gas charge. The energy is returned to the system when required. In addition to energy storage capacity, accumulators serve other functions. Many hydraulic systems are subject to rapid or sudden flow changes where the dynamics of fluid (incompressible) in motion can create high levels of system shock. In these situations, accumulators act as shock absorbers. They are able to reduce the severity of the system shock by allowing the instantaneous pressure rises to be taken up by the compressible mechanisms within the accumulators. This helps reduce the potential for line breakage and component failures in those high flow, high pressure systems that are subject to abrupt changes in flow. They can also be used to smooth out flow by absorbing pump pulsations, maintain constant pressures for long periods of time, such as in clamping operations, and make up for system internal leakage. Most hydraulic applications use gas-pressurized accumulators. Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. IV. ACTUATORS A. Hydraulic Cylinders At the point of use, hydraulic energy must be converted to mechanical energy and motion. The device most frequently used is the hydraulic cylinder (Figure 7.24), sometimes called a reciprocating motor. Hydraulic cylinders may be single-acting (pressure applied in one direction) or double-acting. They are made in sizes ranging from less than an inch to several feet in diameter. Working strokes of up to several feet and pressure ratings of up to several thousand psi are available. Single-rod-end cylinders, as shown in Figure 7.24, are most common, but double-rod-end cylinders (rod extends through both heads, as shown earlier: Figure 7.9) can be obtained. By means of flanges, extended tie rods, and adapters, cylinders can be mounted in many positions. Cylinders are usually made of steel tubing, bored and honed. Rods are often hard chrome-plated steel, ground and polished. All sur- faces in contact with seals and gaskets are finely finished. Typical double-acting cylinders are illustrated. The left-hand view of Figure 7.24 features a cartridge-type rod gland that can be removed without dismantling the cylinder. Accessibility of the rod gland is impor- tant from the maintenance point of view. The piston is sealed by means of cast iron piston rings, which are very satisfactory where a small amount of leakage can be permitted. O- ring static seals, which are very popular for hydraulic service, are used. The right-hand view of Figure 7.24 is cushioned at both ends. As the piston nears the end of its stroke, the cushion plunger enters the head counterbore, shutting off oil flow. The stroke can then continue only as fast as oil can flow through passage A and past the cushioned valve. At the beginning of the next stroke, oil flows past the spring-loaded ball-type cushion check valve and builds up pressure against the full face of the piston. Unless other means are provided to decelerate the piston, cushioning becomes more and more necessary at higher piston speeds. The piston in this case is sealed by means of cup-type seals, which permit little or no leakage. V-ring packings are similar in this respect. Figure 7.24 Hydraulic cylinders. Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. In addition to single and double-acting cylinders described earlier, and the ram and double-rod cylinders also discussed earlier in this chapter, several other types are com- monly used in industry. These are spring return, telescoping, and tandem types. Spring return cylinders are components of the single-rod type with pressure to activate the work stroke, and the spring in the nonpressurized end returns the piston to its original position when pressure is released. Telescoping cylinders are used where long strokes are required but space limits the length of the cylinder. These applications also have lower load require- ments as the cylinder rod extends. These are made of multitubular rod segments fit into each other to provide the telescoping action. Tandem cylinders are used when high force is required and length is not limited but larger diameter cylinders cannot fit in the available space. These are mounted in-line with a common piston rod. B. Rotary Fluid Motors Rotary fluid motors are used instead of hydraulic cylinders to convert fluid energy to mechanical motion, especially when rotary motion is required or continuous or long move- ments in one direction are involved. They compete with electric motors under the following conditions: (1) when a variable speed transmission having a wide range of closely con- trolled speeds and torques is required, (2) when space limitations demand a very compact power source, or (3) when torques or loading might occasionally be severe enough to overload an electric motor. Rotary fluid motors can be of gear, vane, or piston (radial or axial) design. They are similar to their counterparts in pumps but differ in certain details that affect efficiency. In fact, some radial and axial piston pumps are designed to act as both a pump and a motor, as described in connection with Figure 7.11, but in these designs, they do lose some efficiencies. Rotary fluid motors are supplied with oil under pressure and rotate at a speed and torque dependent on the available volume of oil that flows through the motor. Radial and axial piston motors are usually of the constant displacement design but are available in variable displacement forms. In a constant displacement axial piston motor (Figure 7.25), the motor shaft is supported by ball bearings A and B. The cylinder barrel Figure 7.25 Axial piston fluid motor. Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. is supported by the motor shaft and is keyed to by means of a splined joint. The drive plate is supported at an angle by ball bearing C. Ports are arranged so that oil under pressure starts to enter a cylinder when its piston is at the end of its inward stroke. The pressure causes the piston to push against the drive plate. Part of this force is perpendicular to the drive plate and part is tangential. The tangential force causes the drive plate, cylinder barrel, pistons, and motor shaft to rotate. The torque developed depends on oil pressure and the motor dimensions. The speed depends on the rate of oil flow. Axial piston motors exhibit high volumetric efficiencies and excellent operation over a wide range of speeds. They can be used where torque requirements are up to more than 17,000 ft-lb or speeds up to 4000 rpm. Radial piston motors can attain several hundred thousand foot-pounds of torque or speeds up to 2000 rpm. Gear-type motors will provide up to 6000 ft-lb of torque or up to 3000 rpm. It is important to recognize that since the torque of hydraulic motors is inversely proportional to rotational speed, their highest torques will occur at low speeds. V. HYDRAULIC DRIVES Hydraulic drives are classified as hydrostatic or hydrodynamic. These can be designed to produce power in three ways: variable power and torque, constant power and variable torque, and variable power and constant torque. Hydrostatic drives use oil under pressure to transmit force while hydrodynamic drives use the effects of high velocity fluid to transmit force. Engine-driven transmissions widely used in main and auxiliary drives of mobile construction and farming equipment is an example of hydrostatic drives. Torque converters (sometimes referred to as hydrokinetic drives) are commonly found in automo- tive applications but are finding increased use in industrial applications. Another form of a hydrokinetic drive is the hydroviscous drive. This form uses the viscosity characteristics of the fluid rather than the energy from fluid in motion to develop the drive torque. Hydrodynamic drives include hydrokinetic and hydroviscous drives. A. Hydrostatic Drives In a hydrostatic system, power from an electric motor, internal combustion engine, or other form of prime mover is converted into static fluid pressure by the hydraulic pump. This static pressure acts on the hydraulic motor to produce mechanical power output. While the fluid actually moves through a closed-loop circuit between the pump and motor, energy is transferred primarily by the static pressure rather than the kinetic energy of the moving fluid. The hydraulic pump in a hydrostatic system is of the positive displacement type. Either fixed or variable displacement is acceptable, but the majority of systems use variable displacement. Axial piston pumps are the most commonly used, although radial piston pumps are used in some applications. The motor in a hydrostatic system can be any positive displacement hydraulic motor. Axial piston motors are usually used for most drives, but both gear motors and radial piston motors are used for specific designs. The motor is usually of fixed displacement type, but variable displacement is acceptable. The motor is reversible, with the direction of rotation dependent on the direction of flow in the closed- loop circuit from the pump. Figure 7.26 shows a diagram of a typical hydrostatic drive. Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. VI. OIL RESERVOIRS The oil reservoir is also a very important component of the hydraulic system. It contains the oil supply, provides radiant and convection cooling, allows solid contamination and water to drop out, and helps reduce entrained air from circulating to the critical control components. In addition, in relation to oil levels and pump location, the oil reservoir facilitates easy flow of the oil to the pump suction, reducing the potential for cavitation or starvation conditions. Proper design and care of oil reservoirs will help assure satisfactory component service life and long oil life. A. Reservoir Design When possible, the oil capacity of a hydraulic reservoir should be at least 2.5 times the rate of oil circulation (pump capacity) at full operating conditions. This rule does not apply to closed-loop systems, such as the hydrostatic drives, in which the oil flows back and forth between the pump and motor without returning to the reservoir. In these systems, the reservoir is used only for oil makeup to the system necessitated by internal leakage (or other system leakage). Figure 7.27 shows a typical reservoir configuration for non- closed-loop systems. The proportions shown are suitable. If the reservoir is too shallow, there may not be enough sidewall area in contact with the returning oil for effective cooling and conditions that allow air to enter the pump suction (vortexing) may be promoted. On the other hand, if the reservoir is too deep and narrow, there may not be enough surface area for separation of the air in the oil. The baffle aids cooling and contamination separation by promoting flow along the sidewalls. The baffle also helps to prevent short-circuiting of hot oil from the return on one side to the pump suction line on the other. The bottom of the reservoir is dished, and the drain is located at the lowest point. This design not only permits complete drainage at oil change time, but allows occasional removal of water and other heavier-than-oil contaminants that separate in the reservoir. Space below the Figure 7.27 Oil reservoir design. Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. reservoir permits the use of hoses, pans, and so on, to permit oil to be drained without spillage. The reservoir cover is welded on in this example, but bolted and gasketed covers are often used. Large clean-out doors are provided at one or both ends. Other design features include a filler hole with cap and screen, a level gage, a breather with a filter, and gasketed seals for clearance holes where pipes pass through the cover. Return lines should be extended well below the oil level to reduce misting and aeration of the oil. The reservoir design discussed is a separate component of most typical hydraulic systems and is exposed to atmospheric pressure. In a pressurized reservoir, sometimes used to positively charge the pump suction, the level of pressure must be considered in the design. The pressurized reservoir reduces the potential for atmospheric contamina- tion such as moist air or other airborne contamination to enter the reservoir. In addition to these designs, a space-saving integral design can be used. In these, the reservoir is built into the machine as, for example, a fluid-tight base or hollow member of the support structure that can hold oil without requiring additional space for a separate oil reservoir. The same rules and precautions apply to these alternate designs. The needs remain for the oil to be cooled, contamination removed, and adequate pump suction supply provided. VII. OIL QUALITIES REQUIRED BY HYDRAULIC SYSTEMS As the discussion of hydraulic system components has indicated, the hydraulic fluid is a very important component that is often casually considered. Most often, satisfying the requirements of only the pump seems to be the primary consideration for fluid selection. Although the costs of hydraulic pump failures are generally one of the more costly occur- rences within hydraulic systems, erratic operation of valves and actuators due to inadequate oil performance characteristics such as oil degradation (oxidation) that causes deposits to form in critical clearance areas, often leads to costly production losses. With the close clearances, different metallurgies, various elastomers, and high pressures and temperatures, service life and performance of all the system components depend on proper selection and maintenance of the hydraulic fluids. Hydraulic fluids perform many functions in addition to transmitting pressure and energy. These include minimizing friction and wear, sealing close-clearance parts from leakage, removing heat, minimizing system deposits, flushing away wear particles and contamination, and protecting surfaces from rust and corrosion. The important characteris- tics of a hydraulic fluid vary by the components used and the severity of service. Chapter 3 dealt with product characteristics and testing in some detail. A number of the physical characteristics and performance qualities of hydraulic fluids commonly required by most hydraulic systems are listed as follows: Viscosity Viscosity index (VI) Wear protection capability Oxidation stability Antifoam and air separation characteristics Demulsibility (water-separating characteristics) Rust protection Compatibility Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. Some specific applications may require the following: Soluble oils High water content fluids Fire-resistant fluids Environmental performance A. Viscosity The single most important physical characteristic of a hydraulic fluid is its viscosity. Viscosity is a measure of the oil’s resistance to flow, so in hydraulic systems that are dependent on flow, viscosity is important with respect to both lubrication and energy transmission. Although viscosity requirements are to some extent dictated by the compo- nents (pumps, valves, motors, etc.) and by system manufacturers, certain effects of im- proper viscosity selection need to be recognized. Too low a viscosity can lead to excessive metal-to-metal contact of moving parts, as well as to wear and leakage. Too high a viscosity can result in excessive heating, sluggish operation (particularly at start-up), higher energy consumption, lower mechanical efficiencies, and increased pressure drops in transmission lines and across filters. Since viscosity decreases as temperature increase, viscosity require- ments are generally specified at operating temperature. If temperatures are higher than those specified for normal operation, a higher viscosity oil may be required to provide long service life of the components. If start-up and operating temperatures are lower than those specified, then a lower viscosity oil may prove to be better for overall system performance. Systems operating over a wide range of temperatures may require oil that exhibit high VIs. Hydraulic systems normally use oil with a viscosity range of 32–68 cSt at 40ЊC (150–315 SUS at 100ЊF). To ensure flow to the pump, most hydraulic equipment builders require that the viscosity at start-up temperatures not exceed 1515 cSt (7000 SUS). Some builders, however, limit the start-up viscosity to 866 cSt (4000 SUS). B. Viscosity Index (VI) The viscosity of all oils varies substantially with changes in temperature. In some hydraulic systems, subjected to wide variations in start-up and operating temperatures, it is desirable to use an oil that changes relatively little in viscosity for a given temperature range. An oil that does this is said to have a high viscosity index. High viscosity indexes can be achieved by using mineral oil base stocks that have been refined through a severe hydropro- cessing technique (Chapter 2) in conjunction with the use of long chain polymers called viscosity index improvers (VIIs). Mineral oil base stocks refined through conventional methods can also achieve wide temperature range performance by the addition of viscosity index improvers. Synthetic hydraulic fluids with naturally high VIs are an alternative for severe application temperatures—both high and low. Hydraulic fluids are subjected to high shear rate conditions, particularly as pressures and speeds rise. Viscosity index improvers are generally long chain polymers and, depend- ing on the type of polymer used, are subject to shearing over time. This shear can result in loss of viscosity and, if severe enough, in poorer hydraulic system performance owing to increased potential for metal-to-metal contact, increased leakage, and loss of some of the friction-reducing qualities. In the selection of a mineral oil based hydraulic fluid with a high viscosity index obtained by the use of viscosity index improvers, good shear stability performance should be required. Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. C. Antiwear (Wear Protection) To assure satisfactory hydraulic component life, the hydraulic fluid must minimize wear. Wear results in loss of mechanical efficiency as well as higher costs due to shorter compo- nent life. In some hydraulic systems, such as low pressure, low temperature systems with gear pumps, antiwear additives are not necessary. Other high pressure, high temperature systems using vane pumps do require antiwear additives in the hydraulic fluid. Piston pumps may or may not require antiwear additives depending on the metallurgy and design. A properly refined petroleum oil has naturally good wear protection (without the use of antiwear additives). This quality, sometimes referred to as lubricity or film strength, is present to a greater degree in oils of higher viscosity than those of lower viscosity. How- ever, certain additive materials are used to improve antiwear performance in hydraulic oils. These additives work by chemically reacting with the metal surfaces forming a strong film preventing metal-to-metal contact under boundary lubrication conditions. The use of effective antiwear additives may allow the use of lower viscosity fluids without sacrificing potential wear. Antiwear fluids are generally required in gear and vane pumps operating at pressures above 1000 psi and over 1200 rpm. Piston pumps may or may not require antiwear additives depending on the specific manufacturer and the metallurgy used. For example, Denison Hydraulics Inc. prefersR&Ooils for their piston pumps, whereas antiwear additives are required for Vickers piston pumps. Denison Hydraulics typically uses bronze piston shoes against a steel swashplate, while Vickers may use steel on steel. Steel against steel at high pressure will always require antiwear formulations. Premium quality antiwear hydraulic fluids are formulated to provide performance in all pumps and hydraulic systems. Because a given industrial plant will use many different pumps and other component, it may be advisable to consolidate the number of hydraulic fluids by using antiwear formulations that meet all the requirements. The antiwear performance of hydraulic fluids is evaluated in several standard indus- try-recognized tests. The major ones include the ASTM D 2882 (Vickers V-104C Pump), the Vickers M-2952-S (Vickers 35VQ25 Pump), and the Denison Hydraulics HF-0 (combi- nation of the P-46 piston pump test and the TP-30283A Vane Pump Test). These tests were discussed in some detail in Chapter 6 in connection with environmental hydraulic fluids. D. Oxidation Stability Oil is circulated over and over during long periods in hydraulic systems. It is heated by the churning and shearing action in pumps, valves, tubing, and actuators. Also, the energy released as the oil goes from high pressure to low pressure in a relief valve is converted to heat, which raises its temperature. Oil can be further increased by convection or conduction heating while performing its work in applications such as the hot molds in plastic injection molding operations and continuous caster hydraulics in steel mills. The oil is in contact with warm air in the reservoir. Air is also dissolved or entrained in the oil. Because of this contact with air, oxygen is intimately mixed with the oil. Under these conditions (exposure to temperature and oxygen), the oil tends to chemically combine with the oxygen, creating oxidation products. The tendency to oxidize is greatly increased as temperatures increase, as agitation or splashing becomes excessive, and by exposure to certain materials that catalyze the oxidation reactions. Catalysts such as iron, copper, rust, and other metallic materials are commonly present in hydraulic systems. Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. Slight oxidation is not harmful, but if the oil has poor resistance to this chemical change, oxidation may become excessive. If this occurs, substantial amounts of both solu- ble and insoluble oxidation products are formed, and the oil gradually increases in viscosity. Some variation of viscosity within a range that has proved satisfactory in service is not necessarily harmful. However, viscosity higher than necessary is accompanied by higher fluid friction and more heating. With many of today’s critical systems using electrohy- draulic servovalves with extremely close clearances, slight oxidation could result in the formation of deposits on the servovalve spools, restricting their movement and resulting in production problems. Low quality oils have poor resistance to oxidation under severe conditions. With such oils, troubles of the kind just described often occur. In high quality hydraulic oils, the natural ability of well-refined, carefully selected base oils to resist oxidation is greatly improved by the use of additives that retard the oxidation process. E. Antifoam/Air Separation Characteristics The positive and accurate motion of actuators within a hydraulic system is dependent on the virtual incompressibility of the hydraulic fluid. Under high pressure, mineral oils can see a very slight reduction in volume (4.0% at 10,000 psi and 140ЊF) and a corresponding increase in density. For purposes of the vast majority of hydraulic systems, this is consid- ered to be insignificant. Introduction of air into the fluid can substantially change the compressibility. Air causes spongy or erratic motion, which will result in poor system performance, particularly during the production of close-tolerance parts. Antifoaming and air separation characteristics are two different concepts, although somewhat connected. ‘‘Air separation’’ means that the entrained air is released from the oil, while ‘‘antifoam’’ means that the air bubbles getting to the surface of the oil are readily dissipated. Both aspects are important to the performance of a hydraulic oil. Contamination can alter both these characteristics so it is not only important to select an oil that will provide good antifoam and air separation performance but it is necessary to minimize contamination in order to maintain this good performance. F. Demulsibility (Water-Separating Ability) Water contamination is sometimes a problem in hydraulic systems. It may be present as a result of water leaks in heat exchangers or washdown procedures, but more commonly it accumulates because of condensation of atmospheric moisture. Most condensation occurs above the oil level in reservoirs as machines cool during idle or shutdown periods. A clean hydraulic oil of suitable type will have little tendency to mix with water; and in a still reservoir, the water will tend to settle at a low point. During operation, the water may be picked up by oil circulation, broken up into droplets, and mixed with the oil, forming an emulsion. The water and oil in such an emulsion should separate quickly in the reservoir, but when solid contaminants or oil oxidation products are present, emulsions tend to persist and to join with other deposit-forming materials present to form sludge. The emulsion may be drawn into the pump and made more permanent by the churning action of the pump and the mixing effect of flow at high velocities through control devices. To prevent such contamination from occurring, it is essential that a hydraulic oil have the ability to separate quickly and completely from water. Properly refined oils have this ability when new, but only oils having exceptionally high oxidation stability are able to retain good water-separating ability over long service periods. In addition to using Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. [...]... of vanes Piston pump: piston to bore Servovalves: flapper wall Actuators 0 .5 0 .5 1.0 0 .5 5. 0 0 .5 5. 0 0 .5 5. 0 1.4 1.0–23.0 1.0– 25. 0 5. 0–13.0 5. 0–40.0 18.0–63.0 50 .0– 250 .0 0.000019 0.000019–0.000039 0.000019–0.000197 0.000019–0.000197 0.000019–0.000197 0.000 055 0.000039–0.000904 0.000039–0.000984 0.000197–0.00 051 1 0.000197–0.00 157 5 0.000708–0.002363 0.001969–0.009843 housings are generally equipped with... formed This type of lubrication is called elastohydrodynamic lubrication (EHL: the acronym EHD is also used) EHL films are very thin, in the order of 10 50 in (0. 25 1. 25 m) thick However, even with these thin films, complete separation of the contacting surfaces can be obtained Any material that will flow at the shear stresses available in the system may be used for fluid film lubrication In most... and load (P) Plain bearings are designed for either fluid film lubrication or thin film lubrication Most fluid film bearings are designed for hydrodynamic lubrication, but increasing numbers of bearings for special applications are being designed for hydrostatic lubrication A Hydrodynamic Lubrication The primary requirement for hydrodynamic lubrication is that oil of correct viscosity and sufficient quantity... Conventional emulsions contain about 95% water, and the inverts contain between 40 and 45% water Fire resistance characteristics are provided by the water, and maintenance of the correct water content is necessary to assure fire resistance Invert emulsions provide good lubrication characteristics, and various products range in viscosity from 65 to 129 cSt at 40ЊC (300– 650 SUS at 100ЊF) The viscosity of... Thin films are not thick enough to maintain complete separation of the surfaces all the time Thin film lubrication includes mixed film and boundary lubrication 3 Solid films are more or less permanently bonded onto the moving surfaces A Fluid Films Fluid film lubrication is the most desirable form of lubrication because during normal operation, the films are thick enough to completely separate the load-carrying... systems Operating temperatures are limited to 60ЊC (140ЊF) to prevent excess evaporation B High Water Content Fluids (HWCF) High water content fluids, sometimes referred to as 95/ 5 fluids, contain 2 5% watersoluble chemicals that impart some lubricity, rust protection, and wear protection of spe- Copyright 2001 by Exxon Mobil Corporation All Rights Reserved cially designed pumps, valves, packing glands,... to full fluid film conditions that exist in thick hydrodynamic film lubrication In the region labeled ‘‘partial EHL,’’ the film becomes progressively thinner, and more and more surface asperities penetrate the film Fatigue life is decreased accordingly This region is one of mixed film lubrication When is below about 1, boundary lubrication prevails and the surfaces are in contact nearly all the time... are such that fluid film cannot be maintained Under these conditions, lubrication is by the so-called thin films When surfaces run together under thin film conditions, enough oil is often present to permit part of the load to be carried by fluid films and part by contact between surfaces This condition is often called mixed film lubrication With less oil present, or with higher loads, a point is reached... referred to as beta ratios () A beta ratio of 75 for a 10 m filter would mean that 98.7% of the particles in the 10 m and larger range will be removed Tables 7.3 and 7.4 give a brief explanation of beta ratios and filter efficiencies Bypass filters, sometimes referred to as polishing filters, generally are installed in an independent system where from 5 15% of the system’s oil capacity (in gpm) is filtered... coefficient of friction in terms of ZN/P, the relationship can be shown by a curve such as that in Fig 8- 15 A similar type curve could be developed experimentally for any fluid film bearing In Figure 8. 15, in the zone to the right of c, fluid film lubrication exists To the left of a, boundary lubrication exists In this latter zone, conditions are such that a full fluid * The expression ZN/P is dimensionless . pump sides of vanes 5. 0–13.0 0.000197–0.00 051 1 Piston pump: piston to bore 5. 0–40.0 0.000197–0.00 157 5 Servovalves: flapper wall 18.0–63.0 0.000708–0.002363 Actuators 50 .0– 250 .0 0.001969–0.009843 housings. inches Antifriction bearings 0 .5 0.000019 Vane pump: tip to vane 0 .5 1.0 0.000019–0.000039 Piston pump: valve plate to cylinder 0 .5 5. 0 0.000019–0.000197 Gear pump: gear to side plate 0 .5 5. 0 0.000019–0.000197 Gear. pump: gear tip to case 0 .5 5. 0 0.000019–0.000197 Servovalve spool (radial) 1.4 0.000 055 Control valve spool (radial) 1.0–23.0 0.000039–0.000904 Hydrostatic bearings 1.0– 25. 0 0.000039–0.000984 Vane