Hydraulic Diaphragm Accumulators

The diaphragm-type accumulator is constructed in two halves which are either screwed or bolted together. A synthetic rubber diaphragm is installed between both halves, making two chambers. Two threaded openings exist in the assembled component. The opening at the top, as shown in figure 9-9, contains a screen disc which prevents the diaphragm from extruding through the threaded opening when system pressure is depleted, thus rupturing the diaphragm. On some designs the screen is replaced by a button-type protector fastened to the center of the diaphragm. An air valve for pressurizing the accumulator is located in the gas chamber end of the sphere, and the liquid port to the hydraulic system is located on the opposite end of the sphere. This accumulator operates in a manner similar to that of the bladder-type accumulator.

Hydraulic O-Rings

An O-ring is doughnut-shaped. O-rings are usually molded from rubber compounds; however, they can be molded or machined from plastic materials. The O-ring is usually fitted into a rectangular groove (usually called a gland) machined into the mechanism to be sealed. An O-ring seal consists of an O-ring mounted in the gland so that the O-ring’s cross section is compressed (squeezed) when the gland is assembled (fig. 7-6).

An O-ring sealing system is often one of the first sealing systems considered when a fluid closure is designed because of the following advantages of such a system:

1. Simplicity
2. Ruggedness
3. Low cost
4. Ease of installation
5. Ease of maintenance
6. No adjustment required
7. No critical torque in clamping
8. Low distortion of structure
9. Small space requirement
10. Reliability
11. Effectiveness over wide pressure and temperature ranges

As stated previously, O-rings are used in both static (as gaskets) and dynamic (as packing) applications. An O-ring will almost always be the most satisfactory choice of seals in static applications if the fluids, temperatures, pressure, and geometry permit.

Standard O-ring packings are not specifically designed to be used as rotary seals. When infrequent rotary motion or low peripheral velocity is involved standard O-ring packings may be used, provided consistent surface finishes over
the entire gland are used and eccentricities are accurately controlled. O-rings cannot compensate for out-of-round or eccentrically rotating shafts.

As rotary seals, O-rings perform satisfactorily in two application areas:

1. In low-speed applications where the surface speed of the shaft does not exceed 200 ft/min
2. In high-speed moderate-pressure applications, between 50 and 800 psi

The use of low-friction extrusion-resistant devices is helpful in prolonging the life and improving the performance of O-rings used as rotary seals.

O-rings are often used as reciprocating seals in hydraulic and pneumatic systems. While best suited for short-stroke, relatively small diameter applications, O-rings have been used successfully in long-stroke, large diameter applications.
Glands for O-rings used as reciprocating seals are usually designed according to MIL-G-5514 to provide a squeeze that varies from 8 to 10 percent minimum and 13.5 to 16 percent maximum. A squeeze of 20 percent is allowed on O-rings with a cross section of 0.070-inch or less. In some reciprocating pneumatic applications, a floating O-ring design may simultaneously reduce friction and wear by maintaining no squeeze by the gland on the O-ring. When air pressure enters the cylinder, the air pressure flattens the O-ring, causing sufficient squeeze to seal during the stroke. If the return stroke does not use pneumatic power, the O-ring returns to its round cross section, minimizing drag and wear on the return stroke.

Hydraulic V-Seals

The V-ring is one of the most frequently used dynamic seals in ship service although its identification, installation, and performance are probably most misunderstood. Properly selected and installed, V-rings can provide excellent service
life; otherwise, problems associated with friction, rod and seal wear, noise, and leakage can be expected.

The V-ring is the part of the packing set that does the sealing. It has a cross section resembling the letter V, (fig. 7-3) from which its name is derived. To achieve a seal, the V-ring must be installed as part of a packing set or stack, which
includes one male adapter, one female adapter, and several V-rings (fig. 7-4). The male adapter is the first ring on the pressure end of the packing stack and is flat on one side and wedge-shaped on the other to contain the V of the adjacent
V-ring. The female adapter, the last ring of the packing stack, is flat on one side and V-shaped on the other to properly support the adjacent V-ring. Proper design and installation of the female adapter has significant impact on the
service life and performance of the V-rings because the female adapter bridges the clearance gap between the moving surfaces and resists extrusion.

The packing set is installed in a cavity that is slightly deeper than the free stack height (the nominal overall height of a V-ring packing set, including the male and female adapters as measured before installation) and as wide as the nominal cross section of the V-rings. This cavity, called a packing gland or stuffing box, contains and supports the packing around the shaft, rod, or piston. Adjustment of the packing gland depth through the use of shims or spacers is usually necessary to obtain the correct squeeze or clearance on the packing stack for good service life.

Two basic installations apply to V-ring packings. The more common is referred to as an outside packed installation, in which the packing seals against a shaft or rod, as shown in figure 7-4. The inside packed installation, is shown as a piston seal in figure 7-5. When V-ring packing is to be used in an inside packed installation, only endless ring packing should be used. Where pressures exist in both directions, as on a double-acting piston, opposing sets of packing should always be installed so the sealing lips face away from each other as in figure 7-5. This prevents trapping pressure between the sets of packings. The female adapters in inside packed installations should always be located adjacent to a fixed or rigid part of the piston.

Hydraulic Pump Performance

Pumps are usually rated according to their volumetric output and pressure. Volumetric output (delivery rate or capacity) is the amount of liquid that a pump can deliver at its outlet port per unit of time at a given drive speed, usually expressed in GPM or cubic inches per minute. Because changes in pump drive affect volumetric output, pumps are sometimes rated according to displacement, that is the amount of liquid that they can deliver per cycle or cubic inches per revolution.

Pressure is the force per unit area of a liquid, usually expressed in psi. (Most of the pressure in the hydraulic systems covered in this manual is created by resistance to flow.) Resistance is usually caused by a restriction or obstruction in a path or flow. The pressure developed in a system has an effect on the volumetric output of the pump supplying flow to a system. As pressure increases, volumetric output decreases. This drop in output is caused by an increase in internal leakage (slippage) from a pump’s outlet side to its inlet side. Slippage is a measure of a pump’s efficiency and usually is expressed in percent. Some pumps have greater internal slippage than others; some pumps are rated in terms of volumetric output at a given pressure.

Hydraulic Seals

Seals are packing materials used to prevent leaks in liquid-powered systems. A seal is any gasket, packing, seal ring, or other part designed specifically for sealing. Sealing applications are usually static or dynamic, depending if the parts being sealed move in relation to one another. Sealing keeps the hydraulic oil flowing in passages to hold pressure and keep foreign materials from getting into the hydraulic passages. To prevent leakage, use a positive sealing method, which involves using actual sealing parts or materials. In most hydraulic components, you can use nonpositive sealing (leakage for lubrication) by fitting the parts closely together. The strength of an oil film that the parts slide against provides an effective seal.

Static Seals. Pipe-threaded seals, seal rings used with tube fittings, valve end-cap seals, and other seals on nonmoving parts are static seals. Mounting gaskets and seals are static, as are seals used in making connections between components. A static seal or gasket is placed between parts that do not move in relation to each other. Figure 2-39 shows some typical static seals in flanged connections.

Dynamic Seals. In a dynamic sealing application, either a reciprocating or a rotary motion occurs between the two
parts being sealed; for example, a piston-to-barrel seal in a hydraulic cylinder or a drive-shaft seal in a pump or motor.

O-Ring (Figure 2-40). An O-ring is a positive seal that is used in static and dynamic applications. It has replaced the flat gasket on hydraulic equipment. When being installed, an O-ring is squeezed at the top and bottom in its groove and against the mating part. It is capable of sealing very high pressure. Pressure forces the seal against the side of its groove, and the result is a positive seal on three sides. Dynamic applications of an O-ring are usually limited to reciprocating parts that have relatively short motion.

To remove an O-ring seal, you need a special tool made of soft iron or aluminum or a brass rod (Figure 2-41). Make sure that the tool’s edges are flat and that you polish any burrs and rough surfaces.

Backup Ring (Figure 2-42). Usually, made of stiff nylon, you can use a backup ring with an O-ring so that it is not forced into the space between the mating parts. A combination of high pressure and clearance between the parts could call for a backup ring.

Lathe-Cut Seal. This seal is like an Oring but is square in cross-section rather than round. A lathe-cut ring is cut from extruded tubes, while an O-ring must be individually molded. In many static applications, roundand square-section seals are interchangeable, if made from the same material.

T-Ring Seal (Figure 2-43). This seal is reinforced with back-up rings on each side. A T ring seal is used in reciprocating dynamic applications, particularly on cylinder pistons and around piston rods.

Lip Seal (Figure 2-44). This a dynamic seal used mainly on rotating shafts. A sealing lip provides a positive seal against low pressure. A lip is installed toward the pressure source. Pressure against a lip balloons it out to aid in sealing. Very high pressure, however, can get past this kind of seal because it does not have the backup support that an O-ring has.

Sometimes, double-lip seals are used on the shafts of reversible pumps or motors. Reversing a unit can give an alternating pressure and vacuum condition in the chamber adjacent to a seal. A double-lip seal, therefore, prevents
oil from getting out or air and dirt from getting in.

Cup Seal (Figure 2-45). This is a positive seal that is used on hydraulic cylinder pistons and seals much like a lip seal. A cup seal is backed up so that it can handle very high pressures.

Piston Ring (Figure 2-46). A piston ring is used to seal pressure at the end of a reciprocating piston. It helps keep friction at a minimum in a hydraulic cylinder and offers less resistance to movement than a cup seal. A
piston ring is used in many complex components and systems to seal fluid passages leading from hollow rotating shafts. It is fine for high pressures but may not provide a positive seal. A positive seal is more likely to occur when piston rings are placed side by side. Often, a piston ring is designed to allow some leakage for lubrication.

Face Seal (Figure 2-47, page 2-34). This seal has two smooth, flat elements that run together to seal a rotating shaft. One element is metallic and the other is nonmetallic. The elements are attached to a shaft and a body so that one face is stationary and the other turns against it. One element is often spring-loaded to take up wear. A face seal is used primarily when there is high speed, pressure, and temperature.

Packing. Packing is a type of twisted or woven fiber or soft metal strands that are packed between the two parts being sealed. A packing gland supports and backs up the packing. Packing (Figure 2-48) can be either static or dynamic. It has been and is used as a rotating shaft seal, a reciprocating piston-rod seal, and a gasket in many static applications. In static applications, a seal is replacing a packing. A compression packing is usually placed in a coil or layered in a bore and compressed by tightening a flanged member. A molded packing is molded into a precise cross-sectional form, such as a U or V. Several packings can be used together, with a backup that is spring-loaded to compensate for wear.

Seal Materials. The earliest sealing materials for hydraulic components were mainly leather, cork, and impregnated fibers. Currently, most sealing materials in a hydraulic system are made from synthetic materials such as nitrile, silicone, and neoprene.

Leather Seals. Leather is still a good sealing material and has not been completely replaced by elastomers. It is tough, resists abrasion, and has the ability to hold lubricating fluids in its fibers. Impregnating leather with synthetic rubber improves the leather’s sealing ability and reduces its friction. Leather’s disadvantages are that it tends to squeal when it is dry, and it cannot stand high temperatures.

Nitrile Seals. Nitrile is a comparatively tough material with excellent wearability. Its composition varies to be compatible with petroleum oils, and it can easily be molded into different seal shapes. Some nitrile seals can be used, without difficulty, in temperatures ranging from -40 degrees Fahrenheit to +230° F.

Silicone Seals. Silicone is an elastomer that has a much wider temperature range than some nitrile seals have. Silicone
cannot be used for reciprocating seals because it is not as tough. It tears, elongates, and abrades fairly easily. Many lip-type shaft seals made from silicone are used in extreme temperature applications. Silicone O-rings are used for static applications. Silicone has a tendency to swell since it absorbs a fair volume of oil while running hot. This is an advantage, if the swelling is not objectionable, because a seal can run dry for a longer time at start-up.

Neoprene. At very low temperatures, neoprene is compatible with petroleum oil. Above 150 degrees, it has a habit of cooking or vulcanizing, making it less useful.

Nylon. Nylon is a plastic (also known as fluoro-elastomer) that combines fluorine with a synthetic rubber. It is used for backup rings, has sealing materials in special applications, and has a very high heat resistance.

Hydraulic Leakage

Any hydraulic system will have a certain amount of leakage. Any leakage will reduce efficiency and cause power loss.
Some leakage is built in (planned), some is not. Leakage may be internal, external, or both.

Internal. This type of leakage (nonpositive) must be built into hydraulic components to lubricate valve spools, shafts, pistons, bearings, pumping mechanisms, and other moving parts. In some hydraulic valves and pump and motor compensator controls, leakage paths are built in to provide precise control and to avoid hunting (oscillation) of spools and pistons. Oil is not lost in internal leakage; it returns to a reservoir through return lines or specially provided drain passages.

Too much internal leakage will slow down actuators. The power loss is accompanied by the heat generated at a leakage path. In some instances, excess leakage in a valve could cause a cylinder to drift or even creep when a valve is supposedly in neutral. In the case of flow or pressure-control valves, leakage can often reduce effective control or even cause control
to be lost.

Normal wear increases internal leakage, which provides larger flow paths for the leaking oil. An oil that is low in viscosity leaks more readily than a heavy oil. Therefore an oil’s viscosity and viscosity index are important considerations in providing or preventing internal leakage. Internal leakage also increases with pressure, just as higher pressure causes a greater flow through an orifice. Operating above the recommended pressures adds the danger of excessive internal leakage and heat generation to other possible harmful effects.

A blown or ruptured internal seal can open a large enough leakage path to divert all of a pump’s delivery. When this happens, everything except the oil flow and heat generation at a leakage point can stop.

External. External leakage can be hazardous, expensive, and unsightly. Faulty installation and poor maintenance are the prime causes of external leakage. Joints may leak because they were not put together properly or because shock and vibration in the lines shook them loose. Adding supports to the lines prevents this. If assembled and installed correctly, components seldom leak. However, failure to connect drain lines, excessive pressures, or contamination can cause seals to blow or be damaged, resulting in external leakage from the components.

Prevention. Proper installation, control of operating conditions, and proper maintenance help prevent leakage.

Installation. Installing piping and tubing according to a manufacturer’s recommendations will promote long life of external seals. Vibration or stresses that result from improper installation can shake loose connections and create puddles. Avoid pinching, cocking, or incorrectly installing seals when assembling the units. Use any special tools that the manufacturer recommends for installing the seals.

Operating Conditions. To ensure correct seal life, you must control the operating conditions of the equipment. A shaft seal or piston-rod seal exposed to moisture, salt, dirt, or any other abrasive contaminate will have a shortened life span. Also, operators should always try to keep their loads within the recommended limits to prevent leakage caused by excessive pressures.

Maintenance. Regular filter and oil changes, using a high-quality hydraulic oil, add to seal life. Using inferior oil could wear on a seal and interfere with desirable oil properties. Proper maintenance prevents impurity deposits and circulating ingredients that could wear on a dynamic seal.

Never use additives without approval from the equipment and oil suppliers. Lubrication can be critical to a seal’s life in dynamic applications. Synthetics do not absorb much oil and must be lubricated quickly or they will rub. Leather and fiber do absorb oil. Manufacturers recommend soaking a seal overnight in oil before installing it. Do not install a seal dry. Always coat it in clean hydraulic oil before installing it.

Circulatory Systems

Pipes and fittings, with their necessary seals, make up a circulatory system of liquid-powered equipment. Properly selecting and installing these components are very important. If improperly selected or installed, the result would be
serious power loss or harmful liquid contamination. The following is a list of some of the basic requirements of a circulatory system:

• Lines must be strong enough to contain liquid at desired working pressure and the surges in pressure that may develop in system.
• Lines must be strong enough to support the components that are mounted on them.
• Terminal fittings must be at all junctions where parts must be removed for repair or replacement.
• Line supports must be capable of damping the shock caused by pressure surges.
• Lines should have smooth interiors to reduce turbulent flow.
• Lines must have the correct size for the required liquid flow.
• Lines must be kept clean by regular flushing or purging.
• Sources of contaminants must be eliminated.

The three common types of lines in liquid-powered systems are pipes, tubing, and flexible hose, which are also referred to as rigid, semirigid, and flexible line.

Tubing. The two types of tubing used for hydraulic lines are seamless and electric welded. Both are suitable for hydraulic systems. Seamless tubing is made in larger sizes than tubing that is electric welded. Seamless tubing is flared and fitted with threaded compression fittings. Tubing bends easily, so fewer pieces and fittings are required. Unlike pipe, tubing can be cut and flared and fitted in the field. Generally, tubing makes a neater, less costly, lower- maintenance system with fewer flow restrictions and less chances of leakage. Figure 2-21 shows the proper method of installing tubing.

Knowing the flow, type of fluid, fluid velocity, and system pressure will help determine the type of tubing to use. (Nominal dimensions of tubing are given as fractions in inches or as dash numbers. A dash number represents a tube’s outside diameter [OD] in sixteenths of an inch.) A system’s pressure determines the thickness of the various tubing walls. Tubing above 1/2 inch OD usually is installed with either flange fittings with metal or pressure seals or with welded joints. If joints are welded, they should be stress-relieved.

Piping. You can use piping that is threaded with screwed fittings with diameters up to 1 1/4 inches and pressures of up to 1,000 psi. Where pressures will exceed 1,000 psi and required diameters are over 1 1/4 inches, piping with welded, flanged connections and socket-welded size are specified by nominal inside diameter (ID) dimensions. The thread remains the same for any given pipe size regardless of wall thickness. Piping is used economically in larger-sized hydraulic systems where large flow is carried. It is particularly suited for long, permanent straight lines. Piping is taper-threaded on its OD into a tapped hole or fitting. However, it cannot be bent. Instead, fittings are used wherever a joint is required. This results in additional costs and an increased chance of leakage.

Flexible Hosing. When flexibility is necessary in liquid-powered systems, use hose. Examples would be connections to units that move while in operation to units that are attached to a hinged portion of the equipment or are in locations that are subjected to severe vibration. Flexible hose is usually used to connect a pump to a system. The vibration that is set up by an operating pump would ultimately cause rigid tubing to fail.

Rubber Hose. Rubber hose is a flexible hose that consists of a seamless, synthetic rubber tube covered with layers of cotton braid and wire braid. Figure 2-22, shows cut-away views of typical rubber hose. An inner tube is designed to withstand material passing through it. A braid, which may consist of several layers, is the determining factor
in the strength of a hose. A cover is designed to withstand external abuse.

When installing flexible hose, do not twist it. Doing so reduces its lift and may cause its fittings to loosen. An identification stripe that runs along the hose length should not spiral, which would indicate twisting (Figure 2-23). Protect flexible hose from chafing by wrapping it lightly with tape, when necessary.

The minimum bend radius for flexible hose varies according to its size and construction and the pressure under which a system will operate. Consult the applicable publications that contain the tables and graphs which show the minimum bend radii for the different types of installations. Bends that are too sharp will reduce the bursting pressure of flexible hose considerably below its rated value.

Do not install flexible hose so that it will be subjected to a minimum of flexing during operation. Never stretch hose tightly between two fittings. When under pressure, flexible hose contracts in length and expands in diameter.

Teflon™-Type Hose. This is a flexible hose that is designed to meet the requirements of higher operating pressures and temperatures in today’s fluid-powered systems. The hose consists of a chemical resin that is processed and pulled into a desired-size tube shape. It is covered with stainless-steel wire that is braided over the tube for strength and protection. Teflon-type hose will not absorb moisture and is unaffected by all fluids used in today’s fluid-powered systems. It is nonflammable; however, use an asbestos fire sleeve where the possibility of an open flame exists.

Carefully handle all Teflon-type hose during removal or installation. Sharp or excessive bending will kink or damage the hose. Also, the flexible-type hose tends to form itself to the installed position in a circulatory system.

Installation. Flaring and brazing are the most common methods of connecting tubing. Preparing a tube for installation usually involves cutting, flaring, and bending. After cutting a tube to the correct length, cut it squarely and carefully remove any internal or external burrs.

If you use flare-type fittings, you must flare the tube. A flare angle should extend 37 degrees on each side of the center line. The area’s outer edge should extend beyond the maximum sleeve’s ID but not its OD. Flares that are too short are likely to be squeezed thin, which could result in leaks or breaks. Flares that are too long will stick or jam during assembly.

Keep the lines as short and free of bends as possible. However, bends are preferred to elbows or sharp turns. Try not to assemble the tubing in a straight line because a bend tends to eliminate strain by absorbing vibration and compensating for temperature expansion and contraction.

Install all the lines so you can remove them without dismantling a circuit’s components or without bending or springing them to a bad angle. Add supports to the lines at frequent intervals to minimize vibration or movement; never weld the lines to the supports. Since flexible hose has a tendency to shorten when subjected to pressure, allow enough slack to compensate for this problem.

Keep all the pipes, tubes, or fittings clean and free from scale and other foreign matter. Clean iron or steel pipes, tubes, and fittings with a boiler-tube wire brush or with commercial pipe-cleaning equipment. Remove rust and scale from short, straight pieces by sandblasting them, as long as no sand particles will remain lodged in blind holes or pockets after you flush a piece. In the case of long pieces or pieces bent to complex shapes, remove rust and scale by pickling (cleaning metal in a chemical bath). Cap and plug the open ends of the pipes, tubes, and fittings that will be stored for a long period. Do not use rags or waste for this purpose because they deposit harmful lint that can cause severe damage in a hydraulic system.

Portable Hydraulic-Circuit Testers

Hydraulic power is an efficient method of delivering HP by pumping a fluid through a closed system. If the amount of flow or the pressure unknowingly decreases, the amount of HP delivered to a working unit will be reduced, and a system will not perform as it should.

Testers. Portable hydraulic-circuit testers (Figure 2-20) are lightweight units you can use to check or troubleshoot a hydraulic-powered system on the job or in a maintenance shop. Connect a tester into a system’s circuit to determine its efficiency. Currently, several hydraulic-circuit testers are on the market. Operating procedures may vary on different testers. Therefore, you must follow the operating directions furnished with a tester to check or troubleshoot a circuit accurately.

Improper Operation. When a hydraulic system does not operate properly, the trouble could be one of the following:

• The pump that propels the fluid may be slipping because of a worn or an improperly set spring in the relief valve.
• The fluid may be leaking around the control valves or past the cylinder packing.

Since hydraulic systems are confined, it is difficult to identify which component in a system is not working properly. Measure the flow, pressure, and temperature of a liquid at given points in a system to isolate the malfunctioning unit. If this does not work, take the system apart and check each unit for worn parts or bad packing. This type of inspection can be costly from the standpoint of maintenance time and downtime of the power system.

Hydraulic Accumulators

Like an electrical storage battery, a hydraulic accumulator stores potential power, in this case liquid under pressure for future conversion into useful work. This work can include operating cylinders and fluid motors, maintaining the required system pressure in case of pump or power failure, and compensating for pressure loss due to leakage. Accumulators can be employed as fluid dispensers and fluid barriers and can provide a shock-absorbing (cushioning) action.

On military equipment, accumulators are used mainly on the lift equipment to provide positive clamping action on the heavy loads when a pump’s flow is diverted to lifting or other operations. An accumulator acts as a safety device to prevent a load from being dropped in case of an engine or pump failure or fluid leak. On lifts and other equipment, accumulators absorb shock, which results from a load starting, stopping, or reversal.

Spring-Loaded Accumulator. This accumulator is used in some engineer equipment hydraulic systems. It uses the energy stored in springs to create a constant force on the liquid contained in an adjacent ram assembly. Figure 2-15 shows two spring-loaded accumulators.

The load characteristics of a spring are such that the energy storage depends on the force required to compress s spring. The free (uncompressed) length of a spring represents zero energy storage. As a spring is compressed to the maximum installed length, a minimum pressure value of the liquid in a ram assembly is established. As liquid under pressure enters the ram cylinder, causing a spring to compress, the pressure on the liquid will rise because of the increased loading required to compress the spring.

Bag-Type Accumulator. This accumulator (Figure 2-16) consists of a seamless high-pressure shell, cylindrical in shape, with domed ends and a synthetic rubber bag that separates the liquid and gas (usually nitrogen) within the accumulator. The bag is fully enclosed in the upper end of a shell. The gas system contains a high-pressure gas valve. The bottom end of the shell is sealed with a special plug assembly containing a liquid port and a safety feature that makes it impossible to disassemble the accumulator with pressure in the system. The bag is larger at the top and tapers to a smaller diameter at the bottom. As the pump forces liquid into the accumulator shell, the liquid presses against the bag, reduces its volume, and increases the pressure, which is then available to do work.

Piston-Type Accumulator. This accumulator consists of a cylinder assembly, a piston assembly, and two end-cap assemblies. The cylinder assembly houses a piston assembly and incorporates provisions for securing the end-cap assemblies. An accumulator contains a free-floating piston with liquid on one side of the piston and precharged air or nitrogen on the other side (Figure 2-17). An increase of liquid volume decreases the gas volume and increases gas pressure, which provides a work potential when the liquid is allowed to discharge.

Maintenance. Before removing an accumulator for repairs, relieve the internal pressure: in a spring-loaded type, relieve the spring tension; in a piston or bag type, relieve the gas or liquid pressure.

Filtering Material and Elements

The general classes of filter materials are mechanical, absorbent inactive, and absorbent active.

• Mechanical filters contain closely woven metal screens or discs. They generally remove only fairly coarse particles.
• Absorbent inactive filters, such as cotton, wood pulp, yarn, cloth, or resin, remove much smaller particles; some remove water and water-soluble contaminants. The elements often are treated to make them sticky to attract the contaminants found in hydraulic oil.
• Absorbent active materials, such as charcoal and Fuller’s Earth (a claylike material of very fine particles used in the purification of mineral or vegetable-base oils), are not recommended for hydraulic systems.

The three basic types of filter elements are surface, edge, and depth.

• A surface-type element is made of closely woven fabric or treated paper. Oil flows through the pores of the filter material, and the contaminants are stopped.
• An edge-type filter is made up of paper or metal discs; oil flows through the spaces between the discs. The fineness of the filtration is determined by the closeness of the discs.
• A depth-type element is made up of thick layers of cotton, felt, or other fibers.