CHAPTER 2

Hydraulic Systems

A hydraulic system contains and confines a liquid in such a way that it uses the laws governing liquids to transmit power and do work. This chapter describes some basic systems and discusses components of a hydraulic system that store and condition the fluid. The oil reservoir (sump or tank) usually serves as a storehouse and a fluid conditioner. Filters, strainers, and magnetic plugs condition the fluid by removing harmful impurities that could clog passages and damage parts. Heat exchanges or coolers often are used to keep the oil temperature within safe limits and prevent deterioration of the oil. Accumulators, though technically sources of stored energy, act as fluid storehouses.

2-1. Basic Systems. The advantages of hydraulic systems over other methods of power transmission are-

• • Simpler design. In most cases, a few pre-engineered components will replace complicated mechanical linkages.
  • Flexibility. Hydraulic components can be located with considerable flexibility. Pipes and hoses instead of mechanical elements virtually eliminate location problems.
  • Smoothness. Hydraulic systems are smooth and quiet in operation. Vibration is kept to a minimum.
  • Control. Control of a wide range of speed and forces is easily possible.
  • Cost. High efficiency with minimum friction loss keeps the cost of a power transmission at a minimum.
  • Overload protection. Automatic valves guard the system against a breakdown from overloading.

  The main disadvantage of a hydraulic system is maintaining the precision parts when they are exposed to bad climates and dirty atmospheres. Protection against rust, corrosion, dirt, oil deterioration, and other adverse environmental conditions is very important. The following paragraphs discuss several basic hydraulic systems.

  a. Hydraulic Jack. In this system (Figure 2-1), a reservoir and a system of valves has been added to Pascal's hydraulic lever to stroke a small cylinder or pump continuously and raise a large piston or an actuator a notch with each stroke. Diagram A shows an intake stroke. An outlet check valve closes by pressure under a load, and an inlet check valve opens so that liquid from the reservoir fills the pumping chamber. Diagram B shows the pump stroking downward. An inlet check valve closes by pressure and an outlet valve opens. More liquid is pumped under a large piston to raise it. To lower a load, a third valve (needle valve) opens, which opens an area under a large piston to the reservoir. The load then pushes the piston down and forces the liquid into the reservoir.

  

  b. Motor-Reversing System. Figure 2-2 shows a power-driven pump operating a reversible rotary motor. A reversing valve directs fluid to either side of the motor and back to the reservoir. A relief valve protects the system against excess pressure and can bypass pump output to the reservoir, if pressure rises too high.

  

  

  c. Open-Center System. In this system, a control-valve spool must be open in the center to allow pump flow to pass through the valve and return to the reservoir. Figure 2-3 shows this system in the neutral position. To operate several functions simultaneously, an open-center system must have the correct connections, which are discussed below. An open-center system is efficient on single functions but is limited with multiple functions.

  

  (1) Series Connection. Figure 2-4 shows an open-center system with a series connection. Oil from a pump is routed to the three control valves in series. The return from the first valve is routed to the inlet of the second, and so on. In neutral, the oil passes through the valves in series and returns to the reservoir, as the arrows indicate. When a control valve is operated, the incoming oil is diverted to the cylinder that the valve serves. Return liquid from the cylinder is directed through the return line and on to the next valve.

  

  This system is satisfactory as long as only one valve is operating at a time. When this happens, the full output of the pump at full system pressure is available to that function. However, if more than one valve is operating, the total of the pressures required for each function cannot exceed the system's relief setting.

  (2) Series/Parallel Connection. Figure 2-5 shows a variation on the series connection. Oil from the pump is routed through the control valves in series, as well as in parallel. The valves are sometimes stacked to allow for extra passages. In neutral, a liquid passes through the valves in series, as the arrows indicate. However, when any valve is operating, the return is closed and the oil is available to all the valves through the parallel connection.

  

  When two or more valves are operated at once, the cylinder that needs the least pressure will operate first, then the cylinder with the next least, and so on. This ability to operate two or more valves simultaneously is an advantage over the series connection.

  (3) Flow Divider. Figure 2-6 shows an open-center system with a flow divider. A flow divider takes the volume of oil from a pump and divides it between two functions. For example, a flow divider might be designed to open the left side first in case both control valves were actuated simultaneously. Or, it might divide the oil to both sides, equally or by percentage. With this system, a pump must be large enough to operate all the functions simultaneously. It must also supply all the liquid at the maximum pressure of the highest function, meaning large amounts of hp are wasted when operating only one control valve.

  

  d. Closed-Center System. In this system, a pump can rest when the oil is not required to operate a function. This means that a control valve is closed in the center, stopping the flow of the oil from the pump. Figure 2-7 shows a closed-center system. To operate several functions simultaneously, a closed-center system have the following connections:

  

  (1) Fixed-Displacement Pump and Accumulator. Figure 2-8 shows a closed-center system. In this system, a pump of small but constant volume charges an accumulator. When an accumulator is charged to full pressure, an unloading valve diverts the pump flow back to a reservoir. A check valve traps the pressured oil in the circuit.

  

  When a control valve is operated, an accumulator discharges its oil and actuates a cylinder. As pressure begins to drop, an unloading valve directs the pump flow to an accumulator to recharge the flow. This system, using a small capacity pump, is effective when operating oil is needed only for a short time. However, when the functions need a lot of oil for longer periods, an accumulator system cannot handle it unless the accumulator is very large.

  (2) Variable-Displacement Pump. Figure 2-9 shows a closed-center system with a variable-displacement pump in the neutral mode. When in neutral, oil is pumped until the pressure rises to a predetermined level. A pressure-regulating valve allows the pump to shut off by itself and maintain this pressure to the valve. When the control valve is operating, oil is diverted from the pump to the bottom of a cylinder. The drop in pressure caused by connecting the pump's pressure line to the bottom of the cylinder causes the pump to go back to work, pumping oil to the bottom of the piston and raising the load.

  

  When the valve moves, the top of the piston connects to a return line, which allows the return oil that was forced from the piston to return to the reservoir or pump. When the valve returns to neutral, oil is trapped on both sides of the cylinder, and the pressure passage from the pump is dead-ended. After this sequence, the pump rests. Moving the spool in the downward position directs oil to the top of the piston, moving the load downward. The oil from the bottom of the piston is sent into the return line.

  Figure 2-10 shows this closed-center system with a charging pump, which pumps oil from the reservoir to the variable-displacement pump. The charging pump supplies only the makeup oil required in a system and provides some inlet pressure to make a variable-displacement pump more efficient. The return oil from a system's functions is sent directly to the inlet of a variable-displacement pump.

  

  Because today's machines need more hydraulic power, a closed-center system is more advantageous. For example, on a tractor, oil may be required for power steering, power brakes, remote cylinders, three-point hitches, loaders, and other mounted equipment. In most cases, each function requires a different quantity of oil. With a closed-center system, the quantity of oil to each function can be controlled by line or valve size or by orificing with less heat build up when compared to the flow dividers necessary in a comparable open-center system. Other advantages of a closed-center system are that-

  • It does not require relief valves because the pump simply shuts off by itself when standby pressure is reached. This prevents heat buildup in systems where relief pressure is frequently reached.
  • It has lines, valves, and cylinders that can be tailored to the flow requirements of each function.
  • It has an available reserve flow to ensure full hydraulic speed at low engine revolutions per minute (rpm). More functions can be served.
  • It is more efficient on functions such as brakes, which require force but very little piston movement. By holding the valve open, standby pressure is constantly applied to the brake piston with no efficiency loss because the pump has returned to standby.

2-2. Color Coding. In this manual, the figures that show oil-flow conditions or paths are prepared with industrial standardized color codes. Table 2-1 lists the colors for the hydraulic lines and passages that are in many of the figures:

2-3. Reservoirs. A reservoir stores a liquid that is not being used in a hydraulic system. It also allows gases to expel and foreign matter to settle out from a liquid.

a. Construction. A properly constructed reservoir should be able to dissipate heat from the oil, separate air from the oil, and settle out contaminates that are in it. Reservoirs range in construction from small steel stampings to large cast or fabricated units. The large tanks should be sandblasted after all the welding is completed and then flushed and steam cleaned. Doing so removes welding scale and scale left from hot-rolling the steel. The inner surface then should be sealed with a paint compatible with the hydraulic fluid. Nonbleeding red engine enamel is suitable for petroleum oil and seals in any residual dirt not removed by flushing and steam cleaning.

b. Shape. Figure 2-11 shows some of the design features of a reservoir. It should be high and narrow rather than shallow and broad. The oil level should be as high as possible above the opening to a pump's suction line. This prevents the vacuum at the line opening from causing a vortex or whirlpool effect, which would mean that a system is probably taking in air. Aerated oil will not properly transmit power because air is compressible. Aerated oil has a tendency to break down and lose its lubricating ability.

c. Size. Reservoir sizes will vary. However, a reservoir must be large enough so that it has a reserve of oil with all the cylinders in a system fully extended. An oil reserve must be high enough to prevent a vortex at the suction line's opening. A reservoir must have sufficient space to hold all the oil when the cylinders are retracted, as well as allow space for expansion when the oil is hot.

A common-size reservoir on a mobile machine is a 20- or 30-gallon tank used with a 100-GPM system. Many 10-GPM systems operate with 2- or 3-gallon tanks because these mobile systems operate intermittently, not constantly. For stationary machinery, a rule of thumb is that a reservoir's size should be two to three times a pump's output per minute.

A large-size tank is highly desirable for cooling. The large surface areas exposed to the outside air transfer heat from the oil. Also, a large tank helps settle out the contaminates and separates the air by reducing recirculation.

d. Location. Most mobile equipment reservoirs are located above the pumps. This creates a flooded-pump-inlet condition. This condition reduces the possibility of pump cavitation-a condition where all the available space is not filled and often metal parts will erode. Flooding the inlet also reduces the vortex tendency at a suction pipe's opening.

A reservoir's location affects heat dissipation. Ideally, all tank walls should be exposed to the outside air. Heat moves from a hot substance to a cold substance; heat transfer is greatest when there is a large temperature difference. Reservoirs that are built into front-end loader arms are very effective in transferring heat.

e. Ventilation and Pressurization. Most reservoirs are vented to the atmosphere. A vent opening allows air to leave or enter the space above the oil as the level of the oil goes up or down. This maintains a constant atmospheric pressure above the oil. A reservoir filter cap, with a filter element, is often used as a vent.

Some reservoirs are pressurized, using a simple pressure-control valve rather than a vented one. A pressure-control valve automatically lets filtered air into a tank but prevents air release unless the pressure reaches a preset level. A pressurized reservoir takes place when the oil and air in a tank expand from heat.

f. Line Connections. A pump suction and a tank's return lines should be attached by flanges or by welded heavy-duty couplings. Standard couplings usually are not suitable because they spread when welded. If a suction line is connected at the bottom, a coupling should extend well above the bottom, inside the tank; residual dirt will not get in a suction line when a tank or strainer is cleaned. A return line should discharge near a tank's bottom, always below the oil level. A pipe is usually cut at a 45-degree angle and the flow aimed away from a suction line to improve circulation and cooling.

A baffle plate is used to separate a suction line from a return line. This causes the return oil to circulate around an outer wall for cooling before it gets to the pump again. A baffle plate should be about two-thirds the height of a tank. The lower corners are cut diagonally to allow circulation. They must be larger in area than a suction line's cross section. Otherwise the oil level between a return and a suction side might be uneven. Baffling also prevents oil from sloshing around when a machine is moving. Many large reservoirs are cross-baffled to provide cooling and prevent sloshing.

g. Maintenance. Maintenance procedures include draining and cleaning a reservoir. A tank should have a dished bottom that is fitted with a drain plug at its lowest point; a plug fitting should be flush with the inside of a tank to allow for full drainage. On large tanks, access plates may be bolted on the ends for easy removal and servicing. A reservoir should have a sight gauge or dipstick for checking the oil level to prevent damage from lubrication loss.

The strainers on a pump's suction line may not require as much maintenance. However, an element in a filter in a return line will require regular changing. Therefore, that filter should not be inside a reservoir. When a reservoir is pressurized by compressed air, moisture can become a maintenance problem. A tank should have a water trap for moisture removal; it should be placed where it can be inspected daily.

2-4. Strainers and Filters. To keep hydraulic components performing correctly, the hydraulic liquid must be kept as clean as possible. Foreign matter and tiny metal particles from normal wear of valves, pumps, and other components are going to enter a system. Strainers, filters, and magnetic plugs are used to remove foreign particles from a hydraulic liquid and are effective as safeguards against contamination. Magnetic plugs, located in a reservoir, are used to remove the iron or steel particles from a liquid.

a. Strainers. A strainer is the primary filtering system that removes large particles of foreign matter from a hydraulic liquid. Even though its screening action is not as good as a filter's, a strainer offers less resistance to flow. A strainer usually consists of a metal frame wrapped with a fine-mesh wire screen or a screening element made up of varying thicknesses of specially processed wire. Strainers are used to pump inlet lines (Figure 2-11, page 2-10) where pressure drops must be kept to a minimum.

Figure 2-12 shows a strainer in three possible arrangements for use in a pump inlet line. If one strainer causes excessive flow friction to a pump, two or more can be used in parallel. Strainers and pipe fittings must always be below the liquid level in the tank.

b. Filters. A filter removes small foreign particles from a hydraulic fluid and is most effective as a safeguard against contaminants. Filters are located in a reservoir, a pressure line, a return line, or in any other location where necessary. They are classified as full flow or proportional flow.

(1) Full-Flow Filter (Figure 2-13). In a full-flow filter, all the fluid entering a unit passes through a filtering element. Although a full-flow type provides a more positive filtering action, it offers greater resistance to flow, particularly when it becomes dirty. A hydraulic liquid enters a full-flow filter through an inlet port in the body and flows around an element inside a bowl. Filtering occurs as a liquid passes through the element and into a hollow core, leaving the dirt and impurities on the outside of the element. A filtered liquid then flows from a hollow core to an outlet port and into the system.

A bypass relief valve in a body allows a liquid to bypass the element and pass directly through an outlet port when the element becomes clogged. Filters that do not have a bypass relief valve have a contamination indicator. This indicator works on the principle of the difference in pressure of a fluid as it enters a filter and after it leaves an element. When contaminating particles collect on the element, the differential pressure across it increases. When a pressure increase reaches a specific value, an indicator pops out, signifying that the element must be cleaned or replaced.

(2) Proportional-Flow Filters (Figure 2-14). This filter operates on the venturi principle in which a tube has a narrowing throat (venturi) to increase the velocity of fluid flowing through it. Flow through a venturi throat causes a pressure drop at the narrowest point. This pressure decrease causes a sucking action that draws a portion of a liquid down around a cartridge through a filter element and up into a venturi throat. Filtering occurs for either flow direction. Although only a portion of a liquid is filtered during each cycle, constant recirculation through a system eventually causes all of a liquid to pass through the element. Replace the element according to applicable regulations and by doing the following:

• Relieve the pressure.
• Remove the bowl from the filter's body.
• Remove the filter element from the body, using a slight rocking motion.
• Clean or replace the element, depending on its type.
• Replace all old O-ring packings and backup washers.
• Reinstall the bowl on the body assembly. Do not tighten the bowl excessively; check the appropriate regulations for specifications, as some filter elements require a specific torque.
• Pressurize the system and check the filter assembly for leaks.

2-5. 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.

2-6. 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.

a. 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 a 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.

b. 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.

c. 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.

d. 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.

2-7. Pressure Gauges and Volume Meters. Pressure gauges are used in liquid-powered systems to measure pressure to maintain efficient and safe operating levels. Pressure is measured in psi. Flow measurement may be expressed in units of rate of flow-GPM or cubic feet per second (cfs). It may also be expressed in terms of total quantity-gallons or cubic feet.

a. Pressure Gauges. Figure 2-18 shows a simple pressure gauge. Gauge readings indicate the fluid pressure set up by an opposition of forces within a system. Atmospheric pressure is negligible because its action at one place is balanced by its equal action at another place in a system.

b. Meters. Measuring flow depends on the quantities, flow rates, and types of liquid involved. All liquid meters (flowmeters) are made to measure specific liquids and must be used only for the purpose for which they were made. Each meter is tested and calibrated.

In a nutating-piston-disc flowmeter, liquid passes through a fixed-volume measuring chamber, which is divided into upper and lower compartments by a piston disc (Figure 2-19). During operation, one compartment is continually being filled while the other is being emptied. As a liquid passes through these compartments, its pressure causes a piston disc to roll around in the chamber. The disc's movements operate a dial (or counter) through gearing elements to indicate that a column of fluid that has passed through the meter.

2-8. 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.

a. 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.

b. 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.

2-9. 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 a liquid at a desired working pressure and the surges in pressure that may develop in a 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 be 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, tubes, and flexible hose, which are also referred to as rigid, semirigid, and flexible line.

  a. 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.

  b. 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.

  c. 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.

  (1) 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.

  (2) Teflon^TM-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.

  d. 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 centerline. 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 tht 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.

2-10. Fittings and Connectors. Fittings are used to connect the units of a fluid-powered system, including the individual sections of a circulatory system. Many different types of connectors are available for fluid-powered systems. The type that you will use will depend on the type of circulatory system (pipe, tubing, or flexible hose), the fluid medium, and the maximum operating pressure of a system. Some of the most common types of connectors are described below:

a. Threaded Connectors. Threaded connectors are used in some low-pressure liquid-powered systems. They are usually made of steel, copper, or brass, in a variety of designs (Figure 2-24). The connectors are made with standard female threading cut on the inside surface. The end of the pipe is threaded with outside (male) threads for connecting. Standard pipe threads are tapered slightly to ensure tight connections.

To prevent seizing (threads sticking), apply a pipe-thread compound to the threads. Keep the two end threads free of the compound so that it will not contaminate the fluid. Pipe compound, when improperly applied, may get inside the lines and harm the pumps and the control equipment.

b. Flared Connectors. The common connectors used in circulatory systems consist of tube lines. These connectors provide safe, strong, dependable connections without having to thread, weld, or solder the tubing. A connector consists of a fitting, a sleeve, and a nut (see Figure 2-25).

Fittings are made of steel, aluminum alloy, or bronze. The fittings should be of a material that is similar to that of a sleeve, nut, and tubing. Fittings are made in unions, 45- and 90-degree elbows, Ts, and various other shapes. Figure 2-26 shows some of the most common fittings used with flared connectors.

Fittings are available in many different thread combinations. Unions have tube connections on each end; elbows have tube connections on one end and a male pipe thread, female pipe thread, or tube connection on the opposite end; crosses and Ts have several different combinations.

Tubing used with flared connectors must be flared before being assembled. A nut fits over a sleeve and, when tightened, draws the sleeve and tubing flare tightly against a male fitting to form a seal. A male fitting has a cone-shaped surface with the same angle as the inside of a flare. A sleeve supports the tube so that vibration does not concentrate at the edge of a flare but that it does distribute the shearing action over a wider area for added strength. Tighten the tubing nuts with a torque wrench to the value specified in applicable regulations.

If an aluminum alloy flared connector leaks after tightening to the specified torque, do not tighten it further. Disassemble the leaking connector and correct the fault. If a steel connector leaks, you may tighten it 1/6 turn beyond the specified torque in an attempt to stop the leak. If you are unsuccessful, disassemble it and repair it.

Flared connectors will leak if-

• A flare is distorted into the nut threads.
• A sleeve is cracked.
• A flare is cracked or split.
• A flare is out-of-round.
• A flare is eccentric to the tube's OD.
• A flare's inside is rough or scratched.
• A fitting cone is rough or scratched.
• The threads of a fitting or nut are dirty, damaged, or broken.

c. Flexible-Hose Couplings. If a hose assembly is fabricated with field attachable couplings (Figure 2-27), use the same couplings when fabricating the replacement assembly, as long as the failure (leak or break) did not occur at a coupling. If failure occurred at a coupling, discard it.

When measuring a replacement hose assembly for screw-on couplings, measure from the edge of a retaining bolt (Figure 2-28). Place the hose in hose blocks and then in a bench vice (Figure 2-29). Use the front or rear portion of a hacksaw blade for cutting. (If you use the middle portion of a blade, it could twist and break.) For effective cutting, a blade should have 24 or 32 teeth per inch. To remove an old coupling on a hose assembly that is fabricated with permanently attached couplings, you just discard the entire assembly (see Figure 2-30,).

d. Reusable Fittings. To use a skived fitting (Figure 2-31), you must strip (skive) the hose to a length equal to that from a notch on a fitting to the end of the fitting. (A notch on a female portion of a fitting in Figure 2-31 indicates it to be a skived fitting.)

To assemble a conductor using skived fittings-

• Determine the length of the skive.
• Make a cut around the hose with a sharp knife. Make sure that you cut completely through the rubber cover of the hose.
• Cut lengthwise to the end of the hose (Figure 2-32). Lift the hose flap and remove it with pliers.
• Repeat the process on the opposite end of the hose.
• Place the female portion of the fitting in a bench vice (Figure 2-33) and secure it in place.
• Lubricate the skived portion of the hose with hose lubricant (hydraulic fluid or engine oil, if necessary).
• Insert the hose into the female socket and turn the hose counterclockwise until it bottoms on the shoulder of the female socket, then back off 1/4 turn.
• Place the female socket in an upright position (Figure 2-34) and insert the male nipple into the female socket.
• Turn the male nipple clockwise (Figure 2-35) until the hex is within 1/32 inch of the female socket.
• Repeat the above process on the opposite end of the hose.

When assembling conductors using nonskived-type fittings, follow the above procedures. However, do not skive a hose. Nonskived fittings do not have a notch on the female portion of a fitting (Figure 2-36).

Figure 2-37, diagram A, shows a female hose coupling. One end of the hose has a spiral ridge (course thread) that provides a gripping action on the hose. The other end (small end) has machine threads into which the male, fixed or swivel, nipple is inserted.

Figure 2-37, diagram B shows the male adapter, and diagram C shows the male and the female swivel body. These fittings contain a fixed or swivel hex-nut connector on one end. The opposite end is tapered and has machine threads that mate with the threads in a female fitting. With a long taper inserted into a hose and screwed into a female coupling, the taper tends to expand a hose, forcing it against the inside diameter of a female fitting.

Figure 2-38 shows the assembly of a clamp-type coupling. If you use this coupling, do not skive the hose. Lubricate the ID of a hose and the OD of a stem. Clamp a hose stem in a bench vice and install a hose. Turn the hose counterclockwise until it bottoms against the shoulder of the stem (Figure 2-38, diagram A). If you do not have a vice, force the stem into the hose by pushing or striking the stem with a wooden block. Place the clamp halves in position (Figure 2-38, diagram B) and draw them together with a vice or with extra long bolts until the standard bolts protrude far enough to grip the nuts. Remove the extra long bolts and place retaining bolts through the clamp. Tighten the nuts until you get the required torque (Figure 2-38, diagram C).

NOTE: You may have to retighten the bolts after the hose assembly has been operating about 10 to 20 hours. Use clamp-type couplings on hose assemblies with diameters of 1 inch or greater. Use reusable screw-type fittings on hose assemblies with diameters less than 1 inch.

2-11. 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.

a. 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.

b. 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.

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

(1) 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.

(2) 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 contaminant 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.

(3) Maintenance. Regular filter and oil changes, using a high-quality hydraulic oil, add to seal life. Using inferior oil could cause 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.

2-12. 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.

a. 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.

b. 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.

(1) 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.

(2) 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.

(3) Lathe-Cut Seal. This seal is like an O-ring 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, round- and square-section seals are interchangeable, if made from the same material.

(4) 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.

(5) Lip Seal (Figure 2-44). This is 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.

(6) 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.

(7) 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.

(8) Face Seal (Figure 2-47). 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.

c. 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.

d. 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.

(1) 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.

(2) 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 (° F) to +230° F.

(3) 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.

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

(5) 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.

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