Hydraulic 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.

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 offer 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 thickness of specially processed wire. Strainers are used to pump inlet lines where pressure drop 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.

Filters. A filter removes small foreign particles from a hydraulic fluid and is most effective as a safeguard against contaminates. 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.

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.

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 check the appropriate regulations for specifications, as some filter a specific torque.
• Pressurize the system and check the filter assembly for leaks.

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:

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.

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 as follows:

• It does not require relief valves because the pump simply shuts off by itself when standby pressure is reached. The prevents heat buildup in systems where relief pressure is frequently reached.
• The size of the lines, valves, and cylinders can be tailored to the flow requirements of each function.
• Reserve flow is available, by using a larger pump, 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.

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.

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.

Series/Parallel Connection. Figure 2-5 shows a variation on the series-connected type. 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.

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.

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.

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.

Hydraulic Regeneration Circuit

A conventional cylinder can exert a larger force extending than retracting because of the area difference between full bore and annulus sides of the piston. The system in Figure 5.41 employs a cylinder with a full bore/annulus ratio of 2:1, and is known as a differential cylinder.

Upon cylinder extension, line pressure P is applied to the right hand side of the piston giving a force of P x A, while the left-hand side of the piston returns oil via valve V 3 against line pressure P producing a counter force P x A/2. There is thus a net force of P x A/2 to the left. When retraction is called for, a force of P x A/2 is applied to the left-hand side and fluid from the right-hand side returns to tank at minimal pressure. Extension and retraction forces are thus equal, at P x A/2.

Hydraulic system

A solution along hydraulic lines is shown in Figure 1.2. A hydraulic linear actuator suitable for this application is the ram, shown schematically in Figure 1.2a. This consists of a movable piston connected directly to the output shaft. If fluid is pumped into pipe A the piston will move up and the shaft will extend; if fluid is pumped into pipe B, the shaft will retract. Obviously some method of retrieving fluid from the non-pressurised side of the piston must be incorporated.

The maximum force available from the cylinder depends on fluid pressure and cross sectional area of the piston. This is discussed further in a later section but, as an example, a typical hydraulic pressure of 150 bar will lift 150 kg cm -2 of piston area. A load of 2000 kg could thus be lifted by a 4.2cm diameter piston.

A suitable hydraulic system is shown in Figure 1.2b. The system requires a liquid fluid to operate; expensive and messy and, consequently, the piping must act as a closed loop, with fluid transferred from a storage tank to one side of the piston, and returned from the other side of the piston to the tank. Fluid is drawn from the tank by a pump which produces fluid flow at the required 150 bar. Such high pressure pumps, however, cannot operate into a dead-end load as they deliver constant volumes of fluid from input to output ports for each revolution of the pump shaft. With a dead-end load, fluid pressure rises indefinitely, until a pipe or the pump itself fails. Some form of pressure regulation, as shown, is therefore required to spill excess fluid back to the tank.

Cylinder movement is controlled by a three position changeover valve. To extend the cylinder, port A is connected to the pressure line and port B to the tank. To reverse the motion, port B is connected to the pressure line and port A to the tank. In its centre position the valve locks the fluid into the cylinder (thereby holding it in position) and dead-ends the fluid lines (causing all the pump output fluid to return to the tank via the pressure regulator).

There are a few auxiliary points worthy of comment. First, speed control is easily achieved by regulating the volume flow rate to the cylinder (discussed in a later section). Precise control at low speeds is one of the main advantages of hydraulic systems.

Second, travel limits are determined by the cylinder stroke and cylinders, generally, can be allowed to stall at the ends of travel so no overtravel protection is required.

Third, the pump needs to be turned by an external power source; almost certainly an AC induction motor which, in turn, requires a motor starter and overload protection.

Fourth, hydraulic fluid needs to be very clean, hence a filter is needed (shown in Figure 1.2b) to remove dirt particles before the fluid passes from the tank to the pump.

One final point worth mentioning is that leaks of fluid from the system are unsightly, slippery (hence hazardous) and environmentally very undesirable A major failure can be catastrophic.

Hydrostatic Transmissions

A hydrostatic transmission (HST) is simply a pump and motor connected in a circuit. Other components are added to obtain certain operating features.

The four basic configurations of hydrostatic transmissions are

1. In-line (Fig. 6.10a)
2. U-shape (Fig. 6.10b)
3. S-shape (Fig. 6.10c)
4. Split (Fig. 6.10d)

Various pump and motor designs can be paired together for the split configuration. Manufacturers use the same designs to build the in-line, U-shape, and S-shape configurations.

Hydraulic hose is available with a working pressure rating of 6000 psi. Use of these hoses allows the split transmission to be operated at the maximum pressure rating of high-performance pumps and motors. Particularly on a vehicle, the split transmission can offer many advantages. Most often, the pump is mounted on a pump mount bolted directly to the engine, but the motors can be placed at the most convenient location, thus taking full advantage of fluid power’s ability to flow power “around a corner.”

The split transmission, because of the flexible hoses between the pump and motor(s), will absorb some deflection caused by dynamic loads applied to the frame. Consider the situation when the pump and motor are bolted rigidly together, the pump is bolted to the engine (not driven with a universal joint drive line), and the motor is bolted to the frame. Now the pump motor housing is subject to the dynamic loads applied to the frame. This introduction of stress  into the housing is undesirable and can lead to reduced reliability.

Hydraulic Bladder Accumulator

Bladder- or bag-type accumulators consist of a shell or case with a flexible bladder inside the shell. See figure 9-7. The bladder is larger in diameter at the top (near the air valve) and gradually tapers to a smaller diameter at the bottom. The synthetic rubber is thinner at the top of the bladder than at the bottom. The operation of the accumulator is based on Barlow’s formula for hoop stress, which states: “The stress in a circle is directly proportional to its diameter and wall thickness.” This means that for a certain thickness, a large diameter circle will stretch faster than a small diameter circle; or for a certain diameter, a thin wall hoop will stretch faster than a thick wall hoop. Thus, the bladder will stretch around the top at its largest diameter and thinnest wall thickness, and then will gradually stretch downward and push itself outward against the walls of the shell. As a result, the bladder is capable of squeezing out all the liquid from. the accumulator. Consequently, the bladder accumulator has a very high volumetric efficiency. In other words, this type of accumulator is capable of supplying a large percentage of the stored fluid to do work.

The bladder is precharged with air or inert gas to a specified pressure. Fluid is then forced into the area around the bladder, further compressing the gas in the bladder. This type of accumulator has the advantage that as long as the bladder is intact there is no exposure of fluid to the gas charge and therefore less danger of an explosion.

Hydraulic Piston Accumulators

Piston-type accumulators consist of a cylindrical body called a barrel, closures on each end called heads, and an internal piston. The piston may be fitted with a tailrod, which extends through one end of the cylinder (fig. 9-5), or it
may not have a tailrod at all (fig. 9-6). In the latter case, it is referred to as a floating piston. Hydraulic fluid is pumped into one end of the cylinder and the piston is forced toward the opposite end of the cylinder against a captive charge of air or an inert gas such as nitrogen. Sometimes the amount of air charge is limited to the volume within the accumulator; other installations may use separate air flasks which are piped to the air side of the accumulator. Piston accumulators may be mounted in any position.

The gas portion of the accumulator may be located on either side of the piston. For example, in submarine hydraulic systems with tail rod pistons, the gas is usually on the bottom and the fluid on top; in surface ships with floating pistons, the gas is usually on the top. The orientation of the accumulator and the type of accumulator are based upon such criteria as available space, maintenance accessibility, size, need for external monitoring of the piston’s location (tail rod indication), contamination tolerance, seal life, and safety. The purpose of the piston seals is to keep the fluid and the gas separate.

Usually, tail rod accumulators use two piston seals, one for the air side and one for the oil side, with the space between them vented to the atmosphere through a hole drilled the length of the tailrod. When the piston seals fail in this type of accumulator, air or oil leakage is apparent. However, seal failure in floating piston or non-vented tail rod accumulators will not be as obvious. Therefore, more frequent attention to venting or draining the air side is necessary. An indication of worn and leaking seals can be detected by the presence of significant amounts of oil in the air side.