The Basic 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 in place 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 environment is very important. The following paragraphs discuss several basic hydraulic systems.
a. Hydraulic Jack. In this system (Figure 2-1, page 2-2), 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, page 2-3, 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, page 2-4, 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, page 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.
Figure 2-1. Hydraulic jack
Figure 2-2. Motor-reversing system
Figure 2-3. Open-center system
Figure 2-4. Open-center system with a series connection
(2) 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.
(3) Flow Divider. Figure 2-6, page 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, page 2-6, 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, page 2-7, shows a closed center system. In this system, a pump of small but constant volume charges an accumulator.
Figure 2-5. Open-center system with a series/parallel connection
Figure 2-6. Open-center system with a flow divider
Figure 2-7. Closed-center system
Figure 2-8. Fixed-displacement pump and 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, page 2-8, 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, page 2-8, 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 variabledisplacement pump more efficient. The return oil from a system’s functions is sent directly to the inlet of a variable-displacement pump.
Figure 2-9. Variable-displacement pump
Figure 2-10. Closed-center system with charging 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.
