A good starting point for any hydraulic schematic is at the power unit. The power unit consists of the reservoir, pump or pumps, electric motor, coupling and coupling guard, and entry and exit piping, with flow meters and return filter. It also might include relief valves, unloading valves, pressure filters, off-line filtration circuits, and control valves. The power unit must be able to cycle all functions in the allotted time at a pressure high enough to do the work intended. A well-designed circuit will run efficiently with little to no wasted energy that generates heat. It will run many years with minimum maintenance if its filters are well maintained and it is not overheated.

When items such as pressure gauges and flow meters are installed, it is easy to troubleshoot any system malfunction quickly and accurately. Flow meters always show pump flow (or lack thereof) and eliminate premature pump replacement. They can indicate impending pump failure well in advance of system failure. Also quick-disconnect plug-in type ports at strategic locations make it easy to check pressure at any point.

Directional control valves

The circuit in Figure 5-2 has only one directional control valve to extend and retract the main cylinder. Pressure-control valves make the hydraulic motor and rotary actuator operate in sequence after the cylinder extends and builds a preset pressure. (This is not the best way to control actuators, but it is shown here to demonstrate the use of different valves.)

An isolation check valve between the pumps keeps the high-pressure pump from going to tank when the low-volume pump unloads. A pilot-operated check valve in the line to the cap end of the main cylinder traps fluid in the cylinder while the motor and rotary actuator operate.

Pressure-control valves

A pressure-relief valve at the pumps automatically protects the system from overpressure. An unloading valve dumps the high-volume pump to tank after reaching a preset pressure. A kick-down sequence pressure-control valve forces all oil to the cylinder until it reaches a preset pressure. After reaching this pressure, the valve opens and sends all pump flow to the hydraulic motor first. A sequence valve upstream from the rotary actuator keeps it from moving until the hydraulic motor stalls against its load. A pressure-reducing valve ahead of the hydraulic motor allows the operator to set maximum torque by adjusting pressure to the motor inlet. (All of these controls are covered in the text of this manual.)

Another pressure-control valve -- called a counterbalance valve -- located in the rod end line of the main cylinder keeps it from running away when the directional control valve shifts. The counterbalance valve is adjusted to a pressure that keeps the cylinder from extending, even when weight on its rod could cause this to happen.

Accumulators

Because hydraulic oil is almost non-compressible, a gas-charged accumulator allows for storage of a volume of fluid to perform work. The expandable gas in the accumulator pushes the oil out when external pressure tries to drop. The accumulator in this circuit makes up for leakage in the cylinder cap-end circuit while pump flow runs the hydraulic motor and rotary actuator. Use care when specifying and using accumulators because they can be a safety issue.

These and other hydraulic components are explained and applied in the following chapters.