Pressure control is a key element in the design of any circuit. Not only is it the key to achieving a given functional objective, it is also the key to safe operation.
Components are designed to operate at a given maximum pressure and will withstand pressure peaks up to some burst pressure. Failure of a component can be dangerous to nearby workers. They can be injured by shrapnel, or they may be injured when they are hit by a stream of high-pressure, high temperature oil. Injuries received when oil penetrates the skin are very difficult to treat and require specialized medical knowledge. Often, there is also potential for worker injury by losing control of a load held against gravity.
The fundamental pressure control problem in circuit design is the limiting of pressure to a level below the working pressure of the lowest-rated component in the circuit. If a piece of hose, rated at 1500 psi working pressure, is used in a circuit where all other components are rated at 3000 psi, then maximum pressure in this circuit must be limited to 1500 psi. Pressure can build to the relief valve setting at all points between the pump and the load.
Six pressure-control valves will be discussed in this section, These valves are:
1. Relief valves
2. Unloading valves
3. Sequence valves
4. Pressure-reducing valves
5. Counterbalance valves
6. Brake valves
Each of these valves works on the same principle; a spring force balances a hydraulic force. The hydraulic force is produced by pressure acting on a given area. When the hydraulic force becomes greater than the spring force, the valve spool moves. There are many different ways in which this principle is used in valve design. The construction of some valves is intricate, but the principle of operation is simple. It is appropriate to re-emphasize the principle; a spring force opposes a hydraulic force.