The symbol for an unloading valve is similar to the symbol for a relief valve except that the pilot line is not connected to sense pressure at the valve inlet. The two symbols are compared in Fig. 3.14.
A circuit with an unloading valve is shown in Fig. 3.15. It is necessary to first discuss the operation of this circuit and understand the function of the unloading valve before studying the operation of the valve.
The accumulator is a key component in the Fig. 3.15 circuit. There are three types of accumulators: bladder, diaphragm, and piston. The diaphragm accumulator is a pressure vessel divided into two compartments by a flexible diaphragm. The top half is precharged with a gas, generally nitrogen, and sealed. The bottom half is connected to the hydraulic circuit. Fluid is pumped through the check valve into the bottom of the accumulator. The bottom half is filled, and extra fluid is pumped in as the diaphragm bulges upward. Pressure increases as the gas is compressed. The accumulator is designed for some rated pressure, and pressure must be controlled to ensure that it does not exceed this rating. The unloading valve accomplishes this task.
An accumulator provides pressure to the actuator (cylinder in Fig. 3.15) at the moment the directional control valve is shifted. Pressure does not have to build from a low pressure as it does in a circuit with a open-center directional control valve . Often, acceleration of the load is a significant issue in circuit design. In most manufacturing applications, profitability is increased when the number of cycles per unit time is increased. The cylinder must extend and retract as quickly as possible. If a large load is being moved, pressure must build to achieve enough force to overcome static friction and provide the inertial force to accelerate the mass. It takes an interval of time to build this pressure. This time delay can be eliminated if pressure is already available at the moment the directional control valve is shifted.
An accumulator also provides another important feature. The extra fluid stored in the accumulator allows the desired actuator speed to be achieved with a smaller displacement and, therefore, a lower-cost pump. An example will illustrate how this can be an advantage.
The actuator cycle has a 5-second active part and 20-second passive part, meaning that flow is needed for 5 seconds, and there is a 20-second interval before flow is needed again. The accumulator can supply 924 in3 of oil. If it supplies this fluid during the active part of the cycle, it must be recharged during the passive part. Pump flow to accomplish the recharge is
The pump and prime mover must be four times larger if the accumulator is not included in the circuit.
The analysis done for this example does not include all factors. Design of a circuit with an accumulator requires knowledge of the pressure vs. volume curve as the accumulator is being filled.