A spring-loaded accumulator is similar to the weight-loaded type except that the piston is preloaded with a spring. A typical cross-section of this type of accumulator has been illustrated in Figure 7.15.
The spring is a source of energy, acting against the piston and forcing the fluid into the hydrauhc system. The pressure generated by this accumulator depends on the size and preloading of the spring. In addition, the pressure exerted on the fluid is not constant. They typically deliver small volumes of oil at low pressures and therefore tend to be heavy and large for high-pressure, large volume systems.
A spring-loaded accumulator should not be used for applications requiring high cycle rates as the spring may lose its elasticity and render the accumulator useless.
These types of accumulators (frequently referred to as hydro-pneumatic accumulators) have been found to be more practically viable as compared with the weight and springloaded types. The gas-loaded type operates in accordance with Boyle’s law of gases, according to which the pressure of a gas is found to vary inversely with its volume for a constant temperature process.
The compressibility of the gas accounts for the storage of potential energy in these accumulators. This energy forces the oil out of the accumulator when the gas expands, due to a reduction in system pressure.
Gas-loaded accumulators fall under two main categories:
1. Non-separator type
2. Separator type.
The weight-loaded type is historically the oldest type of accumulator. It consists of a vertical heavy wall steel cylinder, which incorporates a piston with packing to prevent leakage (Figure 7.14).
A dead weight is attached to the top of the piston. The gravitational force of the dead weight provides the potential energy to the accumulator. This type of accumulator creates a constant fluid pressure throughout the full volume output of the unit, irrespective of the rate and quantity. The main disadvantage of this accumulator is its extremely large size and heavy weight.