Once adjusted, the pressure relief valve opens whenever the pressure goes beyond the value set and allows oil to flow back to the reservoir. The unloading valve is not intended to replace the pressure relief valve. In fact, the pressure setting of the unloading valve is much lower than the setting on the pressure relief valve. An accumulator is used to store hydraulic oil, under pressure, to pressurize the system while the pump is unloaded. This oil is also used to supplement the power pump output during times of heavy use or for limited operations when the pump is not working.
Accumulators also dampen surges within the hydraulic system.
They are used where linear motion is required to move something. Cylinders are ususally double-acting, that is, oil under pressure can be applied to either side of the piston to provide movement in either direction. Single acting cylinders are sometimes used where the weight of the load is used to return the cylinder to the closed position. In terms of how it's built, a motor is like al pump. But, when it's operated oil enters the motor and turns the shaft.