When the fluid pressure in a closed hydraulic system reaches a specified level, it forces the relief valve open, which in turn recycles the hydraulic fluid into the system's reservoir. This relief action prevents damage to the system from too much pressure and helps to maintain consistent force.
The cracking pressure is the pressure at which a relief valve opens. When the valve is fully open, it is in a state called full-flow pressure. The amount of force necessary to achieve this reaction is dependent on the type and setting of the relief valve.
Direct-acting valves, one commonly used type of relief valve, are preset to release pressure by the strength of the compression spring used in their construction. Between the spring and the fluid release hole is a ball held in position to block the hole until the fluid pressure can depress the spring and unblock the hole. On the other end of the spring is a screw that adjusts to change the amount of compression force needed to open the valve.
Some systems have pressure-compensated relief circuits that can run successfully without the need for a pressure-relief valve. However, most systems still have a relief valve as a fail-safe mechanical feature should the electronics malfunction.