Fail-safe circuits are basically designed to prevent injury to the operator or damage to the equipment. In general they prevent any accidental fall or overloading of the equipment.
Figure 10.7 shows a fail-safe circuit in which the cylinder is prevented from falling in the event of a hydraulic line rupture.
To lower the cylinder, the pilot pressure from the blank end of the piston must pilot open the check valve at the rod end, in order to allow the oil to return to the reservoir through the DCV. This happens when the push button valve is actuated to permit pilot pressure actuation of the DCV or with direct manual operation of the DCV during pump operation. The pilot-operated DCV allows free flow in the opposite direction to retract the cylinder when this DCV returns to its spring offset mode.
Figure 10.8 is another example of a fail safe circuit in which overload protection is provided for the system components.
The direction control valve 1 is controlled by a push button three-way valve 2. When overload valve 3 is in its spring-offset mode, it drains the pilot line of valve 1. If the cylinder experiences excessive resistance during its extension stroke, the sequence valve 4 pilot-actuates overload valve 3. This drains the pilot line of valve 1, causing it to return to its spring-offset mode. If push button valve 2 is then operated, nothing will happen unless the overload valve 3 is manually shifted into its blocked port configuration. This ensures the protection of the system components against excessive pressure due to excessive cylinder load during the extension stroke.