Fail-Safe Circuits

Fail-safe circuits are those designed to prevent injury to the operator or damage to the equipment. In general, they prevent the system from accidentally falling on an operator and also prevent overloading of the system. In following sections we shall discuss two fail-safe circuits: One is protection from inadvertent cylinder extension and other is fail-safe overload protection.

1. Protection from inadvertent cylinder extension: Figure 1.13 shows a fail-safe circuit that is designed to prevent the cylinder from accidentally falling in the event when a hydraulic line ruptures or a person inadvertently operates the manual override on the pilot-actuated DCV when the pump is not working. To lower the cylinder, pilot pressure from the blank end of piston must pilot open the check valve to allow oil to return through the DCV to the tank. This happens when the push button is actuated to permit the pilot pressure actuation of DCV or when the DCV is directly manually actuated when the pump operates. The pilot-operated DCV allows free flow in the opposite direction to retract the cylinder when this DCV returns to its offset mode.

2. Fail-Safe System with Overload Protection: Figure 1.14 shows a fail-safe system that provides overload protection for system components. The DCV V1 is controlled by the push-button three-way valve V2. When the overload valve V3 is in its spring offset mode, it drains the pilot line of valve V1. If the cylinder experiences excessive resistance during the extension stroke, sequence valve V4 pilot-actuates overload valve V3. This drains the pilot line of valve V1 causing it to return to its spring offset mode. If a person then operates the push-button valve V2 nothing happens unless overload valve V3 is manually shifted into its blocked-port configuration. Thus, the system components are protected against excessive pressure due to an excessive cylinder load during its extension stroke.

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