Skidthek for air/hydrauli brakes
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SKIDCHEK, the Girling anti-skid system is now available for vehicles having air/hydraulic actuated brakes. Believed, by Girling, to be the first anti-skid device for such systems, its first commercial application is on a Ford fire engine for the Hereford and Worcester Fire Brigade.
The system consists of an air-operated hydraulic de-boost valve which controls the hydraulic fluid pressure at anti-skid conditions. Individual wheel sensors transmit wheelspeed data to an electronic control module which, in turn, actuates a solenoid-operated air shut-off valve plumbed into the air side of the de-booster.
When the wheel attempts to lock, the solenoid valve shuts off the air supply to the debooster and dumps the air between it and the solenoid. A piston in the de-booster valve retracts, relieving the hydraulic pressure to the wheel cylinders and the excessive brake torque. When the "apply brakes" signal is received the solenoid valve opens to reenergise the air system and. cause the de-booster valve piston to shift forward, pressurise the hydraulic fluid and reapply brake torque. Under normal braking the piston in the de-booster valve remains stationary leaving the basic brake system to function normally.
Girling claim that modulating the hydraulic fluid pressure instead of the air pressure provides an improved response time, minimises air consumption and allows retention of the tandem air/hydraulic actuator normally specified for these systems.
Air consumption
The de-boost valve operates on the final, torque generating stage of any brake application rather than over the whole process of covering the shoe/ drum clearance and then generating brake torque. The air consumption of a single-axle system when the anti-skid is in operation is claimed to be around the output of a compressor at engine idling.