Parker Pneumatic solenoid valves are specified for Wheelslide Protection Systems developed by Westinghouse Brakes Limited and supplied for current contracts with Alstom and Adtranz in the UK, and for a completed retrofit contract for London Underground's (LUL) Central Line. Westinghouse Brakes has applied innovative technology to the problem of wheelslide protection and produced a system which provides benefits to passengers, train operators, train leasing companies and train builders - on all vehicle types fitted with pneumatic brakes. The system has a novel control technique which uses maximum available adhesion to minimise stopping distances, air consumption and wheel damage on a wide variety of vehicle configurations from light metros to heavy locomotives.
The system is suitable for OEM or retrofit applications.
The system enables high train frequencies to be maintained in poor weather conditions, by improving adhesion and by improving track condition.
Wheel flats can be avoided, improving ride quality and reducing train maintenance costs - and even avoiding the risk of derailment from a large wheelflat.
This train anti-lock brake system uses axle-speed sensors, an electronic control unit and pneumatic dump valves - each incorporating a specially designed bank of four Parker Pneumatic solenoid valves - which are located next to the brake actuators and are used to control the applied force of the friction brake.
The electronic control unit detects axle and vehicle speed, controls braking forces, drives the speedometer, controls sanding, and carries out diagnostic monitoring.
Sensors - one per axle - can be the passive type (magnetic) or optical, although active (proximity) sensors with a toothed wheel are the most common.
The car-based wheelslide controller monitors and controls all axles independently to compare their performance and detects low adhesion when there is a speed difference or excessive deceleration in one or more axles.
The system's technique for detecting the correct train speed is to release single axles in a controlled manner, allowing them to return to the train speed.
Slip is controlled at 15 to 20% levels, which allows heating to take place through friction between the wheel and rail, reducing rail wetness or contamination ã following axles therefore have better adhesion levels, minimising the stopping distance.
In the case of wet leaves on the line, no such cleaning or track conditioning takes place, but the system will still maximise use of available adhesion to brake the train without the wheels locking up.
Sanding is helpful in wet leaf conditions, but it is vital not to use too much sand, as signalling circuits can be affected.
The Westinghouse system allows sand to be applied behind the leading axle or bogie, with the leading axle or axles being used to monitor the adhesion level and determine when to apply sand.