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Linear actuator helps to test railway points

11 April, 2013

An innovative system for testing railway point-changers using a linear actuator has cut testing times by two thirds while enhancing operator safety.

The actuators that switch railway points are safety-critical components. They perform the simple task of moving railway tracks, but must do so safely and reliably despite the high dynamic forces. Points-changers used in the UK operate from 110V DC supplies with a “throw bar” that moves the rails, exerting a force around 3kN. The sideways axial movement is created by a ballscrew that is driven by a DC motor, and the position is monitored using safety relays.

Proof that these actuators meet exacting railway standards was one of the main functions of a new test system developed by QM Systems, based in Aldershot. The company was also able to cut the time needed for the tests by a factor of three, thus reducing the operator workload and increasing safety.

QM faced the challenge of finding a method of testing point-changers that would be credible to a conservative and safety-conscious market. The test system had to be accurate, and capable of recording and reporting measurements precisely. The test process also had to be faster than the 1½ hours previously required, yet had to provide a step change in safety with a minimum of manual handling. It also had to be capable of testing both trackside and in-track changers, in old and new designs.

The system that QM has devised connects to points-changers that are brought in on wheeled trollies. The points-changers are located accurately on dowel pins, followed by connecting the throw bar mechanically and making electrical connections to the motor. External resistance to the throw bar is provided by a motorised ballscrew linear actuator, controlled to deliver a programmed stroke/force profile.

The points-changer is zeroed and then driven to the closed position. Stroke measurements are made using an encoder, with the stroke defining the changer model for the test report. The operator adjusts the torque-limiting clutch in the changer manually to trip at a force that is dialled up on one of two touchscreen operator stations that run on National Instruments software. The electrical functions of the points-changer, including the safety limit switches, are tested automatically.

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