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Pointwork Control Example EM

Electro-Mechanical Control (for Overview of Other Systems see Pointwork Control Overview)

This is an example of an installation using "traditional" bell cranks and rods. The motor is electrically powered and is of the "stall" type in that the motor simply hits the stops and stalls. There are two main suppliers of this type. The "Tortoise" motor and the "Cobolt" motor. The Cobolt motor has a a number of access connections that can used to trigger other devices (such as signals). Either motor is connected to a 12V line via some form of two way switch or two way relay. There is a variable slide that is used as a fulcrum to alter the "throw" of the operating lever of the device, the maximum is 18mm or +/-9mm.

Cobolt motor

Motors in frame

The motion is either used to directly throw the points, (the motor being at one end of the tie bar) or to move the tie bar from a distance via the rods and cranks. I have successfully used a direct "pull" of 3.5m on a rod which translates to 79m (scale). It is better to "pull" the rods than to "push" them, as this removes any bending . Use some form of return spring on the other side of the tie bar to ensure that there is no "slack" in the system.

Rod length

The worst part of the operation is working out which way the bell cranks should rotate... If you use the premise that the top of the bell crank is horizontal to and points AWAY from the direction, the motor pulls on it then the other side of the crank connects to the rod linkage. You then reverse this for connecting to the tie bar!

Crank arrangement

Using an electrical system involves the problem of water.... Design your motor assembly to be easy to remove at the end of a session. Supplying power to your motors and switching them can be very "simple" or "cunning". Each motor will require a Dual Pole Dual Throw (DPDT) switch to operate the reversal of the polarities. A DPDT relay will perform the same function and can be remotely operated by a simple flick switch. Interlocking the points can be done with a little thought by looping the feed wires through more than one set of motors. Thus hitting a switch "open" moves two points and can be used to "close" other points on the one switch motion!

This is my control panel for my points and signals, points are blue, signals are red, and level crossing gates are yellow. Green is of course the on-off switch.

Display Panel

Power for the system comes from an ATX computer PSU which supplies 12v, 5v and 3v lines.


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Page last modified on February 20, 2018, at 08:15 AM