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Pointwork Control Example Pneumatic

DocRef: JC2a

For other methods of point control see Pointwork Control : Overview

This example of a compressed air driven (pneumatic) installation is in Eastern England and utilizes Sunset Valley Railroad parts in conjunction with a 25 litre Draper oil-free electric compressor. The compressor is fitted with a pressure regulator and for this installation the "line" pressure (as opposed to tank pressure) is set to 40 psi (2.75 Bar). The tank pressure is set to 90 psi (6.2 Bar), which means the compressor will not cut-in (it is noisy) until the tank pressure drops to below 40 psi. The recommended minimum operational pressure for the SVRR system is 30 psi.

The air supply gauge and pipework which which connects to the compressor's output pressure regulator and feeds the system via control panels and manifolds.

The point "actuators" are small cylinders manufactured by the Clippard Corporation (USA) which incorporate a spring which returns the piston as soon as the pressure is released by the miniature switches on the control panel.

A selection of the components employed, including switches, manifolds, air supply line connectors and pressure monitor green "eye".

For efficiency of operation it is recommended that (wherever possible) the actuators are mounted so they are de-activated when the points are in the "normal"/principal route setting. In order to achieve this it is, in many instances, necessary to use a reversing crank in the linkage between actuator and tie-bar (e.g. where multiple tracks are too close for the actuator to fit between). To operate double slips using just two actuators (they have four tie-bars to ensure correct closure of blades) it was necessary to introduce an "equalizing" beam to compensate for the unequal movement and ensure all switch blades close fully.

Advantages of this system include the removal of the need to make any adjustments to linkages (the pneumatic nature makes it self-adjusting) and the fact that it will work in all weathers (even if the actuators are submerged in rain water).

The simplest linkage.

Linkage for a double slip with "equalisation" (all the joints are pivots)

Reversing Linkage for a double slip with "equalisation" (all the joints are pivots). The same "reversing" arrangement is used for simple turnouts but without the equalisation component.

Some difficult locations may require an extended linkage as here (note the intermediate guide "sleeve" to prevent lateral and vertical movement of the operating bar).

Control Panels and Micro Switch Circuits to Show Route Settings.

One disadvantage of a pneumatic system (by comparison with a digital/electro-mechanical system) is the lack of electrical signals to indicate route settings on the control panel. This can be overcome by employing either IP67 (weatherproof) micro switches or magnetic reed switches, to detect the point settings and illuminate LEDs on the panel.

To ensure a positive (locking) movement in both directions, it is necessary to employ two detectors for each slidebar. In cases where the pointwork is readily visible to the signalman, this is probably not necessary but in obscured locations and where "foreign objects" (e.g. debris falling onto an outdoor line) may foul switch blades, it is desirable to have dual detection.

I have elected to use microswitches for the "divergence" setting and magnetic reed switches to indicate the default/normal route setting. Low voltage (6V or 12V) batteries provide the power, via a 5V reducing circuit. The operating arm of the microswitch is set to bear upon the end of the slidebar, while the magnet for the reed switch is fixed to either the slide bar or the operating rod from the pneumatic actuator. It is possible to integrate the output from a sequence of switches to provide a warning signal, as well as an indication of the slidebar position. An example would be to provide a flashing LED warning on the panel, if only one turnout of a crossover pair had properly "locked" in position.

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Page last modified on February 27, 2018, at 10:37 PM