Inside the US&S A-5 Switch-Point Machine

This blog devotes a lot of coverage to the plight of the pneumatic switch-point machine and its long decline since its heyday between 1930 and 1955, but apart from the ka-chug--woosh sound, what makes them distinctive? Well its actually their simplicity compared to their electric counterparts and its that simplicity that made them so competitive with electric point machines despite the overhead of making and distributing compressed air. 

Above we see a Union Switch and Signal model A-5 pneumatic point machine. (An earlier A-1 model also existed and you can tell them in photos because the housing looks like a miniature Quonset hut.) At one end we see the double acting air piston, at the other we see a connection for the control circuits and a blind tube without an air piston, and in the middle we have a fairly plain looking box.

Next lets quickly go over a standard power operated switch layout using some casual terminology. Moving away from the camera we have a stretcher bar that provides rigidity. Next is the throw mechanism that transmits force from the point machine to the points via a rod. This uses a slack coupling so the point mechanism can begin to move and break the static friction. Finally we have the detection rods that accurately detect if the points are hard against the stock rail and communicate that state to the signaling system.

Exposed US&S A-5 pneumatic point machine.

The mechanism can be set up for operation from either side.

Here we see a naked A-5 without its cover on showing off the simplicity of the action. The air piston connects to an action bar which runs through a crank mechanism that transmits force to the point rodding and also its positional state to the circuit controller.

The crank (left) and circuit controller (right).

The coupling between the air piston and the action bar.

The air piston, these came in 3 inch and 5 inch diameter models.

The throw rod couples to the crank assembly. In the photo below this connection is the bolt hole on the stubby arm below the pivot pin. The detection rods sit under the circuit controller have the golden coated nuts on them.

The circuit controller pulls double duty, detecting both the position of the points for the signaling system and also the position of the action bar / air piston, which is sent to the style CP air value unit to cut the flow of air and dump the pressure in the piston. We can see the hind end of the action bar that is given room to push outward under the covering.

The upper two rods provide the actual point detection while I believe the lower rod that extends through the mechanism provides a physical locking function in conjunction with the action bar. When this technology was new that rod would serve the role of the familiar facing point lock.

The goal of the mechanism is to lock the switch point securely against the stock rail even when air pressure in the cylinder has been exhausted, but allow smooth movement of the action bar and points when air is admitted to move the piston. A similar concept is present in firearms where the bolt is locked against moving rearward by the force of gases in the chamber, but is able to be easily unlocked via motion in a different axis.

While I have personally captured video of an A-5 point machine performing a throw cycle, it only showed the points moving along with the admittedly cool sound. However I recently came across a different video from a UK rail museum that has their own flavor of a US&S A-series set up for live demonstrations. Still popular on the London Underground, these instances are commonly used without the protective cover which allows visitors to actually see how the mechanism functions. This flavor of Westinghouse point machine is intended to be set between the rails and has the throw rods moving through the mechanism. The extra circuit controller on the back provides feedback to the pneumatic value.