Signals & Telegraph: servicing Harmans Cross lever frame
The Swanage Railway operates three signal boxes plus the Norden crossing control. Each of these has a lever frame that controls points, signals and other equipment such as facing point locks. The lever frame was first developed in the nineteenth century and changed little until it was finally replaced by electric systems.
The basic principle of a lever frame is very simple: each point and signal is attached to a lever by either rods or steel wire and changes when the level is operated. A system known as 'locking' is provided to ensure that the signalman does not inadvertently set a dangerous combination of points and signals. This locking system uses a combination of interlocking bars moving back to front and left to right. The design of these mechanically prevents levers being pulled or released.
Most of the lever frame is out of sight below the signal box floor with access via a small door at ground level. In addition to the locking, which is arranged in trays, there is also the pulley wheels for the signal wires and cranks for the point rodding. Wire is used for signals since gravity returns the signal to the horizontal position but points need to be pulled or pushed into position using rods.
The S&T department provided the following update on their Facebook page:
Once every ten years the mechanical locking frames in our signal boxes need servicing. Today it was the turn of the Stevens & Sons 22 Lever Elevated Frame at Harman’s Cross. This type of frame dates from the late 19th century. The Harmans Cross frame has two locking trays each full of rods that slide to and fro driven by the notches in the tappets (the curved bars above the trays) interacting with the tabs on the locking rods.
Everything was taken apart, cleaned and oiled and then reassembled ready for a thorough test tomorrow. It is a beautiful piece of machinery that resembles nothing so much as a 3D jigsaw but without it we can’t run trains safely.
More photographs of the lever frame are shown in the gallery below (use the arrows to move between the images) and in the Facebook post.
The Signals and Telegraph Wednesday Gang have their Facebook group where the original version of this this news item and all their latest updates are posted.
Photographs: Swanage Railway's S&T Wednesday Gang.