Unlocking the Lock

 Monday 7th Feb

The following is a detailed description of the resolution of a problem with an electric lock from Toddington signal box courtesy of Neil Carr.

For a while now the lever lock on lever 23 at Toddington controlling the point machine for the North siding has been intermittently sticking requiring the cover to be removed, and a brief bit of S & T assistance to the lock mechanism. Often the lock appeared stuck, but then freed when pulled manually  and worked happily once again.

Since the box is currently closed , whilst much point replacement work is happening, the chance was taken to unwire and remove the lock - a Tyers large lock with contract drum. That in itself isn't a five minute job, as there are many connections,  and the lock is well hidden behind the locking trays. These locks have more travel than a standard lock,  having several locking positions   and are driven directly from the lever via an actuating arm  clamped to the lever shaft; therefore they are usually found behind the normal locking and among the signal wires.

The lock was then brought back to the  S & T Coach to be stripped, cleaned and reassembled. 

Given that the lock intermittently jammed up, and then worked for a period once freed  I was expecting to find some dirt or swarf in the mechanism 

 What I actually found  amazed me. At some point in the last few years a signalman must have dropped a plastic pen down through one of the lever slots in the frame; against all odds this had bounced into the back of the lock mechanism and had been comprehensively chewed up by the locking bar moving up and down as the lever was operated. Now and then bits of plastic must have jammed in the lock bolt mechanism  preventing the solenoid from being able to pull the bolt from the lock. 

Whilst the lock was apart the entire mechanism was cleaned of old oil and rubbish ( and pen) the contacts on the contact drum were stripped and cleaned and the lock was set up correctly with the contacts all operating in the correct positions. It was then reassembled clean and dry of oil and grease

This photo gives a good idea of the physical size of these locks, the locking bar is just over a metre long.

Looking right to left we start with the top of the locking bar (they are mounted vertically) where the actuating arm from the lever attaches. Next we have the coli and lock closed connections, then the lock bolt itself , followed by the solenoid coil with the lock actuating arm above it. Energising the coil pulls the iron block at the left hand end of the coil tight to the coil, and via the arm withdrawsthe lock bolt from the locking bar. This locking bar has three positions; as shown it is locked in "Normal"   where the lever is normal in the frame and the points would lie in their normal position.  Underneath the coil you will see a very long slot cut in the bar, when the lock is energized, and the lever moved away from normal the bar is drawn towards the right ((in this picture , in reality it is lifted upeards) until the lock bolt drops into that long slot. The bar then continues its travel until the lefthand end of that long slot hits the lock bolt. The lever is then in the 'Reverse Check' position, and the contact for operating the points reverse is made.

Once the points have moved and been detected in reverse the lock will again energise (automatically via the ele trial locking) and allow the lever to be moved fully reverse  where it will again be locked

Moving from reverse to normal is the same procedure as above , only going the other way. 

The large casting on the left hand end of the lock body is the contact drum. This has a series of pairs of contacts over rotary contact rings, rotated via a gear wheel and a long angled slot in the locking bar. These contacr boxes can have many contact rings and many different lengths and positions of rings - allowing contacts to be made at various positions of the locking bar and for varying durations of its travel. 

An in-depth description of the varying contact combinations is probably touch detail for here, however, suffice it to say.that this contact drum is configured with four contact rings; providing contacts for the Normal, Normal Check, Reverse Check and Reverse lever positions.

The astute of you will however notice that this drum has eight sets of contacts, although only four are used. These locks can have drums which are much longer and can carryall least twice as many contact rings

I hope you found this Informative!

Curly ( on behalf of Neil Carr - S & T Electrical Engineer)