Motorised Points 22

 In a previous post Curly alluded to work being done on Motor Point 22 at Toddington.  Here is a bit more detail of what goes on with a motorised set of points.

On the GWR we now exclusively use Alstom HW point machines.  These are a fairly simple and robust machine and were (still are) in wholesale use over the national network.  The photo below shows the machine fitted to Points 22.

The machine sits on two timbers and is bolted through them.  The 'business end' of the machine is bolted through the sole or gauge plate (along with the rail chairs that hold the toes of the point switches) and this ensures that none of those can move relative to the other, thus keeping the toe of the points at the correct gauge and the machine at the same distance from them.  The machine is connected to the points by four bars:  The first and heaviest is the drive bar, this links the drive stretcher of the points to the drive portion of the machine and this is the first part that we set up.  The machine has a throw of about 150mm which is not adjustable, the points would normally have movement of 105mm to 110mm between the two positions.  In order to couple these two movements together we use an item called a basket, which is a block through which the drive bar goes.  On the Drive Bar, either side of the Basket, is a drive nut and lock nut, these are adjusted tight for each position of the points, allowing the drive bar an amount of free movement before picking up the points and moving them.  This is the first job in setting up the machine and is achieved by hand winding the machine between the two positions until the blades have been set correctly.  The second bar connects the Lock Stretcher (a bar which connects the toes of both blades together) to the facing point lock within the machine.  The purpose of this stretcher and bar is to lock the points safely in one or other of the correct positions and this is the next part that we set up.

When the machine is operated from one points position to another a clever set of cranks and gears (unfortunately hidden beneath covers in the photo above) take the machine through three distinct stages: 1: disengage locking bolt, 2: move drive rod, 3: re-engage locking bolt.  The locking bolt is hidden (as is so much in these machines) under the contact block in the above photo.  It is removed and inserted into a slide which is connected to the locking bar from the points locking stretcher and as previously mentioned is the main safety device to hold the points correctly in either position.  This is adjusted to be only just clear when the points are fully over in each direction (there is a go/no go gauge which is slipped between to point blade and stock rail to set the tolerance).  This process is again done by hand winding the machine back and forth until the adjustment is correct.

Finally we have the two detector bars.  These are independently connected to each point blade toe and are a backup check to prove that the blade toes are in the correct positions (should the lock stretcher break or become detached at either end).  These detector rods are connected to two more slides in the machine which are underneath the contact block.  The astute of you will probably guess that the position of the closed blade on a point is very important, with little room for error, whereas there is far more tolerance in the open blade (just needs to be open enough not to foul the backs of the train's wheels) and this is the case with some slots in the slides connected to these bars.  For the position where the bar concerned with the slide has its blade closed there is a very accurate cut out in the slide, and we adjust the bar to position this correctly.  For when the blade associated with that bar is open, there is a big wide cut out which does not need further adjustment.

Having now set up all four bars correctly we come to the clever bit with the contacts.  The contact block has a set of contacts down each side and in the middle a rocking contact mechanism.  This mechanism sits on two sets of rollers, one down each side, and is held down by a strong spring.  When the locking block is correctly inserted and the detector slides are correctly aligned then all the rollers on one side of the contact mechanism are presented with cut outs below them, and the mechanism rocks to one side, thus making those contacts.  These contacts control three main things, motor power, clutch power and detection of position.  The detection contacts operate relays under the signal box, showing the position of the points and releasing signals which may be cleared when the points are set in that direction.  The motor and clutch contacts control the operation of the machine from one position to the other.  Correct detection of the opposite points position causes the contact mechanism to rock to the opposite side and operate the opposite set of contacts.

Once the machine is correctly set up then the winding handle can be removed and the machine operated electrically.  The winding handle inserts through three aligned holes in plates attached to the machine body, the inserting of the handle and aligning of these holes operates a switch which disconnects the power to the motor.  This is a safety mechanism to prevent injury to the operator.  The switch is reset by pressing a release button after the handle has been removed.

I hope the above isn't too wordy and gives a rough idea of how one of these machines work.  We've re-installed three of these post the winter works.

Neil C.