The Locking Plate.

The designing of the locking plate is one of the few relaxations permitted to the signal expert.

The correctly designed locking plate provides that the signal and points levers have their movements so controlled and their grouping so selected that no signal lever can be pulled to “clear” unless the correct route is set, no shunt can be signalled unless provided for, no two conflicting movements can be signalled together, no facing point can be left unlocked on the ground for a train route, and no trap point left open on the road signalled.

Some half dozen different types of locking are in use in New Zealand, but the form known as “Tappet locking” has gradually ousted other competitiors, both for mechanical and electric machines, having proved in practice to possess those qualities of flexibility, strength, durability, and simplicity, that are necessary for a first grade safety appliance in railway working.

A Tappet locking plate appears on a mechanical machine as a wide shelf supported by brackets bolted to the frame of the machine and extending the whole length. The shelf consists of parallel troughs like a series of channel irons with their open sides upwards. In these troughs steel bars called “bridles” lie side by side. Rivetted to the bridles are triangular pieces of steel called “dogs,” the dogs on one bridle forming a combination which, when that bridle is moved along the trough, locks up a series of levers. The levers are brought into relation with the bridles by flat lengths of iron (“Sword irons”) pivoted on each lever, and as the lever is pulled, these slide at right angles across the bridles through grooves cut in the sides of all the troughs. These sword irons, therefore, lie across the path of the dogs rivetted to the bridles and have triangular notches cut in their sides into which the dogs fit when a bridle is moved (and the sword irons are in the correct position).

Each bridle is operated by one of the sword irons. In the normal position of the lever (and its attached sword iron), the driving dog stands in its notch. Upon the sword iron being moved, that dog is forced from its notch and so its bridle (armed with a series of dogs) engages the sword irons of conflicting levers and locks them until the operating lever is restored to normal.