131 Operation of twin jaw coupling

With the trailer kingpin uncoupled, the jaws will be in their closed position with the plunger withdrawn from the lock gap between the rear of the jaws, which are maintained in this position by the pawl contacting the hold-off stop (Fig. 1.23(a)). When coupling the tractor to the trailer, the jaws of the

fifth wheel strike the kingpin of the trailer. The jaws are then forced open and the kingpin enters the space between the jaws (Fig. l.23(b)). The kingpin contacts the rear of the jaws which then automatically pushes them together. At the same time, one of the coupler jaws causes the trip pin to strike the pawl. The pawl turns on its pivot against the force of the spring, releasing the plunger, allowing it to be forced into the jaws' lock gap by its spring (Fig. l.23(c)). When the tractor is moving, the drag of the kingpin increases the lateral force of the jaws on the plunger.

To disconnect the coupling, the release hand lever is pulled fully back (Fig. l.23(d)). This draws the plunger clear of the rear of the jaws and, at the same time, allows the pawl to swing round so that it engages a projection hold-off stop situated at the upper end of the plunger, thus jamming the plunger in the fully out position in readiness for uncoupling.

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