Operation of the automatic drawbar coupling

(Fig. 1.26) In the uncoupled position the eyebolt is held in the open position ready for coupling (Fig. 1.26(a)). When the truck is reversed, the jaws of the coupling slip over the towing eye and in the process strike the conical lower end of the eye bolt (Fig. 1.26(b)). Subsequently, the eye bolt will lift. This trips the spring-loaded wedge lever which now rotates clockwise so that it bears down on the eye bolt. Further inward movement of the eye bolt between the coupling jaws aligns the towing eye with the eye bolt. The spring pressure now acts through the wedge lever to push the eye bolt through the towing eye and the lower coupling jaw (Fig. 1.26(c)). When the eye bolt stop-plate has been fully lowered by the spring tension, the wedge lever will slot into its groove formed in the centre of the eye bolt so that it locks the eye bolt in the coupled position.

To uncouple the drawbar, the handle is pulled upwards against the tension of the coil spring mounted on the wedge level operating shaft (Fig. 1.26(d)). This unlocks the wedge, freeing the eyebolt and then raises the eye bolt to the uncoupled position where the wedge lever jams it in the open position (Fig. 1.26(a)).

1.4.2 Ball and socket towing bar coupling for light caravan/trailers (Fig. 1.27) Light trailers or caravans are usually attached to the rear of the towing car by means of a ball and socket type coupling. The ball part of the attachment is bolted onto a bracing bracket fitted directly to the boot pan or the towing load may be shared out between two side brackets attached to the rear longitudinal box-section members of the body.

A single channel section or pair of triangularly arranged angle-section arms may be used to form the towbar which both supports and draws the trailer.

Attached to the end of the towbar is the socket housing with an internally formed spherical cavity. This fits over the ball member of the coupling so that it forms a pivot joint which can operate in both the horizontal and vertical plane (Fig. 1.27).

To secure the socket over the ball, a lock device must be incorporated which enables the coupling to be readily connected or disconnected. This lock may take the form of a spring-loaded horizontally positioned wedge with a groove formed across its top face which slips underneath and against the ball. The wedge is held in the closed engaged position by a spring-loaded vertical plunger which has a horizontal groove cut on one side. An uncoupling lever engages the plunger's groove so that when the coupling is disconnected the lever is squeezed to lift and release the plunger from the wedge. At the same time the whole towbar is raised by the handle to clear the socket and from the ball member.

Coupling the tow bar to the car simply reverses the process, the uncoupling lever is again squeezed against the handle to withdraw the plunger and the socket housing is pushed down over the ball member. The wedge moves outwards and allows the ball to enter the socket and immediately the wedge springs back into the engaged position. Releasing the lever and handle completes the coupling by permitting the plunger to enter the wedge lock groove.

Sometimes a strong compression spring is interposed between the socket housing member and the towing (draw) bar to cushion the shock load when the car/trailer combination is initially driven away from a standstill.

1.5 Semi-trailer landing gear (Fig. 1.28) Landing legs are used to support the front of the semi-trailer when the tractor unit is uncoupled.

Extendable landing legs are bolted vertically to each chassis side-member behind the rear wheels of

Fig. 1.26(a-e) Automatic drawbar coupling
Fig. 1.27 Ball and socket caravan/trailer towing attachment
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