13 Fifth wheel coupling assembly

The fifth wheel coupling attaches the semi-trailer to the tractor unit. This coupling consists of a semicircular table plate with a central hole and a vee section cut-out towards the rear (Fig. 1.22(b)). Attached underneath this plate are a pair of pivoting coupling jaws (Fig. 1.22(a)). The semi-trailer has an upper fifth wheel plate welded or bolted to the underside of its chassis at the front and in the centre of this plate is bolted a kingpin which faces downwards (Fig. 1.22(a)).

When the trailer is coupled to the tractor unit, this upper plate rests and is supported on top of the tractor fifth wheel table plate with the two halves of the coupling jaws engaging the kingpin. To permit

frequsnty (Hi)

(c) Relationship of frequency of vibration and damping resistance with conventional rubber and hydroelastic type mountings

Fig. 1.20(a-c) Hydroelastic engine mount relative swivelling between the kingpin and jaws, the two interfaces of the tractor fifth wheel tables and trailer upper plate should be heavily greased. Thus, although the trailer articulates about the kingpin, its load is carried by the tractor table.

Flexible articulation between the tractor and semi-trailer in the horizontal plane is achieved by permitting the fifth wheel table to pivot on horizontal trunnion bearings that lie in the same vertical plane as the kingpin, but with their axes at right angles to that of the tractor's wheel base (Fig. 1.22(b)). Rubber trunnion rubber bushes normally provide longitudinal oscillations of about ±10°.

The fifth wheel table assembly is made from either a machined cast or forged steel sections, or from heavy section rolled steel fabrications, and the upper fifth wheel plate is generally hot rolled steel welded to the trailer chassis. The coupling locking system consisting of the jaws, pawl, pivot pins and kingpin is produced from forged high carbon manganese steels and the pressure areas of these components are induction hardened to withstand shock loading and wear.

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