| British
standards require that there should be sufficient room behind the
pin of any towing jaw to allow a bar of 31.75mm (1.1/4") to
be passed through. This ensures adequate articulation of the eyeshaft
in both vertical and horizontal planes. It is also important that
this gap is not too great, thus allowing the back of the eye to
hit the front of the pin before the front of the eye contacts the
throat of the jaw. |
| A
jaw opening that does not comply with the standards and therefore
restricts articulation upwards and downwards, is likely to bring
about stress fractures on either the coupling or the trailer chassis
drawbar. This may be particularly pronounced if the towing vehicle
has a long overhang behind the rear wheels and, in such cases, the
fitting of more robust, higher capacity equipment than the gross
trailer weight might suggest, is often recommended. |
| The
above standards also apply to the jaw on a combination jaw and towball
unit. This type of product provides the flexibility of being able
to tow trailers fitted with either an eye or 50mm coupling head,
without having to change the fittings on the tow vehicle. Where
the ballpin is separate, it is also preferable to use a unit that
is fitted with a primary locking device to stop the pin/ballpin
working loose or rattling unduly. A secondary device such as an
"R" clip is also often used, but if this is the only means
of securing the ballpin it is insufficient, due to the stresses
imposed by the alternating lifting/dropping forces brought about
by road conditions and/or poor load distribution. |
A
hook has no separate pin, but a safety catch should be fitted to
prevent inadvertent detachment of the eye. Where a hook has no "chin"
or limiting pins on its outer face the towing eye may swing round
and underneath the hook if the trailer has been reversed into a
jack-knife position. In such a situation, as the towing vehicle
moves forward, the eyeshaft and/or drawtube can be severely bent.
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