SMMT Guide on Couplings > Pin / Hook and Eye Couplings and Combination Units
| 50mm Ball Brackets and Couplings Correct jaw/eye and hook/eye set up Parallel Jaws |
Pin / Hook and Eye Couplings and Combination Units Insufficient and Excessive clearance 40 mm Din eyes |
Pin / Hook and Eye Couplings and Combination Units
The Pin and Eye arrangement comprises a jaw with a removable pin.
The hook arrangement has no separate pin, but a safety catch should be fitted to prevent inadvertent detachment of the eye.
No specific legislation applies, but there are a number of things to consider. The jaw or hook should be robust enough not to flex and should allow sufficient articulation of the eyeshaft in both vertical and horizontal planes.
Compliance with British Standard BS AU24a: 1989 is strongly recommended. This Standard specifies the pin/hook and eye dimensions, together with throat clearances to ensure adequate articulation.
Ensure that the diameter of the eye fitted to the trailer coupling is matched to the diameter of the pin such that there is enough clearance to permit towing on the pin and braking on the throat of the jaw.
If the internal diameter of the eye is too small, or the clearance between jaw and pin is too great, then both towing and braking loads will be put onto the pin, which may lead to premature failure. Similar care to match eye to hook should be taken when using this type of coupling arrangement for much the same reasons.
Combination Units
A combination unit offers the option of towing when either a ball or an eyeshaft coupling is fitted to the trailer.
A 50 mm. ball may be formed as part of the removable eyeshaft pin or it may be an integral, and thus fixed, part of the jaw. A primary locking device should be fitted to prevent the pin working loose or rattling unduly. A secondary device such as a retaining “pig-tail” clip is often provided but this alone is insufficient to secure the pin due to the stresses imposed by alternating positive and negative forces brought about by road conditions or poor load distribution.
Similar considerations to those for the pin and eye should be observed and, as EC Directive 94/20/EC applies to combination units, you would be wise to ensure that all units fitted to new vehicles from 1 January 1998 have been type approved to this Directive.
Eyeshafts
The rings found on eyeshafts in the UK usually have an internal diameter of 30mm, 40mm, 50mm., or 76mm, NATO style.
These British eyeshafts, which have a round or toroidal section eye, must not be confused with a DIN pin and eye arrangement where the pin has a bulbous or reinforced section and the bore of the DIN eye is fitted with a special parallel sided hardened steel bushing to match.
Under no circumstances must a DIN style eyeshaft be used with a British, parallel, style of pin, as accelerated wear of the pin caused by the sharp leading edges of the bush will occur, with potentially catastrophic results.
Care should also be taken with the NATO type of eyeshaft which, owing to its large internal diameter of 76mm., will not fit the majority of commercially produced 'jaws and is usually paired with a military style pintle hook. See the illustrations overleaf If in doubt about any particular application, consult one of the coupling manufacturers who will be happy to advise.
