In the case of an incorrectly fitted or maintained bolt-in tow-eye, the large number of stress reversal tension and compression cycles they are subjected to can cause thread pitches to become worn, loosening the coupling shank or nut. This in turn allows the nut to be pulled over the thread on the shank which causes the towing eye to pull out of the drawbar, resulting in the trailer becoming detached.

Failures to bolt-in tow eyes most often occur within days of the tow-eye securing nut being re-tightened.

HVSC requirements

When involved in the repair or recertification of draw bars fitted with bolt-in tow eyes, the HVSC must take the following into account:

• Tightening the nut of a loose bolt-in tow coupling is not acceptable and may increase the potential for earlier catastrophic failure. The complete towing eye and nut must be replaced.
• If there is any sign of it being loose (eg fretting, shiny, rusting, etc) the tow-eye and nut must be replaced – under no circumstances is it to be tightened.
• When the tow-eye and nut is replaced, the nut must be torqued to the manufacturer’s recommendation (including a greased thread) and then if necessary taken to the next castellation – in no circumstances backed off.
• Instructions should be given to the operator so that following initial installation; the nut should be re-torqued at the  lesser of 5000km or as directed by the manufacturer. If it moves it must be re-torqued (Note: this is the only time that re- tightening is permitted).

Inspection requirements

Evidence of looseness of the securing nut, split pin or washer, including wear marks or fretting or evidence that the original nut has been re-tightened are stated as reasons for rejection in this manual and also the VIRM: In-service certification.

• See also the Transport Agency's Safety alert: Bolt-in tow-eye security