Coupling Head Bolt Failure

[Mac2]

After arriving home from holiday with my caravan, I discovered that one of the two high tensile steel bolts that attach the coupling head to the caravan drawtube had sheared. The sheared bolt was still held in place by the rubber boot and the bolt head was stamped 10.9 (high tensile steel grade).

On investigation I have been told by a local caravan dealer that they only fit high tensile steel bolts grade 8.8 (lower tensile & yield) and have never heard of one of these bolts failing in the past.

As the grade 10.9 has a higher tensile and yield rating than the 8.8 it's a bit worrying why this failed.

I have found the following specifications:

8.8 grade = tensile 800 N/mm2, yield 640 N/mm2

10.9 grade = tensile 1040 N/mm2, yield 940 N/mm2

Is there something that I'm overlooking ? Could it be that some other factor makes 10.9 grade unsuitable for this application?

Your opinion would be greatly appreciated.

 

[Gary_5039903]

8.8 is the "standard" high tensile grade. 10.9 grade are far stronger both in tensile and shear. BUT bolts of a grade over 8.8 are more susceptible to hydrogen embrittlement during plating, which will make them more likely to shear. If it is hydrogen embrittlement, the shear will be clean and flat. If your bolts are zinc plated (silver in colour) then this is probably what has happened and is becoming more common with the market being flooded by cheap quality imported bolts.

Replace them both with 8.8 grade. If its a new caravan, contact the manufacturer.

 

[Ogre]

Afew years ago there was also a problem with the actual tow-ball castings, a large number of which were coming from dubious third world areas. Probably a few of these still kicking around too.