Printable View
It all depends on the bolt size. Grade 8.8 bolts in 12mm would be entirely adequate in my opinion. Removing them and checking them for deformation and rust would be my advice, then replace with new equivalents if necessary. Tighten them well so that no flexing occurs during normal service. Fatigue is the enemy of HT bolts.
With softer metal (low grade bolts) if you get a stress concentration, you get plastic flow of the metal so as to reduce the stress concentration. This may loosen the bolt, allowing movement, which will eventually result in failure - but you have a chance of seeing it before it fails - could take months or years. With harder (stronger) steel, this does not happen and if the stress concentration is enough, or cyclic, a crack eventually starts - and failure may be only a few hundred more cycles away. (one cycle = one corrugation!).
Of course, if the strength of the bolt is not enough, it will fail anyway. But fatigue is the enemy on towbars, and if properly designed there should be no call for anything higher than 8.8.
that means they ARE a grade 8 bolt, all thats required is periodic checking with a spanner to make sure they havent loosened, if they are loose, pulll them out, inspect them for stepping and damage and then replace as requried.Quote:
Originally Posted by Mick_Marsh