Hi ive just had a tow bar fitted to my mondeo the fitter tells me to have the bolts that hold the ball on need changing every year as they tend to stretch, ive been caravanning some 20 years now and never heard of this has anyone else . Tom
towball bolts
- Thread starter the other tom
- Start date
No never heard that one before. It does not make much sense, what if you only tow a couple of times a year compared with some-one who tows weekly or even regular long distances. The potential difference in miles could be thousands.
Since when have steel bolts stretched? Especially given the diameter of the ones holding on the tow ball. If this was the case, you whole car would need re-bolting every year. Speaking as a technical engineer, he's talking complete cods-wallop. One way of trying to make a few extra bob each year I suppose.
Hi Guys , thanks for the replys .I dont think i will be changing the bolts as suggested by the fitter Mick your reply made me giggle how very true. Many thanks Tom
at the risk of boring everyone on this subject but also speaking as an engineer bolts do stretch but only when they are first fitted and only if they are over tightened. Think about it if you keep tightening a bolt ( and it would take some doing on a 16mm hi-tensile tow bar bolt ) it will eventually shear due to reaching the end of its tensile limit ie. over stretched, bearing in mind to do this you would need to use a scaff tube on the end of the spanner, this is why we use a torque wrench but they don't need changing yearly, trust me I'm an engineer.
Following on from Roy's comments, a bolt has a maximum recommended torque and in the case of the 'standard' 8.8 towball bolt this is 143ftlb's or 190nm and equates to 60% of it's 'proof' load.
Proof means the point at which the stretch on the bolt will not return to that it was before once the torque applied is released, or put another way...the limit of it's springiness or more correctly it's 'yield' point.
Once a bolt is put into yield it's further use is extremely limited and should never be reused however if correctly torqued it's use is for the most part unlimited given that it is being used well within it's capabilities.
Where this all falls down is if the history of the bolts is unknown? but once replaced and correctly torqued... it's fair to say towball bolts will last indefinitely.
Proof means the point at which the stretch on the bolt will not return to that it was before once the torque applied is released, or put another way...the limit of it's springiness or more correctly it's 'yield' point.
Once a bolt is put into yield it's further use is extremely limited and should never be reused however if correctly torqued it's use is for the most part unlimited given that it is being used well within it's capabilities.
Where this all falls down is if the history of the bolts is unknown? but once replaced and correctly torqued... it's fair to say towball bolts will last indefinitely.
Yes, over torquing a bolt can distort it slightly, but not to the point of failure as you say, especially given the diameter of a towball attachment bolt. I don't think that a person would stand much chance of shearing one of those. Most of the time over torquing will result in stripped threads, which of course is just as dangerous.
Mick you're right what you say but it's not uncommon for bolts to shear I've sheared quite a few 16mm bolts but it's mainly been taking old bolts out of machines using the scaff tube method (not recommended).
"I don't think that a person would stand much chance of shearing one of those"
Not met many farmers then!!!
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