Bolt Torque

[CuriousElectron]

Hey All,
Is there a reduction of maximum torque that should be applied to bolts due to the age of the bolts? If bolts have been in service for a long time, is there a possibility of damage to them if they were re-torqued to their maximum spec value?
Thanks,
EE

 

[BrianE22]

It's a problem if you don't get the bolt preload you need. The joint needs a certain amount of preload. Bolt torque is related to preload but not really nicely. A corroded joint will change the preload/torque relationship. If the bolt or thread can't take the torque needed to get the preload you need you then have a problem.

 

[IRstuff]

Is there a reduction of maximum torque that should be applied to bolts due to the age of the bolts?

The bottom line is NO! If the bolt cannot deal with the torque required, it must be replaced. Such bolts' raison d'etre is to provide a clamping force for some mechanical requirement.

By the same token would you allow, say, a TV scanline timebase to drift its requirement because of its age, regardless of whether it can even generate the correct timing for a TV signal?

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[dvd]

This is too vague - plus, bolts are tensioned, torque is relatively meaningless. Some bolts cannot be re-used, others are allowed to be re-used - here is a short discussion on the topic.

Rules for Reusing Bolts

 

[desertfox]

Hi CuriousElectron

Bolts should only be tensioned to achieve the correct bolt preload for the joint it’s holding together.
A bolt that has deteriorated for example by rusting; then you would replace it because clearly it couldn’t be retightened to its original preload. A good rule of thumb I remember is that bolts tightened at 70% of their strength capacity can be reused but those tightened to 90% of capacity should be replaced. The bottom line being if in doubt replace the bolt because that will be far cheaper in the long run when compared to the disaster a failed bolt may cause.




“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[EdStainless]

Back in the day I worked for a company whose equipment was built in sections and bolted together in the field.
The joints were similar to piping flange connections, but with very restricted clearances.
The bolts were custom made 12-point flange head cap screws in Monel K-500.
Even back then they were over $15 each, with anywhere from 8 to 16 in a joint.
We prohibited re-use of bolts in the field.
If a field service guy had some bolts in his toolbox they had better be in the bag with the original lot traceability information.
It is easier to never reuse bolts than trying to decide when they are too bad to use.

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[drawoh]

CuriousElectron,

It appears to be common practise to work out bolt torques based on 90% of yield or proof stress. Given the very poor correlation of bolt torque to bolt tension, we must assume that many such bolts have been torqued past their yield stresses, and they cannot be safely torqued that way again. If removed, they must be discarded and replaced.

If you are designing a joint for reusable bolts, you must work out torque based on a much lower percentage of yield. Seventy five percent? Fifty percent?

--
JHG