Return to index: [Subject] [Thread] [Date] [Author]

RE: Loose Bolts with DTI Washers

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Title: RE: Loose Bolts with DTI Washers

>Just wondering just how accurate torque readings are in relationship to bolt
>tension.  Are they +/- 10 percent or perhaps +/- 20 percent?

It depends totally upon the condition of the fasteners. If they are well lubricated and maintained in "factory condition", there will be very little variation. That's why auto manufacturers can get away with specifying every fastener they install to be installed toa certain torque value.

However, if the RCSC requirements for protected storage are not followed -- look at the bolt kegs on the next job site you visit -- or if the bolts came from the bottom of a bin in the fabrication shop a few years later or if the lubricant broke down from heat or moisture, the results of a single torque reading can vary significantly. The same torque that might break a well lubed bolt in good condition can correspond to a very low pretension for a degraded bolt. Bob Shaw's bolting handbook givs a formula that can be used to show the variation based upon frictional characteristics. But it's at the office and I'm not.

RCSC has requirements for storage, preinstallation verification and installation that, if followed, will ensure that none of the foregoing conditions will cause improper bolt pretension. Of course, I am not recommending that bolts be stored improperly or that the first-in last-out system of bolt usage become the norm.

>Stated another
>way, just how accurate do they really need to be given the, relatively, wide
>range between minimum bolt tension and "necking down"of the bolt?

This is the beauty of bolt installation. That long plateau after you start to deform the bolt during pretensioning covers a lot of the slight variations that will occur. But to be sure you're getting into the range in which you need  to be for a properly pretensioned bolt, I recommend that one of the methods in the RCSC Specification be followed.

I should also harp on it that snug-tightened installation is acceptable for the majority of cases. The specific instances when pretensioned installation is required are given in the RCSC and AISC Specifications. They mostly involve impact, fatigue and slip resistance.

>Another query, why are "lock" washers or "split" washers, as they are
>properly termed, not used between a hardened washer and the nut in cases of
>vibration and cyclic loading? This method seems to be the accepted solution
>to other applications where bolt loosening is a problem.  I have never seen
>the RSCS address this solution.  Is there a reason why?

Lock washers are useful for mild fasteners like the old A307s that could not be pretensioned. In the old days before high-strength bolts, A307s were probably used in a few applications we wouldn't think of using them in today. And lock washers were probably a solution when it was a concern for the nut backing off an A307.

But for high-strength bolts (A325 or A490), a lock washer would be unnecessary. If snug-tightened installation is acceptable, there is no condition that would cause the nut to back off. If pretensioned installation is required, the very characteristic of that installation method is such that the nut would not back off. Even if you put the lock washer in there, all it would be doing is taking up space, since the pretension in the bolt would quickly overcome any of the spring force it could provide.