Verifying True Residual Torque when Auditing Bolts

When carrying out an audit on a bolt that's already tight, the number you get back might not be the true residual torque.

Especially if you're measuring with peak torque.

It's worth understanding, because a lot of residual-torque audits are done exactly this way.

To audit a tightened fastener, you traditionally put a wrench on it and pull until the joint gives. Take the peak of that pull, and that's accepted as the residual torque.

But watch what you've just done. To make the joint move at all, you had to load it past the torque that was holding it. And the moment it moves, you're tightening it again, climbing back up the curve. The peak you record sits above the torque that was actually in the joint. Every time.

So a peak result overstates the residual torque.

And it always errs high, which means a loose joint can still pass. Say a joint is sitting at 45 Nm and the peak comes back at 52. You haven't measured the joint, you've measured how hard you pulled.

At minimum, you've measured the break-away torque caused by static friction. And you've masked at least some, if not all, of any joint relaxation.

There's a better way that many manufacturing plants use. On a digital wrench running a proper residual-torque algorithm, you don't rely on the peak at all. It reads the true residual from the shape of the pull, and reports both numbers, the peak and the residual, so you can see the gap for yourself.

Does your team use a peak torque wrench for auditing joints?

-Sean Gynane
Sales Director at Crane Electronics

For more articles about Crane products Click Here


Contact us with any questions or for more information