Manual Torque Wrench vs. Impact Wrench

[Montana1]

This question applies to bolted structural connections. Can a manual torque wrench apply the same tension to a fastener as an impact wrench qualified on a skidmore? (Assume bolts are to be tensioned to 39,000 psi.) In other words, does a properly operating impact wrench apply more effective rotational force due to its "dynamic" impact action?

 

[Qshake]

Unless I'm missing something here, the answer should be yes.

Any properly working machine will be more effective than manual work.

One of the big issues is with too much torque. It is very common to over-torque bolts in a connection and after a while the over torqued bolts will fail and pop off. Or, they may pop off while QA/QC with a manual torque wrench.

There are those impact wrenches with auto shut-off but I've not found them to be reliable. And there is a multitude of indicator bolts on the market too.



Regards,
Qshake

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[MikeHalloran]

Skidmore-Wilhelm asserts, or at least implies, that use of their calibrator gives you some assurance that the impact wrench applies torque equivalent to that applied by the manual wrench. The manual wrench is the standard of comparison.

I'm not prepared to assert that the impact wrench and the manual wrench leave the bolt in exactly the same stress state, but the calibrator allows you to establish a correlation.

If the bolt knew the difference, S-W would be out of business.

On the other hand, an impact wrench depends on the dynamics of the joint.

Example: I was getting a flat fixed at a tire shop right before closing, and all the lifts were busy. I started jacking the car with its own cheapo scissor jack, and the tire tech came up with the bright idea of zipping the jack up with his impact wrench. This was a big wrench in good repair, perfectly capable of breaking off a wheel stud. It hammered away like crazy, but it couldn't lift the car as fast as I had been doing it with the jack handle. It could barely lift it at all until the load got serious. We were both surprised.


Mike Halloran
Pembroke Pines, FL, USA

 

[StephenA]

The tension in the bolt is measured by the torque applied by the wrench. The big unknown / variable is the friction factor in the threads which can vary enormously depending on the type of lubricant (or lack of lubricant)used. Otherwise both systems should give the same answer unless the impact wrench is using sliding friction and the manual wrench is using static friction.

 

[GregLocock]

We use what is essentially a manual torque wrench to calibrate the impact wrenches. Not the other way round.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[bridgebuster]

In many cases it would be impractical to use a manual wrench in lieu of an impact wrench, because of the time required and the size of the wrench. Let's assume you want to tension a 7/8" diameter A325 bolt manually, the wrench would be about 3.5 to 5.5 feet long. An impact wrench is heavier but more convenient.