torque wrench and extensions

It is a subject that is confusing to many people.

If you have rusted or fouled threads, then you won't get the correct torque transfer no matter how you apply the force. Friction forces within the threads force the bolt to absorb the energy which should have gone into bolt tightening, so we do not get the desired clamping force.

So the first provision is that the threaded area must be clean. What does clean mean? It means whatever the designer, engineer, and manufacturer specify...We often take it to be clean unlubricated threads, but it does not have to be that way so long as we specify exactly what the conditions are.

And it doesnt make any difference at all whether you use a quarter inch, half inch, or three inch extension on your torque wrench, or even a flexing u-joint connector, as long as you dont exceed the elastic limit.

The extension could, for example, take a turn and a half wrap (or twist) and it would make no difference at all unless you had pushed it past the elastic limit. The proper torque would still be transmitted.

If you use a u-joint at an angle, or get the wrench on at an angle, then errors will be generated. Small angles are okay...there is little or no error, but with larger angles the error can become significant.

I believe most torque wrenches claim accuracies in the range of 2-5%. Your statement about lugnut accuracy of 10% being good enough is probably correct.

yes, in theory. In the real world, we have friction. You are still applying 100 foot-pounds at one end, but you're getting less than that at the socket end. Now I'm wondering - how much energy is dissipated in the torque wrench and does that mean you have to "overtorque" the torque wrench to compensate for that? (although the reality is that's probably negligable... 99.875 foot-pounds vs 100.000 foot-pounds is probably close enough for lug nuts.) ;)

And I do realize we're effectively splitting hairs, so I'll shut up...

Ray

Where does your friction come from, Ray? Why are you getting less force at the driven end of the bar or rod than at the drive end? If you aren't touching anything, and the rod and bolt are in a direct line, what is the explanation for your loss of force?

In your model, will friction cause a loss if the movement of the system is zero?

The energy in "dissipated" in the torque wrench should have been calculated by the manufacturer and compensated for. It's not something the mechanic should have to compensate for. And even the accuracy of the auto mechanics torque wrench would probably exceed that value by at least an order of magnitude. You're talking about something like, how much does the bearing in the torque wrench heat up when the wrench clicks.

What everyone is talking about is that 100 ft-lbs at one end of an extension is still 100 ft-lbs on the other end of the extension.

sheesh

I think that you may come close to the most important issue.

The extension acts like a torsion spring and stores energy temporarily. If you suddenly increase then suddenly decrease the torque at the wrench's pivot, you may cause the wrench to click because the torque at the pivot achieved the desired value momentarily. Yet that torque may have gone to twisting the extension momentarily and the bolt never experienced that torque because the torque on the wrench was quickly removed.

Here is a linear analogy: imagine a spring hung vertically from a man's hand and a weight on the other end of the spring. The man may be able to raise and lower his hand quickly, applying a varying force to one end of the spring. The force goes to stretch and compress the spring so that energy stored in the spring varies. Thus it may be that the spring's force on the hanging weight doesn't change very much and the weight doesn't move much.

So the extension is only a problem in the dynamic case (e.g. with the click-type torque wrench). The problem doesn't come up with the simple type torque wrench for which you just measure the bending of the wrench and hold it steady for a second at the desired torque.

Yes. See also my reply to Jerry Foster on this thread.

Find a bolt that _is_ accessible. Select an appropriate value of torque for the bolt. When using the extension, make sure the extension remains aligned (colinear) with the bolt.

Experiment 1: 1) loosen the bolt 2) tighten the bolt using the torque wrench without the extension 3) tighten the bolt using the torque wrench with the extension. Note whether the bolt turns.

Experiment 2: 1) loosen the bolt 2) tighten the bolt using the torque wrench with the extension 3) tighten the bolt using the torque wrench without the extension. Note whether the bolt turns.

Please report your results.

I had the same experience when Discount Tire rotated my tires. They brag about the care they use, the torque stix, etc. But the fact is, their tightening has resulted in warped rotors for me on two separate and well documented occasions.

Good idea. Thanks for the insight.

"Brian Stell" wrote

Anyone who wants to prove that the "output" torque is reduced purely by using an extension. The claim is that the torsional flex in a certain length of extension will reduce output torque by a measureable amount. Therefore, logically(?!?) there will be a magical length where output torque is zero.

Ever tried to get to the top bolt on a Ford 5.0 starter? 100 feet is about right. :)

Energy is irrelevant. The operator (your arm) supplies WHATEVER amount energy is required to generate the correct torque reading. If its a beam torque wrench, energy is stored in deflecting the beam but it doesn't affect the torque reading because your arm supplies enough energy to deflect the beam. In a "click" wrench, energy is dissipated in the friction mechanism, but again your arm keeps supplying sufficient energy to maintain the correct torque while the mechanism dissipates some energy. Torque is a FORCE only, not energy or power.

Friction?!? You have to have relative motion for friction to come into the equation. Nothing's moving- we're talking about STATIC forces here.

That was my point too, Steve. There is some heating of the torque wrench related to the actual displacement, but it is minimal and would not be a factor in practical measurements. That is why I mentioned it originally, and why I specified at equilibrium...after all other factors, movement, heat etc have been offset...and the torque transmitted is then exactly the same.. It is almost exactly the same even if equilibrium is not met.

Yeah, you're right, I tried to answer Ray's question using his terminology and energy is not the right term.