# How does a torque wrench "know"

…when it has reached the dialed amount of torque? And how fragile is that internal device?

Despite having lots of experience wrenching on bicycles and other gear, I am a newbie with a torque wrench. The instructions are pretty simple, but they leave me wondering just HOW DOES IT KNOW when to give the signal to you? Is there a spring inside? The instructions state that the characteristic click signal may not occur when using it at low torque settings.

The kayaking connection: I am assembling a new trailer and the manual calls for using a torque wrench. And I already dropped the \$80 torque wrench, though only the distance from my truck bumper to the garage floor. That’s why I asked about fragility.

…but from the torque wrenches I have used, there is simply a calibrated beam that deflects a known amount under a given bending load.

In other words, you could make a bathroom scale out of a 2x12 and a yard stick. You just have to calibrate the deflection of the 2x12 to a certain number of pounds.

The digital ones have a little device that measures the bend in the wrench and converts that to a torque.

\$0.02

D

long story short
there is a little man in there

http://en.wikipedia.org/wiki/Torque_wrench

Used one today
to torque cyliner heads on a Detroit to 190 lbs/ft. Important to stop pulling after you hear the first click…

I doubt if you hurt it
I use mine all the time on my travel trailer.

You should be able to check it, by torquing your trailer wheels.

Set it to some where between 80 and ninty FT/pounds, and then try to tighten a few of the lugs. You should hear the click on most of them around that.

Then loosen one and reset the wrench to about forty, and you should hear the click with not much pressure at all on it.

Mine has a ratchet in it that prevents you from going over the set value, and it will just keep clicking with out tightening the lug any more

jack L

This is just the first of 289,000 hits http://www.tegger.com/hondafaq/torque_wrench/index.html

Calibration?
I hear that torque wrenches need to be calibrated to be accurate over time. A yearly process is suggested. I have bever calibrated mine and so I’m not sure how accurate it is. But “by feel” it seems close enough. The important thing in some cases is not the exact amount of torque delivered but that it is the same amount over say half a dozen bolts to prevent the thing being bolted down from warping due to uneven torque in different areas.

Also, obviously, one needs to be within the range of the wrench scale, which means you probably would end up with two - one for delicate stuff (like spark plugs, transmission internals, or oil pan gaskets that need something between 7 and 20lb/ft torque) , and one for other stuff that needs more than 40lb/ft and upwards of 200…

And if the needed torque exceeds the spec of the wrench considerably, I’d use a breaker bar instead of an extension to the torque wrenches handle -;(

Except Wiki is wrong
You’ve got to stop pulling when the wrench goes “click”. There’s nothing in there that prevents further tightening. If you keep pulling you will over tighten.

Anyway, the mechanism is a ball detent and a spring. Not tremendously delicate, and dropping it probably won’t seriously alter the calibration (which is never that accurate to begin with).

Back it down to zero after use to lessen the need for calibration. If you leave it set at a high value the spring will eventually take a permanent set and the wrench will undertighten.

but wiki is never wrong!

The “clik” wrenches are convenient
but if you really care that you have the correct torque use a beam or dial type.

Torque Wrench
The wrench is made so that if “slips” when a specific torque is reached. I have an expensive one and the instructions state that it should be returned to the “0” setting at the end of the day so that the thing will remain in calibration. They also claim that the calibration is pretty important, so it’s a good idea to treat it with care.

you just get a long wrench and tighten it as tight as you can get it. I’m thinking the trailer wont care. Over tightening doesn’t seem possible.

Ryan L.

Over tightening…
When younger and dafter… within reason, I rarely worried about over-tightening anything.

A year in Ghana cured that: we saw LOTS of evidence of over-tightened nuts leading to catastrophic failure!

Mostly cars on which the wheels had fallen off.

Fortunately, most such failures were at low speed with no-one badly hurt… but the vehicles in question were commonly the self-same taxis in which we journeyed.

I’ve never let anyone else tighten a wheelnut for me since!

A Related Issue on Proper Torque
When tightening wheel nuts, if they are a bit rusty it’s very common for them to resist higher and higher torque, and then suddenly “let go” and turn quite a bit too far as the vibration of the rough, rusted, slipping surfaces seems to “lubricate” the motion. I’ve seen it happen a lot: clean or slightly lubricated threads (also the seating surface of the nut against the wheel) will gradually become harder and harder to turn, but rusty ones will stop turning at a lower torque value and then continue to resist further rotation, then suddenly spin an additional half a turn or more once the dirty contact surfaces let go and the “screeching” starts.

Instructions call for calibration
Time interval depends on amount of use. Too bad the wrench must be sent to one of only a few calibration centers. Wonder how many people never do so.

The instructions also clearly say to STOP as soon as the click occurs, because the torque wrench will allow the user to keep cranking on the wrench, possibly overtightening. It is a torque wrench, not a torque-limiting wrench.

I was hearing the click so am relieved that the “when” signal occurs near the lower end of this 20 to 100 ft-lbs device.

Still curious what’s inside it; instructions warn against opening it, ever.

It IS a dial type, and it clicks (nm)

Overtightening very easy
Trailer is aluminum.

Ball detente and spring
That makes sense. And explains why it would be good to back the dial down to zero at day’s end. Funny that my instructions don’t say to do that.

Headin’ out to the shed to dial it down…

It does both?

– Last Updated: Mar-12-11 2:53 PM EST –

I think the dial type that was just mentioned is just a pointer that moves across a semi-circular scale as you apply more "pull" on the handle. It's a very simple device, and one that cannot be calibrated. I haven't seen one in a long time, but as I recall, the bending action of the handle allows another bar, attached only to the far end of the handle, to register the amount of bend via the semi-circular scale. If in fact it has a dial as well as a break-over (click) feature, there's be no need to worry about calibration, since you'd have visual verification of that the click occurred at the proper setting each time you used it (it would be sort of like using a digital bathroom scale while it was stacked on top of an older, spring-dial scale and reading both of them at the same time when you weighed yourself (of course in that case, the scale on the bottom would record your weight plus the extra four pounds or so of the extra scale, but other than that difference they'd read the same if both were working properly)).

Okay I just looked it up, and I see that the "dial" type incorporates a mechanical dial gauge rather than directly registering the amount of bend in the handle. Still, I'd be surprised to see a break-over feature included with that kind of wrench. I'm not saying it doesn't exist, just that I'd never have expected it.