How does a torque wrench "know"

also pika , overtightening is …
… extremely easily accomplished by simply having the bolt or it’s tapped reciever bore (either one) having some form of lube on it . Many critical torque specs. are of the “wet” torque type because the fastners need to be lubed for different reasons , one is corrosion resisitence . Some fastners are plated and that is another torque spec. different from either dry or wet torque .



To understand better if you want to , spend a little time researching “wet” and/or “dry” torque specs. .



It’s also interesting just what is taking place when torquing down fastners … not exactly what you might think .

not fragile
The wrench has a spring that presses a ball into a V shaped cam detent with a fairly sharp edge. When you reach the specified torque value the cam pops across and makes a click. Release the torque and it pops back.



I have torque wrenches that I’ve been using for thirty years, and they are still in the ballpark. You can slack them after each use, but you’ll be one of ten people in the history of the planet that consistently do that.

I guess I’m one of those ten
I always back the pressure off a torque wrench after using it.

Wander who are the other 8 -:wink:
I too do that when I use mine. I suppose I don’t use it often enough though - may be a dozen times a year if that, so it staying in the relaxed position most of its life makes sense to me…

tq
I work on helicopters from day to day and they have some really really big torque wrenches. But if you’re only going to be using one every now and then, I would say go to sears and get a digital socket that tells you how much tq you’re putting on the nut. They’re fairly cheap. I have one for my 3/8ths drive and it works pretty good for tqs from 1’’ lb to 50’’ lbs. Idk if anyone has said this but when torquing something never tq it from the bolt head always go from the nut, the head of the bolt most often times gets seated on something and it can throw of the tq and says it torqued before you really reach the tq.

Number 3 here (raising hand)