Torque wrench errors
 

I've always assumed torque wrenches were pretty accurate. Today I was reading a report and found out that it's generally accepted that a torque wrench (a professional, calibrated and documented one, not any old cheapo one) has a +/- 25% error, compared to +/- 35% error for operator "feel". Not sure how much that helps anyone, just remember a wrench isn't completely accurate, just a bit better than doing it by feel.

ive used mine (a halfords quite expencive one) and stopped tightening before it click's out thinking it might be wrong :? sometimes it never seems to click out and on a honda that might be ok but on a suzuki its a big problem.

I have a Britool one and haven't stripped a thread yet - (touches wood). I also tend to use it wherever torque is specified (including sump bolts).

Those % seem a little odd - I always thought torque wrenches were more accurate towards the higher end of their scales rather than the lower, but if +/- 25% is correct, then that would mean they get less accurate as the values go up :?

Don't understand those figures :? Where was that report and what was the test they did? If you check a "break" type wrench on a free low friction low inertia fixture you can get a spike reading when it breaks which doesn't actually exert any additional tightening in a real fastener (no energy contained, only impulse).

Torque alone often isn't a very precise way of achieving optimum fastener loading, but is the most practical method outside of an industrial environment. Often critically stressed joints are done by tightening to yield (with electronic torque/angle devices) or more commonly in workshop manuals a "torque + angle" method (e.g. 120Nm plus 90deg) for things like cylinder head bolts, though this usually results in a limited re-use for the fasteners (3 times typically the maximum).

My Britool is within 3% across the whole range. Even allowing for some operator error (usually going too far past the "break") you shouldn't be more than 10% out at worst.

I've checked a couple of cheap (Sealey etc) wrenches and found them well within 10%, and more like 5% optimistic (when set to a figure they actually break at a true 10% lower value so no risk of overtightening). 35% really makes no sense.

Both of mine claim a +/1 3%...

I never use a torque wrench when tightening a bolt into Alloy. I just don't trust them.

Generally speaking the allowable tightness for any given bolt only needs to be within a fairly wide tolerance, so if your torque measurement is a bit off it won't matter too much, where torque wrenches really score is, even allowing for measurement error, all the bolts on an assembly will be the same, so reducing distortion.

You can buy torque wrenches that are manufactured for the aircraft industry where the calibrated tolerance is very low,but the price is very high. These tend not to have the scale of Britool,Mac or Snap-on and you would probably need 3 or 4 to cover the same range scale,

i tend to go by feel more when tightening into suzuki alloy.

All depends on what experience you got doesn't it? I tend to go by feel now- especially into plasticine. But I wouldn't let my missus tighten a bolt. And the guy/gal who previously tightened some of the bolts on my SV needs working over with the rough end of a pineapple. :o) oo!

With a bolted joint like a cylinder head/ sump/ or rocker cover you do need even torque on all the bolts though. Can't get that just with feel I guess.

John