Torque wrench errors

[SVeeedy Gonzales]

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.

[kwak zzr]

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.

[lukemillar]

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 :?

[embee]

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.

[northwind]

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

[Stig]

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

[Sid Squid]

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.

[Dicky Ticker]

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,

[kwak zzr]

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

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

[ejohnh]

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

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

[northwind]

See, I don't quite get teh "don't trust torque wrench" argument, since it's not as though you don't have your sense of feel as a backup... They can be risky if you turn off your brain, but then what isn't?

[ejohnh]

See, I don't quite get teh "don't trust torque wrench" argument, since it's not as though you don't have your sense of feel as a backup... They can be risky if you turn off your brain, but then what isn't?

Trouble is Norfie you dont really get sense of feel with a massive torque wrench - at least not with the 25 year old one that I have got.

[21QUEST]

See, I don't quite get teh "don't trust torque wrench" argument, since it's not as though you don't have your sense of feel as a backup... They can be risky if you turn off your brain, but then what isn't?

I hear you but maybe one might as well not be using the torque wrench in the first place. A bit like fitting a gear indicator wotsitthingy and wondering if it's showing the right gear.

I do still use a torque wrench for the odd stuff but haven't sold my soul to the torque wrench god as some people seem inclined to do.


Ben

[hovis]

never used one :shock:

[ejohnh]

never used one :shock:

Sometimes that can make you walk funny.. :shock:

[northwind]

I hear you but maybe one might as well not be using the torque wrench in the first place. A bit like fitting a gear indicator wotsitthingy and wondering if it's showing the right gear.

Hmm, I can see what you mean, but if I was to set mine to, say, 23nm (have the bolts on an SV are at 23nm it seems) and it doesn't click, I'm going to know it's not right by 30. But I can't set 23nm by feel, put it that way. So I tighten to the click, or until it feels like I've gone too far. That way I'm no more likely to snap a bolt than I am doing it by feel.

[kwak zzr]

Ive threaded a few in alloy so i think Ive just learnt the hard way.

[embee]

For smaller fasteners into questionable alloy I always work up towards the specified torque in steps.

For example if it says 23Nm I'll probably set it to 15Nm first and see what it feels like. If you don't get a repeatable firm feel then be very cautious. Fine series threads are a pain too.

I've found brake caliper fixings to be especially delicate in this respect, and often use "low torque then feel" so you know it's at least a minimum value.

When I was involved in specifying fastener tightening in production, most commercial fastener suppliers used a "lightly oiled" condition though with environmental issues this practice is more or less phased out. The lubrication and plating etc affects the torque/tension characteristics significantly. If you're using a grease then you might want to reduce the torque by typically 25% anyway (no responsibility accepted! :roll: ). This doesn't apply to major fasteners like con-rods or cylinder heads though.

Loctite when still wet doesn't lubricate anything like as well as oil/grease, and can sometimes improve the "feel" as you tighten it due to the extra friction.

[Fearg]

I'm not sure I understand this 'I don't trust torque wrenches' concept. I have been involved with aerospace engineering all my working life. Torque settings are quoted by manufacturers for a reason. Its not a case of the general public expect it, there is a sound engineering principal behind it. Designers and stress engineers spends hours calculating what the required torques should be. If the concern is in the accuracy of general torque wrenches then thats another thing. You should always slacken off a torque wrench after use otherwise it will stretch the spring inside and will not be accurate any longer. In the aviation industry torque wrenches are calibrated before every use and the calibration guages are strictly monitored and maintained.

[ejohnh]

I'm not sure I understand this 'I don't trust torque wrenches' concept. I have been involved with aerospace engineering all my working life. Torque settings are quoted by manufacturers for a reason. Its not a case of the general public expect it, there is a sound engineering principal behind it. Designers and stress engineers spends hours calculating what the required torques should be. If the concern is in the accuracy of general torque wrenches then thats another thing. You should always slacken off a torque wrench after use otherwise it will stretch the spring inside and will not be accurate any longer. In the aviation industry torque wrenches are calibrated before every use and the calibration guages are strictly monitored and maintained.

Well you just answered it yourself. I've a torque wrench in my garage which I haven't used for 20 years. Without getting it recalibrated I wouldn't trust it. When I was working on car engines I always torqued the large area joints to prevent distortion. I had no idea how accurate it was though.

[rictus01]

Perhaps it's just me,but most of the stuff that I do up, I've previously undone, allowing for any replacement new bolts and a degree a ware, I should know how tight it should be ?

It's always worked for me, although if I'm ever showing someone how it's done I always go threw the whole thing properly as it does take time to know this stuff.

Cheers Mark.

[northwind]

Assuming it was done up right in the first place ;) Or not seized or damaged, or fitted by a previous owner with less clue than muscle. My front sprocket nut was finger tight, for instance.