Torque wrench calibration

[OliverSedlacek]

I don't suppose many of us check the accuracy of our torque wrenches, but as I clearly had a head clamping problem, I thought I would. I filled a bucket with water to give myself a 24 lb weight and adjusted the wrench at distances calculated to be 35 and 70 ft-lb. The dial needed to be set at 42 and 82 ft-lb respectively. I think it's time to buy a better wrench.

As an aside I've also learned that the wrenches used by BMW are rated to stay within calibration for 100,000 operations, after which they are discarded. They get through that many operations in a week!

 

[Jonathan Kay]

That sounds pretty well calibrated to me. Did you check repeatability at the same time?

Jonathan

[John Vine]

Interesting.

Your two readings suggest that your wrench dial over-reads by approx 17-20%.

Given that a decent everyday wrench can achieve +/- 4% (or even +/- 3%), I'd agree that yours is getting tired.

As JK says, did you run the experiment several times?

JV

[colindavies56]

borrow a mates and torque a nut up,now put yours on.i'm sure halfrauds used to send them to get recalabrated for you.give them a buzzz.

[elie boone]

important is to unwind after every use, this way your spring will stay longer within calibration range

[Jonathan Kay]

important is to unwind after every use, this way your spring will stay longer within calibration range

This is widely recommended and I unload mine. But is there any evidence for it? Even an argument from plausibility?

Jonathan

[Jonathan Kay]

Given that a decent everyday wrench can achieve +/- 4% (or even +/- 3%)

For common or garden varieties? That sounds very tight, is it new or across working life?

Thanks

Jonathan

[John Vine]

For common or garden varieties?

Some examples:

Teng: +/- 4%

Halfords Pro: +/-3%

Norbar (the one that I have): +/- 3%

 

Mind you, I did say "decent"!  Good question about working life.  My Norbar comes with a certificate but doesn't quote a validity period.

JV

 

[Roger Ford]

This is widely recommended and I unload mine. But is there any evidence for it? Even an argument from plausibility?

Spring creep, or spring relaxation.  When stressed, a slow movement of molecules in the metal can occur which causes plastic deformation, and therefore reduces the spring power.

It's a tiny effect. My torque wrench has been left at 55 ft lbs (wheel nut setting) for 11 years now. Must get around to calibrating it some time, but I'll bet it's not far off.

ps. Anyone know where BMW leave their rubbish bins?

[Jonathan Kay]

John: Now I've got to go and look how they calculate that tolerance!

Roger: Yes, that's plausible, and I agree about the likely magnitude.

Thanks x2. 

Jonathan

[Jonathan Kay]

As an aside I've also learned that the wrenches used by BMW are rated to stay within calibration for 100,000 operations, after which they are discarded. They get through that many operations in a week!

Fascinating, thanks. Any more information on that... I'm surprised that they aren't made so that the bit that gets tired can be replaced. But maybe it's like Henry Fords's perfect design...

Jonathan

[AdamQ]

This might or might not be of interest (from ESDU 14001, "Applying, measuring and maintaining pretension in steel bolts").  Note that the accuracy figures relate to the pre-load induced in the bolt and not to the actual torque applied.

http://www.blandings.net/images/14001t31.JPG

[John Vine]

This is widely recommended and I unload mine. But is there any evidence for it? Even an argument from plausibility?

Here are two points of view (for click-type wrenches) from the same manufacturer, Norbar:

• Slacken off to minimum scale value
• Slacken or leave as you wish - their tests found that it makes no difference

You pays yer money and....

Personally, I always slacken off to the minimum scale value (not zero), but I was brought up that way.

JV

 

[Jonathan Kay]

Mmmm... evidence! Thankyou.

Jonathan

PS: 30s ago I was questioning the evidence for benefits from staff viewing previous medical records in hospital EDs....

[Roger Ford]

"Further to this, if a torque wrench has not been used for a day, it should be exercised about five times before use to redistribute any grease that had dried up or been squeezed out. For safety, critical or high volume applications it is common to check the wrench every day or every week."

So what does "exercised" mean?

[OliverSedlacek]

Thanks for the links. It confirms my suspicions that spring set is a myth, and it's all to do with wear , corrosion and grease. I repeated my tests and there was a bit of scatter in the measurements, but only a couple of percent.

The big unknown in the relationship between torque and tension is friction, which is why all the methods that measure stretch directly do so much better.

[alan c]

Depending on the make you can get them recalibrated but I suspect this is only worthwhile on more expensive units. I know that we do this for some of the brands within the group I work for as I have seen the process in action. All the units I saw were either very expensive or very large.

[Nigel Riches]

One of the "wages subsidies" of working in aviation is access to a decent calibration lab, and helpful friendly petrol head people there, I get my 2 wrenches cal checked every year, adjusted as necessary and a cal chart printed off. All for the price of a choccy bar.      .

Nigel.   

[abbot]

The big unknown in the relationship between torque and tension is friction, which is why all the methods that measure stretch directly do so much better. 

I am just starting my 998 Mini engine rebuild after rebore, crank grind, etc.  I had a quick look at some engine build videos this morning to back up what I am reading in the Haynes manual.

I was very surprised to see the the guy doing the build applying thread lube to big end and main cap bolts.

Going back some 40 years, when I was given a task as an apprentice at Massey Ferguson, I did some studies on effects of finishes and lubrication on torque and clamping forces.  I do not remember the details but there was a clear memory that you should not go applying lube to bolts.

 Is it simply a question of like everything else today that life and opinions move on and change or was he talking bo77cks

[Jonathan Kay]

I think the most important rule is that the conditions in which you set the torque (or load or stretch or whatever) must match the conditions envisaged when that setting was specified...

Jonathan

PS: What's the destination of the Mini engine?

[abbot]

I think you are right. As I recall the study i did was also looking at the effect of various plating and finishing effects on tractor wheel bolts.  

Mini engine is going back into my classic Mini project 

[ECR]

Jonathan has it right ...

ARP for instance specify a tightening torque for their big end bolts when using their special anti scuff paste/lubricant.

[Mechanical Moz]

Some interesting stuff on the ARP website, particularly about the effect of lubricants on preload repeatability.

http://arp-bolts.com/p/technical.php#p7TPMc1_3

Going to back to the little A series and its big ends, in Vizards "Tuning the A series engine" there is a section on ARP fasteners and the recommendation of using their supplied lube on the threads and all mating surfaces of washers and nuts to increase the clamping force for a given torque. 

As an example of how the torque specification can differ for fasteners doing seemingly the same job, the ARP rod bolt kit torque specification is 55 lbft using their lube. Although they recommend you measure the stretch instead if possible. The Rover service manual makes no mention of whether the fasteners should be lubricated or not and specifies the torque as 37 lbft for big end bolts, 33 lbft for big end nuts.

[ECR]

In my experience, where torque figures are given it is assumed that the joint is LIGHTLY oiled (unless otherwise specified) That is to say not bone dry.

I'm sure you realise that the torque figures you give in your example above are different not because lubricant is used on one and not the other. The higher torque figure for the ARP bolts is  required to take advantage of the higher specification of the steel used and results in a greater clamping force giving the bolted assembly a greater failure limit (higher revs ...)

Sorry if i'm stating the obvious 

[Mechanical Moz]

No worries, it was more to highlight that the correct torque for a fastener can vary by a lot depending on lubrication, material grade, surface coatings and the components being fastened together.  Even when a replacing a fastener which may, on the face of it, appear to be the same.