Calibrate a Torque Wrench

[Iron]

Torque wrenches are pretty good bits of kit. They can last for years and so should be checked every now and then to make sure they are reading close to where they should be.
I used to have one that had a long pointer and a scale, but it was difficult to read as I was straining on the other end.
I also bought a more modern one that is almost impossible to get into without smashing it up with a club hammer to check it.

So, mine are all now like this one. Simple to use and easy to check that they are clicking when they should.

To calibrate the torque wrench you’ll need something like this. A cheap luggage weighing thing. A few squids. But check that it’s reading correctly.

Use some kitchen scales to weigh something. Then hang that something on the luggage weigher to ensure it’s reading right. Or you can be clever and measure out 4 lires of water into a milk carton and use the luggage weigher to show the 4 litres weigh 4 kilogrammes (obviously do the thing with the weight of the carton as well)

.
Here’s the reading/setting scale on the torque wrench - it’s scale is in NewtonMetres. You can use this scale but the other side of the wrench…

…will have the scale M.KGS. or MetreKilogrammes. You may have f.lbs or FootPounds. These are the correct scales for Torque. It is basically a bending moment - a weight times the distance from the point the torque is being applied to.

Don’t be confused by the NewtonMetres scale, it is basically Kilogrammes mulitiplied by 9.81. If all you have is Nm on your wrench then apply the 9.81 to your calculations. The weight you read on the luggage weigher is multiplied by 9.81 to give you Newtons.

So, to check your wrench is reading ok-ish stick a large socket into the vice and tighten up pretty well.

Put the wrench into the socket

Wrap some masking tape around the handle close to the end and mark the tape. Measure from the mark…

…to the centre of the socket. (Where the torque will be applied.)

In my case, it measures 400mm. Or 0.4 metres. (remember we want MetreKilogrammes not MillimetreKilogrammes) I write that on the tape so I don’t forget.

If you have a wrench that measures in f.lbs - footpounds then convert your measurement to feet (footpounds not inchespounds or millimetrepounds etc). (There’s 12 inches in a foot and 25.4 millimetres in an inch - just in case you forgot).

Set the torque wrench to something simple about half way up the scale. In this case I’ve set it to 10 M.KGS.

Put the wrench into the socket in the vice and hang the luggage weigher on the mark made on the tape. Put something through the hook and pull on the luggage weigher. Pull slowly while watching the reading on the weigher.
Try to get the weight reading when the torque wrench clicks. Do this a few times to get the weight reading as close as posible.

This luggage weigher has a max reading thing built in so I can see what the torque wrench clicked at.
In this case 18.70 kilogrammes. I write that down so I don’t forget.

Then some simple maths. The torque is the weight multiplied by the length of the wrench. This time around it’s 0.4 metres multiplied by 18.70 kilgrammes.
Which gives the torque of 7.48 M.KGS. It should be 10 M.KGS.

So, miles too low. Adjustment is required…I’ll get onto that in a bit - it’s dinner time, hooray.

[PatW]

And folks…don’t forget to loosen the wrench off after you’ve finished.

[Octoberon]

I’ve got a torque wrench that I’ve had for many a year, as they say, and I’ve long wondered about how accurate it is after all this time. As I’m in the pub I haven’t read this thoroughly but i will!

Thank you for helping me with something that’s been bugging me for a dogs age!

[ducatitotriumph]

This is great and thanks for sharing!
Great advice about what you are checking.
How was dinner?
As always, no pics means you didn’t eat…!

[BrownMouse]

I use this scales and ruler method to get correct torque when using C spanners or with normal spanners when I can’t get access with a torque wrench.

[Iron]

Roast dinner tonight, luvly it was too No photos, sorry.

Right where was we? Oh yes, wrench reading too low.

As @PatW says, undo the lock and turn the tension off of the wrench (it should be kept like this when not in use)

Hold onto the handle and undo the large lock nut on the bottom of the handle.

Take the locknut off. There’s essentially a spring under the handle that is loaded by either a threaded collar (as this one is) or a collar that is moved by the pair of holes in the bottom. (like the lock ring on a wheel bearing) that type requires a pair of circlip pliers to turn.

As this toirque wrench is reading low I need to increase the load on the spring by turning the collar further into the handle. As long as the torque wrench isn’t loaded it’s easy to turn the collar. So turn the collar in (I expect you could calculate the amount to turn in given the results but I guessed).

Put the locknut back on and tighten up. No need to get potty here just enough to hold it.

Do the luggage weighing pull thing again. No need to remeasure just dial the torque wrench to an expected 10 KG.M again.

And do the maths again. Those of you who are paying attention will notice I got the weight wrong on the calculation but hey ho, it’s still too low.

Do all the adjustments again and try another one

Closer but probably needs another go. Any old end up, you get the idea. Go check that you’re not about the break the studs with a wrongly calibrated torque wrench.
Thank you…welcome…over

[Col_C]

Hopefully @Iron won’t mind me gate crashing his excellent tutorial and adding a few comments.
As can be seen from the example it’s more likely that the torque wrench will give a lower torque than expected due to the internal spring getting tired with age. Hence the already mentioned advice to always store it at minimum setting to reduce the stress in the spring over time. Accuracy of the torque wrench depends on the spring rate remaining correct, so I would suggest calibrating at its mid range setting and then rechecking at max and min settings.
An old uncalibrated torque wrench is more likely to result in a “lose” bolt rather than breaking it. However we are often in the habit of using copperslip or the like when assembling, this should be borne in mind, the recommended torque setting might be for assembly dry. Torque settings are only a means to generating the correct tension in a bolt, lubrication will increase that tension for a given torque. I’ve seen Ducati specify torque settings along with expected lubrication if any, can’t say I’ve seen it from other manufacturers?

[PatW]

Agreed and never grease a stainless fastener even in a ferrous thread, it’s unecessary.

[Andyc1]

One of the things I like about KTM is just that. They specify the torque, grease, threadlock, special instructions etc on every nut and bolt on the bike. All in the owners manual.

[Rocker]

I use Snap On and Facom, lifetime guarantee and free re calibration, the Snap On torque wrench is still going as good as when I bought it some 40 plus years ago.

[Joe]

I have four or five some never even been used yet. My boys got them all for me and all good makes. Must get them out and use them sometime.
Joe.

[Octoberon]

I was looking for any recommendations for torque wrenches and came across this old post. Just what I need. One question that wasn’t asked, how do we know that the luggage weighing thing is accurate enough?

[Iron]

You use the luggage thing to weigh something of known weight - like a few litres of water in a bucket. Note the difference in weight when the bucket is empty to a known number of litres in it.

[Octoberon]

What is my measuring jug is wrong? Supposing gravity is a bit less where I live.

…I’ll get my calculator.

[Col_C]

Had a quick scan through this thread.

@PatW

Agreed and never grease a stainless fastener even in a ferrous thread, it’s unecessary.

I beg to differ. Google “galling of stainless steel” and you’ll see why. Stainless on stainless (eg nut and bolt) is the worst case combination but can happen with other materials.
A design engineer in oil & gas exploration most of my working life before retiring, I’ve seen it happen too many times.

[PatW]

Whilst I respect your opinion, there’s a fair bit of difference between torqueing a gas tight seal and putting an accessory (typically) on a motorcycle

[Col_C]

This is why I didn’t comment first time around…
But as I’ve already stated assembling a simple nut and bolt both stainless and clean, no external pressures involved, I have seen galling occur. So IS relevant.
I will leave it there for others to Google/investigate and make their own decision. My conscience is now clear, there’ll be no further comment from me.

[Octoberon]

Luggage scales ordered. Wierd looks from Rachel when I explained why I wanted one.

Edit: And delivered.