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.