torque wrench test?

[takeru]

i have two click type torque wrenches that i want to check their accuracy. does anyone know a place in the NY metro area that can do it for a good price, or possibly a free way if possible? never had these checked before.

 

[MedicBob]

If they are Snap-On ones they can have them calibrated. It is for a cost and they have to be shipped.

Check around or call you local tool guys. See if they have any suggestions.

 

[Greenman]

Put one on each end of a bolt, see if they both click at the same time. If they do it would be reasonable to assume that they are ok.

 

[Pacfanweb]

Originally posted by: Greenman
Put one on each end of a bolt, see if they both click at the same time. If they do it would be reasonable to assume that they are ok.

Not necessarily.

If you tighten a bolt to 100 lb/ft, then take a torque wrench and set it to 70 lb/ft, it'll click.

So if you have two of them, and one is reading a bit weak, this method won't work if you use it second. If you use it to tighten the bolt first, then use the one that's right second, you'll know you have a problem with one of them.

 

[Howard]

Apply a torque to them?

 

[Greenman]

Originally posted by: Pacfanweb

Originally posted by: Greenman
Put one on each end of a bolt, see if they both click at the same time. If they do it would be reasonable to assume that they are ok.

Not necessarily.

If you tighten a bolt to 100 lb/ft, then take a torque wrench and set it to 70 lb/ft, it'll click.

So if you have two of them, and one is reading a bit weak, this method won't work if you use it second. If you use it to tighten the bolt first, then use the one that's right second, you'll know you have a problem with one of them.

You misunderstand. There is one wrench on each end of the bolt. The point is to see if they both click at the same time.

 

[Howard]

Originally posted by: Greenman

Originally posted by: Pacfanweb

Originally posted by: Greenman
Put one on each end of a bolt, see if they both click at the same time. If they do it would be reasonable to assume that they are ok.

Not necessarily.

If you tighten a bolt to 100 lb/ft, then take a torque wrench and set it to 70 lb/ft, it'll click.

So if you have two of them, and one is reading a bit weak, this method won't work if you use it second. If you use it to tighten the bolt first, then use the one that's right second, you'll know you have a problem with one of them.

You misunderstand. There is one wrench on each end of the bolt. The point is to see if they both click at the same time.

That'll test their precision, but not their accuracy.

EDIT: Put a cap screw in a vise and put the wrench on such that the wrench is horizontal. Clip a vise grip at either 1 foot or 2 feet away from the center of the screw. These distances will make calculations easier. Hang a known weight from the vise grip. T=Fl, where F is force (combined weight of the vise grip and the load) and l is the distance between the axis of rotation and the point of application of the force. Use a good scale to measure the weight of the mass you use, and something like fishing line so that you don't have to include its weight in the calculations.

If your vise grip is located 2 feet away and weighs 0.5lb and you attach a 10lb weight on it, your torque is (10+0.5)lb*2ft=21lb-ft.

EDIT2: For greater accuracy using this method, include the weight of the torque wrench itself and include it in the total torque like so:

Ttw=weight of your torque wrench * half the length of your torque wrench (this is a good approximation)
Tt = Tw+Ttw, where Tt is total torque, Tw is torque from vise grip with weight, and Ttw is torque from the torque wrench's own weight

So, if your torque wrench is 2lb and is 2.1ft long, your vise grip is 0.5lb and you hang 32.5lb from it at 1ft from the screw, the torque is:

Tt=Tw+Ttw
Tt=(0.5lb+32.5lb)*(1ft) + 2lb*(2.1ft/2) = 35.1 ft-lb

You can calculate an error of ~6% by not including the torque from the wrench's own weight, but the error goes down as you use a heavier load. There's no need to get the wrench perfectly horizontal; at 5 degrees from horizontal, the torque calculated using the above method will be cos(5) = 0.38% lower.

 

[Gillbot]

I was told to keep a beam type since they are most accurate. We compare our clickers to the beam type for a quick check.

 

[Pacfanweb]

Originally posted by: Greenman

Originally posted by: Pacfanweb

Originally posted by: Greenman
Put one on each end of a bolt, see if they both click at the same time. If they do it would be reasonable to assume that they are ok.

Not necessarily.

If you tighten a bolt to 100 lb/ft, then take a torque wrench and set it to 70 lb/ft, it'll click.

So if you have two of them, and one is reading a bit weak, this method won't work if you use it second. If you use it to tighten the bolt first, then use the one that's right second, you'll know you have a problem with one of them.

You misunderstand. There is one wrench on each end of the bolt. The point is to see if they both click at the same time.

Okay, but even with your scenario, you will then know if one wrench is reading "weaker" than the other, but not by how much, and you won't know which one is wrong. Only way to get them truly tested is to send them off.

 

[Pacfanweb]

Originally posted by: Gillbot
I was told to keep a beam type since they are most accurate. We compare our clickers to the beam type for a quick check.

You were told wrong...the beam-type torque wrenches are the LEAST accurate. Nobody uses them for precise torquing.

I'd say the dial-types are the most accurate. Don't have any experience with the digital ones, though.

 

[alkemyst]

my head assploded

 

[herm0016]

what howard said will work. we use to use a similar method to this to test compression strength of fiberglass/balsa wood/ plywood/ laminations.

 

[Gillbot]

Originally posted by: Pacfanweb

Originally posted by: Gillbot
I was told to keep a beam type since they are most accurate. We compare our clickers to the beam type for a quick check.

You were told wrong...the beam-type torque wrenches are the LEAST accurate. Nobody uses them for precise torquing.

I'd say the dial-types are the most accurate. Don't have any experience with the digital ones, though.

Thanks, noted! :thumbsup:

 

[takeru]

thanks for the directions Howard

Clip a vise grip at either 1 foot or 2 feet away from the center of the screw.

can you explain this a bit more? not quite sure what you mean.

i understand the idea of turning the cap screw against the known weight hanging on the vice grip, but i'm not sure how you mean to rotate the bolt held in the vice?

 

[alphatarget1]

I can probably build a device that measures torque with some strain gages and open my own torque wrench calibration business . I don't know how to adjust the torque wrenches itself, but measuring torque accurately shouldn't be that hard.

 

[Howard]

Originally posted by: takeru
thanks for the directions Howard

Clip a vise grip at either 1 foot or 2 feet away from the center of the screw.

can you explain this a bit more? not quite sure what you mean.

Sure. Because the torque (if you want to find it in ft-lbs, and you probably do) is found by multiplying a force that twists something about an axis by the distance between the force and the axis of rotation, making the distance = 1 ft will make it such that the torque (in ft-lbs) is simply equal to the weight in lbs. For example, a 10lb force applied 1 foot away from an axis (can be anything) causes a 10lb*1ft = 10 lb-ft. If you don't understand that, I suggest reading the wiki article on torque.

i understand the idea of turning the cap screw against the known weight hanging on the vice grip, but i'm not sure how you mean to rotate the bolt held in the vice?

The whole point of putting the screw in the vise is to keep it from moving. By the way, don't use a good screw; you'll probably crush the threads. The torque wrench, when installed on the screw, should not move until it clicks. This will tell you how far off it is from nominal. Let's say that you set the wrench for 50lb-ft and then you apply a torque of 50lb-ft. The wrench should click. Adjust the wrench for, say, 49lb-ft. It should not click. Conversely, if you set it for 50lb-ft and it doesn't click at 50 lb-ft, set the wrench to click at 51 and see if it does.

Error = (actual - nominal)/nominal
Error = (51-50)/50
Error = 2%

 

[Howard]

IMG_0875.jpg

If you REALLY want, I can put text on it.

 

[takeru]

Originally posted by: Howard
IMG_0875.jpg

If you REALLY want, I can put text on it.

please do, i am a bit lost now as to how its assembled.

 

[Howard]

I'm too lazy to do that right now.

So, pretend that my hand is the screw and the metal tube is the torque wrench. The head of the bar "fits" over my hand, right? That's the pivot point. My forearm is in a vise. BTW my forearm should also be horizontal; this just gave the picture a better angle. The distance between the center of my hand and the point where the vise grip is attached is the distance used in the torque calculation. You can see that I attached a weight to the vise grip.

EDIT: Fuck it. Put a screw in a table vise and tighten it while screw is horizontal. Put the torque wrench on the screw. Take a vise grip and clip it on the shaft of the torque wrench at a distance of 1 foot away from the screw. Choose the torque measurement on the wrench that you want to test. It might be 50lb-ft. If so, hang a total of 50 lbs (MINUS the weight of the vise grip) from the vise grip. Does it click? OK, unload the weight and set the wrench to 49lb-ft and put the same weight back on. It shouldn't click.

At this point I'm just going to forget about this thread.