torque wrench calibration

[Tazman001]

I just read the home brew manometer thread and decided to post about another problem I have here. I bought a inch-pound torque wrench at a yard sale last summer and do not trust the calibration. Now I've read on several websites about making a home brewed torque wrench calibrator. Some of the websites said to find a Snap-on guy and Snap-on will calibrate it. Well I don't want to spend a fortune on a $2.50 yard sale torque wrench. Don't get me wrong, the torque wrench was a quality item(probably $130-150 new) if it wasn't I wouldn't have bought it. Paying some guy on a Snap-on truck $30, 40, or 50 bucks jus goes against the grain. So has any GSR subscriber made a home built torque wrench calibrator? any suggestions?

 

[Joe Nardy]

It's actually pretty simple to calibrate a torque wrench. Clamp the square socket holder in a vise with the handle parallel to the floor. Hang something of a known weight from the handle at a known distance from the center of the socket holder. Then multiply the weight by the distance. The easiest way is to use a distance of 12 inches (for foot-pounds). For example, hanging a ten pound weight at a distance of 12 inches yields 10lb X 1ft=10 foot-pounds.

Thanks,
Joe

 

[posplayr]

And that is all she wrote

And that is all she wrote

Great idea. I have a small one as well I dont trust.

Posplayr

 

[bwringer]

Yup, what Joe said. This ain't rocket science.



Man, all I ever see at yard sales is baby clothes. :-s

 

[srsupertrap]

It's actually pretty simple to calibrate a torque wrench. Clamp the square socket holder in a vise with the handle parallel to the floor. Hang something of a known weight from the handle at a known distance from the center of the socket holder. Then multiply the weight by the distance. The easiest way is to use a distance of 12 inches (for foot-pounds). For example, hanging a ten pound weight at a distance of 12 inches yields 10lb X 1ft=10 foot-pounds.

Thanks,
Joe

Joe,
I have a question about the technique. How are you hanging the known weight from the handle? Because if the weight isn't in line with the center of the socket holder you have just created a Moment Arm and then the calibration would be off. Perhaps you are using dumb bell weight and slipping it on the end of T-wrench . . . I am interested.

 

[Joe Nardy]

Joe,
I have a question about the technique. How are you hanging the known weight from the handle? Because if the weight isn't in line with the center of the socket holder you have just created a Moment Arm and then the calibration would be off. Perhaps you are using dumb bell weight and slipping it on the end of T-wrench . . . I am interested.

I have always hung a dumbbell weight from a string. I don't know the physics of this but, as far as I can figure, a weight hanging straight down would apply the same pressure as the same weight resting on top of the handle at the same point. Am I wrong in this assumption?

Thanks,
Joe

 

[rustybronco]

You are not wrong in your assumption, one is pushing down from the top, the other is pulling down from the top.

 

[srsupertrap]

I have always hung a dumbbell weight from a string. I don't know the physics of this but, as far as I can figure, a weight hanging straight down would apply the same pressure as the same weight resting on top of the handle at the same point. Am I wrong in this assumption?

Thanks,
Joe

I would have to pull out the old Typler Physics book to be surebut I do recall the following: When you start applying force at angles other than zero you have to account for the Sine & Cosine of those angles in the calculation. The formula T = Force X Distance works without any mention of angles because the Sine of Angle 0 = 0 & the Cosine of Angle 0 = 1 so the net result is the Angle of zero nether mutiplies nor subtracts to the overall result. So if the calibration weight is applied directly to to the axis of the torque wrench at X distance that's certainly valid.

Looking at your technique with the string hanging straight down at 90 degrees you have applied a moment arm but the net result is the same as for 0 degrees because the Sine of 90 = 1 and the Cosine of 90 = 0. The values reveresed but the net result is the same. Looks good.

 

[posplayr]

OK the mathematical relationship in laymans terms

OK the mathematical relationship in laymans terms

The formula for Torque involves Force "crossed" into a distance (or simple multiplication by a moment arm) . 8-[


What that means is the effective distance that a force acts through is computed by the length of a perpendicular distance (see the pic) between the point of rotation and the closest distance to the force vector. In this case the force vector is in line with the taught string.

So using the concept of the "moment arm", you don't have to worry about the cos theta terms, although it can be computed that way as well.

Hope this helps. Realize if you are going to calibrate the wench at different angles, the perpendicular distance is changing and you are bringing into play the cos theta term, because the moment arm is shortening. The distance D (how far the string is tied to the wrench from the bolt center) is always longer than the "moment arm" except when theta is zero and they are the same. This is the cos theta relationship supertrap was describng.

Further note, it is assumed that the bolt lies in the horizontal plane, otherwise you have yet another angle to consider in your moment arm measurement.

 

[tkent02]

Rocket science again.

 

[TheCafeKid]

Leave it to you guys to turn wrenching into a physics lesson. When i wrench, the biggest thing i worry about is whether or not the damn thing is gonna work right when im done! LOL

 

[gs1197]

I've noticed that no one is checking the torque RANGE.
Just because a wrench checks good at 10 lbs. doesn't mean it's correct at 50.
Had a name brand clicker type wrench several yrs ago that checked spot on at lower ft. lb. increments, but would get progressively worse at higher torque settings.
Don't believe it had been used enough to be a wear issue, was probably not calibrated correctly from the beginning.
Stop and think how much time /trouble/expense it is if you pull a cylinder stud out, strip a cam cap bolt...etc...because it was over-torqued or leaky blown gaskets because of under-torque.
A qualified metrology lab will check/calibrate the entire range and it's not that expensive.
Money well spent.

 

[Joe Nardy]

I've noticed that no one is checking the torque RANGE.
Just because a wrench checks good at 10 lbs. doesn't mean it's correct at 50.
Had a name brand clicker type wrench several yrs ago that checked spot on at lower ft. lb. increments, but would get progressively worse at higher torque settings.
Don't believe it had been used enough to be a wear issue, was probably not calibrated correctly from the beginning.
Stop and think how much time /trouble/expense it is if you pull a cylinder stud out, strip a cam cap bolt...etc...because it was over-torqued or leaky blown gaskets because of under-torque.
A qualified metrology lab will check/calibrate the entire range and it's not that expensive.
Money well spent.

I usually check mine from time to time at the torque I am about to use. That is a valid point though. It wouldn't be too hard to check it at several different torques. Even if it was off you would know approximate compensations.

Thanks,
Joe

 

[headsbikesmopars]

One reason I like to use my beam torque wrench to check my click wrench. After I reach torque I put the beam T wrench on there and double check it with that. Sometimes I will reverse that order and set torque with the beam wrench and follow that with the click wrench. It should click without any movement of the bolt. And beam T wrenches are easy to calabrate, just push the needle back to zero if it isn't.

 

[Dark Jedi]

Joe,
I have a question about the technique. How are you hanging the known weight from the handle? Because if the weight isn't in line with the center of the socket holder you have just created a Moment Arm and then the calibration would be off. Perhaps you are using dumb bell weight and slipping it on the end of T-wrench . . . I am interested.

for this method, the wrench being parallel to the gravitational pull is vital. use a level.

(I used to be a Metrologist... a calibrations tech. I couldn't count the number of wrenches I've calibrated, among other things)

 

[posplayr]

Just for reference

Just for reference

The errors associated with a moment arm being "out of level" are summarized below (e.g. e=1-cos(theta)). In other words, the torque applied to the wrench is less than what you would think based on the distance that the weight is tied to the wrench from the axis of rotation and the attached weight.

So being within +/- 10 degrees is equivalent to having +/- 1.5% reduction of applied torque.


Degs Error
1 0.02%
2 0.06%
5 0.38%
10 1.52%
15 3.41%
20 6.03%
30 13.40%

Posplayr