Checking Torque Wrench Calibration

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Fred W

1 Wheel Drive
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Most of us own and use torque wrenches when we are working on our bikes, but not many of us "Do-It-Yourself-ers" will go to the bother and considerable expense of having our torque wrenches sent out for calibration. I don't know about the rest of you, but being a "type A" guy, I've always wondered just how inaccurate my old clicker wrenches are. How much should I trust them?

In a fit of age related insomnia last night, I came up with a plan for checking my torque wrenches to ensure that I won't under, or worse over torque any important fasteners. To begin with, neither of them are particularly expensive wrenches when new, and neither have any means of actually adjusting their calibration settings that I can see. So this is just a check, to know how far and which way they are off. If they are bad enough it might motivate one to replace a clearly defective torque wrench.

Here's what I came up with. I hooked up the torque wrench to an already highly torqued axle nut (90 ft-lbs on my WR250) and then hung barbell weights off of the handle. I could then iteratively adjust the wrench's torque to find the point where the clicker just barely clicks. Then do a little math and you can determine exactly how accurate the wrench is.

Here's a thousand words of the set-up:

100_3197.jpg


In the above picture, due to the angle of the shot it looks like the wrench is not parallel to the leaf strewn floor. It actually is, and that is an important detail. It is also not obvious that the weights are hovering a few inches above the floor. Unfortunately, the case box is empty, so no beer for breakfast today. :angry:

You could use anything you want to hang the weights. I used a length of solid core electrical house wire and a broken screwdriver blade that I had laying around and made a little weight hanger from them:

100_3198.jpg


You'll note that I used the white (neutral) wire. One wouldn't want to get shocked while doing this calibration check! :eek:

(NERDY electrical joke)

To determine how much torque the hung weight is applying you'll want to pre-weigh the barbell weight on a reasonably accurate scale to find its actual weight. I used my package shipping scale (good enough for UPS!) and found that my 2 weights marked as 25 lbs are actually 24.5 lbs each. Then you'll need to multiply that weight by the length of the torque wrench (in feet to get ft-lbs) from the center of the square drive to the point on the handle that you hung the weight from.

In the pictured example, I have two 24.5 lb weights (49 lbs total) hanging 17.5" from the drive end. 17.5 / 12 = 1.458 feet x 49 lbs = 71.46. So that tells me how much torque is actually being applied to the wrench. Somewhat miraculously, my torque wrench would just barely click over between 71 and 72 on the rotary handle. Sweet! :thumbsup:

I also ran the same experiment, but with a single barbell weight suspended, and the clicker went off at 36 1/2. The math said 1.458 x 24.5 = 35.73, so it appears at lower settings I will be slightly under torquing fasteners, but still within a reasonable degree of accuracy.

I then checked my small 3/8" drive torque wrench using a single 24.5 lb weight, but with the shorter handle I hung the weight only 8" from the drive. It clicked over at ~ 200 inch lbs. And the math says that I was applying 24.5 lbs x 8 inches = 196 inch lbs. :thumbsupsmileyanim:

I now can have good confidence that my old, cheap-*** torque wrenches are measuring my fastener torque settings with adequate accuracy, and should be able to sleep better at night.

How are your wrenches?

.

 
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Fred, you are so anal! JK! I actually had the same thoughts as I was reassebling my bike with the new suspension parts this past weekend. Your methos is simple and will determine if the wrenches are within a reasonable accuracy. Yours looks good with less that 1% in the higher range and just over 2% in the lower. If you can stay withing 5% on a cheapo wrench you are doing pretty well. I may try that setup as well.

 
A couple of points to add:

1. I check the calibration of my torque wrenches by clamping the square drive in my bench vise.

2. An inaccurate torque wrench can still be a good tool if you go to the trouble of characterizing its calibration curve. I'd rather have a precise and inaccurate torque wrench, than an unrepeatable yet accurate (on average) torque wrench. If you have an adjustment screw, great, but it's not necessary.

You can also do a quick sanity check of one torque wrench against another (two clickers might be tough) simply by fitting the appropriate socket to one to fit the square drive of the other. If your standard is a beam type wrench, you can easily check where the other wrench clicks or whether the two pointers are more or less in agreement.

Also, if you don't have barbell weights handy, a liter of water weighs 1kg. A gallon of water weighs approximately 8.34 lbs.

Going by what I know about how things can be non-linear, if you care about accuracy then it behooves you to check your torque wrench at a number of operating points along its range.

 
Fred,

Did you weigh the wire and screw driver and add them to the total?
Sure. And I weighed the handle on the torque wrench and added that too. :blink:

I'm anal, but not that anal. I just wanted to know that my beat up old wrenches are "in the ballpark."

 
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Fred,

Did you weigh the wire and screw driver and add them to the total?
Sure. And I weighed the handle on the torque wrench and added that too. :blink:

I'm anal, but not that anal. I just wanted to know that my beat up old wrenches are "in the ballpark."
If you want to get really anal you could also factor in your elevation, barometric pressure temperature, humidity and zodiac sign.

 
Fred,

Did you weigh the wire and screw driver and add them to the total?
Sure. And I weighed the handle on the torque wrench and added that too. :blink:

I'm anal, but not that anal. I just wanted to know that my beat up old wrenches are "in the ballpark."
If you want to get really anal you could also factor in your elevation, barometric pressure temperature, humidity and zodiac sign.
...not to mention your distance from the center of the earth, so your gravity is nominal!

AND...living in the "Granite State", with the abundance of granite substrate, you're probably dealing with above-average gravity readings anyway.

Be careful!

:p

ps: Oh yeah, forgot to add...you ARE that anal! :D

 
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I wanted to do same so bought one of these. Box had calibration sheet showing higher accuracy than the 4% stated. I've only used it a small amount but it seemed to work O.K. I do not like that if you push the wrong buttons at the same time, you'll recalibrate the unit. Figure you don't really have to adjust the wrench as long as you know the torque offset to compensate and mark it accordingly. Obviously the aforementioned method is pretty much free. For what it's worth, Toukow.

https://www.harborfreight.com/digital-torue-adapter-68283.html

 
Fred, that is fucuking brilliant!!!! :clapping: :clapping: :clapping:

Ignore those other asshats, they're just jealous they didn't figure it out first!

Looks like I have a mission this weekend :yahoo:

 
Fred, that is fucuking brilliant!!!! :clapping: :clapping: :clapping:

Ignore those other asshats, they're just jealous they didn't figure it out first!

Looks like I have a mission this weekend :yahoo:
I have no need to be brillant since I still have friends that work in the Class 1 Calibration Lab over at the local Nuke Plant. My dirt bike riding buddy calibrates all the torque wrenches, micrometers and all the other precision measurement tools used by Mechanical Maintenance (referred to locally as the Knuckle Dragers). I just give him my torque wrenches, digital venier calipers, etc. and I get back a calibrated tool with a nice computer generated graph showing me the calibration data. Now what can be simpler than that??

 
I wanted to do same so bought one of these. Box had calibration sheet showing higher accuracy than the 4% stated. I've only used it a small amount but it seemed to work O.K. I do not like that if you push the wrong buttons at the same time, you'll recalibrate the unit. Figure you don't really have to adjust the wrench as long as you know the torque offset to compensate and mark it accordingly. Obviously the aforementioned method is pretty much free. For what it's worth, Toukow.

https://www.harborfre...pter-68283.html
Yeah, that box would be accurate when it is new, just like my torque wrenches theoretically were (I'd never checked them before). But eventually, you'll want to re-check it too. That would mean either sending it out for recal or doing it yourself.

I failed to mention it because my wrenches can't be re-calibrated, but you could use my method to recalibrate a wrench that does have that capability. In that case (if you will be recalibrating, not just checking) you probably would want to weigh the test weight with a highly accurate scale. My postal scale is OK, but is still analog / springs technology. You would also want to factor in the things I dismissed (weight of the hanger and the weight of the hanging wrench) in coming up with the total torque applied number.

...since I still have friends that work in the Class 1 Calibration Lab over at the local Nuke Plant.
So that explains your glowing personality, Billy!! :p

 
I also have one of the electronic torque adapters. It may be used to check and compare my two "clickers". I figure that they will all be right or all be wrong - but at least the same as each other. The "clickers" are of reasonable quality but AFAIK, they are not adjustable - I would have to apply some sort of offset based upon a calibration run.

 
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