Car Analogy

I always wondered about the accuracy of my 3/8 drive Harbor Freight torque wrench. The dial on my 1/4 & 1/2 are spot on. But the 3/8, its always hard to tell if you have it set right. For example, if you want, say, 40 ft/lbs, you're supposed to crank it till the dial is just reaching the line for 40, then line up the 0 line on the dial with the 40 line on the handle. But on mine when you do that, the handle dial is smack in the middle of the 40 line and the 45 line. A manufacturing defect. So I never knew if I should ignore the numbers on the handle, and just line the edge of the dial up to the 40 line, or assume the 0 on the dial is correct. But then should I line up the 0 with the dial between the 40 & 45 line, or the 35 & 40 line?

So I decided to test it. I have this digital luggage scale I got from Amazon for like $15. Useful to make sure you don't go over 50 lbs, so you save having to pay overweight baggage fees when you fly. It will record the max weight, so you don’t have to watch as you measure, and it lets you zero out the scale to a given load. Handy for our purpose here! I know its accurate, as I tried it with several different barbells, made by different companies, and it always read exactly what the barbell stated.

I set the wrench to the middle of its scale, which was 40 ft lbs (I guessed at where 40 should really be, setting it between 40 & 45). Then I put the wrench in my vise, clamping the square 3/8 drive end securely, and so that the wrench was level to the ground. Then I measured out 1 foot from the center of square end, and placed the scale on that spot on the handle. Then I zero’ed out the scale, with the weight of the wrench on it. I just pulled up enough to unweight the wrench, but not enough to move the ratchet to the next click. It read 1.1 lbs, and I zero’ed it to that. Now the wrench weight is not going to mess with our torque readings. Note that 1.1 lbs is not the full weight of the wrench. We don’t want that. We want the ‘unsprung’ weight. Hence zero’ing it when its hanging from vice.

Then I just lifted up on the scale, slowly, until I felt the click. Then I noted the reading on the scale. It was like 43 ft lbs. Not so good 

So then I just tried it with the handle dial set to align the 0, but between the 35 & 40 lines on the handle. Retried, 39.6 lbs! Nice. 1% accuracy. Now I know to rely on aligning the 0 on the handle, but to the lower side of the line on the handle for the value I’m looking for, and my wrench will be accurate. Previously I had to guess which was correct, and I was guessing wrong.

So the cheat sheet to measure calibration using luggage scale:

1. Set wrench to some value, near the center of its scale
2. Place wrench in vise, level, clamped on its square drive
3. Measure down handle out 1 foot from center of drive, that is where to place scale
a. If your handle is too short, you can just measure how many inches out from center you are placing the scale, and we’ll use that number later
4. If scale can be zero'd, then pull up just enough to unweight handle, not enough to get it to click the ratchet - zero scale
a. otherwise just subtract 1 lb from whatever reading scale shows for the rest of the testing
5. With scale holding handle so it doesn't fall, flip the ratchet so now pulling up is the direction for tighten - make sure wrench stays level
6. Lift up on scale slowly, until the wrench just clicks
a. Be careful not to keep pulling after it clicks, or you will read higher than the real calibration number
7. Check the scale to see what the max pull was on the last reading
8. If you had the scale at exactly one foot down the handle, this is your ft/lb reading – compare it to what you had wrench set to, see if the numbers jive
9. If you had the scale at some number of inches down the handle, use the formula below:
a. T = W x D
b. Where T is the Torque (in ft/lbs), W is the Weight (in lbs), and D is the Distance (in feet)
c. So if you placed the scale at 8 inches down the handle from square drive, then D is .67 (8/12 = .67, divide your inches by 12, and use that number as D)
d. So if you set wrench to 40 ft/lbs, and scale is at 8 inches down the handle, and to get the wrench to click you had to pull up 60 lbs on scale to get it to click, then your wrench is accurate (60 lb x .67 feet = 40.2 ft/lb)