View Full Version : Homemade torque wrenches

01-28-2008, 09:24 PM
Hey guys thought I'd share one of my more recent/interesting projects. I've been working on re-gearing a ford 8.8 inch rear-end on the weekends. I needed a small torque wrench to measure bearing preloads, one capable of 16-29 in-lbs. The cheapest I could find one was 40$ or so and I just couldn't justify that considering how simple they really are. So I got to work did some calcs and came up with one, then realized that it was wrong, cause i forgot the in-lbs part and made it for ft-lbs, i of course didn't realize it until it was near complete, oh well learned a little and then made the correct sized one with a few additional improvements

I used 5/16 drill rod for the larger and 3/16 for the smaller, i verified the calculations with some calibrated weights and they were spot on. Right now i only have marks for the upper and lower limits of the range I'm looking for; but I'll print out a graduated strip and put on them eventually.


thanks for looking

01-28-2008, 09:28 PM
The picture does not work.


01-28-2008, 09:35 PM

01-28-2008, 09:44 PM
Pinion preload on a 9" Ford is 15- 20 in.lbs with used bearings and 25-28 in.lbs for new bearings.
If your setting them up with spacers instead of crush sleeves you don't need a torque wrench, just set the preload without the seal so it feels the same as it would with a new seal.
Carrier bearing preload once contact pattern is satisfactory is to tighten the spanner nuts one more hole and install the locks.
I've done hundreds of them.

01-28-2008, 10:33 PM
yeah i don't know whats wrong with the picture, i'm using webshots and it only seems to show up when i visit the picture directly then it shows fine. i put up a link sorry about that

and j.r. i'm using a crush sleeve on a 8.8, as far as i know i need a torque wrench

Rookie machinist
01-28-2008, 10:44 PM
Once you have done 30 or 40 rears you can set them up by feel and be dead on. When I worked for GM I was the only guy in the shop that did the rear axle work, easy money once you learn a few little tricks.

Paul Alciatore
01-28-2008, 10:47 PM
Try Photobucket. Read the sticky post on photos at the top of this board.

01-28-2008, 11:06 PM
yeah i don't know whats wrong with the picture, i'm using webshots and it only seems to show up when i visit the picture directly then it shows fine. i put up a link sorry about that

and j.r. i'm using a crush sleeve on a 8.8, as far as i know i need a torque wrench

HTTP 403 Forbidden

The website declined to show this webpage
HTTP 403
Most likely causes:
This website requires you to log in.

What you can try:
Go back to the previous page.

Photobucket is the ticket.

01-29-2008, 07:34 AM
yeah so sorry about that, stuborned german in me said webshots will work! but i got a photobucket now so it should show.

i understand that i probably could have done it more or less by feel, but i'm always in the market for new tools and love it when i can make them myself

01-29-2008, 09:52 AM
That is pretty slick. How did you do the calculations to decide on beam (rod) diameter?

Paul Alciatore
01-29-2008, 10:08 AM
That is neat. Good work.

I had to convert a torque screwdriver handle from in-oz to metric. It had a circular scale below the handle and I made a paste on label with the metric numbers. It worked well.

Paul Alciatore
01-29-2008, 10:18 AM
That is pretty slick. How did you do the calculations to decide on beam (rod) diameter?

I don't know how Dave did it, but you only need to do an approximate calculation. After it is built, you can measure the beam length from socket to hand hold and use weights or a spring scale to calibrate. Foot-pounds mean just that, the number of pounds applied at one foot. The formula is:

Length (in feet) X force (in pounds) = torque (in foot-pounds)

If the beam is longer than a foot it multiplies up so you use less force at each calibration point. So 100 pounds at 1 foot = 50 pounds at 2 feet. With a few weights and some pipe for extensions to different lengths it shouldn't take long to calibrate the scale.

01-29-2008, 10:28 AM
I'm very impressed...home made torque wrench.

Did you harden any parts?

If it's not hardened, don't you have to worry about making sure that no parts of the wrench experience stress greater than the yield strength of the material?

01-29-2008, 10:43 AM
But it's more fun to use energy methods to calculate the deflection of the beam at the end point;) You only need the moment of Inertia I, the length of the beam, the modulus of the material. Saving everyone the trouble of learning energy methods, the formula from Marks handbook for the engineering model (a cantilevered beam set into a brick wall) is:

Deflection = P*L^3/3*E*I

where P is the applied force at the end of the wrench, L is the length of the lever arm, E is Young's Modulus for the material and I is the moment of inertia for the beam .
since torque at the end of the wrench Tau is P*L,

the torque to deflection relationship is thus:

Scale Deflection=Tau*L^2/3*E*I

For a round rod I= (Area/4)*radius^2.
An approximation for steel E=2.93*10^7 psi.

For bonus points, you can also calculate the slight change due to shear in the part where the beam is connected to the wrench socket on the end.

Regards all,


01-29-2008, 12:16 PM
That's cool!

Now I wonder, since you can make decent torque wrenches, whether there is applicability to making special purpose wrenches for the shop?

For example, the gunsmith crowd likes a torque wrench on the tailstock lock. Seems like you could build one in permanently.



01-29-2008, 12:17 PM
ckelloug pretty much hit it on the head, except i used the mechanics of materials approach, it leads to the same answers just derived from different methods. There are tables of formulas in machinery's hand book, i used those to help me choose which size drill rod to use. I made an excel file that calculated the deflections and such, note that the formula ckelloug gave is for the deflection at the point of the force, when what you really need is the deflection somewhere along the beam (where ever your scale is) I was even able to plug in the yielding stress and figure out what the max deflection would be before permanent deformation. (should answer your question rotate, i did not harden anything as i was able to achieve the deflections i needed without doing so.)

I would make the excel file available if anyone is interested, and could tell me an easy way to post it for everyone


01-29-2008, 07:47 PM

Kudos for using the canonical right approach and working it out with mechanics of materials. I would have had to get out the mechanics of materials book but I'm rusty on it and didn't have time. Sounds like time for virtual forces and Castigliano's deflection theorem. . .

Hanging around here is a great way to find problems to keep skills from complete decay.

I was assuming my approach was going to turn out right for a torque wrench where the scale was at the very end of the beam but I didn't stop to make sure I hadn't flubbed the logic and that it was possible to make such a torque wrench.

Good work on the handmade torque wrenches by the way as I didn't think to say it in my last post.

Regards all,