Make a Torque Wrench Extension
Background
I have a decent "clicker" type torque wrench with a range of 15 – 150 ft-lbs. I recently had to pull the front hub from my car. When installing it back on, the spindle nut needed to be torqued to 180 ft-lbs. What to do? My first inclination was to buy another torque wrench, at a cost of about $120 (for a decent one). But how often am I going to use it? Practically never.
An alternative was to make a special extension which will multiply the torque. I DO NOT mean an extension like this:
A regular straight extension like the one above does not change the effective length of your torque wrench, and will therefore not have any affect on the torque setting.
Now look at this:
The big difference is that the extension shown above does increase the effective length of the torque wrench. As a result, the actual torque applied to the bolt will be higher than the setting on the torque wrench.
The extension shown in the above picture is called a crowfoot extension, which can be handy when trying to torque hard-to-reach bolts. You must, however, use a formula to account for the length of the extension:
wrench setting = desired torque x wrench length / (wrench length + extension length)
For example lets say that the length of the torque wrench is 18 inches, and the length of the extension is 6 inches. The desired torque is 180 ft-lbs.
Wrench setting = 180 x 18 / (18 + 6)
= 180 x 18 / 24
= 135 ft-lbs.
If the torque wrench is set to 135 ft-lbs, the actual torque applied to the bolt will be 180 ft-lbs.
You can try plugging in numbers below for your particular torque wrench. When measuring the length of your torque wrench, be sure to measure from the center of the handle to the center of the ratchet head.
Rather than using a crowfoot extension, I decided to make my own extension so that I can use sockets. Then all I have to do is apply the above formula. And actually you don’t even need to do that. All you need to do is apply the above multiplication factors.
Making The Extension
The extension must have a female fitting on one end, for attaching to the torque wrench. On the other end it needs a male fitting to attach sockets. Since my torque wrench takes 1/2" sockets, all the examples below are for a 1/2" extension. If your torque wrench is 3/8" or 1/4", then resize as needed.
Parts List:
5/8" drill bit ($7 at Menards)
1/2" socket extension, 6" long ($7 at Menards)
3/8" x 1.5" x 12" steel bar ($9 from local steel supplier)
$23 total
Step 1: Get a steel bar 1.5" wide
Your first decision is what thickness to get. I recommend 3/8 to 1/2", to help hold the fittings straight during welding. You want the fittings to be 90° to the bar. If you use a thinner bar, be careful to keep the fittings straight while welding.
You will be drilling 5/8" holes in the steel, so I would not go much narrower than 1.5" wide.
Menards sells a 1/4" x 1.5" x 36" bar for about $10. You could cut two pieces and tack-weld them together to make it 1/2" thick. Instead, I visited a local steel supplier and had them cut me a 12" piece of 3/8" steel for a total cost of $9. Look in the phone book under "Steel Distributors".
Another decision is what type of steel to get. Admittedly I know nothing about steel. There are numerous grades of carbon steel, and lots of different alloys. And then there's hot-rolled steel and cold-rolled steel. After some research, I concluded that regular carbon steel is inexpensive and will work just fine. No need for any special alloys. Bottom line - buy something cheap. If it is 3/8" to 1/2" thick it will be plenty strong!
The length of the extension is up to you. Make it long enough to handle the highest torque you think you might need. I chose 10" for mine, which will allow my 150 ft-lb torque wrench to handle jobs up to 240 ft-lbs.
NOTE: When I say a 10" extension, I mean that the holes will be drilled 10" apart. To accommodate that, my bar is 12" long.
Here is a calculator you can use to help choose the length of the extension:
The calculator will tell you how far apart the fittings must be. To determine the overall length of the extension, add about 2 inches. This will allow for 1" of extra material at each end of the bar.
Step 2: Get a regular straight extension and cut it in half
Get yourself a 6" socket extension like this:
NOTE: Not all extensions have the same diameter shaft. It varies with the manufacturer. Drill bits only come in fixed sizes. Menards has a 9/16" bit and a 5/8" bit.
Please remember that the extension is chrome plated. Welding on chrome will create toxic fumes, so you should lightly grind away the chrome plating before welding. This will slightly reduce the diameter, so get an extension with a diameter slightly larger than the drill bit.
After checking at a variety of stores, I found that the best extension was at Menards. After grinding away the chrome plating, it fits nicely into a 5/8" hole.
After you have chosen an appropriate extension, cut it in half like this:
In the above picture you can see that I have already ground off the chrome plating.
Wait until after welding before you cut these any shorter.
Step 3: Drill holes in the steel bar
Hopefully you have a drill press, so that the holes will be nice and straight. Here is my bar after drilling 5/8" holes 10" apart:
Step 4: Weld the ends onto the steel bar
You must be careful when welding the female end. This is because when you are using the extension, it should be aligned with the torque wrench (see "Sources of Errors" below). You do not want the extension to be cocked at an angle. To align the extension to the wrench, you would typically ratchet the wrench around until the two are lined up. BUT, the ratchet mechanism has distinct steps. If you are unlucky, it will not ratchet into a position where the extension is lined up to the wrench. You do not want to discover this after welding the female fitting into position! You should weld the female end following these steps:
Insert the female fitting onto your torque wrench.
Insert the other end of the female fitting into the extension.
Insert the end of the fitting into a vise and let the torque wrench "hang" against the ratchet mechanism. This will remove any slack.
Put a level on the torque wrench, and adjust the wrench until it is level. You may need to loosen the vise to adjust for level. The wrench should be level when it is hanging against the ratchet mechanism.
Put a level on the extension and then turn the extension until it is level.
Mark the location of the female fitting on the extension.
Remove the torque wrench and tack-weld the female fitting into position.
Now check your work:
Clamp the extension into a vise.
Put a level on the extension and adjust it to level.
Plug the torque wrench onto the extension and let it hang against the ratchet mechanism.
Check that the torque wrench is level.
If everything looks good, then permanently weld the female fitting.
If you need to make an adjustment, then grind-off the tack welds and repeat the process.
Take your time so that the extension and the wrench line up properly. Don't worry, it doesn't have to be perfect. You might want to review the section on angular errors. Click <here>.
After you have welded the female fitting into position, weld the male fitting. The position of the male fitting is not critical. Here is my finished extension:
That's it! Your extension is now ready for use.
Using Your Extension
Using your extension is very straightforward:
Attach the appropriate socket to the extension.
Set the torque wrench using the multipliers from the previous text (see <here>).
Attach the torque wrench to the extension.
Align the extension and the torque wrench. If it does not align very good then unplug the wrench, turn the ratchet 90°, and try again.
Torque the bolt.
Sometimes a range is given for a torque specification. In Step 2, set your torque wrench for the middle of the range so that if you have a small error, the applied torque will be within range.
Sources of Errors
Worried that using an extension might cause inaccurate torques? I don't blame you. Here are the various errors that can be introduced when using an extension:
If you do not accurately measure the length of the torque wrench.
If you do not accurately measure the length of the extension.
If the extension is not perfectly lined up with the torque wrench.
The above errors are in addition to the calibration accuracy of your torque wrench. If you are reasonably careful, each of the above errors will be negligible. But just how careful do you need to be? See the following:
A & B - Wrench Length and/or Extension Length is Incorrect
Try plugging numbers into the following boxes to see what happens if you are sloppy when measuring the length of your torque wrench or the length of the extension.
You can see that 1/4" does not cause much of an error on the wrench length. A 1/8" error is not bad on the extension length. You should have no trouble keeping these errors to a minimum.
C - The Extension is Not Lined up With the Wrench
If things are not lined up, you will have an angle between the extension and the wrench. This will reduce the effective length of the extension as shown:
Dimension "R" represents the actual length of the extension
Dimension "X" represents the effective length of the extension
Try plugging numbers into the following boxes to see what happens if you are sloppy when aligning the extension with the wrench:
An angle as big as 10° does not make a very big error. But be careful -- it is not linear. The error at 20° is a lot more than double what it is at 10°. Moral: keep the angle at 10° or less.
Notice that if the angle reaches 90°, the effective length of the extension becomes zero and the applied torque is exactly the same as the wrench setting. It's like not using an extension at all. This means that if you ever use an extension at 90°, just set the torque wrench to the desired torque and then torque the bolt.
You might be wondering what a 10° angle looks like. This will give you an idea:
Plug in the overall length of your wrench (from the center of the ratchet head to the end of the handle). Also plug in the angle. Then click on the Calculate button to see how high your wrench would tilt:
You should have no difficulty keeping this error to a minimum.
CONCLUSION
Making a torque wrench extension is relatively easy and inexpensive, and is a viable alternative to purchasing a separate torque wrench that will get very little use. With a little care, potential errors can easily be kept to negligible levels.