changing the range of a torque wrench, use of extensions

Changing the range of a torque wrench; and adjusting for extensions
IncTorqWr.htm

71C

There are only a few places on an BMW Airhead motorcycle that require torque settings over 72 foot-pounds. Common ones that are over 72 foot-pounds are the transmission output flange nut; the ring nut and input nut on the rear drive, and the exhaust finned nuts. The finned exhaust nuts should be done by feel, and I do NOT torque them as tight as the factory says to! Cheap beam wrenches are adequate for the higher torque areas.

It is possible to increase the torque from a common tubular clicker type wrench, which is the type most Airhead owners have. It is also possible to increase the range of a beam wrench. It is all done the same way.

***In a few instances, one being the driveshaft U-joint bolts at the transmission output, you need to use a special or modified wrench, and when you do that, and decide to use a torque wrench, that extension will multiply the torque by a factor. You CAN set the extension at 90° and avoid this, but I prefer not to get into that, although it IS true. This usage is common, where one needs to adjust the torque wrench setting to give the proper value of final torque to the bolt or nut.

I will discuss both extending the torque wrench range; and, the other use, what reading on the torque wrench to use when using the wrench to torque a fitting at less than the maximum range of the torque wrench, but using the torque wrench directly is not possible, due to the accessibility of the bolt or nut. This is the situation at the above noted transmission output.

In order to increase the output of a torque wrench beyond its maximum setting, you must increase the leverage. This is done by EXTENDING the working end of the torque wrench...and NEVER by extending the handle in ANY way. To make this as simple as reasonably possible, you should extend it straight out, that is, parallel, in-line, with the wrench. The only needed calculation is to know by how much the torque reading/setting is INcreased. You may want to know how to set a specific torque, which is typical. I will have the information below for you.

Beam type torque wrench design ASSUMES that the point of pressure is a 'pivot' point that is centered under your hand. Many of these types have an actual pivot, that the outer handle is affixed to. Some do not. The center of the hand applied force is assumed, however.

Tubular 'clicker' torque wrench design is different for SOME types, but the assumption remains for that same pivot point, whether an actual pivot, or not.

1. Measure the distance from the middle of the handle (so-called handle pivot point) to the center of the 1/4" or 3/8" or 1/2" square male drive. This distance is the lever length of the wrench, the center of the grip to the center of the drive. Call that measurement "L".
You can do all these things in the metric system, but I shall show how to do it in the American measuring system. Measure the distance "L" in INCHES. You can use decimal for fractions or fractions, whatever makes you happy.

2. Measure the distance of the extension, from the center of the drive to the center of the output. The output will likely be another male square drive. Call that measurement "E". As in the previous measurement, do it in inches.

3. Write down L and E.

4. The formulas follow. THINK!! If the torque wrench is extended straight-out, then, whatever reading is on the torque wrench, that reading is LOWER than the ACTUAL applied torque. This means that in setting the wrench for a specific torque you want to apply to a bolt or nut, the torque wrench must be set to LESS than the value the books or literature ask for, since the extension is a multiplying factor.

Torque SETTING as read on the wrench needs an adjustment factor. The adjustment factor is found by L divided by (L+E). When you get that factor, divide it into the wrench setting. Example: L is 13 inches, E is 4 inches. The formula gives use 13/17, which is 0.764. This means that whatever the wrench is set at, dividing it by 0.764 will give the true torque applied. If you set the wrench to a 29 foot-pounds, and divided that by 0.764 you would be applying 38 foot-pounds. Very few instances will be seen where you are using teh formula like this. Mostly it is when you want to MAKE a tool, and want some specific adjustment factor.

Much more commonly, is wanting to know what setting you use on a torque wrench to get some specific torque.

The formula for that is just a re-arrangement of the first formula by simple algebra. The Torque applied, which is what you want to happen, is found by dividing L into the sum of L + E.
Let is say you WANT 29 foot pounds, and L is, again, 13 inches, and E is, again, 4 inches. L + E is 17. Dividing that by L is 1.31. You WANT 29 foot-pounds. So, divided 29 by the 1.31 factor. The answer is 22 foot-pounds to be set on the wrench.

Rev:

07/16/2006: Slightly revised for clarity
06/13/2011: Because of math-challenged folks who have contacted me (3); I have revised this article extensively.

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