Changing the range of a torque wrench and adjustment correction when using extensions
© Copyright, 2014, R. Fleischer
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 center nut; the ring nut and input nut on the rear drive, and the exhaust finned nuts. The swing arm locking nuts can be 72 to 75 ftlbs, not critical.
The finned exhaust nuts should be done by feel (IMO), and I do NOT torque them as tight as the factory says to.
Cheap beam wrenches are adequate for most higher torque areas. NEVER use a cheap beam wrench for the cylinder stud nuts or other critical places.
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, unless you are doing the tightening by feel ('good grunt' on a 6 inch long box end wrench) you will need to use a special or modified wrench, and when you do that, and decide to use a torque wrench with it, the extension, if used straight out, will multiply the torque by a factor. You CAN set the entire extension at 90° and avoid this, so if you use the extension at 90° to the torque wrench, you can just set the wrench for 25-29 footpounds. BUT, if the wrench is used straight-out, then you must compensate for the added leverage, by setting the torque wrench to a lower value than the acceptable range of 25-29 footpounds. This article explains how to use an extension for the purpose of INcreasing the range of any torque wrench; and, this article also explains how to use an extension for such as the U-joint flange bolts, where you simply cannot get a torque wrench withOUT an extension to fit properly, due to the limited room between the bolt and the U-joint body.
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. In most instances the EXTENSION is used straight out, that is, parallel, in-line, with the wrench. The only needed calculation is so you will know by how much the torque is INcreased. TYPICALLY, however, you will want to know how to set the torque wrench for a specific torque value. In the instance of such as the U-joint bolts, you are not interested in extending the torque wrench beyond its maximum setting; rather, you are interested in calculating what setting to set the torque wrench TO, for a specified actual torque on the bolt. The very same process and calculations are used for both of these ideas.
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; that is, the handle moves angularly a bit. Some do not. The center of the hand applied force is assumed, however, and in every instance I know of, it is the CENTER OF THE HANDLE LENGTH. In some instances the owners manual tells you where there is a mark of some sort on the handle.
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 in INCH MEASUREMENT 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. Call that measurement "L". Write that down.
2. Measure the distance of the extension, from the center of the drive to the center of the output. The extension output will likely be another male square drive (although it could be such as the box end of a box end wrench, etc). Call that measurement "E". As in the previous measurement, do it in inch measurement. Write that down.
3. THINK!! With the torque wrench added extension extended straight-out, whatever READING YOU SET on the torque wrench MUST BE LOWER than the ACTUAL applied torque that you want!! RESTATING: 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 straight-out extension is a multiplying factor.
4. Most commonly you want to know what setting to use on a torque wrench to get some specific torque, when using an extension.
The torque you wish to apply is found by dividing L into the sum of L + E and then applying the result as a correction factor.
L + E
Example: You want to torque the U-joint 10 mm bolts. The value of torque you want is the value of 29 ftlbs which happens to be the maximum specification.
You measure your torque wrench from the center of the handle to the center of the 3/8" square drive and find it to be 13 inches. L is then 13".
You are using an adapter/extension that is being used straight out from the torque wrench, just as in the example sketch.
Center-to-center of that extension is 4 inches. E is then 4".
L + E is 17. Dividing that by L gives you 1.31.
You WANT 29 foot-pounds, so, divide 29 by the 1.31 correction factor. The answer is 22 foot-pounds to be set on the wrench.
Calculations can be done to calibrate your wrench using a known weight. Those used with gym barbells are fine for this, hang on a rope from the handle center, square drive locked into a vice, handle horizontal.
07/16/2006: Slightly revised for clarity
06/13/2011: Because of three math-challenged folks who have contacted me; I have revised this article
10/06/2012: Add QR code; add language button; update Google Ad-Sense code; simplify article once
more, clarify a few details.
10/19/2012: Revise some for clarity. Remove one multiplying formula, condense and clarify methods, add
© Copyright, 2014, R. Fleischer
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