Understanding Torque Values

Check out www.cditorque.com. Under the heading "Torque Facts" are formulae for calculating the addition of adapters.

The value of the torque or moment applied to anything is equal to the distance from the point of contact of the object (center of the bolt head, for example) to the point at which the force is applied multiplied by the force and also by the sin of the angle between the line along which the force is imparted and the line connecting the center of the bolt head with the point at which the force is applied. If your pushing on the wrench at 90 degrees to the line between the point of contact and point of force application, then the sin would be 1 and the torque would just be force times distance. With a crows foot, the actual moment arm is not the length of the wrench but the distance between the center of the bolt and the point at which you apply force. If you apply a force 90 degrees to that line, then the torque is force times distance. If you apply a force at 60 degrees to that line, then the torque is force times distance times sin 60. Only the force applied perpendicular to the line connecting the point of force application with the center of the fastener counts towards rotating the bolt, and thus, counts as torque. Sorry for the lousy explanation but I think think this is correct. It works in physics and weight and balance of aircraft.

1- torque wrenches:

let us call the distance between the middle point of the handle and the center line of the wrench square as Lo and the distance in the crowfoot between the input square (where you introduce the wrench square drive) and the middle of the fork as L1.

The torque you introduce in the assembly is the set torque of the wrench alone multiplied by the factor kt= 1+L1/Lo.

There is never the less a problem which is valid for ALL torque wrenches: the torque depends of the point where the force is generated. If the operator hand moves along the handle the torque will not be the same for all positions. Differences can be relatively important if the accepted dispersion of tightening torque is narrow.

2- Torque amplifiers

In general those fixtures are planetary gears with the sun gear connected to the square drive of the wrench and with a reaction arm fastened to the crown.

The output shaft is carrying the planets. The internal friction is variable so that the out put torque will not be always the same. If dirt polluted the meshing gears and/or the bearings the torque losses can be up to 15...20%. The risk is that the resulting torque is less the expected and thus the pre-load is reduced which can lead to a total loss of pre-load under vibrations or under external loads which would not affect the assembly under correct tensioning. IT IS NOT possible to predict the friction evolution since it depends from the way the tools are used and of the environment.

IT IS COMPULSORY if you assembly critical parts to set the COMBINED tool (wrench + fixture) to the requested OUTPUT torque. More than that if critical joints are assembled you should have a periodical check and assure a traceability of your calibration tools ans working tools. This the base of a quality assurance in torquing.

I hope you have now what you expected to get. If other explanations are required please feel free to ask.

Thanks to all for the extremely helpful advice.

CR4 rocks and this info has been passed around to my peers.

Thanks again!

Torque wrench is not the best way to preload bolts. Due to different in friction between mating surface preload will vary. Best way to have accurate preload is actually preload the bolt by pulling and stretching it with the force required, spin the nut on and release the load. There are equipments available to do that which will result in prefect preload every time.

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There is no perfect solution. Stretching is a good technical solution but it has a very low productivity. It is used in most cases for bolts > M20 ( 3/4") and only if the number of operations is limited. There are even with torque wrenches tactics which allow a good repetability of preload. The most known being the Torque-Angle approach. Advantage is the relatively high productivity for a good "quality" and the very short dad time. Soultions have to be considered depending on the problem.