Torque-wrench adapter offsets

jimgoose wrote: »

Greetings.

For those of you who, like me, occasionally find themselves confronted with a fastener on a fuel line or a HVAC refrigerant hose armed with nowt but a torque spec and a regular torque-wrench. A socket can't be got on the thing, and you have a crowfoot adapter but, yikes, the Leaving Cert. physics nerd in you knows that this will screw up the torque setting. Well fear not, for this is how to correct it.

The first thing to know is that if the wrench is fitted to the adapter at a 90-degree angle, there is no adjustment required. Zip. Nada. Bubkes. All well-and-fine until you need, for space/access reasons, to keep the whole tool straight, or maybe even at 45 degrees.

So, the equation goes like this:

TS == (TR x DW)/(DW+(DA*cos(Z)))

...where:

TS == torque setting at the wrench
TR == torque required, i.e. torque spec of the fastener
DW == Distance from the wrench pivot-point to the centre of the handle
DA == Distance from the centre of the adapter "jaw" to the wrench pivot-point
Z == Angle made by the wrench and the adapter

Say we have a fastener rated at 50 lb. ft. The distance from the centre of the square-drive on our wrench to the centre of the handle is 18 inches. The distance from the centre of the adapter "jaw" to the centre of the wrench square-drive is 6 inches. We want to keep the whole lot straight, i.e. the angle of the adapter to the wrench will be zero degress. So:

cos(0) == 1

Torque setting to use == 50 x 18/(18+(6 x 1)) == 37.5 lb. ft.

Now say we have the same set-up, but this time we want the adapter at a 90-degree angle to the wrench. So:

cos(90) == 0

Torque setting to use == 50 x 18/(18+(6 x 0)) == 50 lb. ft.

...i.e. no adjustment from specified torque is necessary.


As a speadsheet formula (as tested in OpenOffice Calc):

=((TR*DW)/(DW+(DA*(COS(RADIANS(Z))))))

As a Unix "bc" calculation:

TS=$(echo "scale=10; $TR*$DW/($DW+($DA*c($Z*(4*a(1))/180)))" | bc -l)

Happy nerding!!