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 High-School 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:

...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):

As a Unix "bc" calculation:


Happy nerding!!

 

Jimgoose, I have to deal with torquing stuff all day long at my job. I used to be a quality assurance specialist and had to know more about torquing things than I care to admit to. You are correct in that you can turn your adapter 90 degrees and that will cancel out any change in length of the torque wrench.

The other thing to keep in mind is that the addition of a crows foot does not significantly change the length of the torque wrench. So, no recalculation is really needed. Nothing is so critical that even being off by 5% will cause significant damage. The nearest thing to that is torquing Ford head bolts where you are doing angular torquing (ie, torque using a torque wrench to a given value and then turning the bolt an additional number of degrees to get the necessary torque-yes, this does work). But, with the case of the Ford head bolts, you are actually taking the bolt to its yield strength (ie, you are stretching the bolt and by doing this, you are lengthening the stud which actually causes it to become stronger and take out any elasticity that it may have to ensure the head remains firm against the block). But, most bolts are not stressed that bad.

You are more likely to have more error in making up a joint by putting on the wrong lubricant (or not using a lubrication when one should be used) than the difference adding a crows foot to a torque wrench. If you want to learn more about this, let me know.

 

Hi Thermo. Yes, thanks - I'd be very interested in further information that you may have.