[s-cars] Re More on Wheel Torque

[Paul Heneghan]

>BTW, how or where does one get their torque wrench calibrated?

Three ways:
a) Bring it to a calibration centre.
b) Calibrate it by connecting it to another torque wrench whose calibration you trust.
c) Calibrate it yourself by using a known force at a known distance.

a) is not particularly cheap and requires finding a calibration centre (I couldn't find one near me using google - I would have to post it).  I would guess the cost to be around $30 or £30.

b) is a bit iffy as the other TW might not be that accurate.  You just connect the two together (I've used a 17mm socket on one and a 19mm socket on the other and a 19/17 hex adapter to connect the two).

c) is nice because it can be as accurate as you like.  I usually use a lead-acid battery of 25kg - this needs 250N to lift.  I connect my TW through a 10" extension to a 17mm socket to a 17/19 adapter to a 19mm socket to a t-bar.  This contraption sits on top of the work bench with the t-bar sticking out over the edge.  The 25kg battery sits on the floor and I tie some cord to it (like wrapping a parcel) and from the centre at the top the cord goes up to the t-bar where it is tied in place exactly 20cm from the 1/2" drive.  Then I measure the torque required to lift the battery off the floor.

NOTE: The T-bar MUST be very close to horizontal while doing this (or within a couple of degrees of horizontal), the cord must tied to the t-bar at exactly 20cm from the centre of the 1/2" drive, and the battery must be lifted clear of the floor.


The torque due to the battery should measure 250 x 0.20 = 50Nm.
Being pedantic, I also need to add on the effect of the t-bar.
Assuming that it weighs 0.5kg and is 25cm long, that requires a torque of 5 x 0.25/2 = 0.625Nm.
Total torque registered on the torque wrench should be 50.6Nm.

If you're working imperial, then using a 60 lb battery with the cord tied onto the t-bar 9" from the 1/2" drive, you would get the following:
The torque due to the battery should measure 60 x 0.75 = 45 lb*ft.
Now, the torque due to the weight of the t-bar.
Assuming that it weighs 1 lb and is 10" long, that requires a torque of 1 x 10/12/2 = 0.42 lb*ft.
Total torque registered on the torque wrench should be 45.4 lb*ft.

I can adjust the calibration of my TW using an allen key.

This condition requires that the t-bar is horizontal (or within a couple of degrees of horizontal), the cord is tied to the t-bar at exactly 20cm from the centre of the 1/2" drive, and the battery is just lifted clear of the floor.

Prepare to enter the strange world of ASCII art (it's been a long time since I've done this, so it's not going to be very pretty)!


         Torque Wrench
   ----------------------------
   -------------------------- -
                            |X|
                            \ /
                             X
                             X
                             X
                             X
                             X  10" extension
                             X
                             X
                             X
                             X
                            |X|
                17mm socket | |               XXXXXXXXXXXXXXXXX
                            |X|               X               X
              17/19 adapter  X                X               X
                            |X|               X               X
                19mm socket | |               X               X
                            |X|     T-bar     X               X
                            - --------------------------      X
                            ----------------------------      X
                             | - - - - 20cm - - - - - |       X
                                              X               X
                                              X               X
                                              X               X
                                              X   Battery     X
                                              X               X
                                              XXXXXXXXXXXXXXXXX


Paul



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