Torqueing knock sensor/crow's foot

[Eric_R_Kissell at whirlpool.com]

Here we go again... (see the crank bolt tightening thread of a few month's
ago)

The torque applied to the bolt is the force applied times the total length
between the force application (the handle) and the center of rotation (the
bolt, i.e. knock sensor).

T_bolt = F_handle x L_handle to bolt

The torque wrench measures torque based on the force applied times the
distance between the force application (the handle) and the center of the
wrench's pivot point (the center of the square shaft where the socket or
crows foot attaches).

T_wrench = F_handle x L_handle to square shaft

Note that anything that extends the length of the lever arm beyond the
length of the wrench will result in the application of more torque on the
bolt than is indicated by the wrench, which is reading the same applied
force times a now shorter lever arm.

L_handle to bolt  >  L_handle to square shaft

because,

L_handle to bolt = ( L_handle to square shaft + L_square shaft to bolt )

therefore,  T_bolt  >  T_wrench

To keep T_bolt close to T_wrench you need to minimize the length of
whatever is beyond the length of the wrench, which is why turning the
crow's foot 90 degrees reduces the "error" introduced by the crow's foot.
In this case the new length of the lever over which the force acts is the
hypotenuse of the right triangle formed between the wrench handle, the
crow's foot, and the new length of the lever arm, which is not too
different than the length of the wrench, unless the wrench is really short.
My bending-beam torque wrenches do not qualify as really short compared to
a typical crow's foot.

HTH,
Eric Kissell
1991 200q 20v Avant, Koni Sports (yellow), 2.4 Bar (MRC), eBay Surprise
1989 200q 10v Avant, Boge Turbogas, 1.8 Bar (qlcc) - FOR SALE
1986 5000cstq Sedan, Bilsteins, 1.8 Bar (SJM)