Re: [Phys-l] Torque and wrenches

[trappe@physics.utexas.edu]

The torque wrench measures the amount of torque you apply to the bolt.  
 The R is the  distance from the center of the bolt to the center of  
the location of your hand on the torque wrench handle. The F is the  
Force you have to apply. If you choose to apply it at an angle other  
than 90 degrees, the measurement is still based on R X F at 90 degrees.

Like most things related to cars, looking at early versions of torque  
wrenches gets the point more easily. Before the ratcheting slip torque  
wrench came one with a pointer.  This has a small thin rod fixed to  
the end where the socket attached, but free at the handle end. Located  
just in front of the handle grip was a scale.  The scale was  
calibrated based on the amount of bending of the torque wrench rod.   
Since the pointer was free at the handle end, it did not bend, but the  
main rod did bend.  If the main rod deflected 1/4 inch over its 12  
inch length, that might correspond to 50 Ftlbs of torque, etc. The  
pointer was not bending because it was only connected to the socket  
end of the wrench.

Some of the early torque wrenches even had a pivoting handle so that  
the point of application of the force was exact.

If you apply your force at some off angle, the torque to the bolt is  
still the 90 degree applied R X F because the calibration mechanism is  
only measuring the amount of bending of the solid handle rod.

The ratcheting style uses a counter torque adjustment calibrated to  
slip at the equivalent 50 ftlbs of torque, etc.  I honestly don't know  
how it works since my old torque wrenches work fine...except for being  
calibrated in ftlbs instead of Newton-meters or Kg-m (ignoring gravity).




Quoting Paul Lulai <plulai@stanthony.k12.mn.us>:

> Hi.
> I've got a lack of understanding regarding torque wrenches.
> My understanding is that torque is the product of the force and  
> radial distance of the force.
> Torque wrenches will stop turning / start ratcheting when the torque  
> is higher than the settings on the wrench.
> My question is, at what radius does the torque wrench measure the torque?
> Is the radius that of the bolt head?
> If the radius is that of the bolt head, would simply changing the  
> head of a torque wrench (and the bolt you are using) affect the  
> actual tightness of a bolt?
>
>
> Paul Lulai
> Physics Teacher
> St Anthony Village S.H.
> 3303 33rd Ave NE
> St Anthony Village, MN 55418
>
> 612-706-1146
> plulai@stanthony.k12.mn.us
> http://www.stanthony.k12.mn.us/hsscience/ ;
>
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