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Re: [Phys-l] Torque and wrenches



-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Paul Lulai
Sent: Monday, March 14, 2011 4:37 PM
To: Forum for Physics Educators
Subject: [Phys-l] Torque and wrenches

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.

[Bill Nettles]
There are 2 main types of torque wrenches, metered/scaled or snap/slip.
The older types are metered and use a long rod attached to the handle, pointing to a mechanical, etched scale. As the handle is pulled, the resistive torque of the bolt or nut will cause the handle to twist relative to the relaxed position, causing the rod to point to a "non-zero" value on the scale. Some will use a dial indicator. These wrenches, once calibrated properly, are extremely accurate, but require the user to pay attention to the dial and use good technique (slow and steady).

The snap/slip type have a cam which will click (snap) or release (slip) when the cam can no longer provide the counter-torque. These are usually cheaper, although they seem more "advanced" than the scale-type wrenches. They are not as accurate and will go out of calibration much easier. You also have to "dial-in" the torque you want and are susceptible to sloppy setting. They are not as easy to use in doing progressive torque operations (take everything to 35 ft-lbs, then 65) because you have to reset the wrench. If you don't back the wrench down to ALMOST zero (if you go to far, you lose the calibration), it will weaken the cam mechanism and become uncalibrated.

My question is, at what radius does the torque wrench measure the
torque?

[Bill Nettles]
The wrenches are calibrated to read torque, not force. Think about this. If I put a cheater bar on a wrench, I make the moment arm longer so that I get the same torque with less force, or more torque with the same. Basically, there isn't "a" radius at which you have a particular torque. It's all in the calibration of the wrench. The force application point is actually at the corners of the socket/bolt interface, so you can figure the force by taking the torque and dividing by the bolt head radius. But the torque is transferred to the bolt shaft itself, so the tangential force on the bolt is the torque divided by the bolt diameter.
Is the radius that of the bolt head?
[Bill Nettles] To calculate the force on 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?
[Bill Nettles] The rotational torque is the same at all points on the wrench. What you are actually doing that is important regarding bolt dimensions is creating a mechanical advantage swapping distance for force.


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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