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View Full Version : Help calculating torque extension for RF connectors

nheng
12-23-2010, 03:41 PM
I know that torque on an extension such as a crow foot wrench is pretty straight forward but I've managed to confuse myself.

For a 1/4" drive torque screwdriver (calibrated in in-oz) is there a way I can calculate torque applied to a small "crow foot like" extension. In this case, it's a small plate with a 1/4" square drive hole and a 5/16" machined open end, centered a short distance "d" away from the drive hole.

It seems like you'd want to let the driver sort of float in your hand when using it this way.

Actually, if the driver were floating in space (perhaps impossible without a reference point for the applied torque from your hand) then the extension length wouldn't even matter .. or would it.

As I said ... confused :)

Den

12-23-2010, 04:59 PM
Here's a calculator for a straight torque wrench.
Calculator (http://www.belknaptools.com/extcalc.asp)
Don't forget, if the extension isn't along the axis of the wrench, things will change.

In your case the wrench length is the radius of the driver.

For critical stuff I usually rig up a torque tester with a fulcrum and weight just to be certain.

oldtiffie
12-23-2010, 05:23 PM
Try these:

http://i200.photobucket.com/albums/aa294/oldtiffie/Clamp_and_torque/Torque7.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Clamp_and_torque/Torque6.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Clamp_and_torque/Torque4.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Clamp_and_torque/Torque3.jpg

Jpfalt
12-23-2010, 08:18 PM
As long as you don't side load the screwdriver and use it like a handle on a crank, the torque you apply to the handle is the same torque that is applied by the crowsfoot. Just remember to twist the handle but not pull it sideways.

nheng
12-24-2010, 11:44 AM
Thanks guys. I'm thinking that until I have the time to test a crowfoot style extension, I'd be better off with a cutaway cylindrical extension that will surround a cable and turn the nut, all on axis with the screwdriver.

12-26-2010, 10:24 AM
As long as you don't side load the screwdriver and use it like a handle on a crank, the torque you apply to the handle is the same torque that is applied by the crowsfoot. Just remember to twist the handle but not pull it sideways.

Do you have a diagram or something that makes this more obvious?
I can see how it's possible to improperly use the driver as a handle so the torque setting would be meaningless.
I just can't see how the applied torque has nothing to do with the distance between the driver axis and the fastener axis.

Jpfalt
12-26-2010, 10:43 PM
Dredging up the course on statics and dynamics, the thing you need to get used to with the screwdriver is the notion of a moment, also called a force couple.
http://i155.photobucket.com/albums/s319/jpfalt/torque.jpg

The first figure shows a force couple. The sum of forces in x and y is zero, so all it tries to do is spin the object it is applied to. The second figure shows a body with a moment or torque applied to it.

The third figure shows the body, or the crows foot, with two couples applied. Call the one on the left the torque from the screwdriver. The one on the right is the moment applied by the nut to the other end of the crows foot. This time, the sum of all forces in x and y are still zero, but the moments are also equal and opposite and the crows foot is stationary and doesn't want to go spinning off into the sunset. What you should also see is that it doesn't matter where the moments are applied to the body as long as they are equal and opposite. This means that if the torque screwdriver indicates 25 inch pounds going into the crows foot, the nut on the other end also sees 25 inch pounds.