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Physics/Mechanics Question
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Mar 31, 2013, 11:42 PM
 
Please help me out and either confirm my thoughts on this or tell me where I'm wrong... This came up in a discussion that started with how an old camera tripod's legs were adjusted for length, and veered from the concept of the tripod's legs to weirdness from "the mechanic with years of experience" on all manner of stuff that just didn't seem mechanically sound.

Let's say you have two cylindrical, tubular parts, with one threading into the other coaxially like a pipe into a fitting, and the inner part is secured by a jam nut that turns up against the end of the outer part. To apply a specific amount of torque to the nut, you could secure a) the outer tube, b) secure the inner tube, or c) it doesn't matter. I am fairly certain that the answer is c, and that's how I stated it.

Similarly, if your inner part slips into the outer part and is held in place by a nut that threads onto the outside of the outer part to secure a flange on the inner part against the end of the outer part like a plastic pipe drain trap, I think that it doesn't matter which part is secured when the torque is applied to the nut, as long as the nut is tightened enough to make the inner part seat properly against the outer part.

Finally, if in the second example the inner part has a locating tab that fits a notch in the outer tube, it still shouldn't matter which part you secure, right?

Glenn -----OTR/L, MOT, Tx
     
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Apr 1, 2013, 12:37 AM
 
My knee-jerk response is it doesn't matter directly.

It could matter indirectly, as applying torque from the inside is likely more difficult than from the outside. One is working by compression, and the other expansion, so there may be a materials science angle.

This is assuming I understand you properly. Yo dawg, I hear you like parts.
     
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Apr 1, 2013, 02:43 AM
 
Originally Posted by ghporter View Post
Let's say you have two cylindrical, tubular parts, with one threading into the other coaxially like a pipe into a fitting, and the inner part is secured by a jam nut that turns up against the end of the outer part. To apply a specific amount of torque to the nut, you could secure a) the outer tube, b) secure the inner tube, or c) it doesn't matter. I am fairly certain that the answer is c, and that's how I stated it.
This is easier to picture if you simplify the model. You have an inner threaded object, screwed into an outer object with interior threads. Dump the "cylinders" part, and picture it as a bolt screwed into a nut. Then you add the jam nut on the end.

If you hold the bolt, the first nut may turn when the jam nut arrives. Or it might not. So the best answer is to position the main nut first (the "outer cylinder") then hold the main nut while tightening the jam nut. So the best answer is (a).

Similarly, if your inner part slips into the outer part and is held in place by a nut that threads onto the outside of the outer part to secure a flange on the inner part against the end of the outer part like a plastic pipe drain trap, I think that it doesn't matter which part is secured when the torque is applied to the nut, as long as the nut is tightened enough to make the inner part seat properly against the outer part.
Assuming the drain model is right here (description is a bit shaky) then you hold the outer part. On a real pipe, you can successfully hold the inner part -- because the outer part is already held by whatever it's attached to. Or you might break the next joint. Try it without the outer part held somehow, and you'll just spin. Correct answer is (a).

Finally, if in the second example the inner part has a locating tab that fits a notch in the outer tube, it still shouldn't matter which part you secure, right?
Assuming the locator tab can take some torque, then you could hold either part while tightening. But the outer part remains the safer choice.
     
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Apr 1, 2013, 08:27 PM
 
Thanks for the much better mental model. I got stuck thinking in terms of The Expert's pronouncements based on the tripod legs.

It later came to me that it would make a difference if the outer part had a stop inside it, like a closed nut. In that case the "bolt" would stop and essentially lock up in the "nut," allowing the jam nut (now much easier to picture) to be torqued against the assembly rather than a potentially moving part. This would be similar to the last example, except the inner contact between bolt and nut would be substantially stronger than simply a tab. Does that make sense?

Glenn -----OTR/L, MOT, Tx
     
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Apr 5, 2013, 01:49 AM
 
It improves the odds that the main nut (outer cylinder) would not move when the jam nut arrives. But (a) is still the safer answer. Holding the outer part always works in this model, vs 'maybe' on (b) - holding the inner component.
     
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Apr 5, 2013, 08:52 AM
 
Got it, and (more importantly) I see the logic. Thanks.

Glenn -----OTR/L, MOT, Tx
     
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Apr 7, 2013, 11:16 AM
 
Gorilla Glue is easier.
To know your Enemy, you must become your Enemy.”
Sun Tzu
     
   
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