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Old 19th Aug 2012, 10:31 am   #41
John_BS
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Default Re: Removing a seized screw.

Quote:
I never said you can get more torque at the tip than you can put in!
Now I'm really disappointed: we were all convinced that you'd invented the elusive perpetual motion machine......

John
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Old 19th Aug 2012, 1:34 pm   #42
brenellic2000
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Default Re: Removing a seized screw.

.... fear not, I'm working on that one! There are of torque multipliers, but that's another story.

Last edited by brenellic2000; 19th Aug 2012 at 1:37 pm. Reason: spelink
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Old 19th Aug 2012, 4:47 pm   #43
wireful3
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Default Re: Removing a seized screw.

I have puzzled myself with this effect. If the tip is fixed and the top of the shaft has torque applied to it the shaft will twist. Does this store energy as mentioned earlier regarding torsion bars? If so can this extra energy be used to advantage?

I wonder if there is something missing in the theory? After all, aerodynamicists used to joke that theoretically bees could not fly. I would like to think that a detailed study of just before and just after static friction is overcome, might explain how the stored energy in the shaft is useful.

It obviously works and I don't think it is by magic. My favourite reason is helping to keep the tip properly seated in the screw head.
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Old 20th Aug 2012, 8:24 am   #44
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Default Re: Removing a seized screw.

Yes John, it is stored energy in the twisted bar - 'wind-up' in the bar (quite different to winding up experts on this forum)!

'Torsion' goes straight down the shaft (axially) regardless of the angle of attack. When it meets resistance it distorts the srew/bolt (it stretches it to put it under tension using the mechanical advantage of the screw thread) - the screw/bolt has a limited amount of stretch before it shears. The torque will also twist the shaft, the longer the shaft the greater the torque it will carry, evenly distributed along its length (hence drilling oil wells).

The stresses set up by the torque contain the energy (rebound) applied in the shaft (very little torque is lost through heat (friction)). Once these stresses overcome the breaking resistance of the rusted in screw/bolt the torque is neutralsied throughout the shaft.

When using a turnscrew, all the action is by leverage in the wrist. The size of the handle is determined by the maximum torque a screw will take and the size of an average man's hand - thus a No.0 Posidrive has a smaller handle and shaft diameter than a No.3 Posidrive. The size of the handle remains the same regardless of the length. A No.0 3" shaft has the same handle as a 12" long No.0.

Likewise to prevent shearing a stud/bolt, a spanner's length is usually no more than 16 times the diameter of the stud/bolt - thus, using a standard 40-lbs pull by a British Standard Man it is almost impossible to shear a stud. Use a longer spanner at your peril!

This is technology understood by simple country bumpkins using 19th century machinery but not one used or understood by those brought up on electric-screwdrivers and A quadruple star GCEs! Me? I'm BA (Calcutta) (failed).

I bet someone will now quote Wilkipedia!

By 'eck
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Old 20th Aug 2012, 1:07 pm   #45
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Default Re: Removing a seized screw.

But doesn't the 'stored energy' reduce the actual transmitted force by reacting against the applied force...?

To obtain an additional twisting force in the same direction it would have to be wound up in the opposite direction first.

There again, I failed A-level physics...
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Old 20th Aug 2012, 2:31 pm   #46
brenellic2000
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Default Re: Removing a seized screw.

Yes and No! Think of it like a wind-up Mickey Mouse clock. You apply torque to the winding stem, nothing happens until the torque has overcome internal resistance (that is 'lost' torque) and it starts to twist the clockwork spring into storing sufficient energy to release same energy through the escapement allowing Mickey's hands to move around the clock face, indicating time. This works equally well on other animated clocks.

Don't forget that physics only works because a fizzisist found a hypothesis that happened to consistently work. He thus became a Physicist. 20 years on another fizzisist found another obscuce hypothesis which likewise worked and gave the same results. He too became a Physicist. Both theories are right because they work... but come from different approaches.

Physicists are like economists - as two the same question and you'll get three answers all four of which are right .... until proven otherwise!!

I rest my case, M'Lud!
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