[NAFEX] Apple pressing

Mark Angermayer hangermayer at isp.com
Sun Dec 7 17:56:10 EST 2008

Claude wrote:

"I will now put my mechanical engineer's hat... The general relation for
torque vs axial force in a screw is:
Torque = 0.2 x Axial Force x Screw Diameter
This is a simplified, but fairly accurate, formula for standard friction
coefficients between screw and nut."


I'm just a country bumkin, so I can't discuss friction coefficients.  I do
know how to figure mechanical advantage, assuming 100% effeciency, because
the math is grade school level.  That is pitch (threads per inch) X
circumference.  One would think from the simple formula above, a larger
screw (therefore larger circumference) would produce more mechanical
advantage.  Hence get a bigger screw to deliver more thrust.  But it is not
so,  the bigger screws naturally have less threads per inch, so mechanical
advantage is actually less with a larger diameter screw.  I know Claude, you
know all this, I mention it for the possible interest of someone else.

How did I come up with an 1 1/4" screw producing 4 tons of force with 100
ft.lbs of torque?  I simply used Roton's website.  For each screw they sell,
they list the drive torque ratio, so the customer knows how much torque it
takes to move the load, they want to move.  For an 1 1/4" screw with 4
threads per inch, they list a drive torque ratio of  0.128.  Taking that
times a 4 ton load, gives 1024 inch lbs. of torque required.  1024 divided
by 12 gives 85 ft. lbs. of torque to move the force.  It had been quite a
while since I had looked at these numbers, so I guess I must have rounded up
to 100 ft. lbs. in my mind.  But it would actually only take 85 ft. lbs.
But again, that figure assumes a 100% effecient thrust bearing.  Without a
thrust bearing, maybe you could answer how much more torque would be needed
to produce 4 ton of force.  My guess is that it would be over 100 ft. lbs.
of torque.

Incidentally, for anyone whose interested in how much work it is to apply
100 ft. lbs, and had never torqued a crank shaft bolt, 100 ft. lbs. is about
how tight you crank a lug nut on your pickup.  Another way to think of it is
that if you have a 1 foot long lever, and hang 100 lbs. on the end, you have
100 ft. lbs. of torque.


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