North American Fruit Explorers mailing list at ibiblio

[Claude Jolicoeur <cjoli@gmc.ulaval.ca>]

Mark wrote:
>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.

There is approximately a 2 to 1 ratio between your drive ratio and my
formula. This is because the screw formula takes into account the friction
at the collar of the head while your drive ratio doesn't. The friction loss
at the collar would be approximately the same as the one from the end of
the screw on the plate if contact is steel on steel. I agree that if a
thrust bearing is used, then much less torque is required than given by my
formula. 
I think it is not easy to apply a torque exceeding 80 ft-lb on the handle
considering the position - this would probably be the practical higher
limit unless the press is very heavy or bolted on the floor, and the lever
is very long.

A few additional points re screw vs hydraulic cylinder:
After using a screw for 15 years and switching to hydraulic, I found the
following advantages with hydraulic
-stability: the piston plate has much less tendency to tilt.
-quick to go up - no need to unscrew all the way up.
-no wear, no grease necessary.
-no rotation or displacement of the press as when a large torque is applied
to the handle.
-very inexpensive (15$ for 4 tons). Just a thrust bearing will easily cost
twice this price, and you still have to add the cost of the screw, nut,
handle, plus some welding to hold things together.

one point against it: the stroke is often too short and it is necessary to
release the cylinder, add blocks, and tighten again.

Claude