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Thread: acme tread torque and force
01-05-2006, 10:55 AM #1
I am building a new lift system for a sawmill and plan to use a 1"- 5 t.p.i. acme thread. I need to know how much force (pull)I will get for each lb. of torgue I apply to the nut. I realize there is a friction factor also. The nut will thrust against a bearing.
01-05-2006, 11:29 AM #2
A screw is a lever. It has a ratio. one turn gets you .200 of travel. I'd think with the same ratio that one ft lb at the screw would get you 5 lbs of lift or something close to that
01-05-2006, 11:48 AM #3
The quick-&-dirty way is to look in "Machinery's Handbook" in the section entitled "Mechanics". The basic formulas, per "Machinery's Handbook" for force produced by threads depends on whether the thread is jacking/raising or lowering the load.
A screw thread is basically an inclined plane wrapped around a cylinder. The force of the thread against the nut is the action of a wedge. Figuring the mechanical advantage produced by it amounts to stuff you may have learned in HS Physics- having to figure the angle the inclined plane makes to the horizontal and playing with some trig and vectors along with the coefficient of friction. The formulas in "Machinery's Handbook" cut to the chase.
The formulas are too lengthy to type out here, but you will need to know the coefficient of friction between the nut and screw (obtained elsewhere in the Machinery's Handbook). the formulas calculate the force, F, required on a lever of length R, to move a given load Q with a screw thread, having a coefficient of friction Mu- (no Greek letters on this computer) between the nut and screw.
If you do not already own a copy of "Machinery's Handbook", I would strongly reocmmend you get hold of one. Even an old edition will have what amounts to priceless and timeless information.
01-05-2006, 11:48 AM #4
The Nook Industries web site has torque vs. lift for a wide variety of acme screws.
These values assume a hardened 4130 steel screw running a bronze nut or plastic nut, your choice.
01-06-2006, 12:04 AM #5
You probably know this, but I believe that the nut is usually of a somewhat softer material-like the plastic or brass previously mentioned. This is so that the smaller (and cheaper) nut will wear out faster than the more expensive acme-threaded shafting. Try to make access to change the nut easy for the poor guy who has to go in and change it when it wears out! Designing to facilitate all the maintenance actions will ensure guys looking to "kill time" on a friday afternoon at 3 or 4pm or so will look to lavishing attention on your easy-to-maintain lift system. Just a thought...
01-06-2006, 12:43 AM #6
Thanks for the link lbender!
I've been looking for an easy way to compare various acme threads.