View Full Version : minimum lead screw size and support bearing sizing
is there a calculation which allows one to choose the minimum lead screw diameter based on the maximum load applied? I've got a 6kg z-axis 'gantry' which will be affected by upward forces up to the maximum of 100N. Currently I'm planning to use an M12 lead screw, but due to various reasons (mostly trying to use low profile miniature linear rails and carriages) I was thinking of dropping this to M10 or M8.
Second question which someone might know an answer to is, what's normally used for miniature lead screw support? Looking at big bearing manufacturers like SKF or Timken, the smallest ID available is 10mm.
Any help is massively appreciated.
29-03-2013, 08:13 PM
Since the Z-axis weighs 6g=58.8N and the maximum force is in the opposite direction, the magnitude of the maximum force (from the information given) is 58.8N, as |58.8-100|<58.8N. That's a very small force, so I doubt you need to worry about the diameter of the leadscrew, unless there are other axial forces on it, which I guess there would be since this is probably for your coffee grinder? The easiest way to be sure is to check the manufacturers load ratings for the screw in question.
For small screws with low forces it's common to use the smaller angular contact bearings, which are often called magneto bearings. For example:
Angular Contact Ball Bearings - Arc Euro Trade (http://www.arceurotrade.co.uk/Catalogue/Bearings/Angular-Contact-Ball-Bearings)
You would almost certainly need a pair of them.
I've seen some calcs which include Euler's buckling load equations (column load), but unit-wise they don't make much sense. I think I will go the usual path of 'make it big enough so that there's no way anything can go wrong' that most people seem to take. Safety factor of 100 will do:)
For the lead screw I'm using standard M10 or M12 threaded rod and I will stick to standard ID 10mm angular contact bearings. It seems like going miniature can cost an arm and a leg.
30-03-2013, 11:59 AM
I would go for a trap (square) type thread, threaded rod at that size would strip the nut very quickly regardless of the calcs.
30-03-2013, 03:06 PM
You can try the 'column strength calculator' from Nook (http://www.nookindustries.com/EngineeringTool/Index)
I thought of using trapezoidal screw at the beginning, but I couldn't find anything with a fine enough pitch. M12 standard thread fine pitch is 1.25, smallest I could find in ACME screws is 1.5.
I'll try Nook, when they fix their website:) (seems to be down at the moment).
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