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I'm working on a lifting system that uses a ball screw and linear shaft as guide and I would like to determine which shaft diameter would be enough for my application.

So here is a sketch of the system that uses 3 linear shafts and 3 linear bearings. The grey transparent huge block weighs 40 kg and its dimensions (which I guess are important to determine radial loads?) are 90 cm (width) x 70 cm (depth). How can I determine (approximately) the diameter required for the 3 shafts and also determine whether 2 shafts would be enough?

sketch of the system

After reading this guide: http://www.nookindustries.com/LinearLibraryItem/Linear_Components_Design_Considerations I found that using a diameter of 1/2" with a length of 35 cm for the shaft would create a deflection that produces a misalignement angle around 0.25° (which is acceptable if I refer to what is written on that guide).

But I am really not convinced that a single shaft with such a small diameter could support that 40kg load.

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  • $\begingroup$ There's almost certainly a standard table of shaft size/thread depth per lifting weight capability. Have you looked? $\endgroup$ – Carl Witthoft Dec 14 '15 at 13:12
  • $\begingroup$ If there's a table, I did not find it. The only thing I found are formulae that I'm not good enough to understand, and actually I hoped that someone could help me with an example (probably using similar formulae). $\endgroup$ – LiohAu Dec 14 '15 at 13:55
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If I interpret this correctly this is a broadly similar arrangement to a screw jack with additional guide rods (green) . If you are talking about lifting the load with a single screw (and no other support/guides) then the situation is a little bit different.

Really you have two separate considerations. First is the force required to lift the load itself and secondly is the requirement to keep it flat (ie. avoid bending forces one the screw) . In an ideal case these are the same and you just have a single lifting screw directly below the centre of mass. In practice there will always be some bending moment on the screw from small variations in the load distribution.

So in practice you have the 'active' part of the system which is lifting the load and the 'passive' part which is constraining it to move only in the vertical axis.

It looks like bending moments from any misalignments and variations in load distribution are going to be more of a factor than the axial loads on the shafts in this case.

In practice this sort of condition tends to be empirical rather than something which ends itself to straightforward analysis so I would say that going with supplier tables for load is a sensible way to look at the problem.

In terms of the load on the shaft 40kg for a 12.5mm thread diameter seems perfectly reasonable even for a relatively low strength thread.

The most important thing is how well the liner bearings take bending stresses from any misalignment of the load which is down to their fit and stiffness.

Personally I would use 4 guide rods with the screw in the centre so that they are all seeing equal loading.

Having said that my practical experience says that the arrangement is absolutely fine.

If you want to support the load with a single screw and nothing else then I would say go to maybe 25-30mm diameter to be absolutely safe and reliable ( almost identical arrangements are used by stone carvers and sculptors for adjustable height work stands)

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