Do I need a keyway?

[Jonathan]

in other words, john, jonathon or Jazz. rest of us need not apply

You forgot Irving - I'm about to answer this just using information he posted long before I joined the forum.

What do you more experienced fellas think?

I think it's not a matter of experience, it's just applying a simple equation. From experience I can say that there's nothing to worry about, but lets be more precise:

You're concerned that the ballscrew is lifting a 'large' mass and that as a result may be too high, so calculate the torque. Using the standard formula I posted here, the torque required to lift 42kg with a 5mm pitch ballscrew is 0.36Nm, but if you've got a 3:1 ratio the motor only needs to provide a third of that, so 0.12Nm. Either way the motor will output that torque up to a satisfactory speed. There will be a bit more torque due to acelleration, but still not enough to be a problem.

If the torque on the ballscrew shaft is 0.36Nm (as above), then at 5mm radius the force will be 73N, so so long as the grubscrews impose enough frictional force to oppose that, it wont slip... again, the force isn't very big so you'll be fine with two grubscrews.

Edit:

That's handy information! What size grub screws?

I'd use M4 or M5 if they will fit...you'll certainly fit M5 in the bigger of the two pulleys.

[kingcreaky]

what he said....

[birchy]

Fair play Jonathon, you always impress me with your facts and figures. I'm more of an "over engineer it, guarantee it'll work" type person. But then again I used to make parts for massive steel bridges when I was an apprentice!

I have to admit that I'm VERY surprised the load is so small when converted to Nm via the ball screw and pulleys. Obviously I'll have to be a bit careful with the acceleration/deceleration but I'm more interested in accuracy than speed and would rather err on the side of caution than bollock the machine flat out and end up wearing it out prematurely.

Thanks everyone for your replies. Not needing keyways is one less headache to worry about. Just for the record, I'll be using M4 or M5 grub screws at 90 degrees, dimpling the shaft slightly and applying a little Loctite for the final fit.