Rotating lead nuts

[Tom Brown]

I have an assortment of questions regarding fixed lead screw and rotating lead nut configurations.

Everyone seems to use timing belts on lead screws. My local Harbor Freight type store (Princess Auto here in Canada) has an excellent selection of small sprockets and chain. Is there an advantage to timing belts and cogged pulleys? I think I could adjust the backlash out of a chain system.

How much side load can I put on a stepper output shaft? I'd guess most steppers use bushings so they may not stand up well to significant side loading. Do I need to plan for a side load support on the motor side, in the event I go with a rotating nut system?

Does anyone have pictures of a DIY implementation of a rotating nut system?

I really like the idea of direct drive lead screws. I'm just kicking this around. It would really be a sweet idea if someone made a hollow shaft stepper that could act as both stepper and nut both.

[fkaCarel]

Just did a simple sketch as a worst case scenario. Imagine a 4-tooth chain sprocket, not realistic, but is calculates easier. The line of the chain is horizontal. The sprocket is in the "X" position. The chain is now lifted during 45 degrees, if you move the chain. This means that true anti-backlash is not possible with a chain. A timing belt has chords, outside the pulley, that are driving. The tooth are actually just anti slip. So a timing belt gives true rotation transfer, a chain not.

A stepper is a motor, it should be designed to transfer at least it's output.

[Geof]

Originally Posted by fkaCarel Just did a simple sketch as a worst case scenario. Imagine a 4-tooth chain sprocket, not realistic, but is calculates easier. The line of the chain is horizontal. The sprocket is in the "X" position. The chain is now lifted during 45 degrees, if you move the chain. This means that true anti-backlash is not possible with a chain......

This effect is somewhat analgous to the non-constant transmission of rotary motion through a Hooke coupling universal joint running at an angle. The effect can be countered by putting two universal joints out of phase an what I think is called a Birfield coupling. Would it be possible to use two successive chain drives out of phase to compensate for the non-constant chain transmission?

[Jason Marsha]

I saw this pic in one of the forums but cannot remember where, maybe ger21 can help.

Jason

[fkaCarel]

It's the same as the rear drive axle from gearbox to differential (Hooke = Cardan?), double the system and offset the sets halftooth. Invent wearfree chain and put new batteries in your calculator, where you could have used a timing belt in the first place. This is just one the uses of geometry, where you see error introduction. In practice, you may never see it, or on the other side, you may never find it, if it bites you.

[Guldberg]

What about something like this, i guess it wouldn be that hard to fabricate. 2 bearings 30mm inner diameter, a deldrin or aluminium rod, bored and taped to fit the leadscrew, and a standard pully turned to fit the rod

[fkaCarel]

I agree on the double bearing, the picture in post #4 leans on the shaft. I disagree on delrin for a bearing fit or aluminium for a nut. Bronze would be a better suited material. And the design should be expandable for anti-backlash. The difficult part in designing rotating nuts is, that there is almost the need to design space for spare belts, to avoid large scale disassembly.

[Guldberg]

Bronze then, as long as its soft enough to tap:-) My guess is that with a normal router for hobby usage the belt will last "forever", anyway you have to remove it from the rod to change belt so a few screws should'nt make a different. Maybe this inclosure would work

[Geof]

Originally Posted by fkaCarel ....(Hooke = Cardan?)......

Courtesy of Google:
simple universal joint, known in English-speaking countries as Hooke's joint and in continental Europe as a Cardan joint,

So when you communicate in English you have to use Hooke but any other language you can use Cardan.

[fkaCarel]

On a bicycle you have to remove the wheel to change the tyre. Replacing a belt on a rotating nut is the same, with all the accompanying alignment problems.

[fkaCarel]

So when you communicate in English you have to use Hooke but any other language you can use Cardan.

This forum keeps on improving my technical English. I have to thank everyone, and especially you, Geof, for this. Sometimes when reading this forum and at the same time being confronted with people from the American continent asking for directions in the only language they speak and expecting us to speak it too, I tend to misdirect them. I promise you all will get correct guidance from me. Code: "Do you speak Ballscrew?"

[Guldberg]

Maybe this could do as an AB solution, just add a spring in between the 2 parts and tighten. No idea if it would work, but it looks like some of the other solutions i have seen around here