Gear-driven Z-stage instead of Belt-driven

I've been thinking about how to improve the basic design, and had a bit of a brain-flash some time ago. We all agree that the belt is the more painful part of the structural mechanics, as we'd have to cut/sand/glue that thing, add tensioner, etc... But what if we could come up with a fairly accurate gear ratio system that drives all four Z-stage posts from a motor placed in the center of the base (or thereof?)

My inital idea is pretty much one main gear with a second feed gear for each post--the secondary benefit is that we'd have to come up with a more stable base to put the whole RepRap on. (That's a second line of thought, but until I can get a RepStrap going to test the idea, I'm not voicing my ideas on RepRappable frame structs. Too many variables ATM.)

Backlash in the gears would be a problem, it can be solved with anti-backlash gears but it all gets more complicated, although backlash in the Z axis is less of a problem as while printing it always goes the same way.

---

Ian
[www.bitsfrombytes.com]

Along with backlash, you would have to take into consideration gear mounting and clearances. Most gearing requires a clearance of 1mm or less which is easy to do if you have the right equipment, but a real pain to do for most home users.
I'm not saying it can't be done by a home user, I'm just saying it is extremely frustrating.

As for stablizing the base, I have another z axis machine that I built with a gear train on it, and I used sheet metal for the base. I've forgotten the thickness, but it was cut out of a scrap computer case and secured with #6 imperial machine screws.

Luckily I have access to a CNC machine that drilled the holes for the posts for the gears.

Oh, and the belt is much cheaper to buy when you look at the cost of gears and posts.

A better design would be to have a solid base for reprap, move the motor to the center under the z-axis, add an idler pulley to each side of the motor, and have the motor on a slotted mount. You could then run a belt around all 4 corner pulleys, around the idlers, and motor; and adjust the tension by moving the motor in it's mounts.

criswilson10 Wrote:
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> A better design would be to have a solid base for
> reprap, move the motor to the center under the
> z-axis, add an idler pulley to each side of the
> motor, and have the motor on a slotted mount. You
> could then run a belt around all 4 corner pulleys,
> around the idlers, and motor; and adjust the
> tension by moving the motor in it's mounts.

Actually, that's a great way to remove the tensioner arm by building it into the base (or thereof.) In fact, if one has the clearance, you can prolly house the electronics down there, too. I'll give it a try once my hands are on a RepStrap, but already the ideas are coming to me--you can eliminate one of those idler pulleys if you can use the motor to gently push on the belt til it's nice and solid, then lock it down structurally within its housing.

It completely eliminates the need for a tensioner arm and there is plenty of room for the electronics with a NEMA 23 or NEMA 17. I've built a z-axis like that before and it works well. As a matter of fact, I think I have one in my storage closet. If I do, I'll pull it out and post a picture of it and the details.

OK, so here are some pictures of what I was talking about above.

[people.clemson.edu]

And while you could get away with only 1 idler, 2 idlers kept the belt from
moving around (vertically) when changing direction.

The photos are pretty much a general idea of what I have in my head, but my suggestion involves two things: A lip plate on the motor gear, and that, based on the angle of view in your photos, if you moved just one pulley northward and moved the motor a bit southward, mechanically you'd still get the same tension performace with just 1 pulley.

I agree that would probably work.
The reason I use(d) 2 idler posts is to give the belt and pulley
more surface area contact. This lowers the shear force applied to
the teeth on the belt. Essentially, the more belt you have in contact
with the pulley, the lower the chance of shearing off the teeth on the
belt.
I know at the time I didn't sit down and do the math on the minimum
contact angle needed for the belt because it was a rush job.

The lip on the motor pulley is definitely a must. You can't see it from
the pictures, but that pulley does have a lip. Un fortunately the pulley
is also twice as wide as the belt, so the belt was moving up and down
the pulley. That metal pulley can easily cut into those neoprene belts
and break the belt. The obvious correction would be to put the proper
sized pulley on the motor, but again it was a rush job.

I haven't done the calculations for a reprap either, but my gut feeling
is that a 30 degree contact angle would be enough. Which would mean
that one idler post could be used.

If you don't want to do the math for it, I'd say build it with one idler
post. If it works, great. If it shears the belt teeth, then add the second
idler and make your next belt longer. My gut feeling is that it will work
with only one post.

Cris, et al,

IMHO, more contact with the pulley helps prevent slipping (pullye vs. belt), which is a more likely source of error than actually stripping teeth off the belt. So, even if the teeth won't break, having more wrap makes for a more reliable drive system.

-- Larry

That is the rule of thumb in designing with pulleys - get as much contact as you can to prevent slippage.

Since these are toothed belts, slippage is minimized by the teeth and belt tension, but you have to worry about tooth shear.

I was wondering enough about the contact angle, that I went ahead and did them out of curiosity.
The reprap stepper motors have a maximum torque of 1.2 Nm
The reprap 6mm wide MXL belt has a tooth torque rating of 0.182 Nm
So 1.2/0.182 gives a minimum of 6.6 teeth which would be a minimum
mesh of 7 teeth.
7 teeth is about 50% of the darwin capa x or y motor pulley. So you would
need to use a different pulley if you used only 1 idler post. I would go
ahead and order a metal pulley anyway, plastic drive pulleys don't seem
to hold up very well.