Timing Belts Versus Sprockets and sources for either?

Martin_H

A while ago I built a turntable proof of concept using an eBay stepper motor, a lazy Susan, and some tread for a belt. It was kind of a kludge, but proved the concept. So I want to try to build a better version. I plan to reuse the lazy Susan, use a better stepper harvested from an old scanner. But that leaves me with the drive system to couple the stepper to the plywood disk. I have a few thoughts on that.

I've eliminated the idea of using a gear chain because I suspect I'll never get everything aligned and the gears will bind.

The timing belt material people use in 3D printers seems promising. I'd glue some of that material to the outside of the plywood disk to make the whole disk act like a pulley, then use a shaft adapter to get the matching timing pulley to fit on the stepper's shaft. I'll need some way to tension the belt and make it a continuous loop (glue it?). I've seen all this stuff on eBay, so for a few dollars I might have something working quickly.

The second is a chain and sprocket drive. The problem is finding a source for the parts as everything I've seen is too heavy duty for this purpose. Conversely the Tamiya ladder-chain & sprocket set is too light weight and an embarrassingly red plastic. Any good sources for something lighter duty?

The third is extending the thread friction drive using Kevlar fishing line like I've seen done in a few 3D printers. This is likely going to be a headache getting it tight enough for the friction to prevent slippage of the fishing line. But the part count is low.

Note that there's no purpose to this turntable. I just want to build something that I can reliably rotate to desired relative position.

Gadgetman

The timing belt may be the easier solution...
How large is the turntable?

Try finding a complete loop timing belt, not a cut piecce. Something like this:
http://www.ebay.com/itm/New-Synchroflex-T5-525-Timing-Belt-10mm-Width-5mm-Pitch-/200778466712?pt=LH_DefaultDomain_0&hash=item2ebf544598

Loop it around the disc, with the teeth pointing out(it should have enough friction on the other side to grab the plywood), add a spring-loaded tensioner to pull it away from the disc at one point, and place the stepper with pulley near the tensioner so that the pulley grasps the teeth of the belt properly.
If you can get the axle of the turntable, the stepper and the tensioner more or less along one straight line, you should be good to go.

There's a lot of thinner, smooth drive belts around, too.
(It really helps to know sizes needed, weight to pull and all that)

Ttailspin

Google search "Model Railroad Turntable Construction" and see if that gives you any ideas..


-Tommy

Martin_H

Thanks Tommy, I'll do that tonight.

Gadgetman wrote: »

How large is the turntable?
...
(It really helps to know sizes needed, weight to pull and all that)

I'm flexible because the prototype was built from MDF, so I plan to rebuild using 1/4" to 1/2" plywood. The only thing I'm reusing is the 4" lazy Susan bearing. I'll probably keep it smaller than 9" but I don't have any size planned just yet. The weight I plan to turn on it is less than a pound.

kwinn

There should be suitable timing belts and sprockets available from industrial supply stores, and another place to look would be a sewing machine repair store.

kwinn

Try googling toothed belts and pulleys.

ElectricAye

McMaster sells timing belts and so forth. Kinda expensive, maybe, but you'll get them fast.

http://www.mcmaster.com/#timing-belts/=o5kddx

http://www.mcmaster.com/#timing-belt-pulleys/=o5ke91

Zoot

Also here: http://www.servocity.com/html/pulleys___belts.html

garyg

HI Martin
It sounds like you are looking for a gear reduction of some type.
Is your E-Bay stepper motor strong enough to just drill a hole in the center of your plywood base to mount the motor?
resolution will be less than the gears, timing belts etc. but
If the resolution per step is what you need, It would be as simple as
drilling a hole in the center of your plywood turntable after it has been mounted to the lazy susan bearing.
pushing and screwing the stepper motor to the plywood.
After the stepper is bolted to the plywood, a coupling made of silicone caulk and glycerin would stabilize the motor but allow
it to move with any error in alignment.

I hope this suggestion helps.

erco

Bicycle chainrings & cassettes using std. 1/2" chain or

#25 sprockets & 1/4" chain: http://tncscooters.com/index.php?route=product/category&path=63

also see 8mm/T8F chain & sprockets there

Martin_H

@garyg, I tried one of the cheap eBay steppers and they don't have the torque when coupled to the center of the turntable, even when I made the linkage compliant to avoid binding. The thread drive acted like a reduction gearing and then it worked, but was too slow. The salvage stepper should have more torque and be fast enough when acting through a reduction gearing.

@all, thanks for the pointers. I'm leaning towards a pulley on the stepper that uses a 5 mm pitch belt with friction drive to the turntable. As long as I keep the belt taught enough it might work.

kwinn

If you are buying the timing belt material used in 3D printers to glue on the lazy susan you should be able to get a pulley with a matching pitch as well. If that is the case you may not need a belt between the motor pulley and the lazy susan. Simply mount the motor so the pulley presses against the lazy susan. There will be a bit of backlash when you change direction, but that would be there even with a belt between the motor and lazy susan.

David B

If your motor can take the side thrust, and you don't need a lot of drive force, you could put a rubber wheel on the motor shaft and use friction to drive the rim of the turntable. (like kwinn said)

kwinn

With a wood or rubber surface to ride on you could even use a grinding or sanding drum to drive the rim. The early HP inkjet printers used a wheel with a gritty surface similar to a grinding wheel to feed the paper. Never had a paper feed problem on any of those printers.

T Chap

You can order the precise number of teeth on a closed loop belt from Gates-Mechtrol. I use a 5m pitch HTD 15mm wide belt from them, it is a steel belt with a coating on it. I have seen guys on youtube cut the large pulley by drawing the teeth from a printout, then using a band saw.

If you have a method to CNC, laser, or waterjet the larger pulley you can get a cheap program like this to generate the files http://woodgears.ca/gear/

Depending on how much torque is required, you may get by with the bearings built into the stepper for the radial load, otherwise you will need to put bearings on both sides of the pulley so the motor is not carry the load.