Help choosing the correct timing belt and gears

[rick777]

I need help figuring out a suitable toothed timing belt for an astronomy tracking project I’m working on. I’ve got everything up and running with a Uno Rev1 and am using a 12V 0.4A 36oz-in Soyo SY42STH38-0406A Unipolar Stepper Motor controlled with a ULN2003A driver that I ordered from you.

Currently I have an Actobotics 16T Gearmotor Pinion Gear (6mm) Pitch: 32 Teeth: 16T connected to a Actobotics 84T Aluminum Hub Gear (0.5") Pitch: 32 Bore: 1/2’’ (.500’’). I attached the pinion gear to the stepper with a shim from a window curtain rod.

I’ve tested the setup and the tracking is very good.

However, with the pinion gear touching the hub, the vibration from the stepper motor is transferred to the telescope resulting in blurred images. When I disconnect the pinion gear, the vibration is contained to the stepper motor.

I am looking to change the setup to a belt driven one to hopefully reduce the vibration.

The pinion gear gear I am looking to order is the RB-Sct-941, 15T Timing Pinion Pulley (6mm).

I’ve gone to other web sites to do the calculations for belt size and have the following details:

CTC = ~1 7/8”

16T = .563”

84T = 2.68”

This gives me a belt length of around 9” or 250mm.

My questions:

1. With some real fine adjustments, I think I can use the 222mm, 74 Tooth Timing Belt, RB-Ftr-21but am not sure it would work with the 15T gear? I’ve also considered the 125 Tooth Timing Belt, RB-Ftr-38 which is 375mm. (I’m trying to pick in-stock items…)

2. Can I use my existing 84T Aluminum Hub Gear with the 74 tooth belt and the 15T pinion gea or do I have to switch it out as well?

I can move the stepper motor away from the hub to accommodate a longer timing belt, but I can ‘t go much closer.

I like the size of the 84T as it gives me a good gear ratio. Anything less and I would have to see if I could get the stepper to go slower.

I’ve added some photos of the set up so you can see how it all goes together.

Thanks

[igor_X]

Hi @rick777 and welcome to our forum

You have a nice and interesting setup there. But, it’s not really easy to answer your question, without having real parts in hands or at least 3D model assembly.

Anyway, I am not sure that you can use 84T Aluminum Hub Gear with the belt. That gear is not meant for belts because it can slip and belt probably won’t fit.

Will this Rb-Ftr-21 or Rb-Ftr-38 fit Rb-Sct-941 is a good question. Generally , what I’ve seen in past, parts from different manufacturers (in this case it is Servocity and Fingertech) in lot of cases do not fit very well. But in the end, you could buy them, try them, and if it doesn’t work, return them.

[rick777]

I ordered several Rb belt configurations see what fits the 84T as well as a RB-Ftr-25 that can be drilled out to 1/4" for the shaft. For the 84T, if any of the belts fit I have enough room to make and attach a metal guide to either side of the gear to keep the belt on. If the belts don’t fit, I’ll see about another gear.
As an aside, I tried the assembly out with a velcro cable tie to see if it would move the gear. If you get the tension right, and have the tie fuzzy side down, the stepper will drive the gear. Okay for a very short term or testing situation, but I wouldn’t recommend it for any on-going project.
Thanks

[igor_X]

Hi @rick777 !

I think those are good decisions that you made. Of course, we would like to see your final solution in the end.