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alonso
11-18-2012, 07:09 PM
Hey everyone!

I'm making a jumping robot and I needed the servos to move lightning quick so I lasercut some Delrin gears to gear-down my RX-24F and had some questions for you.

What is the difference in achiving a desired gear ratio using a small vs high amount of teeth? For example in the image below I can achieve a 2:1 where the small gear has 12 , 15 or 20 teeth.

4315

4316

The couple of things I was guessing:

Higher amount of teeth could handle more torque and would be less likely to strip

Smaller teeth would be harder to laser-cut because of the fine detail

Smaller teeth might have less backlash?

Any thoughts?

Also, If you don't make your own gears, where do you buy your gears? Are they interchangeable between makers?

Thanks and happy building!

-Alonso

Th232
11-18-2012, 07:22 PM
Higher amount of teeth could handle more torque and would be less likely to strip


I think that's the wrong way around, while you have a larger number of teeth but they're also a lot smaller and you rarely get engagement on more than a couple of teeth at a time anyway.

A couple of other thoughts on gear design:
* Try to have odd numbers of teeth in your gears, so if there's a defect in one tooth it'll cause an even amount of wear on the gear it's interacting with, rather than causing a lot of wear on only a few teeth.
* It's a bit hard to tell, but are you using an involute gear profile for those gears? If not, do so.

alonso
11-18-2012, 08:07 PM
Hey Th232,

Yes, I'm involute designs, but the cutting thickness of the laser cutter messed up most of that shaping. I heard that there might be "cut on outside" feature in the software that I was going to check out later tonight.

I'm using something similar to this website http://woodgears.ca/gear_cutting/template.html

I was taking a look at the gears inside of my MX-28 and the pressure/contact angle is something high like 45. Wikipedia says:

"Increasing the pressure angle increases the width of the base of the gear tooth, leading to greater strength and load carrying capacity. Decreasing the pressure angle provides lower backlash, smoother operation and less sensitivity to manufacturing errors"

Do servos use high pressure angle gears because they need them to have greater strength? I love to see some servos that are more quiet.

Th232
11-18-2012, 08:12 PM
Yeah, I would expect that to be the case. I haven't cracked open any of my servos, but was the angle uniformly high? Would be interesting to see if they could be made more quiet by reducing the pressure angle for the gears closer to the motor, since the torque at those pairs would be lower.

jwatte
11-18-2012, 09:20 PM
Spur gears are categorized by four properties:

1) The number of teeth per inch (imperial) or the modulus (metric) which defines the distance between each gear on a rack (a gear of infinite diameter)
2) The pressure angle between the actual contacting areas of the gears. This is generally 14.5 degrees for older gears and 20 degrees for newer gears. Higher angle runs more noisily but can take larger forces.
3) The profile shift. This means that the bigger gear and larger gear are "offset" from the standard values to make fabrication easier, but you have to use the gears with the proper shift together (positive for one, negative for the other.)
4) The profile depth. This is used for "stub" gears, where the depth of teeth is smaller; this makes the teeth thicker and thus more robust, at the expense of requiring more teeth to make sure you have contact at all times.

Last year, I wrote a C# program to design involute gear profiles, and output SVG and G-code to create gear combinations of various parameters; you may want to check it out on GitHub (you have to build it yourself from source): https://github.com/jwatte/gears/