Taking a 1750RPM 110VAC appliance motor down to about 60RPM, maybe a little less. It's a home project on the cheap, so dropping a few hundred dollars on a ready-made gear box is to be avoided, and I'm working with the kids as a school project and so creating by hand as much as possible.

1/2" plywood for an initial prototype/proof of concept, with a planned move to 3D printed gears afterward. Assuming a tooth pitch of 18-25mm, and that we play with the tooth count between the gears to spread out the wear and tear, which would be better for durability, chatter, noise? Fewer gears with larger reduction ratio at each step, or more gears with a smaller ratio?

  • $\begingroup$ For low torque, it seems like a worm gear would work. $\endgroup$ Mar 1 '18 at 20:34
  • $\begingroup$ I like pulley reducer than gear reducer. Gear reducer is more difficult to design & produce, have big friction loss, and more expensive. $\endgroup$
    – RainerJ
    Mar 2 '18 at 15:07

Gear chatter/noise is caused by all manner of things, but tooth count isn't a primary driver. Tooth count will impact the bending and pitting resistance of your gear teeth, meaning you could make them last longer, although if torque is minimal you probably don't need to worry about this.

To reduce noise you need to reduce transmission error (have involute gears as close to geometrically perfect as possible). Having a good surface finish on the gear teeth will help, 3D printing sounds fine for both accuracy and surface finish in your application. (Might be worth checking the material strength of whatever you choose to ensure it can handle the torque from the motor)

If I were you (assuming the torque from the motor is low enough that you essentially have infinite fatigue life) I would go for the bigger reduction over a single gear (to save space) and focus on making the teeth as accurately as possible, with smooth surface finish. Also look at reducing the backlash, in a low speed application this could be a primary source of noise.

Outside of that, I recommend reading this - it's quite a nice summary for reducing gear noise. There are some sources of noise you won't be able to do anything about.

If you are unsure about durability, go for more gears to prevent one big ratio step. For noise, improve the accuracy of geometry and surface finish on all gears where possible, reduce backlash (but not too much!).

Source: I worked as a gear designer.

enter image description here


According to one source, the tooth pitch is the distance on the circumference of the gear for one tooth and one gap. If your value is correct, you'll have quite a large diameter gear.

On the other hand, if you are referring to pitch diameter, that brings the gears down to almost a tiny level.

If you consider the 1750 rpm of the source, a destination rpm of 40 would mean a 35:1 ratio, a nice integer ratio.

If your starting point is a 25 mm drive gear, the driven gear will be almost a meter in diameter (875 mm). Obviously, if you go larger with the drive gear, the driven gear has to go up.

A compound gear arrangement may be a bit better suited if you have the vertical space to stack gears. Another possible option would be to create and use a planetary gear set. 35:1 is well within the capabilities of such a mechanism.

There are "bunches" of planetary gear models on Thingiverse for 3D printing.

I'm currently working on a project involving a much lower ratio, only 1.5:1 which means an 80T sun gear and a 120T ring gear. Planet gear sizes are meaningless in this application. In order to get the greater reduction you require, you would have to stack at least two stages and that gets out of my current range of understanding.

Thingiverse has a few multi-stage designs, one that looks particularly attractive. The design is based on a 6:1 reduction, therefore two stages would provide for 36:1, nearly the aforementioned 35:1.

multi-stage planetary gear reduction

The output of this design is not shown, but the crank drive could be adapted to take a motor shaft. If you desire to construct this of plywood, you could take the tooth counts for the components and enter them into one of the many gear generator programs available.

The above linked site shows how to accomplish a greater reduction with a compound gear set rather than a planetary gear set, but will still allow you to construct the necessary drawings (and downloads) to build a model.

36 rpm gear reduction

If I did this properly, the above site and the link provided will appear with a 1750 rpm input and a 36 rpm output. A bit lower than you requested, but I had to change only one figure and the result is suitable for demonstration purposes.

To attempt to answer your primary question, fewer gears means less wear, less noise, but much more space used to accomplish this. Planetary gear sets are not known for being quiet, especially when input is so high, but a compound gear set may not be much quieter, although there will be fewer gear components in the latter design.

Design for the largest size you can fit to your application, to spread wear out over a greater area. Lubrication, even on wood, will be of great value at the speed you've provided.

Larger gears will be in the range of hand tools such as a coping saw, unless you have the great resources of a laser cutter. I think I'd have had great fun in your class if I had such opportunities in my youth.


Why use 3d printed gears ?

3D printing is a terrible way to make gears and you can just buy standard ones with proper tooth profiles off the shelf.

For that sort of high reduction ratio a planetary gearbox is generally preferable to long compound gear trains.

However it sounds like you might be better of with a belt drive. Vee belt pulleys are fairly cheap, can achieve the required ratio without too much compounding and are reasonably quiet and importantly lend themselves well to low-tech fabrication as they are reasonably tolerant of misalignment and have a lot of inherent scope for adjustment. Unlike gearboxes which require pretty good manufacturing tolerances.

For all of these reasons belt drives tend to be the preferred option for stationary tools driven by AC motors.


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