Gears - Gear Ratio and Gear Size

[sciencefair mom]

Why have different size gears (teeth numbers) to get the SAME gear ratio? Just to cover different distance in same rotations? What factors would influence your decision to pick a smaller or larger number of teeth to get the same ratio? i.e. When is it better to choose gear combination of 15 teeth to 30 teeth (gear ration 1:2) versus gear combination of 20 teeth to 40 teeth (same gear ratio 1:2). Thanks!

[rmarz]

sciencefair mom - Good question, and as most design decisions have trade-offs, so does this issue. You are right in the overall observation that the ratio is the same for the 15:30 vs 20:40 selection. Here are a couple of other issues to consider.

1) Smoothness of power transmission. As each tooth meshes with it's mate, there is a transfer of power, but it is not perfectly smooth, like a belt and pulley might be. The more teeth, the smoother the power transmission. There will be an
oscillation in the measured power transmitted. More, finer teeth minimize this issue.
2) Torque - In most cases, more torque can be transmitted through a physical gear with fewer teeth (stronger, more robust). There are extremes here, but this is a reasonable rule of thumb.
3) What if your design problem wasn't to create a simple ratio of 1:2, but 33:167. Now you have to use more teeth than than the 1:2 ratio, or more gears in a series of ratio changes.
4) Finer (more numerous teeth) gears may have a cost penalty in being more time consuming to 'hob'.

I hope some of these examples help understand the decisions and trade-offs that engineers and designers make every day.

Rick Marz