A Mechanical Engineer’s Quick Tips for Using Gears

Whether to slow down a movement, speed it up, or change its direction, it’s easy to find technical information about gears. But using them, especially custom cut pieces, can still be tricky because they have to work so precisely together. I’ve learned some lessons along the way — here are a few.

Fabrication

What’s a MOD? The module number of a gear is the ratio of teeth to diameter, when diameter is measured as distance to the mesh point (where the teeth of one gear meet the teeth of another). A MOD 2 gear with a diameter of 30mm will have 15 teeth. Knowing this number simplifies calculating the proper distance between gear shafts. Photograph by Hep Svadja

» Really large gear teeth can be hand cut. I once made all the teeth for a clock with an angle grinder. Large teeth are easier to make because the distance between the shafts is less critical.
» When getting metal parts laser cut, allow a tiny gap between the teeth of a pair of gears (I use a 0.4mm gap for MOD 2 gears). The output gear will be a bit wobbly (backlash), but this is preferable to the gears being too tight (lots of friction).

Gear alternatives

» I often drop the speed of a DC motor by 4 to 1 simply by putting diodes in series with it — the motor loses surprisingly little power.
» Don’t use gears to reduce the speed of an ungeared motor. A motor with a built-in gearbox will be less noisy and more powerful.
» For changes in speed less than 5 to 1, a belt or chain is often easier.

Resources

» Pre-made gears of all types are available through various outlets such as McMaster Carr.
» Designing gears and gear trains is easy and fun with the websites Gear Generator and Wood Gears.
» An exhaustive treatment of gears and gear design, including how to choose the right gears and determining gearing calculations, can be found in the book Handbook of Practical Gear Design from CRC Press.
» The website 507movements offers an incredible assortment of gear combinations, along with various other forms of mechanical motion, from pulleys to levers to linkages.
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