App note: Introductory gear equations


In using motors, we may tackle on coupling these little mechanical devices to something to move or maybe we might need more torque but paying for speed, Precision Microdrives’ app note gives us basics in gear calculation. Link is here.

For quickly experimenting or prototyping, you may wish to build your own cheap gear chain. Here we’ll take a look at a couple of basic calculations to get you started and which should provide an easy reference to return to as you need.

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  1. At what point should you move to metal gears and a bigger motor?
    Say I’m winding a fishing line, with a weight on it. Sure, stepping down with ratios gives me more torque at the expense of speed – but get a decent marlin on the end, and your motor / gear teeth won’t take the strain.

  2. Does anybody know good sources for such kind of gears? I need to replace a broken one from on old Sony device, and Sony doesn’t have it in stock anymore. It is a compound gear with 35 teeth outside (about 29mm diameter) and 12 teeth inside (about 11mm diameter).
    Is there any kind of standardisation about sizes, teeth numbers… ? Or any kind of tools to help choosing the right couple of gears which would fit with a given axes distance?

  3. We in Russia always use lots of grease. Helps a lot. Like vodka.

    Solving that problem, technical speaking with all plastic gears you have mainly three reasons of failure.

    – first the too much force failure, teeth break at the foot. So calculate the shearing, torque etc. (DIN 3990 gives the outline to that, should not be too hard to do.)
    – second the fatigue fracture problem, teeth foot breaks due to long time use under load. (difficult to calculate, often you only get rule of the thumb values…)
    – third: failure of lubrication. not only by technical failure but also be structural problems. You simply can’t ensure that there is always a lubrication film of a needed thickness on a surface. Dirt, etc. leads to pitting, all that stuff. Nasty.

    At the end of the day you end with a lot of material depending factors, a huge pile of experience and some specialized software… I would assume that for winding a fishing line they simply build something and then test it.

    1. Designing a power winch to take up the slack is overkill – if all you catch is a sardine.
      A 3V hobby motor and plastic gears won’t last – if you get a marlin on the line.
      Got something like a trout or salmon? I suppose the fun is in finding the best setup.

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