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I have heard that it is good practice to always chose the number of teeth of two gears to be relatively prime. This is because there can be impurities in the me

I have heard that it is good practice to always chose the number of teeth of two gears to be relatively prime. This is because there can be impurities in the metal, and if you have a small part of a tooth that is harder than the rest it will erode the opposing gear (there is always some friction going on). By choosing the number of teeth to be relatively prime, the wear and tear is distributed uniformly on the gears, and thus they last longer.

Aperocky 7y ago
By relatively prime, do you mean the largest denominator == 1? Or in other words, 15 and 22 should be relatively prime?
- dan-robertson 7y ago
  Yes, relatively prime means coprime means gcd=1
- graphpapa 7y ago
  This is what is commonly meant by ‘relatively prime’, yes. Also referred to as being coprime. Defn: a & b are coprime/ relatively prime iff GCD(a,b) = 1
nbadg 7y ago
That's also what I was taught as a best practice, but it's not always possible. Otherwise, best practice is to index the gears (ie mark a specific tooth on each). That way, when you disassemble the gearbox (it's bound to happen sometime for maintenance), you can make sure it's reassembled so that the same teeth will continue to mesh, preserving the wear pattern. Otherwise, wear is substantially increased.
- fermenflo 7y ago
  That's also what I was taught as a best practice, but it's not always possible. Well of course it's possible -there are infinitely many sets of coprime numbers! Silly engineers
  - _jal 7y ago
    I look forward to trying out your infinite gearbox implementation.
  - D_Alex 7y ago
    Good luck for when you need exactly 2:1 ratio, like eg in clocks.
    - Dylan16807 7y ago
      Just add more gears in between.
      - IMTDb 7y ago
        So it's a bit like violence. Gears: If it's not solving all your problems, you simply aren't using enough of it.
tomglynch 7y ago
This is a similar problem to riding a fixed gear bike. When using skidding as a braking technique, specific areas of the tyre will wear faster unless you select a gear (front chainring and rear sprocket) combo with a large amount of skid patches. This is due to the rider always engaging in a skid with their crankset in a specific position - either left or right foot at 90 degrees, and the other at 270 degrees. Here's a fun calculator for it: https://www.surplace.fr/ffgc/ https://www.surplace.fr/ffgc/ Notice that a very common gear combo, 44/16 only gives 4 skid patches, so if you're after longer lasting tyres avoid this setup.
erikig 7y ago
Anyone that is fascinated by gears owes themselves a visit to This Old Tony's video on YouTube titled "Gears! - But Were Afraid To Ask" https://www.youtube.com/watch?v=Q-XOM4E4RZQ https://www.youtube.com/watch?v=Q-XOM4E4RZQ He goes into the nerdy depths of gear make-up, metallurgy, meshing, machining and mistakes as he manufactured metal gears on a mini-lathe.
- linsomniac 7y ago
  I second TOT, his videos are just fun to watch. Plus, you learn something.
  - nvahalik 7y ago
    Going through his page, I immediately thought of his video on gears. Suprise! when it was mentioned at the end.
steve76 7y ago
Size on diameter, and find something standard. Gears are just like threads. Alignment would be a bigger factor. Then service factor and then lubrication. After that I would say machining the part. Then grain structure and porosity before chemistry. The article needs to add that an involute is a point on a rolling cylinder. Also a torque needs a strain through the material. Then it makes sense when you find equivalent power transmission at a speed. That wrench depicts a moment, an ideal rigid body. Very little work needs to go into stuff like this. Everything is already made on the shelf waiting for use. Maybe for medical applications. Use a pulley, or a bar linkage.

Aperocky 7y ago

By relatively prime, do you mean the largest denominator == 1? Or in other words, 15 and 22 should be relatively prime?

dan-robertson 7y ago
Yes, relatively prime means coprime means gcd=1
graphpapa 7y ago
This is what is commonly meant by ‘relatively prime’, yes. Also referred to as being coprime. Defn: a & b are coprime/ relatively prime iff GCD(a,b) = 1

nbadg 7y ago

That's also what I was taught as a best practice, but it's not always possible. Otherwise, best practice is to index the gears (ie mark a specific tooth on each). That way, when you disassemble the gearbox (it's bound to happen sometime for maintenance), you can make sure it's reassembled so that the same teeth will continue to mesh, preserving the wear pattern. Otherwise, wear is substantially increased.

fermenflo 7y ago
That's also what I was taught as a best practice, but it's not always possible. Well of course it's possible -there are infinitely many sets of coprime numbers! Silly engineers
- _jal 7y ago
  I look forward to trying out your infinite gearbox implementation.
- D_Alex 7y ago
  Good luck for when you need exactly 2:1 ratio, like eg in clocks.
  - Dylan16807 7y ago
    Just add more gears in between.
    - IMTDb 7y ago
      So it's a bit like violence. Gears: If it's not solving all your problems, you simply aren't using enough of it.

tomglynch 7y ago

This is a similar problem to riding a fixed gear bike. When using skidding as a braking technique, specific areas of the tyre will wear faster unless you select a gear (front chainring and rear sprocket) combo with a large amount of skid patches. This is due to the rider always engaging in a skid with their crankset in a specific position - either left or right foot at 90 degrees, and the other at 270 degrees. Here's a fun calculator for it: https://www.surplace.fr/ffgc/ https://www.surplace.fr/ffgc/ Notice that a very common gear combo, 44/16 only gives 4 skid patches, so if you're after longer lasting tyres avoid this setup.

erikig 7y ago

Anyone that is fascinated by gears owes themselves a visit to This Old Tony's video on YouTube titled "Gears! - But Were Afraid To Ask" https://www.youtube.com/watch?v=Q-XOM4E4RZQ https://www.youtube.com/watch?v=Q-XOM4E4RZQ He goes into the nerdy depths of gear make-up, metallurgy, meshing, machining and mistakes as he manufactured metal gears on a mini-lathe.

linsomniac 7y ago
I second TOT, his videos are just fun to watch. Plus, you learn something.
- nvahalik 7y ago
  Going through his page, I immediately thought of his video on gears. Suprise! when it was mentioned at the end.

steve76 7y ago

Size on diameter, and find something standard. Gears are just like threads. Alignment would be a bigger factor. Then service factor and then lubrication. After that I would say machining the part. Then grain structure and porosity before chemistry. The article needs to add that an involute is a point on a rolling cylinder. Also a torque needs a strain through the material. Then it makes sense when you find equivalent power transmission at a speed. That wrench depicts a moment, an ideal rigid body. Very little work needs to go into stuff like this. Everything is already made on the shelf waiting for use. Maybe for medical applications. Use a pulley, or a bar linkage.