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Don't Fear the Gear
O.K.
As I have seen in this post, several of us FIRSTers don't like gears and only swear by chain. Well, I am the complete opposite! We try to use gears everywhere; because, if designed correctly, they will last forever. Gear design entails a lot of approximations and uses a lot of derived equations, but it really boils down to 2 things:
(1) Gear Tooth Bending Stress
(2) Gear Contact Stress
For machines that need a finer precision (like NC machines and Industrial Robot Arms), we try to use a finer pitch gear (large Diametral Pitch), but because of their fine pitch, they have bending strength limitations. Contact strength is directly related to the type of material you use and the surface hardness as well as the lubrication (heat and friction are the enemy).
Gear Contact strength is the limiting factor for machines that run thousands of hours and the teeth actually wear themselves thin and has never been a source of failure for our robots.
Bending stress is also known as "We shattered our teeth into a million pieces" stress and applies to many FIRST robots. Material strength, tooth facewidth, and Diametral pitch all contribute to the tooth bending strength. Larger gear teeth hold up better(smaller Diametral Pitch, or DP), face widths up to 3/4" are good, and steel is better than brass in most instances.
I will let you in on a little secret: Brass is heavier than steel
So why would you use brass?
Steel gears from Boston Gear do come width keyways standard.
Face width is your friend, but don't get carried away. 1/2" face width for the last stages in a drivetrain are common.
Lastly, the reason the little gears (finer than 36 DP) in the drill transmission don't break is because the load is shared between 3 or 4 gears at the output. The main advantage of a planetary gear train is the ability to handle large loads in a small area.
Don't fear the gear, because it can be your best friend.
-Paul
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