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same ratio dfferent gears

6344 Views 64 Replies 19 Participants Last post by  stoner
hy would some one like to explain the different combo,s, and why. take a 3-1 ratio, the standard is 9-27. going to extremes, what about 7-21 or 11-33 there must be a difference and i,d like to know. a euro sport runs 6-1 6-36 why. john
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You called?

Several things going on here:

stress level and material properties - contact pressures, loads

sliding friction

error robustness

First point, it is not just about the number of teeth. If you use a different gear modulus then you can have the same diameter contrate with different number of teeth. Just look at some of the early brass gears, fine pitch without large diameter.

The error robusness is the bit that really determines the size of teeth that should be used. Misalignment and tolerances of build need larger teeth. A small inaccuracy should not lead to the gears disengaging and jumping teeth.

Too large teeth gives a pinion with too few teeth, you don't really want less than 8, the involute profile is compromised and you end up with more of a sliding action as the teeth engage and rotate, so friction gets higher and transmission losses increase, noise, heat, wear all the cost to you of speed and power.

Plastic teeth need a reasonable cross section to resist the high torques of modern motors. This can be achieved by increasing the depth of the tooth, but again that can lead to more rubbing. Look at the gears on the AutoArt cars, same pitch but really thin. Put a NSR motor in one of those cars and the gears are going to fail really quickly...

So 9:27 gives decent size plastic teeth, allows for reasonable mismatch, gears are not too large and the tranmission efficiency is good.
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QUOTE (Abarth Mike @ 24 Apr 2012, 10:33) <{POST_SNAPBACK}>11:33 requires a larger spur gear so torque is multiplied. 8:24 is the same but the torque to the axle is reduced.

No, torque multiplication is simply a product of the gear ratio (excluding losses) not to do with gear sizes
But fundamentally that can't be right
Offset gears have a sliding action and that means energy going into heat/noise. Bizarre that they run so well. I do use offset gears in the Larkfield class, and I am class leader, but I do a lot of maintenance to keep the gears in good condition.
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Nice marathon!

The bit I was thinking about didn't take into account rubbing you get with cheap manufactured V cut gears. In an ideal world the tooth shape is involute - that gives a sort of rolling motion without sliding, so the contact line moves inboard but there is no real friction.

That works if the gears are both on the same axis - no offset. The contact point stays at the same position relative to the motor spin axis. With offset gears as the gear rotates the contact point moves along the motor spin axis so has to slide against the gear.

No matter, the offset is pretty small so the effect is clearly negligiable. Try without any lube and it might be a different story though...
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