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HelicopterMain Discussion › Lets talk gear ratios part Deux
03-12-2003 05:04 AM  14 years agoPost 1
The_Dave

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Las Vegas / Pahrump, Nevada

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OK, here is a question for all you gearing experts/ mechanical engineers...

Lets say we have a gear ratio of 3.5:1. There are multiple gearing combinations that we could use to achieve that ratio. For example 14:49:1 or 12:42:1, both ratios are 3.5:1.

My question is this: Would one gearing combination accelerate faster than another? If you had a 14-tooth pinion would it require more torque to turn it? Would a 12 tooth pinion spool up faster? If overall gear ratio is the same how does the gearing combo's affect performance?

Mark McAlpine - 2005 - We will never forget.

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03-12-2003 05:52 AM  14 years agoPost 2
Phil Cole

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Menlo Park CA

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I'm not a mechanical engineer. However I can say that if the gears had no friction, no mass, and were infinitely stiff, and a whole bunch of other that are impossible in real life, there would be no difference in performance.

The answer depends on the details of the gear design more than anything else. Diameter (hence tooth pitch), material, width of the gears, etc. will all determine the efficiency of the gearing.

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03-12-2003 09:20 PM  14 years agoPost 3
spincycl

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Marietta, GA

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When you ask about them "accelerating faster" what do you mean by that?

I can say the following :

Generally the larger pinion/spur combinations will have a greater mass, which will definately make them "accelerate" slower in start/stop situations. (the difference should be minor in most circumstances, though)

But, in the case of course pitch gears, there are definate advantages to using larger combinations. The reason is that in cases with relatively small pinions, there is an amount of "tugging" that happens. This basically feels notchy when put under load. This phenomenon is related to what the angle the teeth are to each other on the pinion. For any given pitch, the larger the diameter, the smaller the angle of the teeth. Low tooth count pinions never run as smooth as larger ones.

When I raced RC cars, I could get the same ratio with several combinations, but I generally tried to use largest diameter combination possible. The gears ran smoother, quieter, and lasted longer.

It's almost always preferable to increase the diameter of the driven/spur gear than to decrease the diameter of the pinion/driving gear - whether it be heli's or RC cars.

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HelicopterMain Discussion › Lets talk gear ratios part Deux
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