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Old 06-15-2016, 09:05 PM   #1 (permalink)
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Default Prime numbers of teeth on gears

In the main forum guys were talking about wobbly tt gears and matching high spots with low spots on meshing gears. Why don't manufacturers match gears with a prime number of teeth on one of the gears so it will share the wear and isn't this normal engineering practice?
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Old 06-16-2016, 05:08 AM   #2 (permalink)
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That isnt what they are talking about. They are talking about gears that are not concentric with their axis- they wobble due to manufacturing errors. So for gears that are 1:1 (perhaps a tailcase), they try to position the two gears such that side of one and low of another are meshed- so that you get less radial force on the gears as they rotate.

As regards tooth count, that is one of those areas where you design properly when you have lots of control (i.e. ground-up design on a jet turbine/stator setup), but in some cases, you simply don’t have control. Example, if you require a variable gear ratio with an electric heli, with a single reduction stage, and you cannot alter tooth count on one of the two gears- then the tooth count on the changeable gear must be varied. To have reasonable resolution there, you cannot rely on pure primes, where 11 is about the lowest prime you could use for a pinion, but then you have to go 13, 17, 19.

So, you don’t look for prime numbers per se, you look for mutually prime integers. That is just a pair of numbers which have only the number “1” as a common divisor. The classic example there is 14 and 15 as mutually prime, but 14 and 16 are not (each are divisible both by 1 and by 2). With mutually prime tooth counts, you will have each tooth touch all others before the pattern repeats( if I recall my math on this correctly- please double check).

But even there, you just may not have any option, where the application may require 11, 12, 13, 14 and 15 tooth pinions to get the headspeed range required without requiring a motor (kV) change
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Old 06-16-2016, 04:43 PM   #3 (permalink)
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Extrapilot
I did understand those guys trying to compensate for eccentricity in the 'paired' gears.
I was more interested in why,for example on my align 600 the tail gears have 20 teeth on each of the bevel gears, no prime numbers , so no 'hunting' tooth and no shared wear?
Perhaps there is so little wear on 'plastic' gear teeth it's not a commercial consideration.
I do accept your reason/answer has to do with initial design considerations and I had overlooked what happens when there are multiple motor pinions but only one main gear option.
Thanks for your insight.
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Old 06-16-2016, 09:00 PM   #4 (permalink)
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Tropic-

IMO, there is often here an assumption that people who design these machines all know what they are doing. Some of the errors we have all seen in design are beyond explanation, so it is pretty clear that many designers/engineers are not degreed, and don’t know even basic rules about mechanical or aero engineering.

But in this particular example, I think you are probably correct. I believe most gear manufacturers (or consultants to manufacturers) have to know the conventions about tooth count- but for us wear is probably distant third to structural failure from crash or overtorque/RPM. And I think that is probably a fair approach, because you often read posts from casual scale/pattern pilots with 1000+ flights on a machine without failure/incident. There is no way in hell a typical 3D machine survives 100 flights without incident.

Enjoy your weekend-
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