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  • Scale is important when looking at plastic vs metal gears.  For a typical model size heli plastic wins.  Even for an "industrial" machine it is more than strong enough.  It's lighter, self lubricating, cheaper, and requires less maintenance.  Get to the size of a manned heli and you need metal.  You've basically increased the load so much that you are in a different realm.  Again, even though a model heli may look like a manned one they are different enough in scale that what's best for one might not be for the other.

  • Couple things though:

    The actual load on the servo gears isn't that high.  Up the load, and you'll see the plastic gears wear faster.  I think there's a cross-over point with wear and loading.

    Second, servo gears are not in an oil bath.  There's a bit of grease, but it's not very effective.

    Lots of gearboxes in my plant, none of them use plastic gears.  They are either oil bath gearboxes.  Exposed roller chain or metal gears that get greased regularly.  Or belt drives running dry.

  • T3

    Karbonite servos wear less than metal.

    Nylon servos wear less than metal.

    But it is cheap metal, and the choice of plastic is 'special'.

  • Where between metal and plastic, it's a complicate subject and I couldn't even begin to explain it all here.  And I'm not an expert by any means.  But a few key points I believe are true:

    1) If the gears (or whatever) cannot be oiled, must be run dry, then plastic can actually last longer than metal. If the wear is metal on metal, you really need to have oil.

    2) The gears are probably going to break if you crash no matter what they are made of.  Hobby helis crash frequently.

    3) A gear case would be quite expensive, and quite heavy. A plastic gear for a heli weighs what, 2oz and costs about $10?  A metal geartrain in a case would weigh maybe a pound or more, and cost $1000 at least.  

    Therefore, it only makes sense to use plastic gears in a Hobby heli.  Even for "industrial" stuff, I'd think a case could be made for plastic gears.  They have a performance advantage in that they're light weight.  Maybe the user would want to save the weight and say "Ok, so I replace them every 100 hours.  It's easy and they're only $20."

    Personally, I think belt drive offers the best of both worlds.  They're light, quiet, durable, and may even survive a crash.  If I was designing a heli of this class, that's what I'd do.  I think the reason most don't is just due to complexity.  The single gear system is about as simple as it gets.  Also, belts do require tension which places extra load on bearings.  No big deal though, just increase the bearing size in the motors. That's why I have to do.  Specify that it's a belted application, and they put roller bearings in the nose of the motor rather than balls.

  • That seems counter-intuitive to me that plastic wears less than metal, especially as Robert says if they are sealed and oiled.  Of course, I have no training in mechanical engineering, so I don't really have any experience in that kind of thing.

  • T3

    "The idea is probably to replace the gears regularly. They are probably considered consumable parts, after all, that's why they're made of plastic mostly. "

    I would say this is because plastic doesnt wears as much as metal and suppresses noise. At the same time when you crash, you have to replace the gears in both cases with equal probability.

  • Robert, interesting.  So have any of the industrial grade heli frames addressed these problems? As for the exposed gears, it does make it easier to spot the wear.  The idea is probably to replace the gears regularly.  They are probably considered consumable parts, after all, that's why they're made of plastic mostly. 

  • T3

    I was hoping to get some initial materials in order to do market search by myself, instead got market digest right on my desk. Thank you all, waiting for more.

  • I think it comes down to what the design was intended for.  Hobby grade helis tend to be designed for extreme performance.  Very light weight in some places, and then overbuilt in others.  For example, the head on a 600 is strong enough to easily swing 800 size blades when used in a gentle application.  But then the main shaft bearings are undersized for a carrying a heavy load for any length of time.  Any time you have an axial load on a ball bearing, the design cannot be "industrial".  Ball bearings shouldn't be loaded that way, they wear out way too fast.

    Also, things like, exposed belt drives are ok.  But exposed gear drives are not, they really should be in a gearbox with oil.  

  • Well, seems like there's quite a bit of experience here. I'm still interesting in hearing what features make an "industrial" airframe better than say a hobby TREK 600 frame for instance.

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