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metal gear servos - press fit gears

Old 05-12-2015, 11:05 AM
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dbsonic
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Default metal gear servos - press fit gears

Foks,
Recently, I became aware of an issue, insidious in nature, that I would like to understand more fully. I've been in this hobby for a while and this is something I never realized or didn't think much about - in metal gear servos, many manufacturers press fit gears together to make the gear stack. This is even true of a few servos in 300 to 400 oz-in category from a few different manufacturers from what I can gather.

For the machinists and MechE types out there, what are you thoughts on this. Does this work for well for steel gears? But what about titanium? Can you press fit titanium well?
What about reliability for such an approach to making gears? difficult to QA?
and would this be more of an issue for 3d heli and 3d aircraft vs jets?
As we move up to HV servos with even more torque capacity at higher voltage, is this assembly method running out of steam?

The following is an old thread but illustrates one possible shortcoming to this assembly method and resulting failure mode( I am not picking on one manufacturer and don't want to see this turn into a mfg vs mfg war, I only point to this thread because it has pics and gets to the point. I use this manufacturers servos in the CARF Flash and trust them completely after hundreds of flights. There are other more recent failures with other manufacturers as it turns out..)

http://www.rcuniverse.com/forum/jr-r...r-problem.html

As with any type of model we don't want to find this out in the air...(should it result in a crash, you will probably never know the cause)

Last edited by dbsonic; 05-12-2015 at 01:18 PM.
Old 05-15-2015, 01:45 PM
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I guess from the response there is not a lot of opinion or experience with this in the jet community. That's probably a good sign.
From my research it looks to me like this has happened to virtually every manufacturers servos but rarely. Of those, it typically happens to the the 3D guys where extreme throws may be the reason.

I'll leave it with a pic of an old Futaba gear I tried to break in moment. it simply would not with something on the order of 15 to 20 ft-lbs before it slipped off the vice. that's like 3000 oz-in.
I popped it out with a hammer and punch after that.
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Old 05-18-2015, 07:01 AM
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Until you posted I had never heard of this. I am very glad that it seems to not be a chronic widespread issue.
Old 05-18-2015, 07:59 AM
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I guess Im not understanding what you mean by press fit? Gears should have spacing between them, the only permanent contact should be at the Pressure Diameter.
Do you mean press fit at the Pressure Diameter?
Old 05-18-2015, 01:21 PM
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No I think he means the interference fit between the pan gear and pinion gear as seen in the picture but obviously not assembled! Never seen this happen in jets, although there have been some issues with 3D flying!
Old 05-19-2015, 10:40 AM
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Correct, As it turns out a lot of the metal gear trains I have examined, either from the ones I have in possession or pictures of, have an interference fit between a small gear and one of larger dia as shown in the pics above.

The first reaction that most have (based on posts in flying giants and rcgroups) is surprise - to think this kind of construction being used for mg high torque servos. But I talked to a machinist friend and it is perfectly good and cost effective way to make composite parts that would otherwise be difficult and expensive to create in one piece. My test above shows it can be quite strong. In this case small gear left an impression on the other.. sort of interlocked to some degree. I could put it back together and it still engaged surprisingly well. In the reported cases of failed parts the smaller gear left no impression and would spin freely. There is an entire science behind this I guess the MechE will know about.
http://www.engineersedge.com/calcula...calculator.htm

The engagement length(between the 2 gears) is not a lot. about 1/16 or 3/32" for example in the case above. So, I think the issue comes down to tolerances. I would imagine +/- thousanth is not good enough. It would seem like precision to ten thousanth are on order here to avoid quality issues on the loose side and not being able to take on the designed torque.

Last edited by dbsonic; 05-19-2015 at 01:37 PM.
Old 05-19-2015, 04:14 PM
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I didn't know this was done until last month when flutter stripped a 500oz servo and I replaced the gears. I don't think this is an issues since the gears will break before you break the fit between the two. If I'm understanding this post?
Old 05-19-2015, 06:18 PM
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On correctly manufactured metal gear servos, I think most are expecting the failure mode to be... none. Expecting them to be bulletproof.
But in reality the failures can range from breaking teeth off any plastic gear in the gear train(some servo manufactures have a combination plastic + metal) to a failure in the electronics since the stalled digital servo will tank so much current and overheat/smoke it.

It's a good question, whether you can break a metal tooth off the metal gear before breaking the press fit of that metal gear stack? Keith, did you pull yours apart to examine what exactly failed on your servo? did you find broken teeth?

Last edited by dbsonic; 05-19-2015 at 06:21 PM.
Old 05-19-2015, 06:48 PM
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I did, the gear on the output shaft the one next to it had one broke tooth each.

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