Teeth gear ring cracks

twistair

Living in the Skies
Joined
Nov 24, 2003
Messages
1,161
Location
Kronshtadt, Russia
Aircraft
none own :( currently flying Calidus mostly
Total Flight Time
Don't know exactly, ca. 3000 hours
Got a message from an owner who recently purchased and started to fly an MTOsport and a Calidus. After few hours he found multiple cracks on the gear ring. They doesn't look like weared, just cracked like a glass. I wonder if anybody else recently got same problem and how it could be solved. Note that at one gear ring cracks are located at the lower and at another one similar cracks are at the upper side of the ring.



IMG-fb2b9450fb072dcc88b1af1f27e8eea2-V.jpg
 

Attachments

  • IMG-575b114360178f1bd5b0d72a539728ef-V.jpg
    IMG-575b114360178f1bd5b0d72a539728ef-V.jpg
    71.2 KB · Views: 6
  • IMG-bb161d1d857faa257aa881b0cd4b7e75-V.jpg
    IMG-bb161d1d857faa257aa881b0cd4b7e75-V.jpg
    69.4 KB · Views: 6
  • IMG-575b114360178f1bd5b0d72a539728ef-V.jpg
    IMG-575b114360178f1bd5b0d72a539728ef-V.jpg
    71.2 KB · Views: 6
  • IMG-bb161d1d857faa257aa881b0cd4b7e75-V.jpg
    IMG-bb161d1d857faa257aa881b0cd4b7e75-V.jpg
    69.4 KB · Views: 5
  • IMG-bb161d1d857faa257aa881b0cd4b7e75-V.jpg
    IMG-bb161d1d857faa257aa881b0cd4b7e75-V.jpg
    69.4 KB · Views: 5
  • IMG-575b114360178f1bd5b0d72a539728ef-V.jpg
    IMG-575b114360178f1bd5b0d72a539728ef-V.jpg
    71.2 KB · Views: 6
  • IMG-5ce368b2819816725b34b329e1766c7f-V.jpg
    IMG-5ce368b2819816725b34b329e1766c7f-V.jpg
    97.4 KB · Views: 5
  • IMG-3c34d989908bfab61257adc00a8ce7e1-V.jpg
    IMG-3c34d989908bfab61257adc00a8ce7e1-V.jpg
    93.3 KB · Views: 5
  • IMG-fb2b9450fb072dcc88b1af1f27e8eea2-V.jpg
    IMG-fb2b9450fb072dcc88b1af1f27e8eea2-V.jpg
    98.3 KB · Views: 5
  • IMG-09a8b86519b7e8abca762806a533e82f-V.jpg
    IMG-09a8b86519b7e8abca762806a533e82f-V.jpg
    88.7 KB · Views: 5
  • IMG-5ce368b2819816725b34b329e1766c7f-V.jpg
    IMG-5ce368b2819816725b34b329e1766c7f-V.jpg
    97.4 KB · Views: 5
  • IMG-3c34d989908bfab61257adc00a8ce7e1-V.jpg
    IMG-3c34d989908bfab61257adc00a8ce7e1-V.jpg
    93.3 KB · Views: 5
Last edited:
Twistair
I'm currently struggling with my ELA prerotator and since the MTO is an ELA clone I'm interested in your problem.

Looking at your photos I cannot see any cracks, is this a problem of translation (I doubt it because your english is excellent) or just a problem of photography (or my failing eyesight)?

Could you try to point out the cracks?

My problem is that (in my opinion) in my brand new rotor head, ELA haven't drilled the holes for the main bearing shaft and the bendix bearings to the correct dimension. This means that there is an excessive clearance (1 to 1.5 mm) between the ring gear teeth and the pinion teeth resulting in one wrecked bendix pinion and one wrecked ring gear.

During my investigations I discovered that, unlike the ELA, the MTO rotor head allows you to adjust the bendix to ring gear clearance (the Germans obviously saw the problem and didn't copy this lack of simple engineering as blindly as they are often accused).

Your ring gear looks similar to mine except that I haven't found any cracks (it only did half a dozen prerotations before the ring gear teeth were damaged and the bendix pinion gear cracked).

Perhaps you could start by looking at this clearance.

Mike G
 
It's likely my clumsy translation, Mike, sorry - I meant,.. well - chinks or chips, in other words, small particles that split off a number of teeth. I saw some damaged German gear rings earlier - some due to misplaced bendix, but there were no lost parts of gear teeth, just extreme wear. In this case damage looks like the teeth are too brittlle (or fragile?).
 
Twistar, I appears to me to be a coating applied to the gears to improve the wear characteristics. Sometimes these applied coatings do not adhere properly to the
base metal. Hence the appearance of chipping. Should be replaced under warranty I would think. I don’t think it is likely that the gear would fail catastrophically, but I am no
Gear expert either.

Dave
 
Contact Autogyro and they can help determine if it’s a warranty issue or a clearance adjustment issue. There were a few Cavalons with soft rings, but they addressed the defect in the process. Perhaps it’s from that batch?
 
Alex
It doesn't sound like my problem. I have metal being rubbed or "smeared off the teeth. This was after a couple of pre rotations.DSC02936 1.JPG
Mike G
 
Yep, Mike, that's opposite problem - too soft teeth. BTW, I recently had a chance to fly new MTO-2017 - it also had too soft teeth which crumpled after just couple prerotations. Germans sent new gear ring. Surprisingly it has same diameter but 3 teeth more. I cannot chew what this change was for - there were no comments from there.
 
Last edited:
I have seen this befor and it usually has to do with the bendix not meshing properly with the ring gear. Its an adjustment problem or some times pneumatic.
 
Desmon, did you mean improper rotorhead geometry? I thought there is no adjustment for bendix there other than slight axle up'n'down movement.
 
Alex
I discovered by talking to a guy at Autogyro that the main bearing axle is bolted on to the rotor head (the part made of two large flat plates welded together) with a bolt that goes into the axle from underneath. The location hole in the rotor head for this axle is 1 mm oversized so you can move the rotor axle back and forth to make the ring gear approach the Bendix pinion gear and hence set the right clearance for the gear teeth. Crude but effective. Better than ELA who don't give you any means to set the clearance. I thought you knew this judging by your reply to my first post on this thread but reading your latest post I'm not so sure.

The number of teeth is interesting. I know of one European manufacturer who used the Tom Milton Bendix (I'm pretty certain it's the one you referenced in my previous thread about Bendix ring gear distance) and he had teeth failing untill he realised that the ring gear he was using was metric!!!!

I heard of another manufacturer who used the Tom Milton Bendix and the ring gear off a Skoda.

Looking at the Autogyro stuff it looks like they make their own Bendix and ring gear so one would hope that these sort of mixed designs wouldn't occur.

Mike
 
Mike, that's very valuable info, thank you. I didn't know about this adjustment since never disassembled German main bolt. Now we have two possible things which could cause the damage and will wait for the owner sending these parts for inspection.
 
To me it appears the large ring gear has not been heated treated, in fact it appears it was not heat treated at all, no way would a gear tooth mush out of shape that much with only two or three prerotation cycles if it were heat treated.
 
WHUBBS;n1125759 said:
To me it appears the large ring gear has not been heated treated, in fact it appears it was not heat treated at all, no way would a gear tooth mush out of shape that much with only two or three prerotation cycles if it were heat treated.

There is at least one more possible way for this: if the steel used isn't treatable at all. I met this once when we've got bolts made by order but half of them appeared to be soft like not treated while they looked absolutely same like "true" treated and cad-plated bolts. Short investigation showed that they were made of low carbon steel which couldn't be treated at all. This is why we always perform at least selective hardness test for each bunch of critical hardware.
 
Last edited:
Yes it is also possible the steel is not proper for heat treatment, Three things are possibly going on that are not correct, improper steel type, not heat treated or improper temp.. also the gear that is chipping at the bottom of the tooth is so hard it is brittle and as the bendix gear strikes the ring gear at the bottom first it is chipping the tooth before the bendix moves on up for full engagement. Test if you try to file one tooth of that ring gear that appears to have worn a lot in only two pre-rotator cycles, you should be able to see filing dust fall. Then use a file on the gear that is chipping you should not be able to even scratch the tooth as it is so hard. The gear manufacturers must know what he is doing for the bendix and ring gear to wear properly as far as material and heat treating.
 
Yes you have another good point on bolt metallurgy, hardware store bolts are not certified to meet aircraft quality bolt standards and I would never use these bolts in my aircraft, Aircraft Spruce has certified hardware of all types and for the most part will have the manufacturer brand stamped on the head of the bolt. These bolts are then used in certified aircraft.
 
I have my ring gears made for me, they are the standard Wunderlich ring gears in use since the 60's. The teeth are hardened to a Rockwell number I can't remember. I also have my Bendix drives made to my specs. The pinion gears on the Bendix drives are softer than the more expensive ring gears on purpose. A Wunderlich Bendix pinion or ring gear has never, to my recollection, stripped or damaged their teeth. I believe it is because the Wunderlich upper units are NOT adjustable and have a very close mesh. Wunderlich Bendix pinions have worn the teeth down on almost every other style of upper unit I am aware of. Most American manufacturers use, or have used, the Wunderlich Bendix and ring gear. I have offered to help those manufacturers with their design, but all were set in their ways. Gears are always trying to push themselves apart under load.
 
Tom
If you have the ring and pinion gears "made to specs", you, or your machinist, must have the dimensions for the pitch circle diameters. With that I can work out the design distance between shaft centres that I asked for some time ago to resolve my long running argument with ELA
If you can remember or find the Rockwell hardness I'd appreciate that as well. I'd like to test mine to see if when ELA weld the ring to the disc they change the characteristics.
You say “I believe it is because the Wunderlich upper units are NOT adjustable and have a very close mesh.”
I don’t understand because the Wunderlich upper units (Bendix??) are bolted on to the rotor head, so are, by definition, adjustable depending on the distance between the rotor main bearing centre and the holes for the Wunderlich upper unit or the clearance on those bolt holes.
When people build their own gyros and buy one of your units who gives them the dimension for drilling the holes for the Wunderlich upper unit??? .
I understand if you don't want to put this data on the forum but please pm me or email me.


Alex
Are the teeth on the AutoGyro ring gear machined into the disc or fitted to the disc by welding, press fit, riveting etc. ???

Desmond
You never answered Alex’s question, what did you mean exactly by:
“the bendix not meshing properly with the ring gear” why and how and what solution did you find?
“ Its an adjustment problem” is that different to “not meshing properly”? What did you adjust and how did you do it?
“ or some times pneumatic.” Exactly what pneumatic problem causes this and what are the solutions?

It doesn't help if you don't give more detail.

Mike G
 
Guys:
I don't know if this will help you but AutoGyro bendix gear that moves up is from the auto industry and used in cars and many of them are mostly made in Brazil from what I had found 2 years ago. They probably make their own ring gear and from the looks of it they either use EDM or water jetting. We make our own ring gears as well using EDM and then case harden them to HRc 45 to 55.

In reality in SAE applications the ring gear is not made like that because a lot of material is wasted this way and their runs are huge unlike us. Instead a square flat medium carbon steel bar is used and the teeth are formed and then the bar is cut to size and folded in a circle and welded to form a ring and then normalized and the teeth hardened to 45-55 HRc.

The Bendix we use is different from AutoGyro, made for a Lycoming starter but without the spring inside.

From what I can gather from the pictures, it seems the ring gear in this thread was not hardened properly and the layer is getting chipped and deformed in short time Usually you would keep the bendix side softer than hardened ring gear as its much cheaper and easier to replace. It is possible that the material cannot be properly hardened because there isn't enough carbon in it or it simply was not hardened enough. The specs should give a range of 45 to 55 HRc and the hardening process is well defined by standard organizations. Obviously the hardening facility definitely did not test the batch for hardness specified. That's a QC issue.

For the AutoGyro ring gear just from the pictures it may also seem that their gear pitch is not standard but that may not matter in this case.
https://www.engineersedge.com/gear_formula.htm
 
Last edited:
Mike G;n1125452 said:
Alex
It doesn't sound like my problem. I have metal being rubbed or "smeared off the teeth. This was after a couple of pre rotations.



Mike G

Mike G. This gear is not hardened enough. You can test with a Rockwell hardness tester. It should be 45 to 55 HRc. Also this does not even seem like EDM or forming or hobbing processes which are common for gear making. Its too rough a finish. May be water jetting or an old EDM. Welding the hardened ring gear to the flex plate will nt necessarily take its hardness away. We weld it post hardening as well in short welds but at 10 different places. You have to weld 180 degrees apart to keep warping under control.
http://www.hardnesstesters.com/test-...rdness-testing
 
Last edited:
Fara
Thanks I know I can test the hardness, that's what I plan to do, but I would like to know what hardness Tom specifies since ELA use his ring (and pinion) gear so this plus the other that is damaged are his I'm still convinced that the problem comes from a machining error by ELA but they have now stopped answering my emails.

How many teeth are there on your Bendix pinion?

You also say you "remove the spring", I've heard this before,what spring do you remove? Is it the one in the photo?
Bendix open resized.jpg
Mike
 
Top