Planet Gears Self Destruction 1

[mfgenggear]

This is a general questions.
and it may be an elementary question.

what are some of the reasons a set of high RPM planetary gear train will self destruct. where the teeth will be totally destroyed.

overload?
metallurgical & case hardness values?
balancing?
contact ratio?
or what?

where is has worked for extended long periods then there is a drastic failures.

Mfgenggear

 

[tbuelna]

mfgenggear,

Your description is a bit short on details, which makes it hard to even speculate. The only insight I would offer is to consider the tooth bending fatigue condition that exists in planets gears. They are similar to idler gears in that the teeth see full reverse bending each rev, which leads to reduced fatigue life.

High rpm planetary gearsets can experience lubrication-related issues, but these problems are more likely to affect the planet bearings/races and not the gear teeth.

Hope that helps.
Terry

 

[leisure17]

if it's worked for a long time then it is probably not the design of the gearbox but some other factor that you & Terry mention.
You'll have to look closely at all the failed parts and see if you can spot a reason.

 

[mfgenggear]

sorry for the too brief of explanation.
The metallurgical looks good
the gear teeth are mfg like master gears.
the design is old, & has worked for years.
bearing races clearance may be excessive, between the shafts or maybe not mfg correctly?
parts are weight balanced but not dynamic balanced(2 plane).
the gear teeth will sometimes self destruct within a short time.

I can not give the gear details because of propriety reasons.

Thanks

Mfgenggear

 

[MikeHalloran]

My friend Wayne spent WW2 rebuilding planetary speed increasers for turbosuperchargers, including selective fit of planetaries for tooth thickness to meet the backlash specs.

He said the activity mostly consisted of fitting new internals to repaired housings, because even tiny failures rapidly progressed into complete self-destruction of all the gears and bearings.

I.e., you _might_ have a chance of doing a forensic investigation, only if the units are removed from service the first time _anything_ shows up on a chip detector.



Mike Halloran
Pembroke Pines, FL, USA

 

[mfgenggear]

Mike

there was detection of chips, when the gear teeth would start to fail.

mfgenggear

 

[tbuelna]

mfgenggear,

If the ferrous debris is from the gears (and not the bearings), it may be from contact pitting/spalling. Excessive contact stress can be caused by misalignment, lead error or torsional wind-up, dynamic loading, poor load sharing between planets, etc. Getting planet gears to load share properly also requires a design with the correct degrees-of-freedom or constraints. Once you go beyond 3 planets, the problem becomes progressively more difficult.

Since the problem did not occur with previous gearsets of the same design, it sounds like it might be a manufacturing tolerance issue. While you noted that your gears have a very high quality class, the carrier and housings must also have tight tolerances. Otherwise the planet gears will not load share.

In my opinion, I don't see planet gear dynamic imbalance being the source of your problem, even with high rpms. There would have been very noticeable vibration during operation even with dynamic imbalance levels below that which would cause the kind of gear failures you describe.

Hope that helps.
Terry

 

[Richar24]

A few simple suggestions, The gear cutters have been known to leave radii off the gear form cutters. Look at the minor diameter of the gears and see if you have sharp edges (their should be a radius).

Possibly a bad lot of steel? Have they changed their supplier.

Are gear teeth overly hardened as opposed to case hardening? I had a lot of stainless steel gears that were overhardened, if you droppped one on the floor they shattered like glass.

Contamination during assembly, Excessive backlash, Are shaft ends supported correctly?

Run Magnetic particle test on gears prior to retesting to ensure that there is nothing wrong with the gears. If you are dealing with a gear dealer, did they change their supplier? Is the supplier now in a different country?

Richar24

 

[mfgenggear]

Terry

You bring up some interesting facts.
If I understand you correctly
if the carrier is not square or true position not correct the
planets will have excessive edge contact.
there causing premature failure.

Richar24

you bring up some good information.


Mike H

can you please explain the planet matching.
and what type of small defects.

Thanks All
Mfgenggear

 

[tbuelna]

mfgenggear,

You understood correctly. It doesn't take much misalignment along the tooth face to cause edge loading and excessive contact stress. Face misalignment in highly loaded planet gear meshes is a very common problem, especially with wide spur gear faces. One of the biggest culprits is torsional deflection in the carrier structure, when the carrier is used as the output member. That is why you see some planet gears mounted on spherical roller bearings.

Terry

 

[monkeydog]

I had a supplier change from cut gears to powdermet gears within their product (motor). We were not notified of the change. We started having motor gears "self destruct" just before acceptance testing of the next higher assembly. Turns out the powdermet gears were a little more brittle then the motor manufacture anticipated.

If you don't have direct insight on the gear material/fabrication you might dig deeper in that direction.

 

[mfgenggear]

Terry

Thanks for the input.
it must be a tolerance issue with wear.
because it happens on used units with replacements.
oops sorry left this info out.
It seems to me that a backlash test would discover this issue.
in other words in-consistency with backlash on each planet gear.

mokeydog

thanks for sharing your experience, the gears where destructive test to make sure it was not a material, heat treat issue.

Mfgenggear

 

[loki3000]

were the gears heat treated? (if they were not only on the teeth sides, but entire planet, you can check the hardness)
which material was used? A good one for gears is 16MnCr5 for example.