Gear Fabrication

[bicolexpress]

Hi Everyone!
Our torque hub for a certain manufacturing machine broke and upon inspection all the gears - sun gear, planetary gears, ring gears has broken teeth. New OE unit is a made-to-order and will be available in 6 months +. We decided to fabricated (in-house) all gears since we have the capability (5-Axis CNC Machines and Wire EDM). Per our analysis, the OE used 8620 materials (carburized). But since 8620 is difficult to find here in the Philippines, we opted to use 4340 neutral hardened to 45-48 HRC. We were able to faricate all the gears and re-assembled the torque hub. It runs smoothly in production but lasted only in more than a month. All gear teeth were broken again. Now we decided to try to fabricate again all the gears using 4340 again but is planning to nitride the gears instead of neutral hardening.The supplied 4340 has 28 to 35 HRC. Anybody here with extensive experience in gear fabrication? Is our direction, i.e.,machine finish all the gears (with as suplied materials of 28-35 hrc) then suject them to nitriding will work? Thank you in advance to all who will respond to this query...

 

[Tmoose]

Nitriding is often too shallow for hardworking gears. I picture the hard candy shell on an M&M.
Carburizing can harden way deeper.

All the teeth broke?
I'd be thinking one tooth broke, and took out the rest.

Was the service life of the gearbox acceptable before ?
I'm thinking there may be a new overload condition in service.
What protections against overload are in place ?

 

[mfgenggear]

bicolexpress (Mechanical)(OP)26 May 21 00:43
Hi Everyone!
Our torque hub for a certain manufacturing machine broke and upon inspection all the gears - sun gear, planetary gears, ring gears has broken teeth. New OE unit is a made-to-order and will be available in 6 months +. We decided to fabricated (in-house) all gears since we have the capability (5-Axis CNC Machines and Wire EDM). Per our analysis, the OE used 8620 materials (carburized). But since 8620 is difficult to find here in the Philippines, we opted to use 4340 neutral hardened to 45-48 HRC. We were able to faricate all the gears and re-assembled the torque hub. It runs smoothly in production but lasted only in more than a month. All gear teeth were broken again. Now we decided to try to fabricate again all the gears using 4340 again but is planning to nitride the gears instead of neutral hardening.The supplied 4340 has 28 to 35 HRC. Anybody here with extensive experience in gear fabrication? Is our direction, i.e.,machine finish all the gears (with as suplied materials of 28-35 hrc) then suject them to nitriding will work? Thank you in advance to all who will respond to this query...

Bicolepress

We have no information on the stress and strain loads on the gear train. and what it does. and is a very complicated. first a failure analysis must be done to summarize why this gear train broke a second time.
Failed gear teeth is very bad and can cause damage to other components. there is a deeper issue.
gears are designed for the torque loads, and the RPM. a case harden gear is for contact stresses. to prevent scuffing and chipping. 8620 is for carburized case surface, to have high case harden surface, yet the core is at a softer core. if one looks at the heat treatment of a carburize gear time and temperature, the case will be 58 Hrc min and the core will be approximately 21-48 Hrc.depending on the temper range.
Gears must be inspected for several attributes. Involute error, inspects for correct Diametral or Module, and pressure angle has been machine. and requires an Involute Inspection.
secondly gears need to be inspected for total variation, and tooth to tooth variation. and pitch diameter runout.
4340 alloyed steel normally required a higher hardness than 45-48, should have been tempered to 50 HRc +/- 2 points. to reach higher stress and scuffing resistance.
but if the stress, strain values, and contact stress exceed the metallurgical properties then a case harden gear is required. I suspect that is why 8620 steel was used.

there are to many variables with out engineering drawings to give advise processing instructions. due to geometry of the gears.

 

[bicolexpress]

Hi Tmoose,

Thank you for responding.
Please find below answers to your comments/questions:

Nitriding is often too shallow for hardworking gears. I picture the hard candy shell on an M&M.
Carburizing can harden way deeper.
> Yes I understand that case hardening through carburizing will have deeper surface hardness. But per our local heat treatment contractors, they are not case hardening 4340 - they can only process it by through hardening. We also considered 5919 since it is case hardenable but the size of dia 450mm for the ring gears is not readily available and also no heat treatment contractors can give us how big will be the distortion is after hardening. Because we need to know what the expected deviations on the part so we can provide correct machining/finishing allowance that will also guarantee us a certain depth of case hardness say at least 0.5mm

All the teeth broke?
I'd be thinking one tooth broke, and took out the rest.
> Yes, this is also what we assumed... because with planetary gear assy, only one tooth broke then it will ruin everything.

Was the service life of the gearbox acceptable before ?
> I believe it was on service for more than 30 years. Our findings why it broke because the maintenance team forgot to monitor and maintain the lubricating oil level.

I'm thinking there may be a new overload condition in service.
What protections against overload are in place ?
> Currently the system has VFD.

 

[bicolexpress]

Hi mfgenggear,

Thank you for your comments and suggestions.

For now we already have a team working on the root cause analysis.

As for the gear nomenclature, we used gear design software, generated the 3D model and used it in CAM/CNC machining. For internal gears, we machined it through wirecut. I understand the accuracy of gear teeth profiles depends on the software and on the actual machining. We are not into mass pro of gears so we don't have gear shaver, hobber and gear grinder. We usually do lapping to smoothen gear teeth meshing.

 

[mfgenggear]

biolexpress

quote "gears - sun gear, planetary gears, ring gears has broken teeth "unquote

I really want to help you, being that your company is in a tough situation.
if the software produce accurate model
I believe because it was wired cut it was very accurate tooth involute profile. question is if was the correct geometry on the gear teeth involute.
if this is a high rpm gear set the involute can be important if have been a modified involute to not cause interference. if it is slow rpm not as important.
heat treating a ring gear thru carburize and harden can cause bad distortion, and can render the part scrap.
for a 2 parts up to 10 parts, a quench plug can be used, but a 17 inch (450 mm) internal ring gear will be very difficult.
for a first run a it may take 3 lots of two parts each to obtain correct size for gear grind, a carburized gear will have to much gear teeth distortion.
the first lot will have teeth rough cut, and only machine teeth in two parts. process through heat treat, measure the gear before after heat treat, roundness
must be held to properly grind the gear teeth, if to badly distorted the case depth and hardness will be violated. also the quench plug diameter will effect size.
the diameter of the quench plug will require adjustment. if the diameter is to small, it will require remake or repair to the correct size.
I could write a book on carburizing gears.
a through harden and then nitrided gear will not take the shock loads like a carburized gear , nitrided surface is hard, brittle and and will fracture easily.
but a very high for wear resistance., a better steel for nitriding is nitraloy 135 nitriding steel. it's made for this, it can obtain a deeper case
and the steel has a high ultimate and yield strength. .020 effective case depth can easily be obtained.
and has very very little distortion.

the core can be heat treated to 40 rockwell then semi finished machine, the bearing diameters can be ground after nitride. only a very small amount of stock
can be left on nitrided surfaces for grinding. as not to compromise the hardness and case depth.
a through harden gear will be easier to fabricate with wire cut but there may be issues with the core being to hard for shock loads, and the surface of the gear teeth
since it is not case harden have scuffing and wear issues, as you have found out.
the surface finish of the gear teeth is also important, must obtain a very fine finish of 16 micro or better of the gear train has high velocity.
but in either case will last longer. and there is a lot of analysis and free body diagrams in designing gear trains.
there is a lot more as well in manufacturing gears.

 

[3DDave]

One source of failure is having too small a root radius. A tiny radius is easily able to be made with EDM and can drive stress concentrations up 10X without much trouble, leading to fatigue fracture or just failure from momentary overload. Internal gearing magnifies the result if a fragment gets caught in a pinion - seen as a series of tooth destruction at the ratio of the gearing.

Gear crowning can be a help as well - moving the contact to the central area does potentially increase the contact stress, but it also prevents the tooth from getting stress concentrated at the free edge. You can see this in glass panes where a large force can be applied to the middle without damage, but only a small one at the edge can initiate a fracture.

Photos would help.

As to hardening? It doesn't sound as if the teeth wore out. I've seen gears function with nearly 60% of the original tooth thickness gone. It was a side effect of the customer demand for not using lubrication. Sigh.

So, if the teeth are powdered sludge, then surface hardening may help. If they are snapped off, surface hardening may not help unless it's to provide surface preload to try to prevent tiny surface cracks progressing in a compressive layer. But if it's thin, then that won't help much.

 

[mfgenggear]

3DDave a min. fillet rad is important and I agree, but I have seen carburized gears fail because of high torque and high rpm in short amount of cycles, same scenario as op.
wrong modified involute was the cause of failure. same with rapid gear scuffing.

 

[bicolexpress]

Hi All!

As I mentioned at the start of the tread, we tried already 4340 hardened to 45-48 HRC but the gears lasted only for more than a month.
We assumed only portion of one tooth of one gear broke but of coarse it affects all of the gears inside.
At 45-48HRC, 4340 might be brittle already for the purpose so we are planning to use pre-hardened 4340 (28-36 HRC) to maintain its ductility but nitride the surface for added wear resistance....

Thank you all to all your comments and suggestions.
They are all informative.
I will share with you guys whatever experience we will have on this proj...