Can you use Carburized Pinion with Nitrided Gear? High Speed & Power Application 2

[ewillia13]

First...I am not a gear design 'expert' and am even less of a materials expert when it comes to gear design, but I'd say I know enough to be dangerous.

So I'm looking at a high speed, high power helical gear application. Using AGMA 2001 and 6011 for ratings. Gear ratios between 7 to 22, pitchline velocity up to 32,000 ft/min, Diametral pitch between 7 to 12 teeth per inch, gear OD >50".

I am most familiar with using AISI 9310e carburized for the driven high speed pinion material and AISI 4140 or 4340 thru hardened to a core hardness of 320HB for the large driving low speed gear. I am also aware of nitrided pinions used with either a thru hardened gear or a nitrided gear.

For the special application I am looking at I need to pack a lot of power into an existing gearbox and the current thru hardened gear won't be strong enough. I can design a new gear but it has to fit in the existing gearbox. My first thought was to make a new gear and carburize it but it is to large to carburize (OD>50"), there would be to much thermal distortion. My next thought was to nitride the gear.

Nitriding isn't as strong as carburizing (allowable bending and contact stress) so the pinions have to be carburized.

Is anyone aware of using a nitrided gear (made from 4340 or 34CrNiMo6) meshed with a carburized pinion?? This seems to be the solution which provides the strongest gear and pinion but I am not aware of this being done.

Any thoughts would be much appreciated!!

 

[CoryPad]

I am aware of a planetary system that uses a nitrided 5130 steeel ring gear that mates pinions made from carburized 8620 steel. I think your proposal merits consideration.

 

[tbuelna]

ewillia13-

It is common with aircraft high-performance planetary gear drives to use carburized (9310 or X-53) sun/planet gears in combination with a nitrided (nitralloy N) ring gear. The main reason for using a nitrided ring gear is because the ring gear is often a large diameter with small cross section, and since there is no quench involved with nitriding there is minimal tooth flank surface distortion/displacement that requires correction by removing case stock during finish grinding. The reason this particular combination of carburized and nitrided gears works is because the tooth flank contact stress levels tend to be lower at the planet/ring mesh than they are at the sun/planet mesh, mostly due to contact ratio. So you can get away with a thinner case on the ring gear teeth.

One drawback with nitriding versus carburizing is that the nitrided case is usually much thinner, and the hardness of the nitrided case falls off fairly quickly below the surface. This means you must be very careful about how much material you remove from a nitrided surface.

With your particular case of a 7:1 ratio helical external (with DP somewhere between 7 and 12) gear mesh, where a 7.14" pinion drives a 50" gear, you can probably get a good match in contact fatigue life using a nitrided gear and carburized pinion. With a single mesh/single stage 7:1 drive the pinion teeth will experience 7x the number of fatigue load cycles as the gear teeth.

Lastly, you noted that the PLV in this mesh can reach 32K ft/min which is extremely high. In order to get your gear mesh to operate reliably at this very high PLV, you will need the highest quality gears (ie AGMA Q14 or better) and you will need to pay close attention to details of your gear geometry, dynamic tooth loads, and dynamic structural modes in the gear rim/web/shaft.

Hope that helps.
Terry

 

[ewillia13]

tbuelna: Thank you for the info regarding case depth. The gears are machined to AGMA Q13. The pitchline of this particular gear is more like 27,000ft/min which is still pretty high. I should have mentioned that this is an external gear but it looks like you assumed this already.

Traditionally the bullgear is made from a solid forging...these things are definitely not meant to go in an airplane, weight is never a concern other then how it impacts cost. The only difference compared to the planetary ring would be all the mass in the middle. Not sure if that would matter though, with regards to the heat treatment process that is.

Do you know of any references, such as white papers, that discuss or mention a gear design with a nitrided gear and carburized pinion?

You feedback is greatly appreciated.

 

[tbuelna]

ewillia13-

AGMA 911-A94 covers materials/heat treatments used for aerospace gear applications, including nitriding and carburizing.

I've attached an old chart listing recommended case depth-vs-DP for carburized and nitrided gear teeth, and also recommended contact stress limits for a range of fatigue load cycles. Remember that with your 7:1 mesh, the pinion teeth will be subject to 7X the number of load cycles as the gear teeth.

An excellent source of technical information on high-performance gear design is the NASA technical reports server. Try doing a search for "gear materials" or "helicopter transmission design".

Out of curiosity, if there is no weight consideration with your 50" helical bull gear, why not just carburize it? The quench distortion produced in a solid steel disc will be minimal. You will need to finish grind the bull gear to get the quality/precision level needed for this application regardless of whether carburizing or nitriding is used, and carburizing will cost far less than nitriding. It will probably require something like 30-40 hours in the nitride furnace to produce the .020" (plus grind stock) case depth required for a 7DP tooth.

Regards,
Terry