Superfinishing 4

[geesamand]

I've read some recent articles on superfinishing of gears and bearings. It seems the academic studies are showing substantial improvements and it's not a gimmick. I have a few questions for those of you who have used it:
1) Does superfinishing benefit parts that have already broken-in to a decent polish, or is it mostly of value for OEMs who need to get maximum efficiency with minimal break-in?
2) Can superfinishing be used in conjuction with shot peening and not causing excessive damage to the gear profile?
3) It would seem that chemically accelerated vibratory finishing is being used for these types of parts. Is that the state-of-the art in superfinishing for a reasonable cost? (My intended use is racing, but my budget is more value-driven)

 

[Lowlife]

Our results were not done using a 30 minute oil out run, but a 2 hour aux lube test, based on our transmisson design. After completion of all testing, the power gears and most accessory gears were Magnetic Particle Inspected, with no defects found on any teeth. Visual inspection of the power gears were also performed and found no significant surface distress despite the severe operating conditions. In all cases, these parts showed no evidence of scuffing and no excessive pitting at the end of two hours of reduced lubrication testing. For this testing, test success is defined as completion of the two hour run with a transmission that is fully functional at the test
termination. The transmission was run at 50% torque for 2 hours using only auxiliary oil for lubrication. The oil in the sump reached a maximum temperature of 284.4°F during the test. An oil sample was removed from the transmission after the auxiliary oil testing. Subsequent laboratory evaluation of the oil sample indicated the transmission was operating in the normal range, at the end of the 2 hours auxiliary oil testing per NAVAIR document 17-15-50.3.

 

[tbuelna]

Lowlife,

If your gears (and bearings) survived your qualification test procedure that ran 2 hours on an auxiliary lube system, then it sounds like your auxiliary lube system design is good. Designing an auxiliary lube system that will function for 2 hours is no easy feat. The 30 minute auxiliary lube systems I have seen are normally total loss systems. Usually with some type of air/oil mist delivery to the critical bearings and meshes. I can only imagine the amount of auxiliary oil required for a 2 hour system. I'd be very interested to see the details of your system.

A total loss of lube condition will normally result in gear scoring, due to the fact that there is insufficient oil present to produce a satisfactory EHL oil film to support the tooth contact loads. And severe scoring will produce catastrophic gear failure within a few minutes, not hours. With no oil present, an ISF'd gear flank with a surface roughness of 2AA will fail in scoring almost as quickly as a conventional ground tooth flank with a finish of 8 AA.

I started to read that NAVAIR oil analysis paper you noted, but it is 300 pages so I didn't get through it all. I haven't seen it before, so thanks for the reference!

Regards,
Terry

 

[mfgenggear]

tbuelna

Very impressed with you back ground and discussion.

To all others this is very good, good Topic & very good discussion & Imput.

Generally as a gear manufacture, we give our customers what they want.
However I discourage any process that effects the tooth Involute, & size, It’s not uncommon to Grind gears upto AGMA class 15. However, It’s not a lot of tolerance to play with.

If a gear is finished after heat treat. there should not be any pits or corrosion
If manufactured properly.

 

[tbuelna]

mfgenggear,

"Generally as a gear manufacture, we give our customers what they want."

Sounds like you know how to keep your customers happy! But of course, as an experienced gear house, I'm sure you still occasionally help your customers "decide" what they really want, right? Experience is probably the most valuable commodity with regards to gear design and manufacturing.

The AGMA quality classes are mostly defined by allowable profile, lead, pitch and index errors. I don't believe working surface roughness is specified for the various classes. The tight pitch and index tolerances required by the higher AGMA quality classes (ie. 11-15), while costly, are definitely necessary for meshes operating at high pitch line velocities. Any profile, lead, pitch or index errors can significantly increase dynamic loads at the mesh.

You stated that you would discourage any process that affected tooth profile and size. However, for the gears I design (mostly high performance aerospace power gears), virtually every gear profile has both face and tip profile modifications. We also sometimes use pitch diameter and addendum mods to achieve a full recess action mesh. Each gear drawing we produce has very detailed profile charts defining the tolerance limits we require for the profile mods that deviate from a true involute profile.

Producing high quality gears now days is much easier than it used to be. Especially with the current models of NC gear grinders that can grind the teeth, dress the wheel when required, grind the adjacent bearing journals, and continuously inspect the teeth, all without removing the gear from its set-up on the machine.

Regards,
Terry

 

[mfgenggear]

Tbuelna

You are right! most of our customers designs are with modified involute, generally very close tolerance .0002 -.0003 tolerance. In addition also have crowns on their helical gears.

I am amazed at some of the gears we manufacture for Helicopter Power Trains, where designed & manufactured in the late 1950s & all of the 1960s.
I tip my Hat to those that could design & manufacture such tight tolerances with the technology available at that period. I can only imagine the complexity of the designs.

These gears even today if you do not have the knowledge
are really easy to scrap. It takes years and years of experience.
Heat Treat is always the key and if you make it through Heat treat it’s a mile stone.

You can have the newest machinery in the world but that will not help you though heat treat. Believe me when I say this I know.
But it will help with efficiency & quality once the gears make it through heat treat.

You the designers can also make it tough or semi tough on your designs. But Generally Most gear designers have it on the ball and do make it easier for us who are doing the manufacturing. : >)

Keep up the good work

Take Care