Type III (Hard Anodize) Finish On Gear 1

[JonnyK]

I have an application with a gear on a human interface device that currently uses a Type II anodized finish. However, after some testing under heavy vibration there was excessive wear on the gear face (finish was completely removed).

I am working on several solutions (including addressing the assembly dynamics), however I am also considering switching to a hard anodized finish on the gear. I noticed in our internal spec it says to mask threaded holes unless explicitly stated otherwise. I am assuming this is due to the increased thickness of the finish.

My question is if threads are sensitive to hard anodizing, would a gear profile also be sensitive to this finish? I would assume the answer is yes, but I wanted to get some feedback from other engineers befor shooting this idea down.

My gear quality is AGMA 9 (TTCE=.0007 & TCE=.0014), and the gear is driven by a human finger. No large torques or high speeds to deal with. Backlash is a concern from the stand-point of “feel”.

TIA

 

[israelkk]

If it is "driven by a human finger. No large torques or high speeds to deal with" how it experienced "heavy vibration"?

Are these custom made gears or are catalog gears?

If you will check all catalog precision aluminum gear (PIC, Berg, SDP, etc.) you will find that the teeth are not coated.

First, it will practically hard or impossible to get the AGMA 9-10 accuracy after anodic coating.

Second, the anodic coating is a thin hard and brittle layer and the aluminum hardness underneath is not enough to support it under the contact (Hertzian) loads.

To my opinion and experience the Type II (Sulphuric anodize) or type III (Hard anodize) are not appropriate for gear tooth coating because the contact between tooth are not pure friction but contact Hertzian loads that will break the coated layer.

 

[TVP]

Hard anodizing is typically in the range of 35-65 micrometers thick. Does this sound too thick for your application? I agree with the comments by israelkk regarding anodic coatings and gears-- this is not a good combination for contact stresses, bending stresses, etc.

 

[drawoh]

JonnyK,

I am not sure about hard anodizing, but most other anodizing processes result in a net removable of material. This is due mostly to the acid bath they use to clean the parts. Fabricators take this into account, which is why you specify that dimensions and tolerances apply after finishing.

I would be surprised and impressed if a fabricator and anodizer could maintain AGMA_9 tolerances after hard anodizing.

Does the gear really have to be aluminium?

JHG

 

[unclesyd]

I would give Tiodize a call to discuss your requirements. have a print or sketch ready. You may not need a new anodize only improvement in lubricity to extend the life of the existing anodizing. If your working envelope allows an addition of Teflon would be an immense help. The addition of a dry film might work.

I'm also trying to locate a supplier that did a lot Electroless Nickel on Al for us.

http://www.tiodize.com/aluminium_anodizing.html

 

[mfgenggear]

maybe use bronze gears instead of aluminum

 

[unclesyd]

If you stay with Aluminum you may want to consider this coating from Sub-One.

http://www.sub-one.com/diamondoidCoatings.html

 

[JonnyK]

Vibration comes from the environment; the assembly is installed in a fixed wing cargo/transport aircraft. Although the testing that destroyed my gear was part of accelerated life testing, and with that said the levels were much higher than those actually expected in the aircraft.

The gear is custom and the only reason it is aluminum is so that it could be anodized black (kind of ironic reading all these responses). The gear (and teeth) need to be black per our requirements. I thought anodizing was a better solution than painting. But if there are other alternatives for getting a robust black finish on a gear of any base material, please let me know.

I will look into the dry film coatings on the anodize, but I doubt the Sub-one coating will work. If our approved gear suppliers don’t have access to a particular coating, for propriety reasons, then it will most likely be a no-go since we cannot eat the cost of any additional fees they would occur by using such a technology

 

[drawoh]

JonnyK,

Could you use a black material, such as black plastic? The plastic is not as strong, but you can get it fibre reinforced. A plastic with a low coefficient of friction like Nylon or Delrin might perform better in wear conditions.

JHG

 

[JonnyK]

We had considered plastic gears early in the design, but aluminum chosen as a more robust solution. Based on the test data there may be argument for reevaluating that decision. Are there any concerns or limitations with a plastic gear driving a steel pinion?

 

[dinjin]

If you are experiencing a lot of vibration, can you eliminate the backlash and have the mesh under load?

What size module or dp are these gears?

What aluminum are you using?

How wide is the face?

What lubrications are you allowed to use?

Many coatings can have a supplementary dye
added to get a black finish.

 

[JonnyK]

I don’t know how to get rid of any more backlash in my design without going to a more complicated design (although I am very much inexperienced with gears). I have done what I could from a dimensional and tolerance stand-point to manage backlash while still making it producible. To implement some kind of anti-backlash method I would have to add more gears or additional mechanical parts, at least that is what I found during my research, and am I too far into my development to do that (due to space, cost, and assembly constraints).

The gear in question has a DP of 80 and 135 teeth, it is ¼ inch wide and made of 2024-T351, tt drives a SS303 gear with that is 1/8 inch wide.

I don’t know if I have any limitations on lubrication other than 1) certifiable for aerospace use and 2) doesn’t leave residue on the operators fingers.

I started Googling for black coatings on SS and my consider going to a SS because it may fix some other problems that I didn’t mention. It was a combination of mechanical failures in the gear that led to the problem stated in the OP. Found a MIL-C-13924 Black Oxide Coating for SS…going to look into that as well.

 

[unclesyd]

I would contact Nye to discuss the use of Peenzane based lubricants for your application for lubrication suitable for aerospace applications.

http://www.nyelubricants.com/pennzane/

For the 303 SS gear I would give Nitromet a call and the discuss the possibility of using the QPQ process. This will give a black finish and one of the best antigalling and antiwear finishes around.

http://www.nitromet.com/

There are some other possibilities also.

 

[sreid]

Black Electroless Nickel Plating might be the answer. You'll get the black color you need and a hard surface on the gear teeth.