Load Calculations For Dry Running Gears

[gearcutter]

Does anyone know of standards available for load calculations of non lubricated steel gears?

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

 

[gearcutter]

Due to the extremely low safety factors I'm calculating for the nylon pinion's resistance to surface & bending fatigue (around 0.14 for both) I doubt running the nylon with another material such as acetal is really going to make all that great a difference.
The customer has tried other materials to run with the steel gear in the past such as bronze, cast iron & resinoid materials. While these materials slightly improved longevity the overall cost in replacing all the nylon gears (around 50 of them) compared to continuing to use the nylon wasn’t worth the change.
Once again, I look at industry and see a large amount of unlubricated steel/steel gear sets running satisfactorily for years and I question how it is the designers calculated these sets. I’m surprised I can’t find any literature on the subject other than what is in literature on Tribology. This is probably the closest reference to dry running sliding surfaces I can find.
I’ll let you all know the results of the tests.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

 

[tbuelna]

gearcutter,

The limiting factor with your unlubricated, steel-on-steel gear mesh is not one of bending or contact stress. So core strength or case properties are not really an issue. With steel-on-steel contact and no lubricated interface, the coefficient of friction will be very high (ie. >.30). That means a large percentage of the force (work) being transmitted at the pitch line will be converted to heat in the gear teeth and rim. Unless you have some way of removing that heat, the (thermally isolated) gear teeth and rim will heat up and lose their temper, and ultimately ending up in a very soft, annealed condition. So unless you have designed your gears to have adequate durability in an annealed condition, they will likely fail fairly rapidly.

Having said that, to maximize life in a steel-on-steel mesh without lubrication, I would do everything I could to minimize friction. You could try low friction coatings like hard chrome, DLC (Diamond Like Coatings) or Titanium Nitride. You could also try superfinishing the gear working surfaces. I would also recommend modifying the gear geometry to get full recess action at the gear mesh. Recess action has less friction loss than approach action meshing.

 

[gearcutter]

I thought I might give you all an update on the non lubricated trial steel/steel pair.

I made a few changes to my original proposal:
Changed the material from carburised case hardening steel to a “Nitralloy” type high tensile steel that we had Plasma Nitrided to a depth of 0.50mm @ around 65 Rc.
No gear grinding after heat treatment.
Changed the geometry from near balanced specific sliding ratio, tipped toward recess action, to all recess action (200% addendum on the driver); fortunately the gears always run in one direction.

The result after around 200 hours of continuous operation are quite promising. The tooth surfaces have developed a highly polished finish with machining (hobbed) marks still slightly evident. The noise level is quite tolerable. No evidence of overheating.
The results are promising because the customer would have gone through at least 2 or 3 sets of nylon pinions over this time frame.
I’m hoping for 5000 hrs life or around 6 months of service.
I’ll keep you posted on the outcome.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[Matt51]

Ron,

This is truly fascinating, be sure to keep us posted on how the gears work.

Matt

 

[tbuelna]

gearcutter,

If you nitrided the gear, it is usually necessary to at least remove the "white layer" (gamma prime layer) produced during nitriding. It is very brittle and will usually spall if not removed. It can be removed with a simple honing operation.