Nylon/Acetal gear lubricant choices?

[strokersix]

I need to specify a lubricant for gearboxes with plastic gears. Some of the gearboxes have acetal/nylon gear pairs and some are nylon/nylon all gears neat material. The gearcases are 30% GF polyester with ball bearings supporting the gears. Gearcases are sealed enough to keep dirt out but could allow some water intrusion. The overall configuration is crossed axis helical with 45 degree helix angles. Surface speed is in the 50 feet per minute range. Environment is outdoor machinery that needs to operate from 20-120 degrees F. Typical temperature 50-80 F. Gearbox operating temperature has been shown to be 20-40 degrees above ambient. Gearbox life needs to be 1000 hours continuous running with no additional lubrication.

We are currently using Mobil 28 grease which performs well but seeems to be separating. This causes undesireable seepage which is more of a cosmetic than functional problem but needs to be fixed. Can anyone give any general or specific recommendations on appropriate grease selection? I am interested in low friction to minimize gear temperature rise and eliminating the seepage problem.

Thank you.

 

[drwebb]

How about something like Silicone brake caliper grease? Pricey, but definately won't separate.

 

[romke]

i think you should consider a somewhat less consistent grease. the grease mentioned has a consistency somewhere between NLGI 1 and 2. I would opt for a NLGI 0 or 00 variety, with roughly the same characteristics as the grease currently used.

although at first hand it may seem strange to use a "thinner"grease when you already have some seepage with the current grease, you should realize that at the moment the seepage is due to "bleeding" of the grease (separation of fluid and thickenmer) and therefore, when you use a lower NLGI grade it might well be that seepage is less of a problem, since the oil stays better within the thickener.

there are specialized NLGI 0 greases on the marker designed for use in gears with non-oiltight housings.

 

[plasgears]

We have standardized on PAO synthetic, NLGI 2. Oil sep is minimal, and the VI is very good for cold/hot applications.

 

[strokersix]

plasgears,

Could you please share some of the issues that were considered when choosing to standardize with PAO sythetic NLGI 2? Also, what other grease types were considered and why were they not selected? What are the gear materials you are using? What is the gearing configuration and in what environment?

I have to select a grease for my application and to be honest, I don't know enough to make an intelligent choice. I do have a test stand under construction that will be used primarily for grease selection. The factors I have to consider are: Gear temperature rise and mechanical efficiency, dynamic drive system performance (positional accuracy), cost, and grease seepage through the case faying surfaces. We have evidence that grease selection has an effect on all of these factors.

Thanks in advance for your comments.

 

[plasgears]

Stroker,
For a slight increase in cost the PAO has superior VI, which means small change in visc with temp. Oil sep is nil. We use acetal, nylon, and PM gears. Mechanical effic is good for NLGI 2, and you may want an increment of tacky additive for open gear performance, ei, adhesion to the gears. Positional accuracy is best controlled by anti-backlash design.

We use Aerospace Lubricants in Columbus, OH. They serve a diverse customer base.

 

[strokersix]

plasgears,

Yes, it's a bit odd that grease would have an effect on system output positional accuracy but the application has some complicated dynamics that defy explanation. Grease selection has been shown to have an effect on output accuracy in this application.

I have already made contact wth Aerospace Lubricants per your suggestion and I will be getting some samples. A PAO thickened with PTFE NLGI 2 (high $$ stuff, unfortunately) has shown low temperature rise and no seepage but output accuracy has suffered. In contrast the Mobilgrease 28 NLGI 1.5 has greater temperature rise and exibits seepage but has better output accuracy.

I am very interested in some specific successful gear configuration examples. Gear types, gear materials, environment, duty cycle, etc.

Thanks.

 

[plasgears]

Stroker,
Some additional info. several years ago we produced a high volume snack vend actuator that was lubricated in a unique way. The gears were all acetal including the output gear. All gears except the output gear were immersion lubed with Mobil Extra Heavy petroleum oil, then centrifuged. Today I would use PAO oil instead. This met durability requirements, and leakage was not an issue.