Light weight planetary gear set 1

[Rob2010]

I'm new to gearing and have been doing a lot of reading. Any suggestions would be more than welcome.

We are designing a portable system for a military contract. Weight is very critical. In the system is a motor that generates 6 Nm up to 500 RPM. We are gearing this down to drive a cam that experiences up to 28 Nm continuous for 10 minutes at a time, operating on and off depending on load demands.

We've designed an 8:1 planetary gear set with MOD 2 profile, 16/50/116 tooth sun/planet/ring gears. Thickness is 12/13/12 mm to achieve adequate factors of safety. Planets will have lightening holes/pockets.

I'm interested first in possible finishes to protect the gear surfaces. We are considering PTFE filled hard anodize. Any thoughts or suggestions on that?

I'm also interested in a sanity check of our choices for gearing, particulalry if it strikes anyone out there that we're missing something.

Thanks in advance!

 

[TVP]

When you say PTFE-filled hard anodize, do you intend for the mating gears to be manufactured from some aluminum alloy? Generally speaking, aluminum alloys are not very good gear materials, due to their low strength and even lower wear resistance.

 

[Rob2010]

Thanks TVP,

Yes, for weight purposes, my intention is to use 7075-T351 Aluminum alloy gears. This is a hand carried military assembly and lightest weight is extremely important. With a ring gear approaching 10" OD it adds up fast.

I've discussed with some gear manufacturers and have had recommendations for a hard anodize and am investigating filled anodize (PTFE, graphite...) to try to avoid grease in the assembly.

I would welcome anyone's experience with light weight, aluminum (or similar materials) and surface treatments for wear on such materials.

 

[TVP]

If you are already in discussions with gear manufacturers, then I would concur with hard anodizing + PTFE as a starting point. The contact stresses will have to be very low, but the low friction PTFE surface will help with the sliding wear that typically limits aluminum alloys in gear applications.

With regards to the overall design, have you investigated the stiffness/deflection during operation? As you probably already know, aluminum has only 1/3 the stiffness of steel, so deflection on large diameter gears can be an issue. Also, what is the material for the pins that the planets rotate on? Aluminum certainly wouldn't be my first (or second, or third) choice for this part, for several reasons including deflection and wear.

 

[Rob2010]

Planets are mounting to stainless steel hubs using sleave bearings.

 

[tbuelna]

Rob2010,

While your tooth contact and bending stresses might be OK with hard anodized aluminum, you're transmitting a fair bit of power with limited lubrication, high frictions, and gear mesh combinations that will tend to produce some sliding. So you'll want to check for thermal related issues. Most aluminum alloys begin to lose strength above about 250degF.

If cost is no object, take a look at aluminum MMC or FRA for your gears. These materials have excellent wear and strength properties. But they're a bear to machine.

Good luck.
Terry

 

[spigor]

Have you considered plastics? We've had some very good results with them at low RPMs.

 

[Clyde38]

Titanium?

http://www.eng-tips.com/viewthread.cfm?qid=134760&page=28

[link

http://www.linkedin.com/in/clydehancock

]

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[sreid]

Consider talking to General Magnaplate about Aluminum Coatings.

http://www.magnaplate.com/