Spur gear excessive wear in harsh environment 4

[strokersix]

I'm having a problem with excessive wear on an agricultural machine design I'm responsible for and would like advice.

A pair of spur gears, 5/8 inch face width, 22 tooth meshing with a 36 tooth, 6 diametral pitch, 20 degree pressure angle run at approximately 200 RPM and 100 lb-ft torque are showing excessive wear. They are used on a piece of agricultural equipment and are run dry, thinking being grease will attract dirt and promote abrasive wear. Current design is 1045 steel, hardness 45-53 Rc, .060 inch minimum case depth for both gears. Life goal is 1000 hours. Wear of approximately .020 inch is evident at 200 hours. This wear has been observed on field durability machines and also on a lab test stand in a clean environment so I don't think dirt is a significant cause. I have begun testing some samples of 8620 material with Rc harness of 60 but they have not shown much improvement to date. I've also tested the 22 tooth 8620 and 36 tooth 1045 combination.

Can anyone give advice on my problem? I'll probably try greasing the gears if the 8620 doesn't solve the wear issue. I would rather leave the gears dry if at all possible.

Thank you, Mike

 

[strokersix]

An update on the situation: Austempered ductile iron gears did reduce the wear in this application, unfortunately not enough to justify the additional cost over the current steel gears. Greasing the steel gears works beautifully, even when dirt and sand are thrown in periodically. This was undesireable due to the mess it makes but necessary to make the gears last in the application. Recorded temperature of the ductile gears as high as 120F above ambient; recorded temperature of greased steel gears no more than 30F above ambient.

Thank you to all who took the time to share advice! I learned a bit about ductile iron in the process.

 

[Tmoose]

Various bronzes are thrown at friction or wear problems as kind of a standard solution. Also jacking up hardness to reduce sliding wear. The frequent references of Rabinowicz indicate to me if I'm lucky hardness can get me about 2x improvement, and dis-similar materials can get about 10X. But, just as you found, lubrication gets 100 to 1000 X improvement. Heavily Leaded bronze (SAE 660) might be interesting to try on one gear if it has enough strength. Of course the various dry lubes would probably enhance ANY design.

Why is enclosure impossible ?

 

[diamondjim]

strokersix,
thanks for the update
It is nice to see feedback
on someone working thru a
problem.

 

[strokersix]

Tmoose:

Enclosure is not impossible, just not practical in this application. The gears need to be readily accessible by an operator for maintenance (now requiring grease) and for swapping high/low range. The enclosure would have to be free, or nearly so, to justify inclusion on this machinery.

 

[gearman1234]

Hi Strokersix.

Here is some interesting information on

http://www.balzers.com/bhq/.

They have shown properties of different PVD coatings. Balinit C coating (WC/C coating) might be of particular interest to you. This coating is claimed to be suitable for highly loaded precision components with insufficient lubrication. They have shown some case studies where gears are run dry. Different mechanical and triboligical properties like coeff of friction of steel to steel in dry - 0.7, coeff of friction of WC?C and steel 0.2 etc. are also explained.

There is no harm probably in trying it. I have not personally used this but we use their TiN , TiCN coatings on gear cutting tools a lot and thought this might help you.

 

[Buhrmann]

You might have considered it, but why not use a hardened steel pinion against a PA66 (Molly filled) nylon gear? The self lubricating properties of the nylon, and running it against a hardened gear should give very good wear results.

Plastic gears are used extensively in the food industry where lubrication is a problem and cannot be used. Tooling up would be expensive, but would be cheaper than for sintered gears. Otherwise just cut it out of solid.

Regards,

Teo