Rules of thumb for splash lubrication in gearboxes

[EdBundy]

Good day to all

I'm busy designing a small 12kW reduction gearbox and due to space constraints will have to rely on splash lubrication. Are there an rules of thumb to adhere to regarding splash lubrication which you wise folks would mind sharing?

The gearbox is a 4:1 reduction box (110x120x180mm) so will have obvious cooling issues as well. Problem is, by the time the oil level reaches the input pinion (mounted below the gear, and rotating at 8000RPM) the bearing and seal are already partially submerged. I've read elsewhere that submerged bearings = bad idea + heat. The duty cycle of the gearbox can be altered to regulate the temperature, but this is a last resort.

Also, at the speed that the pinion is rotating, would it be effective lubrication or will it just result in a foamy slick in the gearbox?

Any insight in this area is appreciated - there is not a lot info on this topic online...

Regards,
Ed

 

[tbuelna]

EdBundy,

Gearboxes can be made to work acceptably with splash oil lube systems. And as you noted, keeping gear teeth or rolling element bearings with high peripheral velocities above the sump oil level is desirable. If gear teeth or rolling bearing elements are splashing through liquid oil at high velocity, it can create significant windage losses, and also increases heating of both the oil and the gears/bearings themselves.

With your splash lubed gearbox, any heat generated in the gear mesh or bearings must be transferred to the lube oil, then from the lube oil to the housing wall, then across the housing wall thickness, and finally from the outer housing surface to the ambient air. Having a housing material (like aluminum) with good thermal conductivity is helpful in this regard.

As I mentioned, keeping the gear teeth and bearings above the oil level is desirable. But there is also the need to maintain sufficient oil flows over the gear teeth and bearings to cool them. So some splashing or windage of the sump oil would obviously be necessary. A good approach (if possible) would be to design the inside of your housing with guide vanes that direct the windage flows into scuppers, above the gear mesh location and bearings, that collect the oil and then dribble it in a somewhat "controlled" manner to precisely where it is needed.

Hope that helps.
Terry

 

[EdBundy]

Hi Terry

Thanks for the info. We are indeed making the gearbox casing from aluminium, for mass and heat transfer purposes.

We have sized the bearings to give appropriate life and unfortunately, for splash lubrication to even be possible, the oil level will completely cover the lower rolling elements of the bearing.

Another alternative is to use less oil but use flinger discs attached to the pinion, with grooves/scuppers for distro - it all takes space but hopefully the positives outweigh the negatives. I have no experience with flinger discs at 8000RPM so will have to experiment first. An Alu disc will be strong enough but it remains to be seen how much oil can be distributed.

Regards,
Ed

 

[tbuelna]

EdBundy,

If you want to do some research, take a look at intermediate (IGB) or tail rotor (TRGB) gearbox designs on helicopters. Many of these gearbox designs are splash oil lubricated. And they also run at some pretty high speeds and horsepower. Most also use spiral bevel gearsets and rolling element bearings.

http://incolor.inetnebr.com/iceman/data/90box.jpg

Good luck.
Terry