The Home Machinist!

TL;DR - So, what is the purpose of packing it 2/3 (or whatever) full? Once the gear throws all of the grease off of it, the grease is never going to just migrate back. Seems like an awful waste!


I am getting ready to put the top of my newly (last summer) acquired mill back together so I can finally put it to use. I still need to clean the pile of chips out of the knee, but the power feed is repaired, the quill return spring is repaired, and I have the metric dowel pins I need for the top cover of the backgear.

I have read various threads on back gear lubrication. So many anecdotes about worked for uncle Bob's neighbor's first cousin twice removed's coworker who knew a guy from Bangkok. I have yet to find any info on the Newport brand, aside from the company's history. I'm told that it's one of the closest Bridgeport clones there is. Maybe so. Don't really care. Eventually, I plan to rebuild the head with sealed bearings so lubrication there becomes a non-issue. However, for now I need to get it back together so I can see what else needs attention. (I also have some work to do that would be better done on the new mill than the old one.) Anyway, every story or set of instructions I have ever read for greasing a gear train says something like pack the gearbox 2/3 full of "X" grease. Ok, fine, but the grease doesn't stay on the gears. It always gets flung to the walls, the corners, or wherever there isn't a spinning gear. From the Kitchenaid mixer to the top of this mill, there was a thick layer of grease stuck in all places except on the gear.

So, what is the purpose of packing it 2/3 (or whatever) full? Once the gear throws all of the grease off of it, the grease is never going to just migrate back. Seems like an awful waste!

Dave

My understanding is once the gear box heats up, the grease warms up and flows/ works it's way back into the gears. Overfilling the box with grease will actually cause the box to get to hot. I've been told that spindle bearings on Bridgeports, lathes, CNC machines, etc should not be more than 1/3 packed with grease for the same reason.

Nyle

> My understanding is once the gear box heats up, the grease warms up and flows/ works it's way back into the gears.

To see this look closely at those gears that have "thrown all of the grease off". Touch one of the teeth. Greasy.

John Hasler wrote: ↑Tue Apr 21, 2020 9:48 pm > My understanding is once the gear box heats up, the grease warms up and flows/ works it's way back into the gears.

To see this look closely at those gears that have "thrown all of the grease off". Touch one of the teeth. Greasy.

Yep! All that is needed on the gear tooth flank is a very thin film of grease. Fluid dynamics will take care of the rest.

I suspect that if you filled it, heat will cause expansion and the grease will leak out anywhere it can.

Steve

Guys talked about greases here, years ago. Not too long ago. Synthetics green, blue or red. Some were quite compatible with the base metal- bronze or brass. Some greases were at war with the others. It is a good read about these lubes . I found Red synthetic from Carquest /Advance, pretty sticky good stuff.

SteveM wrote: ↑Wed Apr 22, 2020 7:19 am I suspect that if you filled it, heat will cause expansion and the grease will leak out anywhere it can.

Steve

The gearbox will also overheat due to all the extra heat produced by the churning of the grease.

I designed aircraft gearboxes for 30 years, so I can share with you why I specified 2/3 fill for grease filled boxes.

First, grease is just oil mixed with a thickener, usually a soap. Therefore, more grease equals more lubricant. As the gearbox heats up, and is exposed to vibrations, a small amount of the oil will leach out and through gravity, relubricate the gears and other internal components. It doesn't take much.

Second, no seal is perfect. As the atmospheric pressure changes, you can expect some amount of breathing. If this air is moist, there is also the possibility that water will accumulate in the gearbox. More grease volume means less potential water volume.

Finally, it's nice to have a large reservoir of grease for internally generated debris to get trapped. This keeps the debris out of the gears, bearings and seals.

And, as SteveM suggested, too much grease will cause the gearbox to heat up as it churns the grease.

So why 2/3 full, and not 3/4 or some other number? It's historic. Someone specified that amount once upon a time as a guess. It's an easy quantity for the shop to estimate, and it seems to work.

It's interesting, the idea that as the grease warms it will flow down of the walls and lubricate the gears. The back gears in the Bridgeport series 1 j-head i-don't-yet-know-all-the-lingo are at the top of the gear case, except for the small gear when not engaged. So, when they need to be lubricated, they are both at the top of the gear case. If the grease flows down off the walls, it's flowering down under the gears.

Interesting aside, at least to me. My snowblower manufacturer seemed to adhere to convention and fill the worm gearbox 2/3 with grease just like they should and it didn't leak out through the seals that way. However, snowblower gearboxes do not get warm enough in northern Minnesota for any oil to leach out of the grease so eventually the grease all ends up on the sides of the gearbox and the worm and gear run dry. How long does it take for worm and gear running dry for the worm gear to lose all its teeth? How long does it take for the auger to rust weld to the shaft so you can't disassemble it to replace the worm gear?

Aren't worm drives usually lubricated with heavy oil?

[Edit] https://khkgears.net/new/gear_knowledge ... gears.html

John Hasler wrote: ↑Sat Apr 25, 2020 7:04 am[Edit] https://khkgears.net/new/gear_knowledge ... gears.html

That's some pretty good reference material. Thanks for posting.

Ted