Gearbox Lubrication?

[rhitee05]

I was working on my Cyclone motor/gearbox over the weekend. I cleaned out the gearbox with degreaser and a toothbrush and got it as clean as I could without complete disassembly. I bought some new li-moly grease to replace the stock stuff after I got it cleaned and dry. I noticed after I repacked the gearbox with grease that it became significantly harder to turn than while it was clean and grease-free. That got me thinking about what the best lubrication would be for the gearbox, with both efficiency and lifespan in mind.

Obviously I wouldn't want to run the gearbox under any kind of load without lubrication, but that grease is so thick and viscous. Would something thinner than grease still provide enough protection? I used to put 75W90 gear oil in the rear differential of my car, would something like that work here as well? If it's good enough for the differential on a 300hp car, seems like it should handle gearing for a ~1hp electric motor! What about something lighter still? My current vehicle takes 5W20, so I have some Mobil 1 lying around I could use. I was thinking about filling the gearbox 1/3 or 1/2 full, then let the oil splash around and bathe everything.

I'm pretty sure I understand this well enough to know that higher viscosity -> better protection, but higher viscosity -> lower efficiency as well. I assume there's a sweet spot here for optimum results. Can anybody with more machinery experience weigh in with suggestions? I assume roughly the same would also apply to geared hubmotors and gearboxes people are putting together for RC use (assuming the gearboxes are sealed).

 

[317537]

Are the phase wires joined or touching while its apart? Check the phase wires.

 

[rhitee05]

Phase wires aren't at issue here - I'm talking about the gearbox totally separate from the motor (they're fine, btw, not shorted together). The gearbox resistance increases due to the viscosity and stiction of the grease and I'm interested in how best to reduce that while keeping the gears properly lubed and protected.

 

[317537]

Grease that sigmificant?

I saw something similar in my hub but not so significant. It slowed the bearings down a bit but it always comes good when you run it in. I havent googled this so Im just guessing that the oil stuff gets released and the viscous bonds break down in the grease the more its used till the oil dries up. But thats sound a little too sticky.

 

[Burtie]

When I opened up the gearbox on my new 1200w cyclone motor, the grease looked too thick and sparce to be effective. It was all stuck around the outside corners of the box.
I left the exising grease in there but 1/3 filled the gearbox with EP90 gear oil before I put it back together. I figured, like you, that if it is good enough for a 300 hp car -it should work here.
I have ridden the bike for about 700 mile since with no problems and no oil leaks .

I didn't do any efficiency tests, but i guessed the oil would result in lower losses than filling the box with grease, the only downside would be the risk of oil leaks -external, or into the motor.

 

[rhitee05]

I think I might try to run some tests to try and measure efficiency with different lubricants. Or, at least the no-load draw of the motor. If I do, I'll post the results here. I suspect my eventual choice will be the gear oil like you used.

 

[liveforphysics]

It's simply impossible to compare the oils used in a cars pumpkin to the gear box of a little electric motor.

I run Torco RTF in my race trannys, it's thin like water. I run 0-15 weight oil in my race engines, because I designed the bearing clearances around running a thin oil. The same oil put into a loose big block chev would likely result in engine failure in minutes, because it was designed for 20-50.

Anytime you switch from grease to a bathed lubrication system, you are adding fluid dynamic drag onto all bathed gear surfaces. With fine tooth situations at high speeds, you also run the risk of bizarre effects like whipping the oil into a frothy foam (sucks) and other strange things. Some gear oils eat brass, and absorb into certain plastics and seals, causing them to expand and become weak.

Grease should simply coat each contacting surface, then sling off the rest onto the case, where it stays out of the way of causing parasitic drag on the system. If your surfaces aren't showing signs of accelerated wear, then the grease has done it's job correctly, and you would be hard pressed to find any bath lubricant setup that will be causing less drag at speed.

Once that grease shuffles itself around and breaks-in, the drag will drop down to minimal levels. With the giant rotary power conditioners at my job, after greasing the bearings, it's common to see the idle loads increased to 25-30kw for the next week or so, and it slowly tapers back down to around 10-15kw as the grease breaks-in. It's pretty amazing to think that 10-15kw of power is being used just to push around fresh grease in the bearings until it breaks-in.

 

[rhitee05]

Very interesting, I had no idea that grease had a "break-in" period like that. I'll have to give the fresh grease some time before I make any judgements, then. Just as well, since my bike build is moving along at about the speed of a glacier. Who'd have thought that grad school is such a time sink? 8)