Geared Hub Motor Cooling?

[LI-ghtcycle]

I'm curious who has tried increasing the cooling ability of a geared hub motor, I imagine there would be ways to attach fins or maybe some form of a liquid cooling option that would cool through coils wrapped around the axle?

I also imagine maybe you could simply attach some form of heat sink cooling fins to the case of the hub that would travel along the spokes?

 

[amberwolf]

The real issue with a geared hub is that there is even less heat transfer between the stator and the case than in a non-geared hub, which is already pretty poor. So just cooling the casing isn't really going to do all that much--you'd need to ventilate it in some way, or seal it really well and fill it with a thermally-conductive but electrically-insulative fluid, like some oils. Preferably one that does not eat away the varnish on your coils, or the epoxy of the hall sensors, etc.

 

[John in CR]

I got a ventilated geared hub to the final stages before my machine shop buddy screwed the pooch and didn't get the new large bearing properly centered in the cover. Based on the results I've had with venting direct drive hubbies, I know it will work well, though it may require putting some kind of blades on the open spokes of the rotor to force a good flow through the stator. The motors inside spin 4-5 times as fast as a direct drive hub motor, so I can't get away with crude blades like I did in the DD hub covers.

 

[LI-ghtcycle]

I got a ventilated geared hub to the final stages before my machine shop buddy screwed the pooch and didn't get the new large bearing properly centered in the cover. Based on the results I've had with venting direct drive hubbies, I know it will work well, though it may require putting some kind of blades on the open spokes of the rotor to force a good flow through the stator. The motors inside spin 4-5 times as fast as a direct drive hub motor, so I can't get away with crude blades like I did in the DD hub covers.

Sounds interesting, what about weather? I live in Oregon where we judge the seasons by how hot or cold the rain is. :lol:

Does it open up the motor to more potential water issues?

 

[LI-ghtcycle]

The real issue with a geared hub is that there is even less heat transfer between the stator and the case than in a non-geared hub, which is already pretty poor. So just cooling the casing isn't really going to do all that much--you'd need to ventilate it in some way, or seal it really well and fill it with a thermally-conductive but electrically-insulative fluid, like some oils. Preferably one that does not eat away the varnish on your coils, or the epoxy of the hall sensors, etc.

Ahh, good to know, I wonder if adding a fluid wouldn't cause more friction too, thus reducing efficiency. However, if this same fluid keeps things from burning up, maybe in the end it allows for more power and less chance of peanut butter gears!

Anyone heard of anyone trying this? I guess it doesn't have to be a "filled" case just enough to keep the gears cool and lubed, and then I guess you could go with metal and now put much more power into a geared hub.

I hate to take something so simple that works well enough already, but hey, if I'm not getting more power how am I gonna get that "what the hell!?" look when passing car traffic in the bike lane at 25+ MPH during rush-hour?

 

[John in CR]

Yes, talk with MWKeefer about cooling geared hubs using an oil bath.

 

[amberwolf]

Ahh, good to know, I wonder if adding a fluid wouldn't cause more friction too, thus reducing efficiency. However, if this same fluid keeps things from burning up, maybe in the end it allows for more power and less chance of peanut butter gears!

Basically, yes, to all those things. The technobabble version would be: The loss of mechanical efficiency is very likely more than compensated for by the gain in thermal efficiency allowing a probably very significant gain in power handling capacity.

Anyone heard of anyone trying this? I guess it doesn't have to be a "filled" case just enough to keep the gears cool and lubed, and then I guess you could go with metal and now put much more power into a geared hub.

I expect you would want it to be filled so thermal transfer is occuring throughout the entire volume of the hub. If you don't do that, then most of the heat transfer you're filling it to get would still happen only thru the air, rather than the oil. Thermal conduction in a denser material like oil is usually more effective than a sparser material, like air. That probably holds true for most materials.

 

[Whiplash]

So if I understand correctly, it might be enough, just to fill or partially fill the motor with fluid and then just put fins or some sort of heat sink on the outside casing to then expel the heat?

 

[adrian_sm]

Mark_A_W tried it on his MAC Shanghai. Ended up abandoning the idea. From memory the additional mechanical losses were huge, and not worth it.

Here is a link to the start of the results of the testing from his build thread.
http://www.endless-sphere.com/forums/viewtopic.php?f=6&t=15439&hilit=fill&start=90#p251537

And here is where he drain it all out again
http://www.endless-sphere.com/forums/viewtopic.php?f=6&t=15439&hilit=fill&start=90#p251642

I'm going to drain most of the oil out. The motor can only get to 55kmh no load, instead of between 75 and 80kmh. The no load power is over 500w.

- Adrian

 

[amberwolf]

That's disappointing, but I suppose not truly unexpected.

I note that he used Mobil 1 0w-40 oil, and I wonder if it would be any different with other oils, like those used for turbine engines?

Also, I wonder what effect it would have if various parts of the stator were "streamlined" by casting them in a thermally-conductive material that wouldn't dissolve in the oil? Of course, the material would be one more thermal layer to go thru to get heat out, so it might cause more problems than it solves.

It might not be turbulence that causes the drag, but rather tension effects of some type in the former air-gap between magnet ring and stator; that's probably the closest-fitting portion of the whole motor assembly.

I also wonder what would happen if the entire motor were totally filled, with no air in it at all? Perhaps the mixing of air and oil is creating some kind of bubbles within the oil at the air gap or elsewhere, causing drag by surface tension or similar.