Good! Someone finally oil cooled a hubbie!! HS3540

[Alan B]

Nice experiment. Would be great to record a time temperature trace of the two motors on the same course.

[parabellum]

Sorry did something stupid here.

[parabellum]

Hi parabellum, any updates on how this is working out with use ?

You know, I forgot to install temp sensor wires, having 5 temp sensors on the shelf from HK. So temperature measurement is relative.
My system:
BMX 20" bike
15s 9A Lipo pack
50A limited 15 fet infineon controller.
95kg rider.

I discharged the pack going up and down same stall hill repeatedly, WOT up and regen down, in some points i need to pedal uphill to avoid stall the motor and kill controller.
Motor outside gets to hot ho touch, over 70C, thermal shearing works nice since it is not getting hotter on stops.
After short stops I could fry sausages on my HS R5303 under similar conditions on the burned paint stinking motor covers.
I believe this motor is capable of 2kw continuous in low speed condition and 3kw or more at higher speed.

[parabellum]

At 400ml fill, the oil level is just below the axle, where the filler hole is in the above photo,

How do you plan to make breathing, if your motor is half full of oil?

[Burtie]

I dont really see a problem with the motor not being vented.

I guess there may be some small migration of oil out through the axle seals or through the cable cores (see Justins latest video) due to heating.
Or perhaps some small water ingress via the same routes due to cooling.

The oil level is normaly below the axle seals, so this should prevent any major leakage.

At least the metal parts will be protected from corrosion by a coating of oil

[parabellum]

If it is tight and oil can not travel fast enough, there is risk of pushed out seals. Anyway, if you are right I would prefer to close my vent to, now it loses little oil and I need to wipe it out from time to time.

[electr0n]

So what is the verdict on oil cooling a hub motor? This is much easier than installing fans, drilling side covers and using liquid cooling.

[parabellum]

So what is the verdict on oil cooling a hub motor? This is much easier than installing fans, drilling side covers and using liquid cooling.

It improves heat transfer to ambient significantly, extra bonuses are rust protection and physical protection of closed system (as opposite to air/hole cooling). It also makes external cover cooling reasonable. Planing to make this mode to all my hubs after I get some really thin oil, 0w40 ads to much drag when cold.

[Neovin]

I was thinking for future motor designs, why could we not use small diameter copper enameled tube, I doubt you could find anything suitable for a retrofit but if you designed from the ground up you could take the heat away from the center of the problem.

[Burtie]

I got my oil cooled hub fitted, and early tests results look great.

Setup summary:
The hub is filled to just below the axle with 400ml ATF.
The temperature sensor is fitted in the windings at the top, not in the oil pool.
The hall sensors are sitting at the bottom, in the oil pool.
The hub is a 7 turn MXUS running at 66v, 4.5kw peak power.
Ambient temperature during testing was 8 to 10 deg C

My regular 9 mile round trip takes in some sections of 20% gradient. I rode the same route at the same speeds (stopping mid point) with identical motors, exept that only one of them was oil cooled.
This route, when ridden briskly, burns about half of my 15AH battery capacity.
In short:

Max measured winding temp No oil ----> 140 C, with oil ------> 55C
Finish point winding temp .... No oil ----> 106 C, with oil ------> 38C

The small amount of drag due to the oil is not noticeable when riding, and the motor runs significantly quieter.

Seems to work for me

The only downside I can see is that the rear disk brake may be prone to some oil contamination if the LH axle seal weeps.

Burtie

Edit:
Just checked rear disk after last nights ride, and it is now covered in oil

[Spicerack]

You need some form of vent for the hub to relieve excess pressure as it heats up.

Try a little bit less oil and drill a small (1mm) hole in the cover near the axle. Angle it so that it traverses from the central axis towards the outer circumference as you go from outside to inside. That way, when the wheel spins, any oil that happens to splash into the hole will be flung back inside the hub. I've done this on my mac and had no oil leak at all from the hole or axles.

For the record, the outer cases of my mac are alot hotter than without the oil so it certainly transfers heat away from the windings.

[Burtie]

Thanks andy, sounds like a good idea.
I will try drill a small angled vent in the opposite side, behind the sprockets.

[bigmoose]

Thanks for the great data on oil cooling Burtie! I think this will become standard practice for high performance motors as we go forward. I saw how much this improved the "Power Dense" motor system developed at Bowling Green State University about 10 years ago, and strongly felt it was applicable to hub motors. Your data now proves the point.

This should now end any lamination/magnet corrosion issues, and bump power handling a bunch. I will also bet thermal soak hall failures are a thing of the past also.

[Arlo1]

Thanks for the great data on oil cooling Burtie! I think this will become standard practice for high performance motors as we go forward. I saw how much this improved the "Power Dense" motor system developed at Bowling Green State University about 10 years ago, and strongly felt it was applicable to hub motors. Your data now proves the point.

This should now end any lamination/magnet corrosion issues, and bump power handling a bunch. I will also bet thermal soak hall failures are a thing of the past also.

WHat about drag at hi rpm??

[bigmoose]

WHat about drag at hi rpm??

You do the system analysis and see if you have enough AHr to overcome it. It is my opinion that we now have the capacity to accept some losses that were not acceptable just a few years ago. Using gears in the reduction of mid drives is the next area I see coming...

One thing I harp on at the "big lab" all the time is "It's all about the system..." Maximizing individual component efficiencies and capabilities does not always increase the efficiency and capability of the system. It is ALL about the system... hence my support there for multivariate optimization routines and Monte Carlo simulations.

[itchynackers]

I still don't quite understand whether the oil is just delaying thermal soak (by the oil storing the heat), or if the oil is allowing the hub to dissipate more heat.

Edit: After re-reading the thread, it seems as though the oil transfers the heat quicker to the covers (making them hotter) so more heat is carried away with the same amount of windage. Is this correct?

[parabellum]

I still don't quite understand whether the oil is just delaying thermal soak (by the oil storing the heat), or if the oil is allowing the hub to dissipate more heat.

Edit: After re-reading the thread, it seems as though the oil transfers the heat quicker to the covers (making them hotter) so more heat is carried away with the same amount of windage. Is this correct?

The second. Oil helps transferring heat to covers and they dissipate it to ambient.

[parabellum]

Try a little bit less oil and drill a small (1mm) hole in the cover near the axle. Angle it so that it traverses from the central axis towards the outer circumference as you go from outside to inside. That way, when the wheel spins, any oil that happens to splash into the hole will be flung back inside the hub

I did exactly this on Mxus DD inside disc brake screw hole with inclination to outside D inside, then tightened a screw with sprung washer (sprung part in axle/center direction), I only have 150ml of oil but it gets out of the vent anyway(maybe few droops per ride).

[Burtie]

I cleaned the oil off the disc and pads with caustic soda, and drilled a small vent on the opposite side of the hub, close to the axle.

It all works well, the brake stays clean
Just a small drop escapes from the vent each use, so will have to wipe it over occasionaly to stop things getting too messy.

Oil Cooled Test Rig.jpg (154.03 KiB) Viewed 698 times

This seems to me like a massive step forward. I can now practicaly ride this 4kw bike as hard as I like, for the whole duration of the battery pack, with no more worries about cooking the hub!

Thanks to everyone here for their ideas and suggestions
Well pleased
Burtie

[Spicerack]

Good to see. I was going to liquid cool my 9C 8x8 and run quite a few amps through it. The initial plan was copper cooling pipes around the inside of the hub, going out through a modified/fabricated side cover to a little radiator etc etc but I'll just do the oil bath first and see how I go. It only helps a little bit on my current MAC hub but that's because the windings are damaged from overheating them so a large chunk of the amps get converted to heat!

[Burtie]

I guess the MAC would suffer more loss from the viscous drag of the oil, since its rotor spins at 5x the wheel speed.

This loss might be significant enough to offset the gains (of improved thermal conductivity) somewhat?

[Spicerack]

I wondered about that too as the planetary gears will be spinning through the oil as well- but it didn't seem to make much difference during testing. Not that valid a test though as the windings are damaged- I'm putting a new 10 turn stator in it soon so will do thorough dry and wet tests with a set route, amps etc.

[parabellum]

I wondered about that too as the planetary gears will be spinning through the oil as well- but it didn't seem to make much difference during testing. Not that valid a test though as the windings are damaged- I'm putting a new 10 turn stator in it soon so will do thorough dry and wet tests with a set route, amps etc.

Few nice words from 2009 by the pioneer of oil filled geared hubs practice Reid Welch.

WHAT: running geared transmissions in light oil is a universal practice...except in these new, geared-type hub motors.
WHY: light oil cushions gear teeth, actually preventing contact of the pressure surfaces, through a phenomenon known as
hydrodynamic lubrication: the parts surf upon each other, oil as the separating agent. This form of lubrication prevents all wear
((think of your auto's automatic transmission, how quiet its helical gears run, and with no wear-out))

IS THERE LONG TERM HARM? Ans: No, not unless the oil were to somehow degrade hall sensor adhesive (unlikely, it seems)

Q: How much oil to be installed? A: Enough to fill the motor about one third full. Then, when running, centrifugal force
throws the oil at the perifery of the gear casing, bathing the soft steel ring gear and the plastic planets. Plus, oil contantly
washes all the bearings and constantly wets the lip seals of the axle. Some slight weepage at the axle "seals" is inevitable.

About heat and expansion: when the motor warms up, its contained air expands, and will push out a minute amount of oil.
Upon cooling down, the motor will slightly inhale some of this weeped oil.
Now, consider the stock (dry, not oiled) run in the rain: it will gradually ingest small amounts of water, eventually rusting internal parts to some small extent.

Oil is dielectric and has no effect on nylon gears nor synthetic rubber seals, which are DESIGNED to run in oil.
Oil, sloshing around the inside of the hub motor, is not of sufficient quantity to drown the rapidly spinning central rotor
of the motor, itself, so no fluid spray/strain/churn on the copper windings.
The oil will all be at the ring gear, surfing the planet gears, dampening noise, and fluid polishing the parts over time (perhaps)
to make, in time (thousands of running miles) a super quiet motor, with each planet gear tooth of the three planets, shoulder exactly the same amount of the load.
Benefit accrued: maximum robustness of the geared motor: it should never "peanut butter" any teeth, until some long-away day, when plastic fatique finally breaks off a tooth, which then spells instant destruction for all the remaining planet gear teeth.

[gogo]

Gotta love Reid's nitty gritty. I'd love to see some hard numbers on the cooling benefit and the amount of torque that oil cooled gears can take before failure in an oil bath.

[Farfle]

I know with my rc car, (pre 2004 traxxas rustler) the first time I put a brushless motor in, it lunched the Metal idler gear I bought specifically for that motor in less than 3 mins of bashing, in desperation I put the nylon gear back in, and in the process of trading gears I had to refill it with oil, I put some stihl chainsaw gearcase oil in it, and it ran for several weeks like that so imho, a good oil adds alot of durability and longevity to a gearbox.