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Sunday, 8 April 2018

Xiongda 2-speed hub motor breakdown: slipping magnets

Re-gluing the magnets on a motor with oil lubrication
This week I had a new type of electric bike hub motor breakdown. The Xiongda 2-speed rear hub motor on my 26" wheeled bike started to make a harsh scraping sound - similar to a disc brake with pads worn to the metal. The sound only came when when power was applied against resistance (like going uphill), and the bike felt like it was losing power. Stopping at the roadside, I found I could stop the rear hub-motor wheel completely with the brake, run the motor with the throttle, and hear the motor turning inside the stationary wheel, making the awful scraping sound.
Here's a short video of what happened when I powered the rear wheel, off the ground. First it runs in low speed and I apply the brake while leaving the throttle on, then I do the same in high speed:

The sound could be made in high or low motor speed. Limping the bike home (up the mountain…) I found I could run the motor on 3/5 power without it slipping and scraping.
I dismantled the motor, expecting a sheared key on the freewheel plate, or some other sort of freewheel problem.
Here's the tool I made to unscrew the motor side plate
After checking it all carefully, and trying a new freewheel plate which didn’t stop the sound, I noticed some of the magnets in the magnet drum were a fraction out of alignment with the drum edge. I found I could slip the individual magnets out with gentle levering. Evidently the magnets had come unglued and were spinning inside the drum, not turning the drum.
Here are the magnets slipped out of their places, but still holding onto the drum with magnetism
This motor is one of 2 XD 2-speed hubs I’ve put Automatic Transmission Fluid (ATF - a light oil) into, in the hope of improving cooling of the copper coils on steep climbs. It also makes the motor sound rather smoother - the XD 2-speed is a pretty noisy motor, with its complex gears and freewheels. As I’ve discussed elsewhere in my blog, the coils of a hub motor have trouble losing heat when labouring. To escape from the motor windings, heat must travel either by conduction through the laminated core to the axle, or by radiation from the coils and core to the magnet drum, then radiating again to the hub case - either way is very inefficient. Adding some ATF provides good lubrication throughout the hub, and the flow of fluid can be expected to carry some heat from the coils to the case. The drawback is that some leakage of oil can be expected through the axle bearings and elsewhere, especially if the bike is laid on its side. I haven’t yet concluded whether the ATF is making a significant difference to motor temperature but the motors certainly run quieter.
I’m not sure what effect the ATF has had on the magnets coming loose in this case. Clearly however, once they came unglued, the oil would have made it much easier for the magnets to slide around inside the drum. Perhaps without ATF the magnets would stay in place without glue. But did the ATF have a role in ungluing them? This motor has had ATF in it for a few hundred kilometres and a few months (it's my less-used bike), but I have another XD motor which has had ATF for 1000 or 2000km and a few months, without trouble.
The magnet drum has 20 curved high-strength magnets arranged around inside the drum cylinder, that rotate around the 18 poles of the armature. Before removing them, I marked the magnet ends with alternating patterns of file grooves: 1 groove and then 2 grooves. This represented the alternating polarities of the magnets, so I could replace the magnets with the right orientation, polarity and pattern.
To repair the motor, I needed to glue the magnets back into the drum. The biggest challenge was cleaning the oil off the magnets and the drum. I used turps, metho and degreaser in turn.
Here are the magnets drying on a strip of sheet metal after cleaning - you can't just put them in a box or they'll smack together
I left the 2 sealed bearings in the drum, and took care not to get solvent or degreaser into them. After cleaning, I sanded both surfaces, and ground a bit of roughness into the drum surface with a die grinder and disc. My main concern is that there be enough friction to prevent sliding.
The magnets being glued back into place with epoxy. Note the file marks on the magnet edges, so I could replace them in the right orientation and order.
I used runny epoxy resin (like is used for fibreglassing) to glue the magnets in. The magnets hold themselves into the drum very firmly with their magnetism - no need for clamps. I left all magnets a little proud of the drum, and aligned them together by pressing the drum down onto a flat surface.
I recognise that the epoxy will weaken when the motor is hot, but the glue's job is to resist shear forces - stopping the magnets sliding around inside the drum again - so even if it was softened by the heat it should do its job.
Installed back on the bike, the motor is working perfectly again. Of course I can't be sure until it's done 5000km like this...

1 comment:

  1. Hi Bruce. It looks like you have been busy. Thanks for your very informative and detailed blog.
    Just in case you don’t have time to look at my comments on the other blog.
    Bonnie ask me to say Hi from her at Xiongda

    ReplyDelete