crossbreak
1 MW
Hi folks,
The idea is to use the axle of a geared hubmotor as output shaft. No one ever did post anything about this in the past ( I searched a lot), that's why I wanna show how it might work. I use a 24V 250W front driving MBG Goldenmotor for this mod (reviewed the 36V rear version here https://endless-sphere.com/forums/viewtopic.php?f=3&t=4892&p=654910#p652141 . The mod is ONLY possible with outrunner geared hubmotors.
This mod has some advantages over mounting a sprocket to the hub (rather than the axle) for using it as a middrive:
-the gear ratio of the hub gear is increased by one (original: 4:1, modded: 5:1, that's an increase in torque of 25%)
-the hub can be used to mount the motor unit to the frame (to get a light, rigid and NOT ugly looking mount)
-the heat produced by the stator can be conducted to the hub to increase cooling and so the continuous power (by a so called "thermal bridge" / heatbridge)
-possibility of using sprockets with less than 18 teeth (to speed up the motor and thus raise power and efficiency)
The aim is to achieve the double continuous output power of a regular mid-mounted geared hub motor (edit: I know now it's about triple without additional heatsink).
Warning1! The peak output power is NOT influenced by this conversion. If you could drive your Hub@1kW for a minute, that does not mean you can drive it with 3kW after conversion! But you can drive it with 1kW continuously without overheating it.
Warning2! You might think now that you could still double your output power by changing the plastic gears for metal ones, this is true, but high torque leads to low efficiency due saturation of the magnetic path in the motor! It will also likely break the shaft. This is why I do not recommend using metal gears. I simply replace my plastic gears once in a year, they are cheap and run much more quiet.
Warning3! Up-Speeding your motor does raise output power, but it has limits. As a rule of thumb, 250Hz field-change-frequency should not be exceeded for motors with 0.5mm laminations, since eddy current loss raises a lot at higher rpm. This means you should not run your rotor faster than:
*1000rpm for the older MAC, 2000rpm for newer 0.33mm lamination motors, as well as the BMC hubmotor which are identically constructed
*2000rpm for the Bafang BPM
*1750rpm for the Bafang SWXB and SWXH
*3000rpm for a .35mm Bafang CST, but this one has a smaller stator than the BPM, so...is it worth? I dont know.
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EDIT: the conversion described below int his post was abandoned, since I developed a much better one (read second post) this text below just shows how this idea was born. The first succesfull conversion like that was done on a used and very cheap Bafang SWXB front hub motor, but can be done with many others as well or even better
To achieve this,
1.the phase wires are cut and pulled out of the axle
2.the axle is cut through behind the planetary gear
3.the "new" output shaft is centered by a brass guide pin and a brass washer
4.the left side axle is shortened
5.the stator is screwed to the hub (by a so called "thermal bridge" / heatbridge)
6.the "new" output shaft gets another bearing on the output side of the hub (this is necessarily to not over stress the guide pin)
Today I worked 3 hours and got point one, two, three and four done. The remaining points will follow next sunday. I needed no special tools for the mod, except for a brass cartridge and a 15mm inner dia pipe from the hardware store. I used my lathe to get a better result for the output shaft. It could possibly also be done with an ordinary hand drill, a file and sand paper. Enough talk, here are some pics (graphic found on pg 22):
The idea is to use the axle of a geared hubmotor as output shaft. No one ever did post anything about this in the past ( I searched a lot), that's why I wanna show how it might work. I use a 24V 250W front driving MBG Goldenmotor for this mod (reviewed the 36V rear version here https://endless-sphere.com/forums/viewtopic.php?f=3&t=4892&p=654910#p652141 . The mod is ONLY possible with outrunner geared hubmotors.
This mod has some advantages over mounting a sprocket to the hub (rather than the axle) for using it as a middrive:
-the gear ratio of the hub gear is increased by one (original: 4:1, modded: 5:1, that's an increase in torque of 25%)
-the hub can be used to mount the motor unit to the frame (to get a light, rigid and NOT ugly looking mount)
-the heat produced by the stator can be conducted to the hub to increase cooling and so the continuous power (by a so called "thermal bridge" / heatbridge)
-possibility of using sprockets with less than 18 teeth (to speed up the motor and thus raise power and efficiency)
The aim is to achieve the double continuous output power of a regular mid-mounted geared hub motor (edit: I know now it's about triple without additional heatsink).
Warning1! The peak output power is NOT influenced by this conversion. If you could drive your Hub@1kW for a minute, that does not mean you can drive it with 3kW after conversion! But you can drive it with 1kW continuously without overheating it.
Warning2! You might think now that you could still double your output power by changing the plastic gears for metal ones, this is true, but high torque leads to low efficiency due saturation of the magnetic path in the motor! It will also likely break the shaft. This is why I do not recommend using metal gears. I simply replace my plastic gears once in a year, they are cheap and run much more quiet.
Warning3! Up-Speeding your motor does raise output power, but it has limits. As a rule of thumb, 250Hz field-change-frequency should not be exceeded for motors with 0.5mm laminations, since eddy current loss raises a lot at higher rpm. This means you should not run your rotor faster than:
*1000rpm for the older MAC, 2000rpm for newer 0.33mm lamination motors, as well as the BMC hubmotor which are identically constructed
*2000rpm for the Bafang BPM
*1750rpm for the Bafang SWXB and SWXH
*3000rpm for a .35mm Bafang CST, but this one has a smaller stator than the BPM, so...is it worth? I dont know.
[/color]
EDIT: the conversion described below int his post was abandoned, since I developed a much better one (read second post) this text below just shows how this idea was born. The first succesfull conversion like that was done on a used and very cheap Bafang SWXB front hub motor, but can be done with many others as well or even better
To achieve this,
1.the phase wires are cut and pulled out of the axle
2.the axle is cut through behind the planetary gear
3.the "new" output shaft is centered by a brass guide pin and a brass washer
4.the left side axle is shortened
5.the stator is screwed to the hub (by a so called "thermal bridge" / heatbridge)
6.the "new" output shaft gets another bearing on the output side of the hub (this is necessarily to not over stress the guide pin)
Today I worked 3 hours and got point one, two, three and four done. The remaining points will follow next sunday. I needed no special tools for the mod, except for a brass cartridge and a 15mm inner dia pipe from the hardware store. I used my lathe to get a better result for the output shaft. It could possibly also be done with an ordinary hand drill, a file and sand paper. Enough talk, here are some pics (graphic found on pg 22):
