Geared Hub Motor - Question on Clutch Mechanism..

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sonnetg   1 kW

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Geared Hub Motor - Question on Clutch Mechanism..

Post by sonnetg » May 04 2017 6:36pm

Hello,
I have a dumb question, but I was wondering how exactly does a geared Hub motor clutch work? Is the similar to a bicycle Freewheel, or is there a Clutch mechanism similar to what we have in a Manual Transmission?

I tried to find dissected images of a geared hub motor clutch, but haven't had much luck. I did find one image, but i still dont get how the Motor locks up with the wheel when throttle is engaged, but freewheels when throttle is disengaged. If anyone here ever tore open a clutch and could share the images, it would greatly help to understand it.

The reason I am asking is that clutch failure seems to be a very common issue with geared hub motor, yet it is not serviceable. I think it would be great if the clutch was serviceable. I am not sure manufactured completely seal the clutch unit. Any idea?


Thank you
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Icewrench   10 kW

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Re: Geared Hub Motor - Question on Clutch Mechanism..

Post by Icewrench » May 04 2017 8:30pm

Well when my clutch quit working I opened it up and found the small springs had deformed and no longer would push on the roller.
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These are not like car or truck clutches.

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sonnetg   1 kW

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Re: Geared Hub Motor - Question on Clutch Mechanism..

Post by sonnetg » May 04 2017 8:56pm

Icewrench wrote:Well when my clutch quit working I opened it up and found the small springs had deformed and no longer would push on the roller.

These are not like car or truck clutches.
This is interesting. Thanks for the image. I wonder how those round rollers lock when the wheel spins in one direction. I was expecting to see similar mechanism to that of a freewheel, but it is nothing like that. Maybe it is designed such to get rid of the ticking noise of a bicycle freewheel. Oddly there isn't much information out there regarding this type of clutches. Maybe there is, but I am not able to find it. It would be nice if there were any youtube animations out there on how this type of clutch locking mechanisms work.

Thanks for sharing the image though. It seems you were curious just as I am. If it were not a sealed unit, you could probably have fixed it yourself. No wonder why many shy away from Geared hub motors.

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Re: Geared Hub Motor - Question on Clutch Mechanism..

Post by sonnetg » May 04 2017 9:01pm

FYI...i did more googling. I think now I get the full picture. This video seems to show how it all works together..




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Re: Geared Hub Motor - Question on Clutch Mechanism..

Post by motomech » May 05 2017 3:26am

They are called Sprag or ramp and roller clutches and have been around for decades. They were used in some early automatic transmissions.
They are reliable if power levels are kept reasonable .
I too once had chewed-up springs, but it was my fault. I had the clutch apart and didn't get them correctly re-installed.
I think that is usually the case when it happens.
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sonnetg   1 kW

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Re: Geared Hub Motor - Question on Clutch Mechanism..

Post by sonnetg » May 05 2017 9:32am

motomech wrote:They are called Sprag or ramp and roller clutches and have been around for decades. They were used in some early automatic transmissions.
They are reliable if power levels are kept reasonable .
I too once had chewed-up springs, but it was my fault. I had the clutch apart and didn't get them correctly re-installed.
I think that is usually the case when it happens.
Thank you so much for the information. I had no idea on such clutch mechanisms, but it seems this technology is very much in use, and not only in geared hub motors. I was able to dig up more information on the web now.

I think this video does a nice job with animating the inner workings of a roller clutch.

Thank you.


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Re: Geared Hub Motor - Question on Clutch Mechanism..

Post by molybdenum » May 25 2018 11:17am

I'm curious if how one operates the motor could have an impact on clutch wear/failure.

When the motor is engaged, the rollers become locked between the inner and outer races, causing these parts to rotate in unison. Let's say that you are pedaling hard near the unloaded speed with the throttle engaged. This could cause the motor to cycle between 0W and 50-100W as pedaling effort and topography change. In theory, this would cause the rollers to engage and disengage the outer race multiple times a minute.

Would this scenario lead to excessive clutch wear?
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Re: Geared Hub Motor - Question on Clutch Mechanism..

Post by teklektik » May 25 2018 1:22pm

molybdenum wrote:
May 25 2018 11:17am
I'm curious if how one operates the motor could have an impact on clutch wear/failure.
Yep. High torque from hammering getaways or from impact hitting potholes/obstructions is what does in the clutches. The high torque jams the roller or sprag into the contact faces of the clutch and deforms them - essentially leaving a divot. The rollers/sprags then tend to skip over these imperfections in subsequent engagement attempts making them fail to engage (much as your tires skip when braking on a corduroy road surface). Alternatively, the roller/sprag will just jam into the wedge-shaped space and be held there by the deformation, effectively seizing the clutch so it cannot release.

If you never get into one of these torque failure situations, the clutches never seem to give problems, but once they start to pick up damage from hard starts, etc they work worse and they're on the slide to failure.

Using a soft start controller or CA3 ramping is good insurance against pounding the clutch if you are running higher power. Once engaged, they will hold a lot of torque, but that initial hit is a killer.
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Re: Geared Hub Motor - Question on Clutch Mechanism..

Post by molybdenum » May 25 2018 9:52pm

teklektik wrote:
May 25 2018 1:22pm
molybdenum wrote:
May 25 2018 11:17am
I'm curious if how one operates the motor could have an impact on clutch wear/failure.
Yep. High torque from hammering getaways or from impact hitting potholes/obstructions is what does in the clutches. The high torque jams the roller or sprag into the contact faces of the clutch and deforms them - essentially leaving a divot. The rollers/sprags then tend to skip over these imperfections in subsequent engagement attempts making them fail to engage (much as your tires skip when braking on a corduroy road surface). Alternatively, the roller/sprag will just jam into the wedge-shaped space and be held there by the deformation, effectively seizing the clutch so it cannot release.

If you never get into one of these torque failure situations, the clutches never seem to give problems, but once they start to pick up damage from hard starts, etc they work worse and they're on the slide to failure.

Using a soft start controller or CA3 ramping is good insurance against pounding the clutch if you are running higher power. Once engaged, they will hold a lot of torque, but that initial hit is a killer.
Thanks teklektik! The dimpling will be a simple thing to look out for on the next repair ... or post mortem.

Also interesting about the cascading effects of initial damage to the races.
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teklektik   10 GW

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Re: Geared Hub Motor - Question on Clutch Mechanism..

Post by teklektik » May 26 2018 6:14pm

You're unlikely to be able to see anything with the naked eye. If you are running the clutch near it's torque limit, you will just start to notice that the clutch more frequently breaks free on getaways or inclines with that scary GRAKKAKKAK machine gun sound. Turning up the ramping or backing down the controller current or phase current limits is the only remedy - until things eventually get worse.

On the other hand, if you are being civil and not trying to put silly torque through it, the clutch will seem to last forever. These clutches are not really "fragile" as some folks like to claim, it's just that there is a pretty clear line in the sand and they are not very forgiving about crossing it.

Just another episode of "You play, you pay..." :)
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