What the hell?! Cromotor axle twisted

[warrah]

Basically i added a thermistor to the cromotor recently, and while removing the motor from the frame i noticed that one of my custom 8mm torque arms had slipped slightly from where it was attached to the chainstay. I checked if this meant torque arm failure, and found the torque arm holes were still very snug. I couldnt work out why the two torque arms were now out of line with each other, and assumed that perhaps one of the chainstays had actually bent or something. Oh how i wish, that would have been much cheaper. Instead, when you compare both sides of the axle, the two sides are no longer aligned, in the sense that one side has rotated so that its flats are not facing the same way as the others. When you inspect each side individually, there is no evidence of rotation, each side alone is untwisted, meaning the twist must have happened somewhere inside the stator.

Im afraid its very difficult to show with pictures, every time i try to take a picture of the wheel facing forwards to show both sides of the axle for comparison, the camera focuses on the tire.

Could i just weld two slightly out of line doctorbass torque arms to the dropouts and keep riding, would that be safe or is there serious cause for concern? Can axles be replaced?

 

[e-beach]

Do you know if the axle is only twisted, or is it cracked or torn somewhere inside the motor? Personally, I would open the motor and take a good look for safety.

BTW, is your motor under warranty?

 

[warrah]

Sadly my motor is not under warranty.

When i had it open for the thermistor job, i checked the axle inside... couldnt find any cracking. To be honest, it is all a bit weird. I can only think that the twist is isolated to a very specific place, perhaps right in the centre of the stator, or elsewhere but with no visible signs

 

[knighty]

I'd be seriously surprised if the axle has actually twisted

it's one sold piece, running end to end, and iirc it's the thickest axle of any hub motor available ?

without heating it up, it would be pretty damn hard to twist it if you had the motor apart and one side jammed in a vice


I think your only option is to strip the motor apart and see what's going on :-o

 

[Ypedal]

what voltage/amp are you pushing ?

 

[warrah]

Im running 72v 30a.

Axle is definately twisted, i'll try to post photos. Im thinking if i put it in a brand new frame ive got and take pictures of the left and right side, you'll see that they are very out of line with the dropouts

 

[MadRhino]

Very unlikely to twist a cromotor axle running such low power.
IMO it is broken. This could happen hitting hard on one side, or by repeated torque on misaligned torque plates.
You will have to open both sides of your motor. If it shows no sign of twist, it is likely to be broken inside the stator.
In any case you will have to replace the axle.

 

[Spacey]

Can't imagine it was from the low power of 30A, feel for you dude...hope you get it sorted soon.

 

[Chalo]

Somebody tell me again why we're transmitting motor reaction torque through flats cut into an axle thread? 'Cause that doesn't make very much sense.

 

[e-beach]

........ I can only think that the twist is isolated to a very specific place, perhaps right in the centre of the stator, or elsewhere but with no visible signs

If the axle is broken in the center of the stator, it could be very unstable if the motor is disassembled. Please use extreme caution to prevent injury in the event one side of the axle comes out in your hand......

Do you have a big gear pulley for the job?

 

[warrah]

MadRhino: so the axle can be replaced? Thats a thing that people do?

Chalo: youre right, it doesnt make much sense. I've taken this thing to my adventurous motorcycle mechanic to see if he can think of something, and he doesnt trust the concept of flats and threads on a rear hub at all, wants me to seriously consider converting the axle to work with a motorcycle clamping system of some sort.

e-beach: I have a big gear puller, very useful, infact i worked out i could use it in reverse to gentley and precisely reassemble the motor, i highly recommend this method over the 'line it up and carefully let it fly in through magnetism' method that is mentioned on a lot of posts.

 

[warrah]

new frame (you can still see the foam packaging), but cromotor wont fit because the two sides of the axle have twisted in opposite directions

 

[GCinDC]

no explanation for you, but fwiw, i used to have fun testing my hs3540 while lying sideways clamped in the vice, but then i realized that with only one side of the axle secured, and going hard WOT from a stop, that weak (hs3540) axle was twisting slightly. or so i imagined... i don't do that anymore, and don't recommend it...

 

[MadRhino]

MadRhino: so the axle can be replaced? Thats a thing that people do?

Yep, I replaced one on an X 5304. I had one made pretty quickly at the nearest machine shop, and they kindly offered to press it in for me. The bike was off the road only one week. It's been sold 2 years ago and still running fine.

 

[alsmith]

Have you checked the alignment of the slots/plates on the frame? Or the wheel/axle on a different frame to check that the problem is the axle?

 

[www.recumbents.com]

Put one end of the motor axle in a vise and twist on the other with a crescent wrench. Can you twist it?

 

[Animalector]

OFF TOPIC - what is that frame?

Thanks
Andy

 

[warrah]

alsmith: i have checked it in three frames, none fit. More significantly, the difference in rotation is so large that when looking at the cromotor straight on, you can see more of the flats on one side of the axle then the other.

www.recumbants.com: tried what you said after i found the problem, it doesnt twist back.

animalector: the frame is an 'on one inbred', a cromoly hardtail.

 

[e-beach]

The axle definitely looks twisted.....how did it happen without splaying open one of the dropouts on your frame?

 

[warrah]

It did open out one dropout on my chopper frame. The frame in the pictures is a different frame, to show the twist

 

[alsmith]

less likely possibilities-
Maybe there is an internal inclusion in the metal that has weakened it- this is more likely if they are cast then machined but can happen with billets / drawn steel too? Short of X-raying it the only way to see is if it breaks the surface.
Another possibility is if the rear subframe has been flexing/bending when under power. Is it alloy or steel? Any stress marks on that sub frame?

 

[teklektik]

It did open out one dropout on my chopper frame.

Which suggests that the TA on that side was ineffective....

With a non-zero block time, the controller limit of 30A is suppressed on WOT getaway so the motor can essentially draw the battery limit. Perhaps big amps and only one working TA can account for this...

 

[warrah]

But weirdly enough the TA was ineffective in the opposite direction to what i expected ie i had braced the torque arms against the chainstays to take the force of the motor going forward, but i dont use regen so i only used jubilee clips to hold it there. When i said in the first post that the TA had slipped, i mean it was suspended slightly away from the chainstays, as though an opposite regen force had pulled it away and stretched the jubilee clip. Not sure what to make of it. I'll try post some pictures of the setup

 

[warrah]

Im up for an axle replacement; the greyborg guys said they only have v2 axles, and theyre checking whether those are compatible with my v1 cromotor. If its bad news and they arent compatible, maybe someone in the endless sphere has a dead cromotor they could press the axle out of? Ive been quoted £150 by a machinist to have the axle replicated, and in my desire to save on that sick price, i could help someone make a lot of money....

 

[warrah]

Got the axle out, and located the point of the twisting. The cable channel on a cromotor is milled out just a little too far into the 16mm shaft where the threads, a problem some others have encountered with their cromotors. It eats into the 16mm diameter, so a small section is more like 12mm x 10mm. This small area is where the twist has occurred. It is in the regen braking direction, which is annoying because i dont use regen, so im going to guess that braking in general has been the cause of this damage? whatever the case, the lesson here is that the jubilee clip style torque arms wont cut it. If they are braced against your bikes tubing somewhere, that direction is strong, but the opposite direction defeated my jubilee clip, stretching it gradually until it was too late. My next torque arm will need to be bolted to its own little bracket, so it can take force in both directions.

Steep, expensive learning curve! also, replacement axle will be made out of torque steel, wont have the overly long cable channel, and will be M20!!!! (there is room for it)