motor spline failure

[mxs]

We are building EV shuttle buses using a motor with splined output shaft and a spline coupling shaft to the gearbox. There have been a lot of failures of the motor and shaft spline since we switched to using a new gearbox a few years ago.

The old coupling shaft was broach cut whereas the new one is cut with a shaping tool. Because the old gearbox had 24 tooth spline the same as the motor, but the new one has a 23 tooth. There have not been any failure of the gearbox spline, but it's spline is also much longer than the motor side.

The coupling shaft is now sold to us by the motor vendor. In the past we had been using an overseas supplier with a really rough EDM cut, and then went to the gearbox vendor who was... quite literally... cutting the spline in the wrong area of the shaft. Since moving to the part supplied by motor vendor, the failures of the new part have been much lower but they are still occurring and we are not sure of the cause. I'm waiting to hear back on the hardness and material of the shafts. IIRC they are both hardened to around Rc 55.

I think there are two possibilities, either the female spline is being cut wrong in the input shaft, or the motor and gearbox input shafts are not in alignment.

If the shaft spline is cut wrong it would be possible to move to a two-piece shaft where each side is broach cut separately and then bolted together. The broach cut seems to fit better than the shaped cut shafts ever have, although the motor manufacturer says it is within tolerance.

The motor manufacturer recommended removing the locating pins from the bellhousing ("plate") to the gearbox so the coupling shaft itself is used to locate the two shafts together. The old gearbox had a built in locating device and no plate was necessary. (of course, it was a sh*t box with a ton of other issues internally).

We have seen these splines fail from 10,000 miles or sooner. Of course there are some that are over 100,000 without failure. It seems like there are some vehicles that have a much higher chance of failure. To me this would indicate that when the shaft is replaced the mechanic is not installing the motor correctly, creating (or failing to correct) an alignment problem between the two shafts.

I'm attaching some photos of the failed parts, IDK if anybody can tell me from the photo what is causing the failure.
Thanks!

 

[EVFixer]

"This would not ensure the best shaft alignment, since it is only based on "feel" of loose parts and not on shaft alignment measurements."

My feelings exactly.

 

[TugboatEng]

I believe it's SAE J744 that covers the dimensions of mountings for flanged motors and pumps. The clearance is tight, I think on a 6 inch pilot the clearance is around 0.0015" so 0.005 is quite large.

 

[SwinnyGG]

Are you basing that .005 estimate of maximum error on measurements of a single part (or set of parts)?

Depending on real-world manufacturing accuracy of your mounting plates and whatever locates them, your actual window could be much, much larger.

 

[EVFixer]

We sent out three parts, one prototype and two production. I compared the prototype and the worse of the two production and as close as I can get is the .005 based on the holes for the alignment pins.

 

[BrianPetersen]

Maybe it's time to redesign that shaft connection to use a flexible coupling of some sort, which is clamped in place over the shaft ends so that there is no spline backlash. Lots of such couplings are commercially available.

Automotive automatic transmissions have a piece called the "flex plate" involved in connecting the crankshaft to the transmission's input shaft. Take one guess what its purpose is.

Automotive manual transmissions also allow for this. The clutch housing is rigidly bolted to the flywheel, but the friction disk (which leads to the transmission input shaft) aligns itself wherever it wants to, and there is always some sort of spring-loaded mechanism inside it so that drive torque pulsations are taken up.

 

[TugboatEng]

There are plenty of hydrostatic transmissions out there that used flanged mountings and splined couplings. Building the mounting flange to SAE J744 dimensions provides satisfactory alignment for other similar applications and allows the use of off-the-shelf parts. I have also seen DIN 5482 splines used in conjunction with SAE 744 fkanges. (Bosch does this).

I can't emphasize this enough, a splined coupling is essentially one half of a gear coupling and it MUST be lubricated. Grease does not work well as a lubricant in gear couplings as it tends to get seperated into its base components by centrifugal forces. There are special greases for gear type couplings like Kop-Flex by Honeywell. Removing one of the seals and allowing the spline cavity to flood with oil is really the best option for lubrication, though.

 

[EVFixer]

BrianPeterson, we have looked at some flexible couplings but haven't found any that are good for 8000 RPM.

 

[gruntguru]

There are lots of high speed couplings around, mostly for turbo machinery. Flexible disc is one type capable of high speed and torque.

je suis charlie

 

[EVFixer]

gruntguru, it looks like a flex disc coupling would be good if we had an angular misalignment. Our problem seems to be a parallel misalignment.

 

[SwinnyGG]

Flex disc couplings for parallel misalignment are off-the-shelf items.