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Discussion Starter · #1 ·
Hi everyone,

I'm in the process of choosing a coupler to mate my Warp9 DC motor with my Mini Cooper manual transmission and I've come across a couple of options. I'm browsing grainger.com and their Rigid Couplers here and I've come across a few coupling types that I think will work for my conversion. Here are the different types (on the grainger.com website):
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I've heard that these types of couplings have been successful in EV conversions so I thought I might give one a try. I'm leaning towards using either the 1-piece clamp rigid shaft coupling with a 1.125 inch bore diameter:
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Link (https://www.grainger.com/product/RULAND-MANUFACTURING-Rigid-Shaft-Coupling-1-1-8-2ALJ3?Pid=search)

Or using the 1-piece set screw rigid shaft coupling with a 1.125 inch bore diameter:
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Link (Rigid Shaft Coupling: 1 1/8 in Bore Dia., Steel, 1 7/8 in Outside Dia.)

Are their any benefits/drawbacks to using one coupling over the other? I'm thinking of having a machine shop cut and weld my clutch plate onto the transmission side of the coupling. What does everyone think of this? What coupling do you recommend?
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if you make it rigid then your alignment needs to be absolutely perfect and you need to 'clock' the shafts to each other first by putting a DTI on one shaft and rotating it around the other in several places to ensure it is both parallel and concentric. This is not an easy task if your coupling lives hidden inside a bellhousing and even harder if you don't have a shimming/adjustment system. The jaw couplings allow easy assembly plus have a stated acceptable misalignment amount. Any misalignment in the shaft that is forced into alignment by tightening a rigid coupling will cause extra stresses and lead to something failing.
 

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Discussion Starter · #3 ·
if you make it rigid then your alignment needs to be absolutely perfect and you need to 'clock' the shafts to each other first by putting a DTI on one shaft and rotating it around the other in several places to ensure it is both parallel and concentric. This is not an easy task if your coupling lives hidden inside a bellhousing and even harder if you don't have a shimming/adjustment system. The jaw couplings allow easy assembly plus have a stated acceptable misalignment amount. Any misalignment in the shaft that is forced into alignment by tightening a rigid coupling will cause extra stresses and lead to something failing.
What kind of coupler would you recommend?
 

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I'm using direct drive so no. I have used couplings on large engine driven gas compressors and have seen a variety of failure modes due to the things I have tried to outline. Whatever coupling you choose will work as it was intended, if you use it as it was intended. If your guy just has a hunch that it won't work then ask him for evidence/why. Often people can't explain their 'engineering' assessments, sometimes they can and you learn something
 

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Discussion Starter · #7 ·
Ok, I think I'll give it a shot based on what you've told me.
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I will be purchasing the coupling that is specified in these stats (link) with a 1-1/8" bore diameter. Will I need to purchase two steel hubs to make a complete set (versus one) as well as the spider and the steel spider cover, totaling out to: 2(61.13) + 25.58 + 38.45 = $186.29?
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Discussion Starter · #8 ·
I'm pretty set on buying this part but I would like if anyone could approve or disapprove buying this part so I could feel more confident of my purchase if you know what I mean :)
 

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if you make it rigid then your alignment needs to be absolutely perfect and you need to 'clock' the shafts to each other first by putting a DTI on one shaft and rotating it around the other in several places to ensure it is both parallel and concentric. This is not an easy task if your coupling lives hidden inside a bellhousing and even harder if you don't have a shimming/adjustment system.
As soon as you release the clutch pedal, a traditional clutch setup is also a rigid coupling. The alignment of the coupler is reset every time you push and release the pedal, and that's okay because the engine and transmission are properly aligned, and in a traditional setup the short transmission input shaft is supported and aligned on the engine end by a pilot bushing or bearing.
 

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As soon as you release the clutch pedal, a traditional clutch setup is also a rigid coupling. The alignment of the coupler is reset every time you push and release the pedal, and that's okay because the engine and transmission are properly aligned, and in a traditional setup the short transmission input shaft is supported and aligned on the engine end by a pilot bushing or bearing.
Ah, didn't realise he was using a clutch too so yea, those little metal tabs on the periphery of the disc inside of the lining are the flexible coupling. In fact having a flex coupling and a disc would not be a good idea. Otoh, two separate items with average alignment ability joined by a rigid coupling ends in damage eventually.
 

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Discussion Starter · #11 ·
Ah, didn't realise he was using a clutch too so yea, those little metal tabs on the periphery of the disc inside of the lining are the flexible coupling. In fact having a flex coupling and a disc would not be a good idea. Otoh, two separate items with average alignment ability joined by a rigid coupling ends in damage eventually.
As soon as you release the clutch pedal, a traditional clutch setup is also a rigid coupling. The alignment of the coupler is reset every time you push and release the pedal, and that's okay because the engine and transmission are properly aligned, and in a traditional setup the short transmission input shaft is supported and aligned on the engine end by a pilot bushing or bearing.
Sorry if I didn't make this clear before but I'm going clutchless so I won't be using my clutch pedal or a clutch at all. I just want to connect the motor shaft to the transmission shaft with a coupling. Would the McMaster Carr coupling work fine for this application?
 

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It is your project, ultimately you have to make the decision, otherwise you are outsourcing it to randoms on the internet.

What are you using on the spline, are you going to weld the centre of a clutch onto a coupling or how are you doing that part? A solid coupling will be preferred but only if the shafts are in good alignment. Any misalignment in the housing that results in stressed applied to the shaft as it tries to rotate where it wants to go will end up breaking something eventually. Use a solid coupling but also know that it is suitable for a solid first.

This is why an automatic transmission uses a flex plate and why a manual trans uses a flex coupling in the friction disc. These are done by design for a very good reason- automotive parts aren't good enough tolerance in the worst case situation out of the factory to reliably use a solid coupling. Solution: chuck a flex in.
 

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Ah, didn't realise he was using a clutch too so yea, those little metal tabs on the periphery of the disc inside of the lining are the flexible coupling. In fact having a flex coupling and a disc would not be a good idea. Otoh, two separate items with average alignment ability joined by a rigid coupling ends in damage eventually.
Sorry that wasn't clear, I didn't mean that there's a clutch in the proposed coupling, only that a clutch obviously works and isn't compliant. Yes, the springs in the clutch disk allow torsional compliance, but shouldn't allow radial or angular compliance.

But speaking of how things move...
just as with a normal clutch setup, something needs to have a sliding fit and remain free to float axially to make assembly practical. That's normally a splined connection, and the transmission has a male splined shaft, so the coupler rationally should be clamped to the motor shaft and have a matching female spline to slip over the transmission input shaft, with a pilot bore for the transmission shaft end if that's what the original transmission installation does.
 

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This is why an automatic transmission uses a flex plate and why a manual trans uses a flex coupling in the friction disc. These are done by design for a very good reason- automotive parts aren't good enough tolerance in the worst case situation out of the factory to reliably use a solid coupling. Solution: chuck a flex in.
But every production EV in which the motor shaft is not also the transmission input shaft couples the motor to the transmission input (which is exactly what we're talking about here) with a splined shaft connection and nothing flexible.
 

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But every production EV in which the motor shaft is not also the transmission input shaft couples the motor to the transmission input (which is exactly what we're talking about here) with a splined shaft connection and nothing flexible.
I don't know what the tolerance specs for an EV are but if that is their approach then I'd wager a buck that they don't use the same fit and tolerance bands as an ICE. Just don't assume that a DIY EV with unknown alignment has the same degree of accuracy as an EV OEM.

Given that the input is splined, it kind of rules out a jaw coupling anyway. A solid coupling with a female spline and a mount that has known and predictable alignment would be the best situation. Just don't chuck one in, hope that it will be fine, and then be upset later when it fails
 

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But every production EV in which the motor shaft is not also the transmission input shaft couples the motor to the transmission input (which is exactly what we're talking about here) with a splined shaft connection and nothing flexible.
That's true for the Leaf and the legacy Hyundai/Kia family comprising the Kona/Niro/Soul and classic Ioniq. The splined coupling with adjacent radial locating diameter and closely-located bearings each side is clearly an over-constrained design but Nissan seems to have gotten away with it, presumably by precise production machining methods.

Hyundai/Kia on the other hand did not and there have been quite a few issues surrounding the spline, starting with a knocking noise. In many cases only the gearbox or motor are replaced under warranty and the noise returns. It seems that most successful fixes involve replacing both. This has been ongoing for 3 years now and they seem to be slowly addressing the problem. The motor costs around US$6.5k and the gearbox US$1.5k. I have a Kona from 2018 and am lucky, it's quiet.

The new E-GMP platform seems to have moved to the design tactic adopted by Tesla and no-doubt others where the spline provides support for one end of the pinion shaft, so three bearings in total across the motor and gearbox. The Bolt is even more simple, the pinion gear is simply cantilevered off the motor output shaft. This is not a mistake GM's experienced powertrain engineers would make and the entire gearbox design shows good attention to detail. The legacy H/K and Leaf to lesser extent look no more advanced than a 1950s BSA motorcycle.

Used oil analysis from Konas and Leafs show that the Kona has typically twice the contamination of the Leaf regarding iron and aluminium, and at half the distance driven.

It's fair to say that a gearbox to transfer case splined coupling on a typical 4x4 is not dissimilar in principle but generally one side will have a longer distance to the bearing such that a tiny amount of misalignment can be tolerated.

But there's another issue that legacy H/Kia seem to be weak on and that is draining shaft currents from the motor. There is a circular conductive brush at the motor output shaft but still the gear oil turns black in short order, under 10,000 km or miles. A small number of Ioniq and Kona owners have had unexplained bearing failures, tellingly one leaving no significant ferrous debris. The Leaf gearbox design uses a grounding brush assembly on the intermediate shaft but I'm not clear if there is also one on the motor output as well. Perhaps someone here can confirm that one way or the other? The problem I see with circular brushes is that a tiny amount of oil contamination can reduce the effectiveness. The Leaf's brush (pair) is contained in a dry, sealed cavity.
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Note the shagged out spline on this example.

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Kona/Niro gearbox showing proximity of pinion bearing to spline, upper-right.
 

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Ok, I think I'll give it a shot based on what you've told me.
View attachment 127472
I will be purchasing the coupling that is specified in these stats (link) with a 1-1/8" bore diameter. Will I need to purchase two steel hubs to make a complete set (versus one) as well as the spider and the steel spider cover, totaling out to: 2(61.13) + 25.58 + 38.45 = $186.29?
View attachment 127472
Thanks for your nice informative and necessary post.
 

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Discussion Starter · #18 ·
Sorry everyone if I've been all over the place but I've decided to go with a rigid coupling because it would be the longest lasting for my conversion and I'm willing to do the extra tough work to make it work. I just dropped it off at the machine shop with my clutch plate to be welded together, when I realized something not so good...the torque of the rigid coupling wasn't as high as I thought it was. Here's the spec sheet:
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I could've sworn that when I bought the item, it said that it had a torque rating of 2,400 in-lb. I'm thinking there might be a glitch on the website but I'm not certain because there is a chance that I somehow read it wrong. What do you all think? Does this rating look right for this coupling or does it seem like a glitch? Should I be considering a new coupling?
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I'll stick my nose in because this sort of thing was within my profession before I retired. Assuming the Mini gearbox has a conventional input shaft the first issue is that the pilot end of the input shaft must be supported just like it had been by the crankshaft. Gearboxes normally rely on this, that's why they appear somewhat loose when you wiggle the input shaft on the bench.

Then you have to transfer the torque off the keyed motor shaft to the spline without loading the spline unevenly. You rely on the involute teeth under torque for alignment.
A normal clutch disk of course is rattle-loose in the middle so that can happen freely (and cheaply).

A huge problem with a keyway-type coupling half is that the constant back and forth torque can shag out the key if the coupling is "hard", but allowing slight rotational flex will help that, which is what the rubber does, another requirement to add to the list. The clamp style coupling half (as used in the type you've identified) can help or at least delay the issue. But a far better design is what's called a taper-lock hub (see image) where there is a tapered sleeve between the motor shaft and coupling half that is drawn in by screws. These are really effective, far better than what you've chosen.

Generally engineers send the dimensional details of the two sides to the coupling manufacturer with the maximum torque and RPM and they determine the best product for the job. Hacking something together especially involving welding may work for a while but it's not a professional solution unfortunately.

One suitable design would be a rubber-based flexible coupling (see image) with both radial and bending flexibility that is configured with a taper-lock on one side and machined to match the spline on the other, allowing the pilot journal to poke through the center and locate into the end of the motor shaft - with a bushing as needed. Or the pilot support can be part of the motor side coupling half. It's only holding it centered just like a pilot bushing or bearing in a crankshaft. This would allow a radial misalignment error of perhaps 0.001 to 0.003" between the motor and gearbox, entirely practical.

If you want more assistance a useful photo would be of the two physical parts set to the locations you need.

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Discussion Starter · #20 ·
I'll stick my nose in because this sort of thing was within my profession before I retired. Assuming the Mini gearbox has a conventional input shaft the first issue is that the pilot end of the input shaft must be supported just like it had been by the crankshaft. Gearboxes normally rely on this, that's why they appear somewhat loose when you wiggle the input shaft on the bench.

Then you have to transfer the torque off the keyed motor shaft to the spline without loading the spline unevenly. You rely on the involute teeth under torque for alignment.
A normal clutch disk of course is rattle-loose in the middle so that can happen freely (and cheaply).

A huge problem with a keyway-type coupling half is that the constant back and forth torque can shag out the key if the coupling is "hard", but allowing slight rotational flex will help that, which is what the rubber does, another requirement to add to the list. The clamp style coupling half (as used in the type you've identified) can help or at least delay the issue. But a far better design is what's called a taper-lock hub (see image) where there is a tapered sleeve between the motor shaft and coupling half that is drawn in by screws. These are really effective, far better than what you've chosen.

Generally engineers send the dimensional details of the two sides to the coupling manufacturer with the maximum torque and RPM and they determine the best product for the job. Hacking something together especially involving welding may work for a while but it's not a professional solution unfortunately.

One suitable design would be a rubber-based flexible coupling (see image) with both radial and bending flexibility that is configured with a taper-lock on one side and machined to match the spline on the other, allowing the pilot journal to poke through the center and locate into the end of the motor shaft - with a bushing as needed. Or the pilot support can be part of the motor side coupling half. It's only holding it centered just like a pilot bushing or bearing in a crankshaft. This would allow a radial misalignment error of perhaps 0.001 to 0.003" between the motor and gearbox, entirely practical.

If you want more assistance a useful photo would be of the two physical parts set to the locations you need.

View attachment 127751 View attachment 127753 View attachment 127752
Thank you for your input. I may try looking into this but I have already sent both the coupling and the clutch plate to the machine shop to be welded together. I think that I might just try to align the couplings the best I can to ensure the most minimal misalignment and keep it on for maybe a year or 2 before changing it.
I have pictures of the clutch plate and the rigid coupling above. Could you send links to the 2 different coupling that you recommended so I could look into it? Thanks!
 
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