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Finding a Good Coupler

4449 Views 50 Replies 7 Participants Last post by  57Chevy
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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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... transmissions that use a pilot bearing can be a bit wobbly at the input shaft when detached from the accompanying engine.

sifawangiaEV: Can you confirm if your crankshaft has a pilot bearing or not?
I don't think that it has a pilot bearing. I attached a picture of the clutch disc in my starting post so you can see that too for confirmation.
It is hard to determine if your gearbox used a pilot bearing based on pictures of the clutch disc. Gotta have pics of those shafts as KiwiME said, short of us doing research into your car's design.
Rather than hard, it's impossible to tell if the transmission input shaft is supported by a pilot bearing (or bushing) in the crankshaft from looking at the clutch disk. Fortunately...

The other end of the coupler that's hidden fits the motor shaft and part that's welded fits the transmission shaft. Here are the pictures:
View attachment 127822
... this image shows that the transmission input shaft is splined to the end, without a plain section to insert into a pilot bearing. A pilot bearing is normally needed in a traditional layshaft transmission, in which the input shaft is very short, carrying only a gear which transfers the drive to the layshaft and a coupling to the mainshaft for the direct "gear" (1:1 ratio). This Mini Cooper is typical of transverse transaxles which are all-indirect, meaning that the input shaft is long, carrying the input-side gear for every ratio; since the shaft is supported at two widely-separated bearings, the clutch disk can be safely cantilevered on the end so no pilot bearing is needed or used.

A 2006 Mini Cooper should be the last year of the first generation. If the Wikipedia article is correct, that means that - because it is the Cooper S version - it has a Getrag G285 6-speed transaxle (which was also used in the Ford Focus ST170 and SVT). It is a typical "3-shaft" transverse manual transaxle. This is a photo of the internals of a racing version (with straight-cut gears and dog ring shifting), showing that each of the three shafts (one input, two layshafts each driving the diff's ring gear) is supported by bearings are each extreme end:

The bearings visible (on top as it sits in this photo) are in the end opposite the engine, supported in the main case which has been removed; the bearings at the engine end (the bottom as it sits on the bench here) are in the engine-side case which is visible. The input shaft is the nearest one (without any shift elements on it); the differential housing area is just visible on the far side).

Edit note: replaced image with view from more useful perspective
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Maybe it'd be a better shot welding the clutch spline to a stump of shaft the same diameter as the motor shaft, then tighten the coupler on both the motor and the stump shaft how it was intended to work
The problem might be that there won't be enough meat over the splines to get to motor diameter, but that just means reaming half the coupler to a larger ID...but only if they don't already stock couplers in two different ID's.

No welding needed..
So the problem with that I think is the fact that the coupler works with a key so if I used it as intended, I would need to make a keyway in the clutch hub and I don't know if that would be very reliable/practical. Is there a way where I can have the machine shop lathe the coupler to have a bigger diameter and weld the clutch hub so that it sits in there? Or would tightening the coupler around the clutch hub without a key be enough to keep it in place?
In response to remy, this coupler brand does not sell couplers with different IDs. Do you think that using this method but welding the clutch piece in instead would work fine?
Why are you married to the coupler brand? Find one that works.

Making a keyway in a uniformly cylindrical (turned to same diameter full length) hub is trivial for a well equipped shop. Takes 10 minutes.
How much length do you have between the motor and spline, i.e. is there enough room to have a stump sticking out of the coupling with the spline inside? If so then you can get the spline EDM cut into a short section of turned bar and not worry about welding the clutch disc on, that way everything is as good as perfectly true to start with. Broaching or milling a keyway is no drama.
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Considering all that you all have said, I have two options to present to the machine shop. I will be presenting one of these options to the machine shop on Monday and ensure that the parts are concentric. I'll have the machine shop torque the screw to the rated toque for 1/4-28 screws which is about 13 ft-lb. Here are my two options:

Option 1: Remove the clutch hub from the coupler and then lathe the bore of the coupler to make it larger and also make the clutch hub diameter as small as it can be (while still maintaining its integrity, of course). Then they'll broach or mill a keyway into the clutch hub and tighten it inside the coupling, torqued to 13 ft-lb.

I really like this option and the fact that I will be using the coupler how it was intended, but in the event that things don't work out the way that they are supposed to, I may go for option two:

Option 2: I'm going to talk to the machine shop about the split in the coupler and ask what could be the possible cause of it. In the event that they figure out what caused the split and know how to fix it, I'll see if they can weld the piece back on and make it concentric. If there are no splits and the part is concentric, will this method work fine?

I will use a DTI to make sure that the part is concentric for both options. Will both options work fine? Please let me know what you think of the options.
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How much length do you have between the motor and spline, i.e. is there enough room to have a stump sticking out of the coupling with the spline inside?
There is enough room for a stump to stick out of the coupler. I'll need an adapter plate that's about 1/2" thick.
@sifawangiaEV, Thanks for posting the photos of the actual parts. Because it's now clear that the transmission input (clutch) shaft is already fully supported within the gearbox you should use a flexible coupling that can tolerate parallel misalignment. If you use a solid coupling your alignment must be perfect. That's a tall order and not something I'd recommend trying if any sort of reliability past a few weeks is your goal. So, when I say "should" I mean like an engineer speaking politely to his manager, I really mean "must". You're shooting yourself in the foot if you try to mickey-mouse this, which is regrettably the path you are already on.

There are a number of industrial coupling types that are suitable but you also have to ensure that the type chosen will be available with appropriate shaft options. Because of the frequent torque reversals I would stick to those that use clamping methods on the shaft(s), not just a split clamp but a proper taper-lock design. There may even be one for the spline according to the literature I'm referencing below. A small amount of torsional flexibility would not be a bad thing either, but not a lot.

Because the radial misalignment could be realistically kept to within 0.5mm TIR, one option that I found quickly is the Rexnord CentaFlex. It's quite complicated to navigate the engineering selection details but it's likely that their application department will do all that for you if you supply a fully-dimensioned drawing of both sides and max torque/RPM specs. You'll need to identify the spline dimensions as well and the desired gap between the shaft ends. When you get a recommendation from them you'll need to verify that it fits within the bellhousing.

There are many other coupling manufacturers but that's up to you to research. Following this advice will provide a reliable solution but I'm going to wish you the best of luck and leave the rest up to you, as I have to move on to other things.
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@sifawangiaEV, Thanks for posting the photos of the actual parts. Because it's now clear that the transmission input (clutch) shaft is already fully supported within the gearbox you should use a flexible coupling that can tolerate parallel misalignment. If you use a solid coupling your alignment must be perfect. That's a tall order and not something I'd recommend trying if any sort of reliability past a few weeks is your goal. So, when I say "should" I mean like an engineer speaking politely to his manager, I really mean "must". You're shooting yourself in the foot if you try to mickey-mouse this, which is regrettably the path you are already on.

There are a number of industrial coupling types that are suitable but you also have to ensure that the type chosen will be available with appropriate shaft options. Because of the frequent torque reversals I would stick to those that use clamping methods on the shaft(s), not just a split clamp but a proper taper-lock design. There may even be one for the spline according to the literature I'm referencing below. A small amount of torsional flexibility would not be a bad thing either, but not a lot.

Because the radial misalignment could be realistically kept to within 0.5mm TIR, one option that I found quickly is the Rexnord CentaFlex. It's quite complicated to navigate the engineering selection details but it's likely that their application department will do all that for you if you supply a fully-dimensioned drawing of both sides and max torque/RPM specs. You'll need to identify the spline dimensions as well and the desired gap between the shaft ends. When you get a recommendation from them you'll need to verify that it fits within the bellhousing.

There are many other coupling manufacturers but that's up to you to research. Following this advice will provide a reliable solution but I'm going to wish you the best of luck and leave the rest up to you, as I have to move on to other things.
I called Rexnord and some of their distributors and they told me that they can’t make couplings with splines because of how precise that they have to be. So, sadly, I won’t be able to use a CentaFlex coupling for this application.
I was working with a local machine shop on making the coupling better and they were able to take off the weld and redo it. The weld that they used was TiG weld. And it has no streamline cracks. The only downside is the TIR rating of .006”. Thé machine shop said that they were getting different ratings based on where they were taking the measurements on the splined side of the coupling. They concluded that it’s most likely because the coupling is used (186 thousand miles in the car) and that it would most likely be fine since I’m putting it on the shaft that it came from and worked with.
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When I tried to put the coupling on the transmission, it wouldn’t slide on all of the way. It just stopped. So basically, the coupling and the splined part shrank due to heat when it was being welded.
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I decided to tap the coupling on the transmission shaft and pull it off until it was able to slide on and off easily. I’ve only gotten it to slide on a little bit more though. If this method fails, I will use a brake cylinder hone to shave down the inside diameter of the splined coupling just a little bit. I will also have to do this for the motor side because that side of the coupling shrunk as well.
My other option is to buy a new clutch plate and have the machine shop weld that clutch plate on my coupling instead (but not letting the coupling shrink). Please let me know what you all think.
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I'm glad you are finding all these little things out early on, far better now than on the side of the road in six months with a thrashed gearbox spline. 0.006" is a lot of runout, you'll likely feel that binding as it rotates. Welding will always cause some amount of distortion, there isn't really any way around it other than old-man skill and knowledge of the welder.

A better way would be to EDM the spline, that way you start with a single piece fully machined and so very little runout.
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Yeah, 006" TIR is pretty horrific for the RPMs and physical scale of the system when using a rigid coupling. It will end in disaster. I'm not surprised that the splined hub warped during welding and that's an unfortunate compilation. But I'll repeat that using a rigid coupling is a bad idea anyway.

Almost all modular (meaning configurable) flexible couplings can be purchased with a blank end (with pilot hole only) and could be machined to suit your requirements. EDM would certainly be ideal, no-doubt pricey but you get what you pay for. I'm a bit surprised that in the US of A you can't get this sort of thing done easily? On a remote island in the South Pacific where I live I'd expect that.

I think your machine shop is leading you astray by telling you it will work. More research into flexible couplings is my recommendation. It would be useful if you can identify the spline size on the Mini gearbox. Additionally, creating a proper engineering drawing that you can send out for quote showing the two sides would make your search so much easier because application engineers can quickly interpret the requirements from that without any verbal or written explanation.
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Kiwi, the problem with a jaw coupling is that the spline doesn't axially locate the gearbox half and it'll separate. The other issue is that a flex coupling needs both halves to be rigidly secured and the spline isn't very compatible with that. A lightly press-fitted spline in a blind cavity would work. A loose spline will hammer out with a flex coupling if there is any misalignment, so may be easier to use a solid and clock it up properly during install. Btw, I'm up the top end of the islands
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