elastomeric donut coupling 1

[dexion7]

http://files.engineering.com/getfile.aspx?folder=89572820-07fc-4a5a-9dd4-6642833ac298&file=120.JPG

i'm trying to connect a bike engine to a diff in a kit car (about 250bhp). the engine is solid mounted and, conscious of the possibility of chassis twist at full torque, need to know the best way to achieve this.

the chassis size and packaging issues mean that there is a maximum of 40-50mm available between the drive flange and diff flange. also, the drive flange securing nut protrudes 20mm into this distance as well.

a dount coupling from a bmw 5 series fits perfectly and to try and reduce chassis twist (and thus stress on the coupling), various triangulation chassis members are being added to increase rigidity.

1. how much missalignment will these couplings take?
2. is it better to mount the diff solid or on rubber mounts?
3. would simply bolting the flanges together (no coupling used) work if the diff was rubber mounted?
4. since engine oil in a bike engine is shared with the gearbox, the drive flange will run at say 110c, will this affect the coupling? (even if some cooling process could be fabricated to duct outside air to the coupling whilst moving forward, the heat would still conduct through the coupling when the car is parked)

any comment /ideas / suggestions appreciated

 

[LionelHutz]

That type of coupler is used in the C5 and C6 Corvettes. The coupler for a 318i manual was used in the LS1 driveshafts and one a little bigger in diameter and thickness with M12 bolts is used in the newer models. It has big centering pins on the driveshaft. Those couplers actually have hard bushings for each bolt and a lot of cord in them for strength. They are a soft enough rubber they should give in all directions enough for the car you are proposing. I'd think you'd be fine with a similar coupler without a centering device. Remember to mount it with the arrows facing the flanges, not the bolt heads.

 

[dexion7]

there are no arrows on the genuine bmw donut or on the genuine (larger) range rover ones i have.

incidentaly, the bmw one (which is the one i will use) has poor manufacturing tolerance for use in my application. the distance across the steel inserts varied by up to .5mm and the faces of some of the inserts were not square to the bores.

this would presumably not be relevant in its normal role on a propshaft where it drives or is driven by a shaft which has two universal joints. in that event the two flanges of the coupling may work ok if not parallel.

my flanges MUST be parallel and to avoid the donut being forced to a permanent state of (probably acceptable) missalignment, i ground the faces of the inserts to be square and the length of each insert to be the same within a couple of thou.

i wont use a centering device but, whist not clear in the photos, the nose of the diff is accurately located relative to the gearbox flange. the coupling was supplied with a spherical spigot bearing as the usual instal for this coupling has a prop with two universals on.

the help given on this thread has given me the confidence to use the coupling and be happy with its ability to cope with the potential missalignment that may be present (more triangulation will help). i'm now lookinq at ways of limiting the heat transfered to it from the drive flange. all other uses discussed here have the coupling in a low thermal stress role

 

[Tmoose]

"the bmw one (which is the one i will use) has poor manufacturing tolerance for use in my application."

Every rubber coupling I've seen has been "bent" to some degree. An early ferrari restoration had a vibration at 1X driveshaft rpm. The oem style rubber coupling had a fairly typical amount of runout, but the trans mount was so soft that the relatively stiff coupling easily "had its way", flinging the transmission around at will. Similar to your machining, careful shimming at each bolt made it better, if not acceptable. Enthusiastic MIT students had designed and built a test rig for something or other. The drive motor was on isolation mounts, and "rubber tire" type couplings were used as u-joints at each end of a short driveshaft. The rpm was toward the upper range of what the coupling manufacturer would allow without centering devices, so the driveshaft could move off center and whirl if it felt like it, and the tires are very vulnerable to being assembled all skew-geed. The vibration of the flexibly mounted motor at higher speeds was impressive.

Both malfeasances were measureable as many thousandths of "runout" with a dial indicator when rotating components by hand. If a quasi static motion/runout check reveals 0.010 inch (10 MILS) TIR/pk-pk motion, there is going to be at least that much at speed. Balance weights are not likely to make the situation any better.

 

[LionelHutz]

I don't follow all your coupler alignment issues. You should be using two flanges with 3 bolt tabs or 3 bolt ears on each flange. 3 bolts should face one direction and pass through the coupler and then thread into the engine flange. The other 3 bolts should face the other direction and pass through the coupler before threading into the differential flange.

I didn't mention it before but I would hope you are not actually expecting to install the coupler as shown in your first picture. I had assumed you knew how these are to be installed and had just done that for a quick mock-up.

The length of each bolt bushing in the coupler will have no effect on the installation, within reason. Since there is only a flange on one side of each bushing the rubber can easily give the small amount required for each bushing to pull-up to the flange. Similarily, I would image that the face being exactly square to the bolt hole really would matter either as long as the bushing is not so bad it's putting a bending force into the bolt.

Also, every coupler I have seen has a little triangle or arrow on the outside diameter corresponding to each bolt hole. The triangle points towards the flange, not the bolt head.

 

[MikeHalloran]

Star for Lionel; good catch!



Mike Halloran
Pembroke Pines, FL, USA

 

[dexion7]

"I didn't mention it before but I would hope you are not actually expecting to install the coupler as shown in your first picture. I had assumed you knew how these are to be installed and had just done that for a quick mock-up.The length of each bolt bushing in the coupler will have no effect on the installation, within reason. Since there is only a flange on one side of each bushing the rubber can easily give the small amount required for each bushing to pull-up to the flange. Similarily, I would image that the face being exactly square to the bolt hole really would matter either as long as the bushing is not so bad it's putting a bending force into the bolt.2

well spotted eagle eyes! i'm well aware of this but the coupling is installed that way at the time of the photo for mock up purposes.

there is a spigot shaft which is an m8 bolt screwed in the gearbox shaft and the unthreaded part of the bolt into an 8mm hole (actually 7.9mm) machined into the diff pinion shaft. the differential has been removed from the diff housing saving 8kg and this means i can hang the diff housing on the spigot bolt. the diff is then perfectly aligned to facilitate mounting fabrication but to relieve the weight on the 8mm bolt i also put the coupling bolts in too, but 'direct' rather than offset as you point out they should be fitted. it was then i realised that when the assembly was turned, the diff rear end made a cyclic rotation and this was due to the variation in the length of the inserts (presumably causing a stress on the spigot).

thus to use the spigot without stressing it for fabrication requires additional support and the additional support was introducing errors which was corrected by machining the dount inserts!

its likley that this small variance of the inserts would not matter in use but for fabriaction it is significant

 

[HRHtex]

Evel - I had a 1965 998cc Cooper (not an "S"). This had the rubber couplings and I think this type of coupling persisted for quite few years after 1965 (in Oz at least). So it just wasn't very early Minis that had the rubber joints. I don't think the "S" types ever had rubber joints.

The Elan I drove would surge badly even at relatively constant speed - it really was terrible. I recently saw an episode of "Wheeler Dealers" where the Elan they were restoring had a similar problem. It turned out that the rubber couplings were starting to break up - so maybe the Elan I drove had the same problem.

 

[evelrod]

My first Mini was a '63 Austin Cooper and it had standard "U" joints...as a matter of fact, they are still in use in the latest powertrain of my '67 "S". As far as I know, at least in the cars modified by John Cooper Works, none have rubber joints. I have never owned a standard, non Cooper Mini or even driven one as they are not very commonly found on my side of the pond. Now for OZ...(-:...I dunno! You chaps do tend to do things somewhat differently from time to time. Sadly, my original Mini Cooper had a rather abrupt end, see attachment.

Oh, and your correct in the evaluation of the Elan's couplings...However, the problem, once it surfaced, should have been repaired with due diligence. I've been lax on occasion and let one fail...Not pretty and NOT cheap to repair. I did learn a valuable lesson as it relates to Rotoflex couplings...I also learned to not like them and to substitute other coupling types whenever I can economically/reliably do so.

Rod

 

[GregLocock]

The rubber band couplings are completely different in almost every respect to the Jurid type the OP was talking about.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?