Homers Favourite Subject :- Donuts

[TonyD]

Guys,

I am experiencing a horrible vibration coming from the rubber Donut in the drivetrain between the propshaft and the gearbox. The Donut is being asked to transmit power in a plane for which it was not designed and even though the propshaft supplier said the rubber donut could cope it isnt up to the job.

I need something more robust to act as a spacer between the gearbox and the propshaft( theres a spigot bearing between so I cant just extend the prop).

Is a simple aluminium spacer carved lovingly out of a billet up to the job or should I go all agricultural and get a simple steel lump profiled out and drill some holes in it for the bolts?

Opinions please. Engine puts out about 300BHP and I intend to use it!! and more........eventually.......

Cheers,

Tony

[Purple AK]

You don't say what setup you've got! But no Rubber dougnut is going to take more than 100bhp! Without shreading! Very Messy!!! You say who have a Spigot bearing, between the box and the prop?, Do you mean a centre bearing, on the prop? I can't see that any Cobra could need a centre bearing! Just go for a one piece prop! Gearbox, UJ, prop, UJ, Diff.

[Miket]

Dave

I have a BMW 7 series gearbox in my AK. this also had a rubber doughnut as a connection to the prop-shaft.

AK supplied me with a metal flange for the end of the box to replace the rubber.

:tu :tu :tu :tu :tu :tu :tu :tu :tu :tu

[russell_ram]

Tony,

Might be Homers favourite subject but is also mine! (Driveline NVH development).

First things first, your 'rubber doughnut' (more correctly an SGF manufactured Jurid coupling) is more than capable of handling the torque (NOT HP note) that you are trying to put through it. They are standard fit on all BMWs (inc M series) and many other high performance vehicles (eg Jaguar V8 SC). They are one of several types of joints used in drivelines to transmit the torque from the g/box output to the diff input while also allowing anglular mis-alignment of those two points and change of that mis-alignment with dynamic engine/axle movement.

An SGF coupling will only work correctly for small (typically < 3 deg) angles before they cause problems due to non constant velocity torque transmission, high articulation stiffness and problems with spigot bearing wear. They require the use of a centring spigot and bearing as they are non self aligning/supporting.

Hookes joints (UJ's) are more tolerant of angle (up to 10 deg max but much more comfortable at 1/2 to 3 1/2 deg) but are always non constant velocity. This is usually dealt with by mounting hooke joints in pairs (one at each end of the prop, ie having the second Hookes joint working to cancel the non linearity introduced by the first.

Third type of joint is a Constant velocity type (like those on front wheel drive drive shafts). These can cope with bigger mis-alignment angles (typically 10 deg in a prop application) without introduction of non constant velocity rotation of the prop (like the name suggests!).

Driveline vibration issues are norammly caused by one of three rotationally related vibration sources.

1) Propshaft 1st order vibration.
Caused by component out of balance (OOB) and/or system OOB. Low frequency vibration typically 30-80Hz, may be worse at at particular speed due to resonance of system components or just 'get worse the faster you go'.

OOB is either poorly balanced bits or run-out of mating flanges. In your case, check the fit of the male spigot of your SGF in to the female bearing - they wear badly and quickly at high angles. Get your prop balance checked - was it done when it was cut and shut?

2) Prop second order vibration
Caused by running a single (or mis-matched pair) of Hookes joints at high angle. You can run only one, but it must be at near zero operating angle (ie 1/2 to 1 deg or so). Same symptoms as OOB but higher frequency of vibration (actually twice).

3) Won't talk about CV generated vibrations cause not relevant to your problem.

4) Prop third order vibration
caused by an SGF trying to self align when run at high angles.

You cannot just replace a doughnut with a fixed aluminium plate - the plate will not allow any angluar mis-alignment between the g/box and prop (you could use a thin carbon disc though) or dynamic change of the mis alignment with engine load (the engine moving on its mounts). Something will break sooner or later.

Your problem is one, or a number of these issues in combination.

Check your driveline angles are 'small', check component balance levels, check wear in the SGF spigot bearing, check run-out of installed components, check UJ bearing wear - it must be one of these.

Russ

Mikes / AK solution of converting a SGF output 'flange' to a 'std' hookes type Jaguar prop is an acceptable conversion.

[BenBaron]

Where did that go??????

OH yeah right over my head :P :P

[Robin427]

Doughnuts, eh? Mmmmm....

Try applying full lock, revving the tits off your engine and dumping the clutch in 1st gear...

It's funny 'cause they're monkeys...

Robin

Drive it like you stole it...
http://www.creffield.com/cobra/index.html

[Steve]

Tony, I believe I happen to have a new unused donut from an m5 in the garage, originally from a BMW motorsport place, It was definately much stiffer than the old unit

It might be useful

let me know if you want to try & I can dig it out for you

Steve
GDEuro/LS6/T56 in progress
sgullick@waverider.co.uk

[Steve]

Tony

I also recall a prior post where you mentioned having "the worlds shortest prop" or something like that.

With such a short prop, and an offset diff flang, eare you sure the donut isn't being forced to flex further than it should?

Steve
GDEuro/LS6/T56 in progress
sgullick@waverider.co.uk

[Steve]

Tony - I found this in an old post of yours.....

"Well in actual fact he cut about 7 feet off it leaving me with less than 8 inches of actual prop with a uj and flange at each end......I therefore claim the prize for the shortest propshaft in the club"

does that mean you have a UJ at each end of the shaft, PLUS the rubber donut. If thats the case I don't think you can do that, 'cos you have 2 parts that flex, which will give rise to uncontrolled flex

Hope that helps

[TonyD]

>
>does that mean you have a UJ at each end of the shaft, PLUS
>the rubber donut. If thats the case I don't think you can do
>that, 'cos you have 2 parts that flex, which will give rise to
>uncontrolled flex
>
>
This is exactly what I have, a short propshaft with a UJ at each end and a rubber donut between the gearbox and the first UJ. I too think that it is the uncontrolled flex which is causing the problem.

Basically the propshaft is at an angle and pushing the Donut in an upward direction which then gets out of shape.

If the donut is kept in the same horozontal plane as it is in the beemer with one end attached to the gearbox and the prop supported by a propshaft bearing it wont get bent but with my driveline being at an angle the rubber donut gets bent out of shape.

Hence replace the now weak Donut ( it bends under hand pressure now) with a solid spacer plate and all should be well.....


Cheers,

Tony