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twin engine crank behaviour. 9

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ivanheow

ivanheow

Automotive
Oct 28, 2005
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[ponder]hello to the collective ,

i am constructing a twin engined 1940's type racer . the 2 engines are straight 6 jaguar lumps, aluminium block 4 litre 245 bhp engines . i am tentatively designing them thus ..

gearbox/clutch/flywheel unit(remote).........10" prop........uj mounted to crank end .....engine.........crank snout to 400 lb/ft jurid donut.............crank end......engine.....snout with stock crank pulley harmonic balancer .


so , i have gone from a trq converter on the end of each crank ,to a prop to a flywheel on one ,and a jurid donut on the other .and from a damper on each to a damper on one .
these engines tend to run very smoothly ,and have good balance as stock .

in the past i have converted a c4 auto to clutch ,created 2 gearboxes back to back ,and the fastest turbo minis use my bespoke clutch design .so have a little transmission experience , but not much on harmonics and acrrued vibration re no flywheel.


thoughts?

regards
robert



 
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thanks chaps ,

re the shrapnel , there will be substantial guardage around all exposed rotating bodies ,being impaled is never fun .

re ignoring advice , oh greg ,my little button , how wrong you are , i am taking it all on board ,and as you may have not noticed ,have added another crank damper ,solely due to this forums freindly warm ecourageing and adventurous input.


re the the keyways and the meat left betwen the root of the keyway slot ,and the threaded hole ,if you study the top fuel and other high 'supercharger bhp draw' engines out in the ionosphere,you csan see they seem to get away with it ,without launching snoutage .


it appears to be a case of , if its not bult , no one will know if it can work ,like so many of my projects , and edisons light bulb.
(the only difference of course is edison knew what he was doing )


regards
robert

 
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If you're referring to the stars, you'll notice they're next to posts by Pontiacjack{{/b] & MikeHalloran. The stars are anonymously given by other members; and pertain particularly to the specific post where they appear, not to the thread in general...

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
 
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re: top fuel cranks.
No Jaguar crank snout has dimensions as robust as a fueler's billet crank snout. "Normal" cranks must find a compromise between number of keyways and integrity of the resulting configuration.
 
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ivanheow-

The crank snout on most engines is not designed to transfer much torque. The damper/pulley commonly use an interference fit, and it's the friction from this interference fit that provides the primary torque transfer capability. The keyway is just there to provide indexing. At the crank/flywheel connection a similar situation exists. The clamping force of the bolts must be sufficient to transfer all of the crank torsional moments by friction, without any slippage at the joint face. At either joint, if there is even the slightest amount of relative motion at the contact faces there will be fretting damage, and surface fretting damage will create fracture initiation points.

Another important thing to consider is the design of the coupling. Even if the two engine blocks are rigidly connected, the crank coupling should allow for some angular misalignment/compliance during operation. If the coupling does not have sufficient compliance in bending it can easily produce enough deflection in the cranks to cause edge loading of the rear main bearing of the forward engine and the front main bearing of the rear engine.

Good luck.
Terry
 
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[2thumbsup]hi terry ,

thank you for the input ,

it mirrors my thinking ,the usage or a drivelive gurid coupling is to take into account exactly what you refer too .

re the pulley , i think that i will make the new pulley an interferance fit ,and also, after a bit more research i am going to add one more 1/4'' keyway , this in addition to the 3/16th keyway ,matches the design used by engines using a large rootes blower and substantial bhp to drive it . in fact , i must get on with a full engine design computer model to see just what sort of bhp the large blowers consume . .

regards
robert

 
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Chain couplings are available from several sources, capable of dealing with reasonable imperfections, etc.

They have one more virtue that may be of interest.
With the chain removed, there is room to sneak a poly-V belt through the resulting gap, so you don't have to remove the front engine to change the rear engine's accessory drive belt.



Mike Halloran
Pembroke Pines, FL, USA
 
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"The damper/pulley commonly use an interference fit, and it's the friction from this interference fit that provides the primary torque transfer capability."

Can't speak for modern V8s, but 60s and 70s Olds and Buick, unlike Chevy used slip fit dampers and big bolt torque to provide end clamping for the friction.
REd block Volvo 4 cylinders thru '95, early Miata, and 1996 Integra non-V-tec are also slip fit, high bolt torque.
 
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It seems to me that one way to engineer a high torque connection to the crank snout of the rear engine would be to go with splines. Obviously there would be some stress & materials analysis needed to determine an appropriate spline pattern, given the crank parent material & OD. No guarantees from yours truly that there is an adequate solution; I'm just speculating. I'm thinking of perhaps a companion flange splined onto the crank snout. Ideally an off-the-shelf companion flange, e.g. from a 3rd member would be selected as the mating component. Precision machining would be needed to modify the crank snout accordingly. Completely separate means for handling axial and angular misalignment would still be needed, as discussed above.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
 
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I'm not picking on SireCustoms' interestingly engineered vehicle. I like it a lot.

At 0:11 it looks like the rear hub attaches to the rear engine using 3 bolts into the damper hub, and a lone bolt clamping the center of the damper, so he's likely back to the OEM attachment of the damper hub to the crank.

The centering stud that engages the "other" hub is common for those style couplings in driveshaft service, since radial stiffness and positioning is required. For reasonable radial misalignment tolerance between the engines I'd lose the stud, although that may be the least of his problems.

This entertaining video of excercising the vehicle a bit is supposedly from 2009.

So it might be interesting to ask how the coupling, crank snout attachment, and shared stressedmember headers have been holding up.
 
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hemi said:
It seems to me that one way to engineer a high torque connection to the crank snout of the rear engine would be to go with splines.
Click to expand...

hemi- Good thought. I imagine it would be possible with most cranks to re-machine the snout with an external spline. The external spline would provide lots of options for attaching couplings. It would also be more economical to rework an existing crank than it would be to have a custom crank made. A spline joint at the snout would greatly benefit in terms of durability/wear from oil lubrication.

You would want to check the modified snout for adequate fatigue life. It would take a bit of work to determine the net torque profile produced at the front flywheel, but determining the rest of the inputs for the analysis should be fairly straightforward. Of course, unless you have fatigue properties for the crank material you would also have to make some assumptions about acceptable stress limits.

Regards,
Terry
 
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hello tmoose ,

i have been in discussion with yannick sire ,on just that topic ,i asked did he use the locating pin ,and he sent me a link to that video .i am thinking along the same lines as you , with two engines in line and securely located , is that pin a help or an unnecssary encumberance,and a litle radial flex would work better?

 
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tbuelna, thanks for the feedback. My thinking is, if there is enough meat on the crank snout to allow for a spline pattern from a passenger vehicle sized transmission output or 3rd member input, you're into a pretty high torque capacity ballpark, considering first gear multiplication of engine torque, i.e. for a given spline design in its originally intended application behind the transmission. This would provide encouragement to go forward with the stress & fatigue analysis as you outlined. The big unknowns, other than the material properties, of course would be the dynamics resulting from the coupling of the two engines.

I see you must be considering a free floating joint, as on a slip yoke. That may well be a common off the shelf companion flange application. I suppose the companion flange could also be rigidly fixed to the crank snout, e.g. with a central bolt & corresponding features that constrain the axial location of the companion flange with respect to the crank. One other configuration that comes to mind is a tapered spline, assuming such a companion flange could be sourced. This would result in a press fit onto the crank, again retained by a fixing bolt. I have seen tapered splines applied to steering Pitman shafts; this may be commonplace but I don't know as I haven't examined very many steering boxes that closely.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
 
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My instinct would be to get a slow taper machined (pref ground) on the rear engine crank nose with a custom corresponding internally tapered flange with lugs to take the donut coupling out of a hide grade steel in one. Sweat the flange onto the crank, you may need to make up a thin split steel ring shim to keep the crank seal away from the hot flange 'til it cools. If you can machine the seal housing to take a bigger id seal as large as the timing cover casting allows, by any means, so that the tapered flange part of the coupler is as large an od as is practible. Getting it all apart for servicing etc will be the problem. At worst it shouldn't shatter, just fret, if it doesn't hold. Put an alignment mark on the coupling/timing case referenced to a known engine timing position (tdc) etc so it can be monitored.

Something like the Hirth oil type coupling or similar, as used as on the Cov Climax flat 16, if still made, may work as your 'flexible coupling', should reduce the torque spikes that made it unsuitable for that particular application.

The coresponding front engine hub speaks for itself, but again high grade billet not welded. That welded stuff doesn't do it IMO.

I also agree with not bothering with the alignment snout/bush, just getting the engines into acceptable alignment.

Normal caveats about 'distance internet info apply.

"It's not always a case of learning more, but often of forgetting less"
 
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