Joining two crankshafts

[normspanners]

When one cylinder fires it will act to tighten the bolt, when the other fires it will act to loosen it. I can't see that holding even with a shallow .

The fly wheel of the engine would be on the non joined side so that would stop it loosening, the joined crank would only be on the tightening side, 'tightening' into the flywheel of the first engine,also a taper+nut is what holds the flywheel on ! have you ever had to take a fly wheel off a small engine, you have to use a puller to split the taper.
A taper is the most accurate secure way of joining two shafts as long as both have been 'clocked' properly before machining they will have to be perfectly in line plus a little stud lock 643 and you would not loosen it.

 

[jinjahbiscuit]

There's no need for threads. Many motor cycle cranks are machined for an interference fit only, pressed together and keyed for alignment.

Once built up they're mounted on vee blocks and trued up using a dial indicator and nothing more than a mallet.

Keep it simple.

The kawasaki guy made up 5 cylinder machines, regularly featured in the bike mags. I think he's based in Berkshire.

 

[jinjahbiscuit]

The Kawasaki builders' name is Allen Millyard.

 

[normspanners]

From my memory of being thirteen and taking my crank to school to use the lathe to set up my 52 matchless 500 crank, the big end pin was nutted on either side-----
Also if using 360 deg you would just need a double ended wasted spark coil with no distributor as it would fire both plugs at the same time off the one set of points.
N

 

[Pete.]

The fly wheel of the engine would be on the non joined side so that would stop it loosening, the joined crank would only be on the tightening side, 'tightening' into the flywheel of the first engine,also a taper+nut is what holds the flywheel on ! have you ever had to take a fly wheel off a small engine, you have to use a puller to split the taper.
A taper is the most accurate secure way of joining two shafts as long as both have been 'clocked' properly before machining they will have to be perfectly in line plus a little stud lock 643 and you would not loosen it.

Yes I've pulled many a bike flywheel and I've yet to see one that relied solely on the taper - they have all been bolted.

If you have a bolted connection and a cylinder either side with the cranks turning anti-clockwise viewed from the left, every time the left cylinder fires it will try to accelerate the right cylinder thus trying to undo the bolt.

 

[KemppiFrog]

Also if using 360 deg you would just need a double ended wasted spark coil with no distributor as it would fire both plugs at the same time off the one set of points.
N

One off a Citroen 2CV would do.

 

[normspanners]

If you have a bolted connection and a cylinder either side with the cranks turning anti-clockwise viewed from the left, every time the left cylinder fires it will try to accelerate the right cylinder thus trying to undo the bolt.

if you read my first post I did say left or right thread depending which suited, it also depends on which end the flywheel is situated, as long as the 'joined' shaft is driving into and through the taper/and a L or R thread (depending on rotation)to the main shaft then through that shaft to the flywheel, it would in my opinion work I have done this on several various different shafts and with no probs the only failure I have had was when a shaft was welded around the join (not by me btw) and the two shafts parted at the weld. The amount of drive transmited through a taper that has been pulled together with a thread is unparalleled (excuse the pun).
Having said all that I have stripped an engine that had an annoying little rattle for months, to find a broken crankshaft. Someone had replaced the harmonic balancer with a different style pulley. The engine was just a little rough at tickover but ran fine at speed.
N

 

[the snooper]

if you intend to weld the cranks i would cut one crank as the picture below (very rough drawing) and cut the other to accept it

 

[mowersman]

I think I'm leaning away from the welding idea and towards the sleeve and pin idea. I'll make up a interference fit steel sleeve, press it onto both cranks and then drill and pin it in place. TBH, I think most things would hold, as it will only be making about 2hp, but I want to be sure.
Andrew

 

[Pete.]

if you read my first post I did say left or right thread depending which suited, it also depends on which end the flywheel is situated, as long as the 'joined' shaft is driving into and through the taper/and a L or R thread (depending on rotation)to the main shaft then through that shaft to the flywheel, it would in my opinion work I have done this on several various different shafts and with no probs the only failure I have had was when a shaft was welded around the join (not by me btw) and the two shafts parted at the weld. The amount of drive transmited through a taper that has been pulled together with a thread is unparalleled (excuse the pun).
Having said all that I have stripped an engine that had an annoying little rattle for months, to find a broken crankshaft. Someone had replaced the harmonic balancer with a different style pulley. The engine was just a little rough at tickover but ran fine at speed.
N

The thread direction maes no odds as it suffers torque reversal at every power pulse. One acts one way - the other acts the other way and the flywheel has nothing to do with it.

A taper may hold, but you have to ask yourself why you never see a crank built up with taper-fit elements.

 

[KemppiFrog]

As earlier, many flywheels are on tapers, and suffer huge fluctuations. The old Minis comes too mind... And, as this is a one time assembly, why not use a shallower angle than morse. They are designed to come apart frequently, whereas the OPs is not.

 

[Gritineye]

I'm loving this thread, seems to me that pictures of the setup are needed in order to for anyone to advise further, how rigid is the connection between the engines?

I was assuming the engines where fixed to a frame, but if they are 'close coupled' then my last suggestion obviously is a no go..

Main question is, how embarrassed will you be if it breaks in public, or are you just experimenting behind your garage door?

 

[mowersman]

Its pretty much just a garage experiment. I shall try and get some photos of the set up at the weekend, however I do not have the coupling at the moment. The connection will be very solid, the front main bearing housing on these engines is removable and the adapter will connect between here and an identical face I will machine on the back of the engine.
Andrew

 

[Gritineye]

Looking forward to pics.....

 

[mowersman]

The other question is, what do I want to be making the sleeve out of. will a bit or EN1A do? I'm afraid I really don't know a lot about material specs, so will need a little educating.
Andrew

 

[jinjahbiscuit]

soz

From my memory of being thirteen and taking my crank to school to use the lathe to set up my 52 matchless 500 crank, the big end pin was nutted on either side-----
Also if using 360 deg you would just need a double ended wasted spark coil with no distributor as it would fire both plugs at the same time off the one set of points.
N

Many small capacity engines have cranks that are just pressed up, no nuts. I'm not disputing the fact that others are more sophisticated. My point is to keep things as simple as possible. It's only a low powered machine anyway.
No matter what the final solution turns out to be, accurate machining is paramount.
Good luck with your interesting project!

And apologies to all for just butting in without an introduction. I'll rectify that in the proper place.

 

[hotrodder]

A bit off topic but still kinda relevant... the SPG Beetle roller cranks that were mentioned earlier origionally had no location whatsoever, just pressed together using a dirty great press. They're legendary for twisting in street use unless pinned and welded but AFAIK they were designed for super vees i.e. light race cars that weren't lugged at low rpm.

Probably wrong but my thinking is that they may have been a copy of the Hirth roller cranks that Porsche were experimenting with on some of the 356s and the 9** series race cars which brings me to the point of the rambling... Hirth joints/couplings although i've no idea how you'd go about machining one

 

[Shox Dr]

Excellent searching as usual HR.

I found this after reading your link

http://www.philspeakman.pwp.blueyonder.co.uk/diary.htm


Scroll to the bottom

 

[Pete.]

Was gonna suggest a hirth-type connection and typed it out but then I remembered the OP has limited access to machining equipment. The hirth built-up cranks are worth reading up on just for the engineering involved, clamped together with differential threads etc.

 

[mowersman]

Right, here we go, hopefully this will clear things up a bit.
These are the 2 crank shafts I'm trying to join. I have already cut them roughly to length and they just need taking down to size, next time I'm near a lathe.



I had discounted the welding idea, however I tried welding the off cuts tonight and they welded fine and bashing them with a hammer in the vice showed no sign of any breaking, despite the shaft bending very slightly. I continue the destructive testing tomorrow

The welding was done with a 150 amp Clarke mig flat out, however if I were to go for the welding option, A mate of mine says with tig, he could get much better penetration, without having to add as much metal. Whether this is right or not I don't know. My worry with welding would be distortion, and I would drill and press a bit of 10mm silver steel in first to try and minimize this. Just to give a sense of scale, the main bearing diameter is 7/8"
Like I said, the other option is to turn up a sleeve (probably out of 2" dia steel) warm it up with oxy and press it on, before pinning it to stop it rotating.

The engine its self, well it started as these...




Stripped




Flywheel



Cowling, will need a good coat of filler after the welds have been ground down, the metal is so thin it is vaporising as soon as the welder touches it. I would cut back, but its all that thin.




Now for the clever bit, or so I though until I hit a snag today. This picture shows the front and back of the block.



My idea was to machine away the back main bearing on one block and machine a matching face on, and then use a 15mm thick aluminum spacer, with bolts through from one block to the other. The issue I have now, is whilst the bottom two holes can have bolts through them, the top 2 cannot, as the bolt heads would foul on the casting of the first block. One of them can be moved, and then I will have to find some other way of bracing the 2 blocks.

This is the spacer I'm considering making, this image shows it with a main bearing, which I may or may not do. I would like to incorporate one as it would reduce the strain on the crank shaft and one will be necessary when the engine sprouts another 2 cylinder on the end!!! (the eventual plan after innocuous comment on another forum the other night!)



Right, I think I have covered everything! I'll stop warbling on now...
Andrew