Built up crankshaft feedback

[john mills]

I work on machines that specified .002"interference fit to hold a roller pin 7/8" dia it needed that to hold it.
fitters reckoned it was to tight so seemed some out to make it easier  of course that made it not tight enough and they quickly came loose.
I had fitted them using to 3/4"UNF cap screws to push them in yes they were tight but they were meant to be.
As for scratching on pressing to get better grip  i have always tried to get good finish and plenty of oil to press things together so parts don't
score or pickup going together as if they do they are likely too fall apart at least the parts will no longer be straight.
John
I was never a fan of lock tight type assembly
I have only made crank shafts from solid

[steamer]

The crank for the 917 has 7mm crankpins with .0025" (64 micron or so) interference. The pins are hardened A2 tool steel, while the webs are AISI 1144SP

Having the crank pin hardened, with a polished leading edge, making the press in go easily, and at that level of interference, the joint is quite sound.   I was able to reliably finish the second crank cheek bearing surfaces with the assembly hanging on ONLY the pressed in crank pin.
I found that I could accomplish this press fit with my 4" Kurt vise clone quite easily and controllably.
.
Doing a bit of engineering here, that joint should stand up to 60 ft lbs of torque....  That'll do for my application anyway.
The calculation states it took 5000 pounds to press together.

I've added my spreadsheet calculation for interference fit sizing where I got this number below.    Alter the YELLOW blocks only   OUTPUT is in GREEN
 

Dave


 

[steamer]

The torsion on the joints is small.  The engine block and bearings will take most of that at the crank pins.   It's just the final drive that is a concern

That's another thing I was mulling in the back of my mind. The strength of the key in the crankshaft (to the prop washer) is easy to calculate in shear. If that somehow sheared, the wind on the whirly end stops. If the axial CS/web pin was equivalent to the key, that might be a good starting point of equal strength. But maybe how & where it fails is also important. If you had some sort of unfortunate kick back & crank arms slipped & displaced & twisted rotationally that would be a problem.

Yes it would be a problem, but if the housing and main bearings are in line, then the likely hood of that happening  is low.  The main bearings would need to displace to make that happen.

Dave

[steamer]

<a href="https://www.youtube.com/watch?v=uW2YFNDjN_o" target="_blank">http://www.youtube.com/watch?v=uW2YFNDjN_o</a>

[gipetto]

crankshafts sure would be a lot easier to make if they put the hole in the crank instead of in the con rod. so a single cylinder engine would have two cranks meeting at the con rod

[crueby]

crankshafts sure would be a lot easier to make if they put the hole in the crank instead of in the con rod. so a single cylinder engine would have two cranks meeting at the con rod

Can you draw that?   

[gipetto]

Can you draw that?   

i guess there's a kind of rocking couple, because the engine load is taken from one side of the crank, but there's also an advantage because you can make the shell bearings twice as wide inside the crank weights.

[petertha]

I was discussing this with a mechanic friend & although he does motorbikes & ATV's for fun vs at work, he said built-up, pin-less, glue-less cranks are the norm in that domain & gave me some specific examples that he had personally disassembled. Aside from a few specialty engines, tearing them down & replacing components is standard fare in the service shops. I would have to think some of the techniques should be transferrable to us.

Now the devil is in the details. Seeing it knocked down & re-assembled from manufactured parts vs shop made parts. By analogy, most commercial liners are coated or plated but only a small percent of model engines can pull that off without a lot more effort. If the surfaces need to be bearing quality to ensure axial & perpendicular fit over shorter distances, that might make for some home shop tradeoffs. But I've been going back to my PDF plagiarism folders & there are indeed some good model engine examples I have forgotten about. Including Dave's 917 which he just posted on. Thanks for the spreadsheet BTW!. Yup, Gemini AI gave me a bogus formula example - right up there with 6-fingered ducks & fictional legal cases LOL

One of many crank teardown / re-assembly vids
<a href="https://www.youtube.com/watch?v=HBLV5sv9nmU" target="_blank">http://www.youtube.com/watch?v=HBLV5sv9nmU</a>

[petertha]

The crank for the 917 has 7mm crankpins with .0025" (64 micron or so) interference. The pins are hardened A2 tool steel, while the webs are AISI 1144SP

Having the crank pin hardened, with a polished leading edge, making the press in go easily, and at that level of interference, the joint is quite sound.   I was able to reliably finish the second crank cheek bearing surfaces with the assembly hanging on ONLY the pressed in crank pin.
I found that I could accomplish this press fit with my 4" Kurt vise clone quite easily and controllably.
.
Doing a bit of engineering here, that joint should stand up to 60 ft lbs of torque....  That'll do for my application anyway.
The calculation states it took 5000 pounds to press together.

I've added my spreadsheet calculation for interference fit sizing where I got this number below.    Alter the YELLOW blocks only   OUTPUT is in GREEN
 

Dave

Thanks so much for this Dave. I have your 917 build series saved & was getting round to review again. In the back of my mind I was thinking it had a higher density 'Schillings-esque' spacing bearing per throw & thus maybe better supported the crank elements from distortion vs my crank. I considered adding intermediate thin ring bearings but from the available sizes, it just didn't play well with the design without the typical design domino effect starting with the crankcase & ending with the spark plug haha. But I am now more committed to make some disposable crank mockups just to test things for myself.

One question. Lets say I had sufficient time to assemble a pre-heated web element, magically perfectly aligned. IOW it was heated to the extent it slid on the shaft with no effort, but the resultant ID/OD interference amount was the same as though I had cold pressed it. Would there be any joint strength difference between the two methods?

[Jasonb]

That is another area where our small sizes don't help as the heat will be lost from a small part much faster than a larger mass. Any press or alignment jig will just act as a big heat sink and suck it away. 

[Niels Abildgaard]

One question. Lets say I had sufficient time to assemble a pre-heated web element, magically perfectly aligned. IOW it was heated to the extent it slid on the shaft with no effort, but the resultant ID/OD interference amount was the same as though I had cold pressed it. Would there be any joint strength difference between the two methods?

The 24.9 hole needs extra 400 degree celcius to grow to plus 25

[steamer]

400C seems a bit high....

[steamer]

That is another area where our small sizes don't help as the heat will be lost from a small part much faster than a larger mass. Any press or alignment jig will just act as a big heat sink and suck it away. 

Agree   in the 6 to 10mm dia range     you won't have much time..   With a hard crank pin and a high strength crank web..the press fit will go well and you will be unlikely to gall  the hole when you press.it together..

[steamer]

On paper the interference fit between press.fit and shrink fit is the same.   BUT... there are some practical things to consider.   I will elaborate this evening perhaps.