Crankshaft help

[Gordon]

I am in the process of building an Upshur Opposed Twin engine. After four failed attempts to make a one piece crankshaft I think that it is time to go to a built up crankshaft. This is small. It has 5/16 dia shaft and 3/16 thick side plates. Every time I get half done and try to turn the second journal it catches and bends something. I have tried soldering a spacer bar between the first journal plates but that has not been too successful. I just ruined try number four so it is time to rethink. My question is on how to join the pieces. On larger crankshafts I have silver soldered the pieces together but I am afraid of warping on these small pieces. Others have used loctite with and without pins. My experience with loctite has not been great. Any advice on this from someone who has built this engine or something similar? I am planning on running the main shaft all the way through and cutting it out after everything is together.

On another forum it was suggested that I use a Keats angle plate. I had never used one but that has possibilities. It holds the shaft instead of putting end pressure when mounted between canters. That also has the advantage of acquiring another gadget.

Thank you: Gordon

 

[Andrew Stroud]

Hello Gordon
I suggest you re visit the Loctite solution. Parts properly sized and prepared will hold together, for belt and braces after the setting you can use pins too. I use 638, get a push fit then lightly score/scratch the surfaces and then clean with a solvent. I have made several crankshafts this way and not had a failure.

Andrew

 

[Tom 1948]

I agree with the loctite. Tough stuff. Just give it enough time to fully cure. It will set in about 20 minutes or less, but full cure is 24 hours. Clean parts real good with Acetone or the likes. Oils and such don't mix with loctite. After full cure it takes heat (600 degrees) to seperate.

 

[Steamchick]

This may sound x a bit stupid, but is the original design for a made-up crank with the 3/16th webs? These sound to be a bit thin, possibly where the lack of stiffness is coming from when you are applying the end pressure between centres? That sounds like your side-catching failures are from a combination of the end force and machining forces on something that simply isn't as stiff as you need for those forces. I'm s re you will have reduced the end force between centres to a minimum, as well as cutting feed rates. So, if the design is for an assembled crank, then you have simply proven the decision for that manufacturing method.
But I have no suggestions on how to do this, Sorry.
Incidentally, my understanding of crankshaft design (for vehicle power plants) is that the big-ends are probably just a bit less stiff than the webs. This is that the smallest diameter is required to minimise friction in the plain bearings, and it is a bit easier to have a bit extra material in the webs for stiffness of the crank. - necessary to maintain journal alignment to avoid wear.
I note the 3/16 thick webs, which will be longer than the journals, will be loads more flexible than the 5/16 journals. Stiffness is proportional to the cube of the thickness, times the length. So on thickness alone you only have 27/125ths of the stiffness, times the relative lengths. I.e. only about 20% !
But I am sure the design is capable in operation, otherwise you would not have plans, or know that it works.
Cheers,
K
,

 

[teeleevs]

This may sound x a bit stupid, but is the original design for a made-up crank with the 3/16th webs? These sound to be a bit thin, possibly where the lack of stiffness is coming from when you are applying the end pressure between centres? That sounds like your side-catching failures are from a combination of the end force and machining forces on something that simply isn't as stiff as you need for those forces. I'm s re you will have reduced the end force between centres to a minimum, as well as cutting feed rates. So, if the design is for an assembled crank, then you have simply proven the decision for that manufacturing method.
But I have no suggestions on how to do this, Sorry.
Incidentally, my understanding of crankshaft design (for vehicle power plants) is that the big-ends are probably just a bit less stiff than the webs. This is that the smallest diameter is required to minimise friction in the plain bearings, and it is a bit easier to have a bit extra material in the webs for stiffness of the crank. - necessary to maintain journal alignment to avoid wear.
I note the 3/16 thick webs, which will be longer than the journals, will be loads more flexible than the 5/16 journals. Stiffness is proportional to the cube of the thickness, times the length. So on thickness alone you only have 27/125ths of the stiffness, times the relative lengths. I.e. only about 20% !
But I am sure the design is capable in operation, otherwise you would not have plans, or know that it works.
Cheers,
K
,

Crankshaft manufacturing is an art, my try was to start with it overlength, centredrill the ends for mains and for the crank journals, set up a 100mm angle grinder in the toolpost, do the crank journals first, run the lathe backwards (if able, forward will work ) very slowly and rough grind as close to finish as game then finish with emery tape on a board, starting course and working finer in steps. Cut the ends off last, Best of luck. Ted from down under

 

[Steamchick]

On the cranks I hack made, from cast blanks, I turned main journals between centres, then chuck mounted the crank, and mounted the chuck on a surface plate in the lathe to give the off-set for big-end journals, and turned those. The lathe tools gave a good surface finish and size, but I lapped them with 600 grade abrasive paper (didn't have 1000 grade). Using a Mole grip (flat jaws) with light pressure, which simulated a lapping tool that I used as a teenager when regrinding and lapping full sized engine cranks. The finished model cranks I made this way were miced and clocked and I reckoned good. My engines run OK, but I feel the sump splash oiling isn't so good, when polluted with blow-by steam condensate and steam oil. Needs proper drilling and oil feed.
Enjoy,
K

 

[Steamchick]

The lathe tool for journals needs to be a parting tool less than half the journal width, re-ground with a tiny central relief and appropriate radius on the corners, then set absolutely square to the shaft and traversed along each journal and back with a small cut, to give a parallel and sized journal. Slowly and carefully, so you get it right first time.
Cheers!
K

 

[Steamchick]

I was machining 1/4in cast iron journals for a Stuart Turner Sun engine... only had 1 casting! Rough initially, till I got to nice metal!
K

 

[Gordon]

I ended up machining the crank using the three jaw chuck with my hex 5C collet fixture with a shim under one jaw. Worked well except I had to go easy because it tended to turn the part in the 5C collet.