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Fabricated crankshaft, racimg motor, viable option ?

Fabricated crankshaft, racimg motor, viable option ?

Fabricated crankshaft, racimg motor, viable option ?

Is a fabricated crankshaft a viable option for a turbo racing engine ? (4 cylinder / 2 litre / approx 800Nm )

By :fabricated" i had in mind :

Bored plates that act as throw/counterweight and the pins and mains would be pieces of round stock, all press fitted together and welded, by tig, multi passes.

Then stress relieved, finish machined, induction hardened, ground.

I dont know exactly how to go aboute stress calculation on this one, there are many variables. Looking for real world examples and if it has been made already ?

Thank you

RE: Fabricated crankshaft, racimg motor, viable option ?

Two-stroke motorcycle engines historically used built-up pressed-together crankshafts (due to the need for the con-rod big-end to use a rolling-element bearing). The Japanese manufacturers continued using built-up crankshafts in their early four-stroke engines due to their familiarity with manufacturing them at the time, although for their larger engines, they have all switched to forged crankshafts a long time ago.

Those engines in standard form never made anywhere near the torque that you are talking about.

RE: Fabricated crankshaft, racimg motor, viable option ?

Why fabricate? It requires all of the same operations as machining from bar stock without any of the pesky development costs. Bar stock cranks aren't as good as forged but they're common for one off cranks. The magazines like to use "billet" but I never cared for the term.

The engine that produces the most torque, 14 cylinder slow speed diesel uses a built up crankshaft. 100,000 horsepower at ~100 rpm.

RE: Fabricated crankshaft, racimg motor, viable option ?

Perhaps you should do a 'simple' calculation on the stress in the welds on the last web at 800 Nm. Wagging 6 mm weld, 60 mm diameter, that'll be 1200 mm^2 by two welds, 800 Nm@30 mm is a shear force of 10 N/mm^2

which seems OK to me. So the easy thing to do is to make up one of these webs, put a long lever on it and see if your welding is up to snuff.

Assuming you can't go forged could you make a wooden one and get them to cast you a sandcast steel blank from ASTM A148, Grade 90-60 (I don't know if that is used for crankshafts but it looks nice and strong). I can't help much with material selection, we used that nodular cast iron stuff, which is not ideal for light weight high torque applications. Mind you 533 Nm is none too shabby in a taxi.


Greg Locock

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RE: Fabricated crankshaft, racimg motor, viable option ?

Tugboat, so how are the parts of those crankshafts assembled into the finished product? You sure would not (want) that crankshaft twisting from using just a press and shrunk fit, splines?
For the OP, there are a few US crankshaft company's that specialize in one off racing/ performance fully machined crankshafts, forged is nice but very difficult for a one off design.
I think most built up motorcycle crankshafts are just an interference fit with no welds and nothing to key them. A certain radial aircraft engine was designed with an almost semibuilt or rather split crankshaft using a face spline at the connection interface and a huge bolt to hold it together, also beginning designs used a straight spline to maintain orientation, yes sure don't need a crankshaft twisting out of shape at 30,000 feet in the air.
The easy more reliable option is to machine it from a solid "billet" bar. I don't like that over used term either.

edited in the ()

RE: Fabricated crankshaft, racimg motor, viable option ?

Assembly is the same as a motorcycle crank. There is a crankshaft slip measurement that is regularly taken throughout the life of the engine. Slip can be corrected.


One manufacturer did experiment with a welded crank. The welds were in the center of the journals. I think the design was successful but offered no benefit over the conventional built up approach and few were constructed. It used a process called narrow-gap welding.

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