Crankshaft Construction Question

[Kevin Renner]

Not about any particular engine but I'm interested on just how multiple piece crankshaft were built. The reason is I'm doing drawings for an inline radial something similar to the Jumo 222 but on a much smaller scale. This will probably never go beyond a paper exercise and is mainly for my own interest. Right now I'm looking at a 36mm x 40mm bore and stroke. I thought about split bearings (sleeve or roller) but instead of splitting the crankcase lengthwise I'm looking at splitting on the cylinder centers. If (an admitted big IF) it ever gets to the cutting metal the case will be a lot easier to machine that way

 

[Hobbes]

Split crankshafts seem to use an interference fit, so you'd need a big press to put the crankshaft together, and you need to align the parts carefully.

If you split the crankcase, your engine consists of a large number of separate parts: a block housing the cylinders, and a crankcase section for each pair of cylinders. That's a less rigid construction than having one casting for the block + top half of the crankcase. You'd have to design the crankcase sections carefully to avoid the sections moving relative to each other.

https://www.highpowermedia.com/blog/3013/split-crankshafts

 

[Kevin Renner]

One reason for the built-up crank is the Master Rod is easier to figure out without having to split it. I'm aware of built-up cranks that use an interference fit on the Pin fits to the throws. The first time I actually encountered this was helping my brother when he tried to did-assemble a Suzuki twin around 1970. One idea I had was assembling the crank and pinning the joints with threaded taper pins or taper pins that have a set screw over them. Just sure if that would be strong enough to maintain orientation. One way to get around the problem of pressing the sections together is to heat the female half of the joint and slip the male half into it. I'm actually looking at either a four row or a six row design. As I stated this most likely will never get to the cutting metal stage (I'm in the process of putting together a shop in the basement) but I find it a nice mental challenge. In order to ensure the case sections stay in alignment they would be pinned in three locations. All case sections would have match marks for assembly. Plus the cylinder banks would act as a bridge across the case sections. The cylinders and crank sections I was thinking 4140 TG&P stock which comes at 30 to 35RC. The cylinders would thread into the case sections and be spigoted into the cylinder head. Studs would screw into the case through the cylinder head and cylinder banks. Viton O-rings to seal the cylinders to the banks. One problem with something like this (and radial engines in general) is the assembly procedure. I'm about to the point of starting to do cad drawings after working out the general layout on paper.

 

[iverson]

I'm not sure if this is any help, but Hirth joints have commonly been used for multipiece crankshafts in the past. See:

https://en.wikipedia.org/wiki/Hirth_joint

 

[Kevin Renner]

I'm not sure if this is any help, but Hirth joints have commonly been used for multipiece crankshafts in the past. See:

https://en.wikipedia.org/wiki/Hirth_joint

As noted machining them in small diameters looks to be a real PITA