Multi-Cylinder Pressed Crankshafts

Someone mentioned that the Harley twins used this crank, could that be why they are so hard to use in a BEC application? You have to build a jackshaft to isolate the engine from the forces of the car transmission.

If you are asking if a pressed crankshaft is possible, the answer is yes. It's been done. But the question, is more: is it worth it?

The problem with a pressed crank is the pressed part. A press fit transmits force by friction. You constrain the metal parts, which creates a pressure in the joint, which creates friction.

This has the double disadvantage of creating a joint that won't transmit a lot of torque unless you put a lot of stress into it, and that won't transmit a lot of torque because it has a lot of stress into it.

For something more efficient, you could design an assembled crankshaft with some sort of joint like the Hirth joint or splines. But these would increase the machining to a point that would make it more complicated than a "billet" crankshaft.

Plus, you have to consider that for a company, engineering time is just as expensive as machining time, and designing a proper assembled crankshaft would require a lot more time than a "billet" crankshaft.

You also have to consider quality control. The more parts you have, the harder it is to keep the tolerances within an acceptable margin. When you make a "billet" crank, your tolerances are solely dependent on the machine that does the machining. That makes it a lot easier to keep good tolerances.

Castings and forgings are really good for mass production. They require a large initial investment, which is impossible to justify for a small batch. But when you produce large quantities, they make sense for the very little machining they require.

This is just my 0.02$

alfadriver wrote: And people are "printing" parts this way? Doesn't seem all that practical- compared to machining a part out of a billet, even.

Not that I am aware of, but it's the analogous way to do it for steel. It'll happen eventually, if not already.

And yes, it does seem awfully wasteful even for a one-off part. But people will have simple brackets CNCed so someone will use it to make something that a machinist could whip up in 15 minutes.

Knurled wrote:

alfadriver wrote: And people are "printing" parts this way? Doesn't seem all that practical- compared to machining a part out of a billet, even.

Not that I am aware of, but it's the analogous way to do it for steel. It'll happen eventually, if not already. And yes, it does seem awfully wasteful even for a one-off part. But people will have simple brackets CNCed so someone will use it to make something that a machinist could whip up in 15 minutes.

Jeez, didn't anyone bother to follow the link I provided?
Metal AM is here folks.
Medical, aerospace, high end tooling, prototypes and short run production (F1!).

Next to roll out are compact AM machines to build metal dental prosthetics, right in the Dentists office, while-U-wait. First use 3D scanner to get a model of your mouth, accurate to 0.001". Next, the dentist (or 'design esthitician') will model your bridge, crown, implant, etc using CAD-like software, so that it will precisely fit your mouth. Send the file to the AM machine, out pops the finished piece.

Anyway, here's the big thing about casting vs forging:

Casting: cheap and easy to do one offs by skilled hands (carve the pattern, pack the sand, pour the metal), can be expensive to migrate that to a repeatable production run.

Forging: expensive to do one offs (DIY forging dies anyone?), but once dies are built, easy to migrate them to repeatable production run. Of course, you still need a furnace to heat the metal, and multi-ton forging presses for the dies, but the dies are the key.

I agree with the possiblities of a pressed crank shaft. Most of the large diesel engines, including shipping engines are all pressed fitted crankshafts. The economics and simplicity of constructing a pressed fit crank will out weight its short-comings; balancing, vibrations. 

Not knowing what Driven5 has up his sleeve, recajiggering built-up cranks is very possible. 

There's a Vietnamese guy named Le Dan who builds all kinds of bananas motorcycle engines - V-twins out of engines that stat life as singles, triples out of twins, etc. His workshop and techniques both look pretty rough and ready, but the engines all work.

If you want something considerably more genteel (to the point that an annoyingly large segment of some videos centers around baked goods and pantomime phone calls), try Allen Millyard. The guy adds cylinders to engines like it's no big deal at all, and often walks you through the process.

In reply to DarkMonohue :

Had. This thread is 11 years old. 

Lord. That's embarrassing. I blame the canoe.

In reply to DarkMonohue :

No no, it's all good.  Thanks for the rabbithole!

In reply to DarkMonohue :

Just saw that this popped back up. Despite the prolonged timeframe, 'has' is still the operative word. Multiple ideas persist, from rejiggering OEM crankshafts to pure from-scratch builtd, but unfortunately still none that could be attempted for years to come. Thanks for the fresh thoughts!

If you want to play with one go to the oldest aircooled VW shop and ask if they have any SPG roller cranks they are using as a doorstop :)

These were sold in the late 1960s  ,