Inline 4, Two Stroke

[PRiggs78]

That sounds pretty good, what size big-end journals?
K

Inner mains 5/16, main ends 3/8 riding in ball bearings. Connecting rod journal size is .1875" diameter.

I've calculated the stresses on all points and this here is minimal size I'd go. This will allow for a more compact size that is more proportional to the stroke.

 

[Tim Wescott]

The danger of taking your crankshaft dimensions off of a glow engine is that the stresses on a typical glow engine crank are so very different from those on an inline engine. Basically, the crank pin carries no torsional load at all, and the only torsional load carried by the crank main bearing is from a single cylinder.

A multi-cylinder crank carries torsional loads from the back of the engine to the front, and it needs to be stiff in bending, and it's long so it'll "ring" in torsion (yes, all discussed already -- I'm going to say it anyway!).

If you're going to copy something, copy a multi-cylinder engine. Even the model A engine -- which produced only about 1/5 horsepower per cubic inch, and needed an overhaul every 10000 miles -- had mains and big ends that were between 1/3 and 1/2 the cylinder bore.

 

[Tim Wescott]

In the process of writing that last bit, it occurred to me that at least for turning the main bearings, you could make a narrow steady rest. Start with a full-length crank blank, turn a main close to one end or the other, then put the steady rest on it. Then turn the next one in (or out), etc.

This doesn't save you when you go to turn the big ends, though. (But big mains and big-end bearings will make the crank stiffer for turning, too).

 

[PRiggs78]

Correct. With a bore of .79" and main ends at .375" and inner mains at .312", this particular design falls above those parameters. The only area concern here is the pin size .187".

The pin size selected at .187" gets me the throw for the stroke. Do you suggest going to .25" on the pin? It will reduce stroke length a bit, but there is room in the design to change things here and there.

 

[Tim Wescott]

If you're taking models from real automotive engines, something from the 1920's is a minimum -- I don't have the figures at my fingertips, but the bearing sizes of -- for instance -- a Chevy small block V8 are much bigger in comparison.

I think that in your shoes I'd try to make the shaft as you have it now and see if it's stiff enough just to make the thing. If your goal is to run the engine to show that it works, then you'll be OK. If it's just too flexible to machine, then make another one.

 

[Steamchick]

In bending, the length of bearing pins is also critical and as mentioned the stress is the compound of bending and torsion. And if you can, try and deduce the bending stiffness overall, as well as torsional stiffness overall and determine resonant frequencies. Almost all "human sized" engines must carry torsional dampers, and need a crankcase structure to support the crank from excessive bending. (Using ladders, ribs and frames). The block alone may not be strong enough. But your model (Naturally) will be completely different, so needs the calculations to avoid "expensive" failures.
I wold need to g back t the text box to d it myself.
K