Crankshaft Diameter

[PaulR]

Another daft question...

For the small bar stock engines I build, I usually pick a crankshaft diameter according to what won't interfere with the crank pin, the weight of the crank/disc and flywheel, what material I happen to have or just on a whim!

What should I be doing? I wouldn't think strength or distortion is an issue on these little engines but I guess the larger the perimeter, the greater the friction due to surface area and increased rotational speed of the contact surface? So is smaller generally better?

[crueby]

For the small bar stock engines I build, I usually pick a crankshaft diameter according to what won't interfere with the crank pin, the weight of the crank/disc and flywheel, what material I happen to have or just on a whim!

If I'm not scaling from a real original engine, thats pretty much what I do! Large enough diameter to be strong enough, small enough to fit, plus whats on the shelf for stock... And what drill/reamer sizes in the toolbox... And what looks good.

[AVTUR]

If the engine is going to do serious work the steel to go for is EN24T. You should not have any difficulty in getting some in the UK.

It is far nicer to machine than mild steel.

AVTUR

[Charles Lamont]

If the engine is going to do serious work the steel to go for is EN24T. You should not have any difficulty in getting some in the UK.

It is far nicer to machine than mild steel.

AVTUR

You must have encountered some horrible mild steel.

[PaulR]

And what drill/reamer sizes in the toolbox.

Oh yes forgot that one, another common deciding factor at the minute.

@AVTUR: I've never had cause to look into different types of steel as I get the engines working (mostly!) then just run them occasionally. At some point I'm sure I'll look into the various options for steel and other metals.

Cheers both.

[Jasonb]

I tend to use precision ground mild steel if it is a built up crankshaft though you may not get it small enough for your engines so silver steel will be on size and a better fit in reamed holes. If cutting from solid then either Black EN3 flat bar or black EN1A round bar does for me on most engines, certainly steam and hot air. Sometimes go upto EN8 on IC engines

Like Chris if I'm not scaling then I go by what looks right, bigger flywheel or distance between bearings and I'll up the diameter. But for your size engines 3 to 4mm is about right

I once took on the completion of two hit and miss engines, the original builder made a big point of the crankshafts being cut from solid EN24. Worst cranks I had ever seen, bent and a terible finish and he was a well respected model and full size engineer.

[PaulR]

I tend to use precision ground mild steel if it is a built up crankshaft though you may not get it small enough for your engines so silver steel will be on size and a better fit in reamed holes. If cutting from solid then either Black EN3 flat bar or black EN1A round bar does for me on most engines, certainly steam and hot air. Sometimes go upto EN8 on IC engines

Like Chris if I'm not scaling then I go by what looks right, bigger flywheel or distance between bearings and I'll up the diameter. But for your size engines 3 to 4mm is about right

I once took on the completion of two hit and miss engines, the original builder made a big point of the crankshafts being cut from solid EN24. Worst cranks I had ever seen, bent and a terible finish and he was a well respected model and full size engineer.

Thanks for this. I've used silver steel for all but one of the engines I've built this year in 3-6mm diameters with reamed holes. Sometimes there's a tiny bit of play between shaft and hole which I find annoying! The silver steel always seems to measure a fraction under the stated size but maybe that's just my vernier. I'm not sure I could get such a good finish by turning down mild steel but it would probably be good enough. Anyway, I'll look into the other steel materials as I know nothing about them atm.

That's a dinky little bent crankshaft in your photo 

[PaulR]

Just had a quick read through this, those EN numbers are not quite as bewildering as I first thought.

https://www.westyorkssteel.com/steel-specifications/en-steel/

[AVTUR]

If the engine is going to do serious work the steel to go for is EN24T. You should not have any difficulty in getting some in the UK.

It is far nicer to machine than mild steel.

AVTUR

You must have encountered some horrible mild steel.

Yes. And I have never tried rebar.

M-Machine's website has a nice "booklet" on steel selection.

AVTUR