Crankshaft Design

Barracuz · Mar 2, 2016

So ive been doing some research and come up a tad short. Im designing a 5 cylinder engine but dont know how big to make the counter sinks or how much to offset from the mains to get a stroke?

Like is there an equation that include the primary and secondary vibrations to establish the offset distance and overall diameter of the crank? How do you guys figure out how much stroke you want?

I tried to use a crank from grabcad and scaled down to my ideal size but the mains and journals are way too big even when scaled down. So in trying to get my own going.

Also ive seen some builds where the crank is built up or machined in one piece using fancy rigs. But i havent seen one using a 4 jaw on a lathe, why arent adjustable 4 jaw chucks on a lathe used? Too much slack? None availvle for your lathes? Easier using an eccentric turning rig?

And how about centerdrilling on the horizontal axis where the individual rod journals would be at. For example 5 holes around the main journal or center of the stock at 60 degree increments. And then just mount on a live center and turn?

Ok so the related tgreads at the bottom of the page shed some light on the crank maching. But still feel free to share your experiences.

gbritnell · Mar 2, 2016

Generally in the automotive world and engine with an equal bore and stroke or one with a shorter stroke than bore is considered a higher revving engine while an engine with a longer stroke relative to the bore size is considered a slower speed or torque engine. There is no formula for calculating bore versus stroke. As far as journal sizes go that would depend somewhat on the stroke of the engine. Here again in the full sized world the journals are sized to withstand the stresses put on them by the operation of the engine.
gbritnell

Parksy · Mar 2, 2016

With regards to using a four jaw vs other methods, there are so many ways to skin a cat, so use which ever method works for you. I'm keen to make a one piece crank by using the mill to remove the majority of bulk from the throws and finish it up on the lathe.
The crank throw distance is half your stroke. Ie, 11mm throw on the crank equals 22mm stroke. At least this is what I used when I created my engine.

stevehuckss396 · Mar 3, 2016

I personally prefer the "centerdrilling on the horizontal axis where the individual rod journals would be at " method. After the basic crankshaft is complete the part gets clamped in a fixture and the counter weights are milled to the final shape.

Barracuz · Mar 3, 2016

stevehuckss396 said:
I personally prefer the "centerdrilling on the horizontal axis where the individual rod journals would be at " method. After the basic crankshaft is complete the part gets clamped in a fixture and the counter weights are milled to the final shape.

This is pretty much what I had in mind! Just use a large piece of roundstock and centerdrill where the mains would be at and drill holes around a radius where the rod journals would line up at. Then just live center it on a lathe and mill away. Or is there more to it?

Also thats a nice crank a 4 cylinder right? But looking at the pics it seems that each piston will fire seperatley unlike common 4 cylinders that fire 2 at a time. Any reason for this?

Parksy said:
With regards to using a four jaw vs other methods, there are so many ways to skin a cat, so use which ever method works for you. I'm keen to make a one piece crank by using the mill to remove the majority of bulk from the throws and finish it up on the lathe.
The crank throw distance is half your stroke. Ie, 11mm throw on the crank equals 22mm stroke. At least this is what I used when I created my engine.

Ah yes makes sense, simple geometry.

gbritnell said:
Generally in the automotive world and engine with an equal bore and stroke or one with a shorter stroke than bore is considered a higher revving engine while an engine with a longer stroke relative to the bore size is considered a slower speed or torque engine. There is no formula for calculating bore versus stroke. As far as journal sizes go that would depend somewhat on the stroke of the engine. Here again in the full sized world the journals are sized to withstand the stresses put on them by the operation of the engine.
gbritnell

Thanks for the info!