Crank operated lever controls rotary valve and fries brain!

[PaulR]

Thanks both, that's what I originally thought - I've clearly been staring at it too long 

Odd that it has the same effect on the valve lever arm though...

[Jasonb]

What will suffer in the valve timing. Having the parts fixed is just like fixing your eccentric at the correct angle to get the engine to run

[crueby]

Thanks both, that's what I originally thought - I've clearly been staring at it too long 

Odd that it has the same effect on the valve lever arm though...

It would not have worked like that in the real world though, it would have flopped around more randomly, and as Jason said the valve timing would way off. That arm needs to be fixed to the crank pin. Just the long connecting arm ends should rotate.

[PaulR]

Right, I think the model finally matches its cardboard and wood counterparts although I omitted the piston rod this time. Getting the relative position of the cranks such that they turn the valve lever through a sensible amount is quite fiddly.

[crueby]

Thats it! 

[Zephyrin]

this arm fixed on the crankpin is the "return crank", present on numerous steam locomotive, as being a part of the Walschaert distribution...
it is also visible on many hot air engines, to set the length of the stroke of the power cylinder.

[PaulR]

this arm fixed on the crankpin is the "return crank", present on numerous steam locomotive, as being a part of the Walschaert distribution...
it is also visible on many hot air engines, to set the length of the stroke of the power cylinder.

Yes, I thought it looked similar to Walschaert valve operation although I don't really understand how that works... I've got a loco book that explains it but it's a bit above my pay grade 

[PaulR]

It also occurred to me that the piston rod (dark green in this clip) could be attached anywhere along the line from the axle to the original crank pin position provided that the long side of the v-shaped link is raised to clear the big end.

[Jasonb]

It will just alter the stroke of the engine.

Do watch your timing, the latest video does not have the piston and valve pins 90degrees apart.

Also I have noticed in the last few simulations that you have the valve rod on the pin that is on the shorter radius and the piston on the larger radius.

Drawing shows piston pin at 7/32" radius and Valve pin on 1/4" radius, you have the opposite which will give less angular movement to the valve.

[PaulR]

It will just alter the stroke of the engine.

Yep, I realise that - could prove handy!

Re the accuracy of the animation, well spotted! I'm not getting to using as a basis for building yet, just fiddling with the components and learning how the software works. I did notice you could swap the valve pin inside or outside the crank's radius and the mechanism still works although I didn't spot any change in the valve lever movement while playing around with it - good catch!

I'm guessing once things are set up ok I can get some measurements and just scale them to suitable dimensions provided everything is kept in proportion?

[crueby]

Yes!  And the size and positions of the openings in the rotary  valve will need to be determined  with the length of the valve cranks, they all need to work together. Nice detective engineering.

[PaulR]

I think this is about right now. The crank pin is at 10mm and the valve pin at 12mm. With the valve centre 80mm from the crank axle, the valve rod comes in at 80.5mm and the lever at 15.5mm, giving an arc of travel of 100 degrees. Putting the centre of the piston travel at 50mm from the crank means there will be little dead space behind the piston at tdc (which I'm guessing is a good idea for a single acting engine?).

[PaulR]

Thinking again about this now the latest project is almost finished, it strikes me that it could probably be simplified by having two cranks rather than the lever arm controlling the valve at the back which would make it easier to get the valve flats (or holes) in the right place. That would make it easier to make the valve. The only downside I can see is the length and extra weight on the main crank. A wrist pin in the piston as per the original post would also be easier than the cross-head I've hinted at.