Eccentric operated rotary valve

[PaulR]

Just thinking about a different rotary valve engine, having a valve crank operated by an eccentric on the crankshaft, ie the valve does a complete rotation in sync with the crankshaft. As far as I can see, the throw of the valve crank could just be some convenient measurement, there would always be a 1:1 rotational relationship. However, I'm concerned about binding.

I asked my AI friend about this and it says the important thing is the length of the valve rod, it needs to be such that the angularity error (deviation from pure linear motion) is less than about 10deg. If the throw of the valve crank is say 6mm, by trig the valve connecting rod would need to be at least ~35mm. Basically, in a horizontal configuration make the rod as long as possible to keep it as close to horizontal as possible at all times which sounds entirely reasonable!

Does that sound ok?


(The timing would be set by a set screw on the valve disk BTW)

[Jasonb]

You will have difficulty getting a full rotation, the valve is more likely to rock back and forth.  Basically as the rod movement from the eccentric gets to the top or bottom of it's movement there is nothing to say it will go over ctr and fully rotate the valve or more likely just move the valve lever back the way it came.

But if you can make do with partial rotation of the valve eg a "rocking valve" then that will work with an eccentric

[PaulR]

You will have difficulty getting a full rotation, the valve is more likely to rock back and forth.  Basically as the rod movement from the eccentric gets to the top or bottom of it's movement there is nothing to say it will go over ctr and fully rotate the valve or more likely just move the valve lever back the way it came.

But if you can make do with partial rotation of the valve eg a "rocking valve" then that will work with an eccentric

I had a nagging feeling that might happen even though I don't know exactly why it should. I guess it'll be a complicated mix of small factors which are probably beyond my understanding 

I was trying to think of a way to get the valve at the far end of the cylinder and have it fully rotate...

For a rocking valve I'd probably go with the 'scraps and solder' type engine I built before as it's a lot easier than making an eccentric.

[crueby]

For a fully rotating  shaft, they would normally  put a crank at either end like a bicycle  foot pedal crank, so as one gets near top or bottom the other is in the middle. Large engines with offset valve shafts did id that way. Or they used an intermediate  shaft with bevel gears, or a chain drive.

[maury]

Try something like this.

[Charles Lamont]

You will have difficulty getting a full rotation, the valve is more likely to rock back and forth.  Basically as the rod movement from the eccentric gets to the top or bottom of it's movement there is nothing to say it will go over ctr and fully rotate the valve or more likely just move the valve lever back the way it came.

But if you can make do with partial rotation of the valve eg a "rocking valve" then that will work with an eccentric

I had a nagging feeling that might happen even though I don't know exactly why it should. I guess it'll be a complicated mix of small factors which are probably beyond my understanding 

I was trying to think of a way to get the valve at the far end of the cylinder and have it fully rotate...

For a rocking valve I'd probably go with the 'scraps and solder' type engine I built before as it's a lot easier than making an eccentric.

Try thinking from the other direction - Why would the valve do complete rotations? What are you providing to make it do that?

Jason says it is just as likely to rock back and forth. I would say it is also just as likely to stop dead, or break.

Maury's post shows the mechanism that will do the job. That mechanism also show you that in this case angularity of the eccentric rods (coupling rods, really) is not relevant.

[PaulR]

Thanks for the replies, seems I'll need to go back to the drawing board on this one.

I did think about gears but as I've got no easy way to make them I put that to one side (I did once try to make some bevel gears in the lathe with a detent and tool on its side set at an angle but it was massively time consuming!). Same goes for the Lees shaft mechanism or a chain drive. Actually the latter gives me an idea... would a simple pulley achieve the same thing? There wouldn't be very much load so slipping might not be a problem... Not an area that I know anything about so will read up on it!

[maury]

I have found inexpensive gears on Ali Express.

My opinion about the time consuming part: If you are in a hurry its no longer fun, which is the
whole purpose of this hobby.

I haven't done the CAD, but at this point I see no reason why you couldn't use eccentrics to drive the mechanism. You will still need a crank for the driven end. As a Plus, this mechanism adds an interesting
and unusual action to your model.

[Jasonb]

The problem for Paul is that he typically builds engines of 6-10mm bore so scaling down that Lee shaft will be quite fiddly and into the realms of watch making.

As for plain pulleys and belts I think some slippage is likely so timing will not remain constant.

[crueby]

Here is a picture of what I was describing in my earlier post. These are screenshots of the CAD model for the big Allis water pumping engine, which has a side shaft to drive the valves from the crankshaft. The side shaft needs to stay in sync with the crankshaft on that engine, so they put a crank at either end of each shaft. The cranks on one end are 90 degrees off the cranks at the other end, just like a bicycle foot pedal crank. That way it can't lock up when one goes near top or bottom center. This is two pictures, one for each end of the shafts, so you can see the 90 degree offset.

[PaulR]

The problem for Paul is that he typically builds engines of 6-10mm bore so scaling down that Lee shaft will be quite fiddly and into the realms of watch making.

As for plain pulleys and belts I think some slippage is likely so timing will not remain constant.

Yes to both of those!

[PaulR]

Here is a picture of what I was describing in my earlier post. These are screenshots of the CAD model for the big Allis water pumping engine, which has a side shaft to drive the valves from the crankshaft. The side shaft needs to stay in sync with the crankshaft on that engine, so they put a crank at either end of each shaft. The cranks on one end are 90 degrees off the cranks at the other end, just like a bicycle foot pedal crank. That way it can't lock up when one goes near top or bottom center. This is two pictures, one for each end of the shafts, so you can see the 90 degree offset.

Thanks Chris, I like this idea using two cranks (or eccentrics I suppose). You threw me talking about bike cranks as they're at 180 (although it would be fun trying to ride a bike with the cranks set at 90/270  ).

[PaulR]

My opinion about the time consuming part: If you are in a hurry its no longer fun, which is the
whole purpose of this hobby.

I don't really want to buy parts for engines I'd rather make them if possible although I did make an exception for a ball race for the Stirling engine! When I tried making bevel gears before it took an eternity (and the result wasn't fantastic) and I just don't want to spend that much of my fairly limited workshop time making a couple of parts.

Thanks for the PDF, filed away just in case 

[crueby]

Here is a picture of what I was describing in my earlier post. These are screenshots of the CAD model for the big Allis water pumping engine, which has a side shaft to drive the valves from the crankshaft. The side shaft needs to stay in sync with the crankshaft on that engine, so they put a crank at either end of each shaft. The cranks on one end are 90 degrees off the cranks at the other end, just like a bicycle foot pedal crank. That way it can't lock up when one goes near top or bottom center. This is two pictures, one for each end of the shafts, so you can see the 90 degree offset.

Thanks Chris, I like this idea using two cranks (or eccentrics I suppose). You threw me talking about bike cranks as they're at 180 (although it would be fun trying to ride a bike with the cranks set at 90/270  ).

Doh!  You are right! Been doing steam engines too long, forgot how my bike worked!       

[PaulR]

Here is a picture of what I was describing in my earlier post. These are screenshots of the CAD model for the big Allis water pumping engine, which has a side shaft to drive the valves from the crankshaft. The side shaft needs to stay in sync with the crankshaft on that engine, so they put a crank at either end of each shaft. The cranks on one end are 90 degrees off the cranks at the other end, just like a bicycle foot pedal crank. That way it can't lock up when one goes near top or bottom center. This is two pictures, one for each end of the shafts, so you can see the 90 degree offset.

Thanks Chris, I like this idea using two cranks (or eccentrics I suppose). You threw me talking about bike cranks as they're at 180 (although it would be fun trying to ride a bike with the cranks set at 90/270  ).

Doh!  You are right! Been doing steam engines too long, forgot how my bike worked!       

I've actually ridden a bike with one crank in the past - not too hard if you can keep it going 

[PaulR]

Here's where I'm at with this idea, seems like it should work. Air enter from the top (blue passage), enters and leaves the cylinder by the yellow passage and exhausts via the red passage through the rotating valve (green).

There's a crank disk on each end of the valve with a matching one at the crankshaft end, with the pins for each pair offset 90 degrees. I've yet to firm up the bearings and piston rod connections and the inlet tube is omitted from the top view. I'll probably have to make the crank disks thinner to accommodate the bearings but increase the diameter so they double up as flywheels.

If anyone can see any obvious problems I'd appreciate a nudge! 

[crueby]

Looks like a good setup, should be fun motions!

[PaulR]

... should be fun motions!

That's what I was thinking. It's basically the same as the rocking valve engine I built last year but with something different to look at.

[PaulR]

Hmmm, thinking about it, that exhaust passage would be better if it passed right through the valve as each end will be partially occluded by the side rods on each stroke.

[Charles Lamont]

If anyone can see any obvious problems I'd appreciate a nudge! 

Not sure how you mean to drill the connecting hole from the blue inlet hole to the valve chamber.

Your phasing of the valve needs thinking about. The radial depth of the inlet and exhaust slots in the valve can be used to give additional contol over the timing, providing such things as lead and cut-off.

[PaulR]

If anyone can see any obvious problems I'd appreciate a nudge! 

Not sure how you mean to drill the connecting hole from the blue inlet hole to the valve chamber.

Your phasing of the valve needs thinking about. The radial depth of the inlet and exhaust slots in the valve can be used to give additional contol over the timing, providing such things as lead and cut-off.

Thanks Charles. Re the inlet, the cylinder and valve block are separate parts so I'll drill down from the top then drill and plug horizontally from the right (the cylinder side) of the block. The other (yellow) passage to the cylinder will be close beside and parallel to it.

Re the flats on the valve, I'll draw a few different options but the essential dimensions are the same as the last engine which seemed to run well - I think a fast running engine wants the ports fully open as soon as possible doesn't it, especially the exhaust for a single acting variant?

[PaulR]

I think I've got all the essential dimensions in place now so it's time to get work and start another build thread. Here's the general arrangement from the top although I might still make the cranks disks a bit bigger to get a bit more momentum.