Control Valve

[Cp489]

Hey guys,

I have been trying to design a replica of James Watt Steam engine, only this replica will be using compressed air. I am having trouble thinking of how I can get a hold of a 4-way butterfly valve that will turn on and off through an attached push-rod with levers on the main beam.  I am wondering if I can get this part machined or does anyone suggest buying one? I have tried to look online for a valve that would work but they all seem to not fit my design. Attached are two pictures for clarity. The size of this valve is small. Right now it is modeled at 1 cubic inch with four 0.5 inch tubes connecting to it.

I am new to this so all help is appreciated. Thanks!

-Chris

[Jasonb]

I would say you will need to make it yourself, the spherical hole in the middle of the block should prove interesting to make!

You may well find it easier to make one with a cylindrical barrel that is drilled for the steam(air) passages as that will be a lot easier to make and seal, another option may be to mill flat into the cylinder rather than drilling.

[Jo]

I have tried to look online for a valve that would work but they all seem to not fit my design. Attached are two pictures for clarity. The size of this valve is small.

Think about a valve design something like an oscillating engine reverser's ports (attached)

A 25mm square is about the normal size for one of these valves.

Jo

[cnr6400]

The ancient 4 way plug valve would do the job as well.

https://en.wikipedia.org/wiki/Four-way_valve     

[Jo]

  that is more likely what was fitted.

Jo

[MJM460]

I am sure that I have read that Watts first engine had a boy to flip the valve at each end of the stroke.  What a horrible job!  A great incentive to invent an automatic valve.  Your diagram is the first I can remember that so clearly shows how it would operate.  I am sure there are many around, I just haven’t seen them.

I suspect you will have to have the valve machined, as most commercial valves are intended for manual operation and have sealing as the priority, rather than highly cyclic operation.  Of your engine runs at even 50 rpm, that valve will operate at 100 times per minute.  That is highly cyclic operation by any definition.  Even though you engine will probably not operate for many hours at a time.

I suggest a cylindrical plug would be quite suitable.  Even if you could find the machinery to manufacture a sphere to the appropriate tolerances to achieve the required sealing, I am sure that such machinery would not have been available to Watt, so would not have been true to the type.  Alternatively, the type suggested by Jo that has flat sealing faces, well proven in oscillating engines, may be easier to manufacture.  Two other posts while I have been typing are in a similar vein.

First think about the steam passages you need.  If you are more familiar with electrical switches, it would be a double pole, double throw.

The attached sketch shows the basic requirements.  You can see the basic idea.  At the top of the stroke, the mechanism will trip the plug anticlockwise, while at the bottom of the stroke, it will be flipped clockwise.  About 90 degrees in each case.  Then define the body around the plug.

The other not so obvious requirement is that you want to isolate the steam/air supply from the exhaust when the valve plug is half way through its movement.  Again the electrical analogy is “break before make”.  And obviously the plug should fit well enough to minimise/prevent leakage from supply to exhaust around the outside of the plug.  This simply requires careful proportioning  of the passages in the plug in relation to the inlet and outlet ports.

I have also made a sketch of a longitudinal section showing a shaft seal to minimise leakage to the atmosphere.  It is not pressure balanced, but has only one leakage path to be sealed.  Commercial plug valves often use a tapered plug for better sealing, but might be  prone to jamming in this operation.  I suggest a cylindrical plug, carefully lapped to a reamed bore in the body would be possible, as leakage at least is limited by the low pressures involved.  However other forum members will be able to suggest other options.

I am looking forward to following your progress so please keep posting.  Just a minor point, may I suggest that you continue by just replying to your own earlier post to keep the whole project in one thread.  Much easier to follow that way.

MJM460

[Jasonb]

They simple valve used on "rocking valve" engines is what I was thinking of and very similar to what CNR posted in it's simplest form. Must get round to making my one of these.

[michaelr]

The Trevithick model dredging engine has a similar valve (I think) plans are online.

Mike.

[ChuckKey]

Trevithick used a tapered 4-way cock.

[Cp489]

Thanks for the help and suggestions so far. I will see what I can do to make it possible and easy to machine.
I'll update when I find a solution.

[Jasonb]

In a simple form just a spindle in a block with the "butterfly" formed by milling a slot on opposite sides. You will need to play about with hole sizzes in th eblock and thickness of the butterfly to get the timing right and stop the incoming air going directly to exhaust.

Also a good example to your students to think about how and if a part can be made given available tooling and a budget when they are designing it.

[Jasonb]

Infact the butterfly can be done if the 4 holes are sized so that their edges touch as if they were mitred then the butterfly shaped as an ellipse with minor diameter equal to the hole size and major diameter 1.404 x hole size and edges rounded to the same elliptical form

[cnr6400]

The butterfly approach might be difficult to seal. Keeping tiny sharp sealing edges engaged to the butterfly may prove difficult. A plug valve with large bearing/sealing areas around the ports, close fitted by lapping, with thick lubricant applied, will likely seal better. (and did, on ancient engines by Watt, Trevithick, etc) Just food for thought.

Boyle's law (p1v1=p2v2) is a good one to remember when designing seals for shafts and valves. 

[Cp489]

Jasonb,

I will look at that design and weigh at the differences. Thanks for modeling that. It's very easy to understand. Here is another valve I made, it would need to be redesigned so that the diameters would be closer in length. I think this is what several people were saying. I am not 100% sure though.

[Jasonb]

That would work and very similar to the first option I posted, just holes instead of a milled away area.