How do you design valve gear

[Bob Wild]

I have to take a short break from my workshop, so I am using the time to consider and draw the details which Julian has omitted from his "looking at" model of Stephenson's Rocket. The biggest challenge is the valve gear, He gives no information about the eccentrics, the port sizes and throw of the valve. I can get a fair idea from Glithero's drawings but they are difficult to scale as they are all in isometric projection. So any help and suggestions would be most appreciated. I do know the size of the cylinder and the throw of the crankshaft, so I assume that would be the best starting point.

By the way, I did get more info from the NRM about these drawings Model of Rocket But they wanted £240 for proper digitised copies and that is a bit beyond my budget! They did say there is an ongoing project to digitise all their drawings which should be done in the next two years.

Bob

 

[GreenTwin]

For simple models, somewhere between 70% and 100% valve travel generally works well (late or no cutoff).

JasonB posted a good summary of this not too long ago, which I have lost.

Many common D-valve steam engines use a simlar or virtually the same port layout, which is ports that are about 70-80% the width of the bore, and ports that are in a 2X landing, 1X steam port, 1X landing, 2X exhaust port, 1X landing, 1X steam port, 2X landing.

Here is a port face from a 20 hp Stanley auto steam engine.

The D-valve rides on this port face, and the valve overruns the ends of the face, to prevent a ridge from building up on the port face.

 

[GreenTwin]

Here is the Stanley D-valve, which is a pretty typical D-valve.

 

[GreenTwin]

Here is the D-valve on the port face.

The valve is shown at its midpoint, and also about at full travel, which is not quite a 100% steam port opening.

 

[GreenTwin]

I basically scale these proportions, and also scale the Stanley eccentric, to use with other steam engines.

For those who are building authentic copies of some of the larger steam engines, those often had an early cutoffs, (with the steam sometimes being cut off at 12% piston travel ?), and so they would be different than this simple design by Stanley.

Here are some sketches I made for the Stanley D-valve and eccentric.
I think these sketches are accurate, and I think they could be scaled and used with most model steam engines.

 

[GreenTwin]

For small steam engines, scaling the Stanley ports down can create ports that are too thin to machine, and so typically the port width is made wider than normal for small model engines.

And often the port opening is 100%, ie: no cutoff is used.

And I think JasonB mentioned that sometimes the lap is eliminated on the valve in small models.

.

 

[Bob Wild]

Thanks Greentwin. That gives me plenty of food for thought

Bob

 

[Ken I]

Here's a diagram I posted on the valvegear off Gerry's Beam Engine

Cylinder heads and anything not involved omitted from diagram.
As you can see the valve motion is 90° out from crank - changing timing from plus 90 to minus 90 changes rotation.
Actual drawings in ACAD or PDF in the downloads section.
Gerry's Beam Engine Metric PDF

Regards, Ken I

 

[Lloyd-ss]

Green Twin and Ken I.
Nice work guys.
Having good drawings to work to greatly reduces the amount of head scratching time wasted at the machine.

 

[Bob Wild]

Thanks Ken. One of the points that I am unclear about is the size of the ports and their separation. Presumably it is a function of the bore and stroke.

Bob

 

[Ken I]

On this beam engine the ports are simply drilled Ø1.5mm holes - 3 of them drilled side by side to emulate a slot. I have done this Ø12mm bore engine with a single hole and it works just fine - its amazing how much air (or steam) can get through a small hole.
Since we are generally interested in slow running rather than high performance, there is no need to worry much about the port size.
However, once you have chosen a port size, everything else about the valvegear follows viz :-

In the above the Ø1.5 port then determines the lands on the "D" valve to be 1.5mm and the pitch between the ports to be 3.0mm.
The cavity in the "D" valve must be 4.5mm for no overlap - it this case I allowed 5.0mm (for 0.25mm overlap either end) this allows ports to be slightly open to exhaust (overlap) at the ends of the stroke helping to evacuate any residual pressure. It also gives 0.25mm overlap separation of the inlet ports from the pressure side to seal (if you made it exactly on size you would need to be impossibly precise to get sealing).

It also determines the valve stroke to be 3mm + overlap = 3.5mm

Hope this helps.

Regards, Ken I

 

[Charles Lamont]

Bob, on a full size engine the width of the steam ports will typically be about 1/10 of the cylinder bore, and the exhaust port about twice as wide. The port bars are usually the same width, or about the same, as the steam ports.

The valve will often be a bit longer that the distance between the outside edges of the steam ports. This is 'lap' and it improves the efficiency of the engine, in full size at least. If there is any lap, the eccentric has to be 'advanced' from the 90° position to ensure the valve is open a crack at dead centre.

For a smaller model that is not going to work really hard, a drilled hole of about 1/10 of the bore will do for the passage. A drilled hole will often do for the port too, but I would mill a slot to give more area as it begins to open. Slotted ports are also better for getting the exhaust away quickly, reducing the back pressure against the piston.