Design Part IV
This is a continuation of Part III where the cylinder casting and the main cylinder parts were described. In this part the components associated with the valve side of the cylinders are described, starting with the valve bushing and valve.
Zeuner Valve Diagram: The Zeuner diagram is useful to show when the various events in the valve cycle occur. The diagram construction procedure is described in the Locomotive Design Part II - Valve Motion pamphlet mentioned earlier. I used a nearly identical procedure described in Peterson's SO YOU WANT TO BUILD A LIVE STEAM LOCOMOTIVE.
The valve travel (or stroke) must be selected before the diagram can be started. A stroke of 0.625" was selected when the eccentrics were laid out. This is slightly larger than the exact scale dimension. The effects of typical valve train wear are more pronounced on the scale model. Using the largest possible valve travel reduces the effect of wear.
The first step in constructing the Zeuner diagram was to lay out a circle with diameter equal to the valve travel. Since the procedure is graphical, when working with a model, the diagram is normally scaled up by a factor of 10 or 20 to increase accuracy. I did the diagram in TurboCAD which maintains accuracy at small scales so I was able to use the actual dimensions.
The diagram construction technique is described in the references so I won't repeat it here. Rather, I'll try to explain what the diagram indicates about the valve operation. The diagram shows the valve operation for one side of the piston. The other side is the mirror image. The diagram is for a full gear setting and 85% cutoff. The 85% cutoff means that the valve closes cutting off steam admission at 85% of the piston stroke. The design is for a 1/32" lead ---- meaning the valve is open to admission 1/32" when the piston is top or bottom dead center. Once the valve travel, percent cutoff and lead are specified, the diagram pictured above can be generated. The first result of the diagram is the Lap Circle, in this case a circle of 0.105" radius. This means that a lap of 0.105" is required.
The valve port width must be selected to complete the diagram. Bigger is better in that a wider port will allow more steam to pass. However, the opening size is constrained by the other valve dimensions, the most important being the need to have ample space between the port and the steam admission port on one side and the exhaust port on the other side. After several sizes were tried, a port width of 0.187" was determined to be near optimum and selected.
The valve cycle begins at point A where the piston is top or bottom dead center and the valve is open 1/32" and beginning to admit steam. The valve port is fully open at point M and remains fully open until point M' based on a port width of 0.187" which was used to construct arc KTL. The valve closes to admission (Cutoff) at point F. The exhaust starts to open at point R and is open for half the cycle ending at point S where compression starts. The valve then starts to open to admission at point G and the cycle repeats.
The effect of wear on the system is to delay the events. The 1/32" lead is meant to compensate some for wear and keep the performance satisfactory.
Both the references show diagrams that explain the effects of hook-up (moving away from full gear) on the valve events and are suggested reading for one interested in the subject. The simple view is that hooking up the valve gear causes an earlier cutoff and increases the compression period. Another good source is the Internet where a search of "Live Steam Valve Motion" will generate many hits including at least one site offering free software that generates interesting graphics of the valve events.
One last result from the diagram --- the 25.82 degree angle between line RS and the horizontal. This means the eccentrics should be set so the valve is centered 25.82 degrees before the piston reaches dead center. As a comparison, Hiraoka uses an angle of 25 degrees on his 3/4" scale Heisler and Schroeder uses 10.5 degrees on his 1.5" scale Shay. Schroeder uses no lead which would reduce the angle some so we're in the right ball park.
Valve Bushing: The drawing below shows the valve bushing. It is very similar to the cylinder bushing and will also be machined from cast iron rod. The bushing will probably be made a tight sliding fit in the cylinder casting and then be secured with Loctite. The steam admission port hole will be drilled after the bushing has been fitted in the cylinder; the port in the casting will be used to guide the drill.
The two sets of eight square holes are the most critical part. The the recesses can be accurately done on the lathe and serve as a guide for the holes. The holes will probably be drilled undersize and then carefully squared with a file. The 16 round holes for the exhaust should be drilled carefully but they are not as critical.
Valve Piston: The next drawing shows a cross section of the valve piston and rod. After screwing around selecting various fills I decided to just use colors.
The 3/16" diameter rod is stainless. The lower end of the rod will be threaded and passed through the valve stem head. I didn't bother determining the correct length at this point. The rod is a loose fit in the piston to accommodate the slight change in angle of the rod as the rocker arm rocks. The shoulder just below the valve is a piece of 3/8" hex rod silver soldered to the stem. The nut and washer on the upper end are also stainless. The nut must be a lock nut. The nut should be tightened so that there is very little slack while still permitting the stem to move side-to-side. There is no need to seal the center of the valve since the two ends are inside the exhaust chambers. In many cases on prototype locomotives the valve center is hollow and open at the ends thus connecting the two exhaust chambers together creating a larger exhaust passage.
The valve body will be bronze (SAE 660). The initial plan was to make cast iron rings following the technique in Joseph Nelson's book motioned above. However, suitable 3/32" wide cast iron rings are available from Otto Gas Engine Works so I decided to buy the finished rings. Many model valves use removable piston ends and a bull ring between the rings to permit sliding the rings onto the valve without breaking them. I decided to make a one piece piston and hope the rings can be slid into place without breaking. (The pistons have now been fabricated and there is no problem sliding the rings on the piston.).
The critical dimensions involve the rings. The rings should have a 0.0015" side clearance. The specified rings are designed to work in a 3/32" groove. The 1.625" distance between the outer edge of the outer rings should exactly match the distance between the outer sides of the ports on the valve bushing. The 1.040" dimension between the inner edge of the rings should exactly match the distance between the inner sides of the ports on the valve bushing less twice the valve lap (0.210").
Valve Heads: The valve heads shown in the following drawing are straightforward and will be fabricated from brass or steel stock.. The 1.125" diameter shoulder fits in the valve bushing to align the head. The lower head has a packing gland to seal the valve stem. The packing gland will be made from a modified 5/16" brass compression fitting silver soldered to the lower head.. A pair of 3/16" ID 5/16" OD Viton O-Rings will be used for the packing.. The heads cover the low pressure exhaust chambers so the sealing task is much simpler than for the cylinder heads.
Exhaust Manifolds: The exhaust manifolds carry the exhaust from the exhaust chambers at the ends of the valve cylinder to the exhaust pipe. These passages are part of the cylinder casting (created using sand cores) on the full size locomotives. On the model, the manifold is attached to the inner side of the cylinder adjacent to the valve cylinder. The passages are formed by a recess in the inner side of the manifold.
Update 7/4/2011: The drawing of the Left and Right Exhaust Manifolds were updated to correct an error in the size of the inner recess.
Left Exhaust Manifold: The drawing above shows the left exhaust manifold.. The inner recess is 0.625" wide and 0.25" deep which will gives free flow to the 5/8" OD - 1/2" ID exhaust pipe. The manifold can be machined from cast iron, mild steel or aluminum. I plan to use aluminum. Once the manifold is installed, it is unlikely that it will ever need to be removed so rather than using a gasket, maybe high temperature Loctite (620) or 510 Flange Sealant should beused to seal the manifold to the cylinder casting..
Right Exhaust Manifold: The design of the right exhaust manifold is identical to the left manifold with the exceptions that that it is reverse (mirror image. The drawing below shows the right exhaust manifold.
This finishes the cylinder and valve design. The pistons and rods are covered in the Engine Design V which is next.