Design Part III
The layout of the cylinders and valves is more complicated than just scaling the full size locomotive. For example, direct scaling of wall thickness is not practical unless very close tolerances are employed. Another complication is the steam passages between the valve and the steam input/exhaust and the valve and between the valve and the cylinder. On the full size cylinders these passages are made as part of the casting process by using sand cores that are shaken out after the parts are cast. The design process must include a technique to machine these passages.
I lack the knowledge and experience necessary to make the rule of thumb judgments that are probably second nature to both mechanical engineers and machinists (my background is electrical engineering). To compensate, I've used Ken Schroeder's Shay design as a model and tried to make all components at least as robust. Also, emphasis was placed on making all components as tolerant as possible of my kind of machining (sloppy).
Another area of design concern is engine efficiency. I've found that my little shay has much more power than needed when running on a flat track or tracks that model a mainline where the grades are on the order of 1.5% or less. However, the geared locomotives typically were used on branch and logging lines where 3% grades were the norm and 6% and greater grades were not uncommon. I usually run my shay at the Mill Creek Central track that has long 3% grades. On that track it's a constant effort to maximize power to climb the hills and then try to down when descending ---- which makes it really fun to run. With this in mind, I made the steam passages and ports as large as possible to improve efficiency in that area. The operating steam pressure of the model will be 100 psi vs. 200 psi for the full size prototype. The correct 1.6" scale pressure would be 27 psi so the model will use much greater than scale pressure. This greater pressure will permit a somewhat smaller than scale steam and exhaust passage. The valve design was done using a reprint of design techniques described in Locomotive Design Part II - Valve Motion. This pamphlet (No 28) published in1910 was originally published in Railway Machinery. The pamphlet was purchased from Nation Builder Books, www.nbbooks.com.
The design process involved numerous iterations where everything was sketched out and then modified to achieve the objectives outlined above and sketched again. Hopefully, this process will minimize revisions after the engine is built.
The table below summarizes parameters arrived at during this iterative process. The decision on the bore and stroke were based on reasonable power and achievable steam as describe in the Heisler Engine I page. The crosshead guide ID decision was also described in the Engine I page. The valve-cylinder spacing and the valve stroke were determined before the crankcase, crankshaft and eccentrics were laid out, also discussed in the Engine I page The rationale for some of the other compromises are described in this page. Note that some values in the MRSR 91 column are preceded by the "~" symbol. These values are estimates based on other measurements or estimates made from photographs. For example, the MRSR 91 cylinder heads were hidden by the head covers. The head OD value of ~ 21" was based on the overall cylinder OD, the head cover OD and the head diameter of a slightly small Heisler engine.
Squeezing it all in: Note from the table above I chose to use a slightly oversize cylinder OD ( 2.91" vs. 2.81" scale). The cylinder length was stretched a little over 5% (3.25" vs. 3.08" scale). This slightly longer cylinder together with a slightly shorter stroke gives enough room to make the piston with a flat top and bottom instead of having a recess in the piston bottom and a bulge in the piston top. The lack of a bulge in the top allows the upper head to be flat and eliminates a bulge in the head cover. Before fixing the cylinder length the overall engine was sketched out and the relationship to the frame and boiler examined as shown in the next drawing.
The cab width of 9.5' scales to 15.28" which was increased to15.5" inches. The inside dimension between the upper and lower frame bars of 40" scales to 5.36" which was increased to 5.437" for the model. The outside edge of the cab should line up with the center of the cylinder head cover and the outer edge of the walkway should line up with the upper corner of the cylinder castings. The drawing above shows this to be the case on the model once the cylinder head and head cover are added, so the esthetics are preserved. Another concern was that there was room between the cylinder casting and the boiler for the exhaust pipe. The drawing shows that there is plenty of room so we should be OK.
Steam Passages: The valve performs the function of connecting the steam input and exhaust ports to the two ends of the cylinder at the correct time --- or more accurately the correct position of the piston. Before designing these passages it's useful to get an general idea of how it was done on the prototype. The following are photos of a smaller Heisler engine that show the general structure of the cylinder, valve and passages.
Update 7/3/2011: The drawings of the left and right cylinders shown below were replaced to correct an error in the exhaust passages on the inside the valve cylinder.
Left Cylinder: The next drawing shows the left cylinder which contains both the main cylinder and the valve cylinder. Both cylinders are oversize to accommodate bushings (sleeves).
The cylinder is basically a chunk of cast iron with two holes for the cylinders and a hole for the admission pipe out the side. The cylinder will be machined from a length of 4.5" diameter gray cast iron rod. The sides of the rod will turned to flats on the lathe. The cylinder holes are the most critical, both in diameter and the spacing between the two holes. The cylinder holes will be bored on the lathe. The rounded sides will be achieved by mounting the block on the rotary table and rotating it under an end mill. The recess for the exhaust manifold will be milled out.
The cylinder heads, valve heads, and exhaust manifold are all attached with 6-32 screws. The depth of the threaded holes for the cylinder heads and valve heads were not shown to reduce drawing clutter. A depth of 0.375" is reasonable for all these holes. The holes for the exhaust manifold attachment screws were also omitted. The locations of these holes are shown on the manifold drawing.
The steam passages add a bit of character to the cylinder casting. The steam admission is via the 3/8" NPT hole in the side of the valve cylinder. The exhaust passage is via the rectangular holes near each end of the valve cylinder. There is a recess in the exhaust manifold that routes the exhaust to the exhaust port on the manifold.
The passages between the valve cylinder and the main cylinder will be machined. The first step (after the cylinders are bored to the correct size) will be to machine the 1/4" wide passages in the side of the valve cylinder. This will be done using a 1/4" wide 1.125" diameter Woodruff key slot cutter. The inner passages are 1/4" deep except in the area under the exhaust manifold where the depth is 1/8". The outer passages used for exhaust are also 1/4" deep except in the area between the cylinders where they are only 1/8" deep. The slot between the outer recesses and the exhaust manifold will be opened using an end mill from the manifold side.
A 1.5" diameter cutter can also be used to cut the recess in the side of the main cylinder. The depth of the recess varies from about 3/16" at the ends to sufficient depth in the middle to provide two passages between the two cylinders. The vertical slots in the side of the main cylinder are cut with a 0.75" diameter woodruff cutter. The cutter is moved vertically to create the slots. A rotary tool can probably be used to deepen the slots without breaking into the head screw holes.
These passages do not require close tolerances; the openings in the bushings are the critical dimensions. The most critical part of machining the steam passages is to avoid cutting through the side of the casting.
Lower Cylinder Head & Packing Gland: The lower head shown above will be fabricated from steel or brass similar to the upper head. The 2.123" diameter shoulder aligns the head with the inside of the main cylinder bushing and the 1.748" diameter shoulder that fits inside the cylinder reduces free space inside the cylinder. . The 0.125" thick flange is sandwiched between the cylinder casting and the flange on the upper end of the crosshead guide. The 1.748 diameter shoulder on the lower side of the head aligns the head with the inside of the crosshead guide. The head bolts pass through the flange on the crosshead guide and the head flange into the cylinder casting. The packing gland is similar to the gland Ken Schroeder used on his shay and will be machined from bronze rod. The packing will be a pair of 5/16" ID, 7/16" OD Viton O-Rings . The socket head cap screws that hold the packing gland will be lined up horizontally. Hopefully it will be possible to access these screws with a long hex driver up through the crankcase.
Update 4/12/2017: The OD of the O-Rings used for packing was incorrectly shown as 9/16". Also, I've switched to PTFE O-Rings for this packing on the Shay and I think I did on the Heisler too. One side of the O-Ring is cut so it can be slipped over the rod and pushed into the recess. Rex Smith who is building a Heisler pointed out the the Packing Gland shown in the photos in the construction page seemed to be larger than the 1.375" OD shown on the drawing. Apparently I made the OD 1.75" using the same material as the Crossheads. The holes retaining the glands were also further apart. Either size is OK, the only critical part is that which holds the packing in the recess in the head.
Update 7/4/2011: After about 100 hours of operation a leak developed in the left cylinder rod packing. It turned out that the one of the packing gland screws came out and the second screw became loose enough so the packing gland was dropping down and hitting the top of the crosshead (which explains one of the engine knocks). After a while that second screw broke in half. So, off with the boiler so that the cylinder could be removed to get at the broken screw. The new screws have lock washers and were secured with Loctite 620. I did the same for the screws on the other side before they backed out.
Head Gaskets: I made head gaskets for the Shay which worked fine but took some effort. This time I'm going to use Loctite 510 gasket eliminator on the Heisler.
The valves and exhaust manifolds are described in Engine Design IV.