90 Ton Mikado, Page 26

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3/8/8 Valve Timing + March is here already.

I am in the process of changing my basic valve timing. I don't like the way the current setup operates, and it's time to try something else. I have built up the right side valve gear, but now it's time to make a new valve piston for the right side. Below is a simulation of the "new" timing.


Image 157 -- Valve Timing

The green line is the piston travel - a total of 3.5" also known as the "stroke". The blue line is the offset for the return crank - about 2" and is not shown to scale. The remaining lines, red for forward, orange for reverse are the position of the valve piston for the given cylinder shown for 4 forward and 4 reverse valve settings plus centered. The 3 grey lines are the position of the steam valve opening, center, and the exhaust valve opening.

When a valve position line is above the "Inlet" line, the cylinder is open to live steam. When it is below the "Exhaust" line, the cylinder is open to the exhaust.

You can see in the "centered" valve setting, the valve piston travel is only influenced by the "lead" (a total travel of .389") and not by the return crank (a max travel of .75"). You can also see that in the centered position, the lead is not enough to open the valve to let the live steam into the cylinder. However, the exhaust side is set to open early, so the lead is capable of opening the cylinder to the exhaust.

I don't know how this compares to prototype practice, but this is what we are going to try next.

3/16/8 New High-Tech Valve.


Image 158 -- New Valve Piston, with rod, nut, crosshead, lever, and part of radius rod

Here it is. The new high-tech valve - made with alloy 2024 "aircraft quality" aluminum. Why? Because it was the cheapest thing I could find. The new valve is designed so that the valve lead does not exceed the valve lap. In other words the lead induced by the union link is incapable of opening the inlet valve with the reverser centered.


Pressure-Volume Diagram
(select diagram to visit image source page for details)

The diagram clearly shows that the inlet is not supposed to open until the piston reaches the very end of the stroke. Likewise, the exhaust does not open until the piston is at the other end of the stroke. This would not be possible if the lead exceeded the valve lap.

I have also done something I almost never do. I have gone back to the rod fastening method of the original drawings using collars pinned to the valve rod. This is necessary because it is not possible to guarantee perfect alignment between valve and crosshead. So the bore inside the piston is oversized to allow the rod to float.


Making the Radius Rod in two pieces was actually a great idea, although this does not adhere to prototype practice. I can adjust the valve or completely remove the valve, rod, and crosshead without taking the reverse link assembly apart.

3/23/8 Branch Pipe Jackets

It amazes me how few people know how a steam locomotive really operates - I mean on the really fine-scale detail, such as how the valves are supposed to operate. (includes myself) Most of those who knew are long gone. And as expected, there is a lot of folklore going around, most of which is either misguided or just plain wrong.

Besides the P-V example, here's another case. Branch pipe jackets -- what are they for? Are they cosmetic? - or do they serve a useful function? The erecting card shows them, but what do they do?

From what I could get on the message boards, these are to protect the branch pipe insulation, which is necessary to reduce heat loss to the weather. It was also pointed out that the cylinder "dress up" sheet is not for looks either, but to cover the insulation over the cylinders for exactly the same reason.


Image 159 -- Branch Pipe Jackets and Flanges and the Tools to make them.

The jackets cover the branch pipes which exit the smokebox at a steep angle. They are bolted to the smokebox with a flange. In the photo above, you can see both the jackets and the flanges although they are not welded together yet.

The flanges are shaped to fit the curvature of the smokebox. I used a trick I learned from Jesse Livingston (alias Uncle Jesse). Use a pair of wood blocks as a former to bend the flanges. - Works great. Also the pipe holes had to be filed to allow the pipes to pass through at the steep angle.

The upper end of the jackets need to match both the angle and the curvature. Try as I may, my computer generated template failed. Oh sure, I could eventually get that to work. But forget it. Make a live template out of cardstock and be done with it.


Image 160 -- What the Jackets Cover + Piston Rod Gland.

Here's the branch pipe, and the Tee from the oiler the jacket is supposed to cover and hide. I made collars that are the inside diameter of the jackets to support them at the bottom (shown here).

Also remember the piston rod glands mentioned in the last page? Here you can see one installed.

3/23/8 Branch Pipe Jackets - complete and installed


Image 161 -- Branch Pipe Jacket -- installed.

These fit so well only 2 screws are required to secure them. I need to sand and repaint them.

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