MORE ON SLIDE VALVES

Case 110

Case 110-Howard Pross of Luverne, North Dakota owns this one. Courtesy of O. R. Aslakson, New Rockford, North Dakota 58356.

O. R. Aslakson

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New Rockford, North Dakota 58356.

I found the Frank Burris story 'How to design a slide valve' in the Sept.-Oct. '72 issue very interesting. I am building my second model, a 40% size of my 25 HP George White, his diagrams may be of help to me, I still have my valve to make. I wish he had included some ideas on suitable 'inside lap' for different sizes and RPM models. I would lean toward too little rather than too much.

Since I have seen more than one instance where model builders using the single eccentric valve gear have had trouble getting the proper dimensions of valve, I will try to explain how I would do this. I hope my efforts will not be a case of 'the blind trying to lead the blind.'

First, there is only one width of valve that is right for single-eccentric gear, woolf etc. I would advise making the valve last, after the gear is completely assembled and adjusted, (eccentric properly located, pedestal correct height, reach rod right length etc.). Now, temporarily fasten a thin piece of aluminum or even thin cardboard to the valve stem so it will move across the valve ports when the engine is turned over. Now, with the reverse lever in one of the corner notches turn the engine to either center, (turn it in the direction the lever is set for) and set your dummy valve for the lead you want at the port leading to the end of the cylinder where the piston is. Now turn the engine to the other center, (lever in the same notch) if the lead at this end is too much your valve is not wide enough, if there is no lead it is too wide.

Now with the lever in the other corner, turn the engine the other direction and check your leads, if they are nearly the same that of course is good, if too far off recheck your centers, height of pedestal, lost motion etc. Possibly you may have to make a compromise in the width of the valve just a trifle short for one motion and a little long for the other. The point of cut-off (best measured in crosshead travel) is very important, if too far off, possibly a little compromise with the proper lead might help. Now for 'inside lap 'you can cut an exhaust cavity in your dummy valve and check the exhaust opening, point where compression starts etc.

There is some precise geometry involved in the design of a single eccentric valve gear. If I tried to go very far into this it would really be a case of the blind leading the blind.

I find articles like Frank's interesting, also ones discussing valve gears, tandem and cross-compounds, double and single simples etc. I happen to like compounds. The Marsh valve gear is my favorite, I am sure there are many engine men that disagree with me. How about some opinions of which would have the advantage as far as backpressure is concerned, the tandem or the cross compound. The big piston being a half-travel when the small one is at one end could be either an advantage or disadvantage. I have run both, liked both. Of course I like any smooth running steam engine.

The fourteenth annual Central North Dakota Steam Thresher show in September 1972 was well attended and the weather was satisfactory. Would like to see you there Sept. 21-22-23, 1973.