Seepage in Small Steam Engines

Seepage in Small Steam Engines

by R.G. Neff

The Live Steamer, March-April 1951

The subject of this article is probably an old story to many of the readers of The Live Steamer, but it may prove helpful to others who are building steam models, especially the very small ones, such as O gauge size, and are having difficulty in getting them to run as well as they think they should.

I never tried to use lead on O gauge steamers because it would be only a few thousandths and very hard to set evenly at all four points of steam admission. Instead, I set the valves to admit steam to the cylinders as near as possible to dead center. The models ran nicely, but not quite as slowly as I thought they should and seemed to "buck" slightly at low speeds. By watching the valves carefully, with chamber heads off (note: piston valve type cylinders) I could see there was a slight seepage of steam around them, not enough to hear it, but enough to show small bubbles although the valves were lapped in with kerosene and had only enough clearance to move freely, which would be only 1 or 2 tenths.

I have discussed this matter with several men who know steam engines, including the division master mechanic of D&H RR, and all agree that in any engine there is a slight seepage of steam to the ports before the actual port opening occurs, regardless of how well the valves are fitted. The volume of steam reaching the cylinders is small and has not effect on an engine of any sice, but when we get down to O gauge cylinders and the tiny clearance at the end of the stroke, just a little bit of steam getting into them too soon will have the same effect as pre-admission in larger engines.

After figuring this out, I built the next model with a longer steam lap, so that the port opening occurred after dead center, about one spoke of the drivers. As soon as this model limbered up, it showed the result of the change in smoother running at lower speed. Another result that I hadn't looked for was a notable increase in the efficiency of the engine. Previous models had run 2-1/2 to 3 minutes per ounce of water, but this model ran 4 minutes per ounce, under normal operating conditions, pulling 15 cars at a scale speed of 45 to 50 MPH. This in turn meant that a lower fire would keep the pressure on the head. The latter result is due to the absence of bucking, in which the engine has to use up part of its power to overcome the effect of pre-admission.

To me the above results proved that seepage could affect the running of an O gauge steamer, and that an allowance should be made for valve design. It doesn't seem to affect 3/4 inch scale models. I drove Ed Bergh's 3/4 inch Hudson and it ran very smoothly at very low speed. Ed said that he had made the lead in exact proportion to that of the B&A prototype. I wonder if it has any effect on 1/2 inch scale models, but have had not chance to ask any of the builders in this size.