Valve Gear & Cutoff

[fredrosse]

The Sidewheeler Margaret S. has a simple double acting American Walking Beam Engine, with steam pressure typically around 80 - 90 PSIG. Running with exhaust to atmosphere presently, a condenser to be fitted later.

The valve gear on the Margaret S. engine is Marshall-Bremme, and the lever has 19 notches, Zero, 9 Forward, 9 Reverse, corresponding to +/-45 degrees rotation = 9th notch. Most of the time I run with the 5-6-7-8-9 notches, (25-30-35-40-45 degrees) which gives 55% to 85% cutoff. The engine will run with much shorter cutoff when on the trailer, but does not do too well in the water with shorter cutoffs, the steam pressure is just too low to make use of the short cutoffs. At full gear (9th notch) the beam driven feed pump can't quite keep up, so I have to use the hand feed pump occasionally, 6th or 7th notch and the boiler level will climb slowly without the hand pump.

I looked at this with an EXCEL spreadsheet, and came up with the Valve Lever Position vs. % Cutoff in the "Head End" and "Crank End" graph attached. This valve gear provides almost equal cutoff in both the head end and crank end, from 30% cutoff all the way to 85% cutoff, and is very nearly the same in forward or reverse. How does this valve gear compare with other popular valve gear with respect to equal cutoffs in full gear and linked up, reversed, etc.?

[fredrosse]

A video showing the valve gear in operation, from 35seconds to 55 seconds on this video.

[Lopez Mike]

That is a GREAT diagram! I intend to print it out and wave it around here in the Northwest. Now I guess I will need to do the same for my Stevenson gear. Sigh. Another project on the list.

Or has someone already done this where I can get to it from the web?

[fredrosse]

Thanks for the compliment. I will have to add the other valve events (Admission, Cutoff, Release, and Compression to the graph. I have to get a "round tuit" first.

The only reason the engine has so many notches on the Forward/Reverse mechanism is that I used a cut down chain drive sprocket (from a GoKart) for the notches.

[fredrosse]

Pressure-Volume Diagram for the beam engine 85 PSIG (5.86 Barg) at 55% Cutoff. Engine produces 0.65 indicated horsepower at 45 RPM here, and this is a comfortable cruising load, making a little over 4 MPH. The other PV Diagram is full gear, 85% Cutoff, 1.05 IHP at 52 RPM, about 5.2 MPH. This is my maximum presently, and what a wasteful use of steam, hardly any expansion, but the exhaust note is music to my ears.

I plan to make the paddlewheels somewhat smaller diameter, and use articulated floats, these two changes will let the engine run at higher speed (if you can call 75 RPM higher speed). This will also let the engine make more power at short cutoff, better engine efficiency. The articulated wheels will make more efficient propulsion as well. This process is much more labor intensive than trying a different propeller pitch!