American Walking Beam Engine

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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Re: American Walking Beam Engine

Post by Steam Captain » Sun Jul 14, 2019 12:42 pm

Hello Fred,

Can it be you used this engine I found on the website of the Mariner's Museum in Newport VA as a reference?

https://browse.startpage.com/do/show_pi ... &t=default
the arduino version steam engine indicator: https://app.box.com/s/b2i0z3gw6ny3rcfdet5xjg8ubrfu799i - app version coming soon
Excuse my occasional long response time. It's caused by the side effects from ptsd.
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fredrosse
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Re: American Walking Beam Engine

Post by fredrosse » Thu Jul 18, 2019 2:34 am

Mike, the owner of this engine will soon be looking for a suitable hull, probably a convert-able sailboat hull in the 60 - 90 foot range, in the New England area, as building a new hull would be far more expensive.

Ramon, the walking beam sidewheel engines of the USA were all very similar, and I basically copied the walking beam geometry of that ilk. I made various compromises, such as Marshall valve gear, slide valves rather than poppet valves, etc. due to the small size of my engine.

The larger engine just posted is much more closely matched to an actual historic wood framed sidewheel engine, but it will use piston valves rather than poppet valves. It is not a copy of the model you posted, but very similar.
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Re: American Walking Beam Engine

Post by TriangleTom » Thu Jul 18, 2019 2:48 am

A little off topic, but what are the advantages of using poppet valves in a walking beam engine?
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fredrosse
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Re: American Walking Beam Engine

Post by fredrosse » Thu Jul 18, 2019 12:16 pm

Not at all "off topic"

Almost all typical steam engines used harmonic motion for valve gears, usually with an eccentric producing continuous motion, mathematically a complex combination of Sine waves. Usually with slide valves or piston valves, with ports opened/closed during the valve continuous motion travel across ports. To get "sharp cutoff" during steam admission (a feature needed for better efficiency) this type valve gear is favored.

Poppet valves are can be favored because they give the opportunity of reliable seating, with rather large flow area available. They can be made double seated, so nearly balanced with respect to valve stem forces. However they have to come to their seat slowly, then stop motion completely for rather lengthy times on every piston stroke. With a fast turning engine, managing sharp cutoff without slamming a poppet valve against its seat was a problem. Large sidewheelers had engines turning at less than 20 RPM, so this technical feature allowed the poppet valves on sidewheelers, since there was plenty of time available to slowly seat the poppet valve, still with relatively sharp cutoff. Dynamic forces in machinery are a function of speed squared, so with an engine turning 20 RPM compared to one turning 200 RPM, the forces are at 10x squared, or 100 times higher, hence typically no poppet valves on typical higher speed engines.

Of course, Internal Combustion engine poppet valve practice has well evolved since that time, well over 100 years ago, and does allow for higher speeds with this type of valve. Note that IC engines do not expose their poppet valves to high pressures while they are opening and flowing, they are completely closed and seated when combustion peak pressures occur.
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Re: American Walking Beam Engine

Post by Lopez Mike » Thu Jul 18, 2019 3:25 pm

Now you are on a subject close to my heart. Cam operated valve motion.

The problem for engine designers of long ago, both steam and I.C., was that a poppet valve has to be stopped and stay stopped for part of the cycle. A piston (spool) valve or slide valve or rotary valve, not so. They can have over travel and thus use combinations of trigonometric motions.

The base problem is vibration and shock loads. There is no escaping the fact that when transitioning from stationary to following a sin curve, for instance, the higher derivatives go to infinity at the transition point. The base equation is the position of the valve during the full cycle. The first derivative of that position being its velocity, the second derivative being its speed, the third derivative being the acceleration and the fourth (the one that can make everything slam and vibrate) has no common name. Clumsily we call it the rate of change of acceleration. It's sometimes called the 'jerk factor'. No relation to politicians.

As two examples chosen from two ends of the design spectrum, going from stopped to in motion following a sin curve produces a spike in the jerk curve that goes mathematically to infinity producing a god awful blow to the valve gear with associated noise, wear and premature failure. Transition from stopped to in motion using a polynomial curve such as a 3-4-5 smooths out the fourth derivative and came into use in the late 19th century in high speed looms.

Why didn't this take over all cam driven linkages forthwith? The cost of computation. To design an I.C. camshaft this way means computing all derivatives through the fourth in perhaps 720 steps. Almost 3000 calculations. For a single loom design, a year's work for a team of ink stained wretches. The method was named Polydyne by it's French inventor. It was only applied very occasionally to I.C. engines until the cost of computation dropped dramatically post WW2. An engine designer named Edward Iskendarian in California copyrighted the name and it is now associated in the public mind with his camshaft designs though ALL modern cam shafts are designed that way. I'm wildly over simplifying this as the weights of all parts of the linkage and their flexibilities are included in the design. The resulting equations are intimidating.

A possible work around for a home steam engine designer might be to disassemble an I.C. engine and use the cam shaft. It will certainly have a superior curve to anything you might calculate yourself. Be sure, however, to include the cam follower if it does not have a flat surface. A curved follower will be included in the calculations. And include the stopped clearance (a few thousandths of an inch usually) in your design. It's important not just to allow for changing temperatures. There is a gentle closure of that clearance included in the original design.

I know. Too much information. Now that you are well and truly asleep, enjoy.
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Re: American Walking Beam Engine

Post by RNoe » Thu Jul 18, 2019 3:42 pm

Mike:
Your description of the calculous of valve designs was fascinating!
So much creative history there. Thanks for sharing.

And now back to machining custom parts for steam machines...
~RN
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