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Steam Engine Balancing - Good Explanation?

Steam Engine Balancing - Good Explanation?

Steam Engine Balancing - Good Explanation?

(OP)
Hello all, this one's definitely hobby-related; perhaps some of you more mechanically-minded types wouldn't mind considering the following and answering the question I've posed at the end.

While preparing a narrative to be used by guides for conducting engine room tours aboard the legacy steamship where I volunteer, somebody who had been Googling around asked me [as if I would know!], "What is the Yarrow-Schlick-Tweedy system of balancing four-cylinder reciprocating steam engines, anyway?"

I did some research of my own - - and was promptly dragged into a milieu of calculus-drenched diagrams that purportedly would make the entire business as clear as mud, something that, alas, did not transpire since I have a higher-maths-challenged mind.

Ripper's Steam Engine Theory and Practice, c. 1932, pp. 485 & ff I found very daunting indeed; Seaton's Manual for Marine Engineering, c. 1918, pp. 778 & ff offered its explanations with [as near as I could tell] only geometry and trigonometry, and was thus not completely beyond me, but still a tough row to hoe; and Sothern's "Verbal" Notes & Sketches for Marine Engineers, c. 1917, pp. 4-8 was much more practical and therefore not nearly as abstruse, but grasping what was being said was still a tall order. [All of the foregoing were [are?] available on line as PDF's at www.archive.org - and this is not promotion; it's merely acknowledgement].

The answer I ended up providing was:

"I've read Ripper's, Durand's, Sennett's and Seaton's books on steam engines trying to research this topic, since it's mentioned on Wikipedia with no real further explanation. I'm no Einstein, so behind all the higher mathematical gobbledy-gook, the following is as close as I've come to being able to understand it...

"At its most basic, the 'Yarrow-Schlick-Tweedy system' is a means or, more accurately, a method of balancing any large four-cylinder steam engine such that no harmonic balancers or other special features need be added to the engine's construction. By avoiding these complications, engine complexity is minimized, maintenance is simplified, and manufacturing costs are reduced.

"The designers of the engine start with a basic form that has [numbering from one end of the engine to the other] the cranks for the first and second cylinders at 180 degrees to each other. Next they put the crank for the third cylinder at ninety degrees to the second, and the fourth at 180 degrees to the third so they end up with two sets of of opposed cylinders that are at 90 degrees to each other. Once they've done that, the designers crunch out and factor together all the inertia forces, couples, thrusts and so on of all the moving parts in the engine to come up with a preliminary force-balance diagram.

"The next step is to calculate what extra and/or different unbalanced forces will be generated by the changes in steam pressure that occur over the course of one engine revolution as the steam expands from its admission pressure to its release pressure in each cylinder at the engine's design loading and speed. Because these large engines are double-acting, the difference in area due to the presence of the piston rod is one of the contributors to the differences in developed force between the top and the bottom of the piston and hence the differences in pressure profile that need to be accommodated...there are lots more, but you get the idea.

"Finally, by changing the masses of the connecting rods, adjusting the masses of the counterweights on the cranks and tweaking the angles between the various cranks slightly to significantly away from ninety degrees, the engine designers come up with a compromise configuration that to a great extent neutralizes the unbalanced forces that cause engine vibration and pounding."

End of explanation.

I do want to add, though, that Seaton explains two other factors in balancing, one being to take note of the fundamental, first and second harmonic frequencies of the vessel itself and use these to assist in calculating where in the vessel the engine should be positioned in regard to the nodes of the hull harmonics, and secondly to endeavour to ensure that the normal running speed of the engine will not coincide with any of the natural vibrational periodicities of the hull.

My question is this: have I got a reasonable rudimentary handle on what's involved?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Steam Engine Balancing - Good Explanation?

Not my field at all, but I suspect optimal phasing between multiple cylinders is compromised by the requirement not to require an external starting device. I.e, there cannot be a shaft angle where the engine cannot self-start. This probably rules out some of the obvious and neat balancing arrangements.

- Steve

RE: Steam Engine Balancing - Good Explanation?

(OP)
Thanks for the reply, Steve; excellent point.

The starting device on such engines was typically bypass valves that expedited getting steam to whatever cylinder was in the optimum position when it came time to 'crank' from rest. Richard Sennett's "The Marine Steam Engine," c. 1899, pg. 222, mentions how in the British naval vessels HMS Powerful and HMS Terrible various crank arrangements were tried in order to reduce vibration, and that one of the designs, although it caused much less vibration, was "difficult to start in certain positions," presumably because it was tough to get the engine to come to rest precisely in one of the spots where cranking was facilitated, also presumably something that would not be desirable in a military vessel...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Steam Engine Balancing - Good Explanation?

My advice is to do your research with those organizations that restore and operate steam trains. In Essex, Ct, you have Essex Steam Train that operates steam train rides and their shop restore these beauties. I know that throughout the USA, there are other similar organizations. I wish that my friend Joe Snopek was still alive as he was an avid train enthusiast that could have helped you.

RE: Steam Engine Balancing - Good Explanation?

You should have replied in a manner similar to the famous "turboencabulator" video.

http://youtu.be/rLDgQg6bq7o

RE: Steam Engine Balancing - Good Explanation?

It doesn't matter how many times I see/read/hear that, it's always funny. I wonder if any non-engineers even get the joke.

- Steve

RE: Steam Engine Balancing - Good Explanation?

(OP)
Marine four-cylinder double-acting steam engines, whether quadruple- or triple-expansion, are a bit of a different animal than locomotive steam engines; for one thing, locomotives have those huge side-rods to contend with when it comes to balancing, necessitiating the use of significantly bigger counterweights to mitigate rail hammer...but that's not at all my area of expertise, and I won't go any further into it.

Nevertheless, I'll take help from anyone who understands what I'm asking, regardless of their background! smile

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

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