Talk:Cutoff (steam engine)

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Terminology for cutoff[edit]

Moonraker88, I am sorry but you definitely have got this the wrong way round in your mind, cutoff is the percentage of the stroke at which the cutoff point occurs, and NOT the amount of the stroke left after cutoff.

I want to add some references later when I have them properly organised. Try:

  • Locomotive Management, Jas T Hodgson & Chas S Lake, Tothill Press, 1954
  • Locomotive Valve Gears, JH Yoder & GB Wharen, TEE publishing, a 1993 reprint of the 1921 2nd edition
  • Walschaerts Valve Gear, Henry Greenly & Ernest Steel, Percival Marshal
  • The Efficient Use of Steam, Lyle?
  • Heat Engines, D A Low

Bill F 10:36, 11 November 2006 (UTC)

Bill: My terminology was clumsy and misleading. You are completely correct. Old Moonraker 11:16, 11 November 2006 (UTC)


To my mind the reverser, as one of the means of altering cut-off, among other things, belongs in a separate article, linked from here and from Steam locomotive nomenclature

Bill F 11:01, 11 November 2006 (UTC)

Incorrect assertion?[edit]

"The steam engine is the only engine design that can provide its maximum torque at zero revolutions."

This assertion in the last sentence of the opening paragraph appears to me to be incorrect, since DC electric motors also provide maximum torque at zero revolutions.

In DC electric motors the torque output is proportional to armature current, which is maximum at zero rpm (so producing maximum torque at startup). I therefore propose modifying it to make it specific to heat engines:

"The steam engine is the only heat engine design that can provide its maximum torque at zero revolutions."

pkt_49 05:28, 24 December 2008 (UTC)

< The steam engine is the only thermodynamic engine design that can provide its maximum torque at zero revolutions per minute. >
I'm by no means convinced that this is true. For a given steam flow, a steam turbine will produce more torque when stationary than when at design speed, as will the power turbine in a gas turbine engine that is configured with a free power turbine. (talk) 15:40, 25 June 2016 (UTC)

Power reverse gear[edit]

The recent L2 header change from "Steam reverser" was necessary, in view of recent additions, but some other changes seem wrong. For example, I don't know why Harry Wainwright chose to use the system, but it certainly wasn't because of the size of his engines: I first encountered one driving one of his H class, a 50-ton 0-4-4. We've also lost the chronology, jumping from 1882 backwards to 1876, then forwards to 1900 or so. Any suggestions? --Old Moonraker (talk) 16:23, 7 June 2010 (UTC)

The fragments you're complaining about are from the old text, preserved unchanged through my edits. The problem with chronology is due to the fact that the example patent (the oldest I could find) clearly documents the technology the British used, but was granted after the British pioneered the system. If someone can find an earlier publication that is as clear as the patent I cited, feel free to use it.Douglas W. Jones (talk) 21:46, 7 June 2010 (UTC)
I'm not complaining of the fragments: I wrote them, and I'm glad that my early contribution to Wikipedia, which leant more heavily on original research than I would now be comfortable with, has found a source in Henszey. The problem is the implication that they are for large engines ("With larger engines, the linkages involved in controlling cutoff and direction grew progressively heavier"), and the implication, from its placing in the chronology, that Henszey's patent was the progenitor. I hope these issues can be fixed. --Old Moonraker (talk) 22:09, 7 June 2010 (UTC)
Found a ref for use in Scotland 1873 and added it. Switched the paragraphs back to chronological order. Happy now. --Old Moonraker (talk) 11:56, 30 March 2011 (UTC)

Proposed article split[edit]

The section on the reversing mechanism has now outgrown the main subject and rather overshadows it. It is time to hive it off to a new page, which could then be merged with Johnson bar. Globbet (talk) 22:13, 8 June 2010 (UTC)

  • Oppose: not yet approaching the cutoff point (sorry!) suggested in WP:SPLIT, when material "non-central" to the main topic may be removed: to me the controls seem an essential feature. Furthermore, the suggested subsequent merge with Johnson bar isn't appropriate: the distinguishing feature of that piece of machinery is its locking mechanism; it has a very wide range of uses beyond steam locomotives, such as securing aircraft landing gear in position. --Old Moonraker (talk) 05:26, 9 June 2010 (UTC)
Having clarified it a bit more in my own mind, my reasoning is this: that this article should be primarily about the thermodynamics, which is completely independent of the means by which cut-off variation is achieved; that altering the cut-off is not even the primary purpose of the reversing mechanism, and that the means by which the Valve gear is controlled is therefore a separate issue, which belongs with the discussion of the kinematics of valve gear mechanisms. Globbet (talk) 17:13, 10 June 2010 (UTC)
I can see your point about the thermodynamics being a separate issue from the mechanisms. Would it be possible to deal with this by expanding the thermodynamics here to give it its proper weight, as it deserves? If enough material can be assembled for a separate piece, this should be possible. For my part, I just don't want the topic sprawling too much: there is already a huge collection of locomotive-related articles to navigate through.
So far there's just the two of us talking about this, but there are many editors with a broader understanding, both of the subject and the way it's dealt with on Wikipedia, than I have. Ideally, I'd like to hear their views before doing anything not easily reversed (sorry, there I go again). --Old Moonraker (talk) 19:51, 10 June 2010 (UTC)
  • Comment -- clearly there is a desire for this new article. This edit to Glossary of rail terminology now links the text 'Reverser' (from the entry for cut-off) to Reverser handle, which looks like it only relates to non-steam locomotives. Although this change seems wrong to me, I am not sufficiently confident of my related knowledge to amend it and the linked article.
    See also the entry for 'Reverser' in the same glossary. -- EdJogg (talk) 12:31, 5 July 2010 (UTC)
  • Support: the discussion of reversing levers, powered or not, does not belong here, particularly as the feedback control mechanisms of 20th century power reverse servos gets the coverage it disserves. The primary reason is that it biases the whole discussion toward steam locomotives and misses the fact there are other major approaches to controlling cutoff. Corliss engines, or rather, Corliss valve gear, and their successors, all but eliminate the throttle valve, used only for starting, and rely almost entirely on feedback control of cutoff to maintain constant speed under wildly variable load. The cutoff web page would be better if there was a subsection referring, roughly equally and briefly, to the different approaches to control of cutoff, each of which would be found elsewhere.Douglas W. Jones (talk) 01:07, 29 July 2010 (UTC)
  • Support I don't understand the relationship between the terms "Cutoff" and "Reverser". Why are they the same article? What does the reverser have to do with timing of the input of steam into the cylinder? --WhiteDragon (talk) 16:45, 12 October 2010 (UTC)
If User:WhiteDragon isn't understanding the connection between "cutoff" and "reverser" the article isn't doing its job. The same mechanism controls the cutoff and the direction of motion and it's not possible describe the topic fully without including both functions. Direction of motion perhaps isn't yet covered sufficiently and I will try and fix this, unless somebody beats me to it. That would make User:Globbet's proposal about thermodynamics the candidate for any possible spinoff, although still believe that it too can be dealt with here. --Old Moonraker (talk) 21:51, 12 October 2010 (UTC)

Merger proposal[edit]

The proposed destination article has recently been expanded (to good effect) and much of the information here is now duplicated; perhaps only a few items of terminology specific to locomotives would need to be carried over. If accepted, I suggest that the main contributor at the new article be asked to do this as well. --Old Moonraker (talk) 11:17, 30 March 2012 (UTC)

  • strong oppose This would be like merging length to tape measure. Expansion valves are only one, largely obsolete, means of controlling cut-off. Andy Dingley (talk) 11:21, 30 March 2012 (UTC)
Per the discussion, I withdraw the suggestion.--Old Moonraker (talk) 20:43, 2 April 2012 (UTC)
Thanks. Now we just need someone who can write an understandable article that manages to explain expansion, cutoff, lap and lead. It's not an easy topic to do full justice to! Andy Dingley (talk) 23:48, 2 April 2012 (UTC)