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Steam locomotive valve gear
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[quote]QUOTE: <i>Originally posted by TomDiehl</i> <br /><br /> <br />Cutoof is controlled by the position of the Johnson Bar or reverser wheel (depending on which the loco has). Cutoff normally is adjustable from 15% to 85% of the stroke. <br /> <br />Steam admitted during the longest part of the stroke will provde the most power, but the same quantity of steam must be exhausted causing back pressure. The back pressure will limit the top speed of the locomotive at the given cutoff setting. A quicker cutoff will admit less steam and give less to exhaust allowing for higher speed. <br /> <br />Another general rule, slide valves won't be used with a superheated engine. The piston valve became more popular, even on saturated steam engines because they were easier to service, repair, and fabricate. All machine shops have at least one lathe. <br />[/quote] <br /> <br />Just to nitpick, the 85% cutoff figure makes sense, but cutoff can go down to 0% when the reverser is in the center. <br /> <br />Also, while it is true that back pressure slows down an engine, in any cutoff position, the force of the forcing steam will be much greater than that of the exhaust steam. Given an infinte amount of steam, maximum speed would be found with the reverser all the way forward or back. The only reason to bring the reverser back is to conserve on steam, since at high speed you would otherwise quickly lower your steam pressure. <br /> <br />About the slide valves and superheating, there is no real problem (as far as I know) with slide valves on a superheated engine. It seems to me that this is the case simply because piston valves were widely adopted before superheating was. <br /> <br />[quote]QUOTE: <i>Originally posted by marknewton</i> <br /><br /> <br />The main difficulty was in properly lubricating the valves. The temperature of superheated steam was well above the flashpoint of the mineral oils in use at the time. <br />[/quote] <br /> <br />Um...oil can't burn when surrounded by steam, only in the presence of oxygen molecules, so the steam temperature doesn't really matter as far as the oil burning. Consider these two locomotives: Grizzly Flats Railroad (ex-Nevada Central) no. 2 and Ventura County Railway no. 2. GF no. 2 was built in 1881, Baldwin 5575, has slide valves and is not superheated. Ventura County Railway no. 2 was built in 1922, Baldwin 55415, and has piston vavles and is superheated. They both use Nathan hydrostatic lubricators, both using steam oil, also known as valve oil (as opposed to journal oil). <br /> <br />Sincerely and respectfully, <br />Daniel Parks
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