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Steam locomotive valve gear
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Looking at the shortening of the cutoff question, trainjunky said that, given an infinite steam supply, maximum speed will be attained with the reverser in the corner. <br /> <br />This is not true. The steam is expanded during the power stroke; with full valve travel, the expansion is less than it would be with the travel reduced. Shortening the cutoff of the valve allows less steam to be admitted, but the steam utilizes its power and loses heat by expanding more. A smaller weight of steam does more work. If this is not true, why use steam? Compressed air would, under that thinking, do the same work. <br /> <br />Not only would full valve travel not produce higher speed because of the steam question, you'd have a much larger quantity of steam to exhaust during the time the port is open on the other end, causing more back pressure. If you are using less steam on the power stroke but allowing it to expand, you have less steam to get out of the cylinder before the next power stroke, during the port opening for exhaust. Your back pressure is going to be much less. <br /> <br />There were two reasons why slide valves went out of fashion. First was the difficulty of lubricating them and the fact that steam pressure had a mechanical effect on them, pressing them into their seats. The second was that port area was limited, because of clearances and the limited width available for the seats. The piston valve ports were all around the circumference of the valve. And steam had no mechanical effect on the operation of the valve. <br /> <br />Old Timer
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