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Machining the Throttle Spool
Thursday, August 29, 2002 by John NiemannThe throttle spool is in our tracer lathe, having just had the two seats cut. They are the shiny bands near the right and left ends. Both seats were cut at a 20-degree angle and you can see the 20-degree template for the tracer in the lower left. We assumed (a dangerous thing to do in the locomotive restoration business) that this throttle was seated when #21 was retired and took the same .010" cut on both the top and bottom seat. Just enough to remove a majority of the rust pits that resulted from sitting so long in a damp environment and no more. This is a Ruston Balanced Throttle and you can get a good idea what "balanced" means in steam terminology from this picture. Notice how close the diameter of the lower seat (the one on the left) is to the upper seat (the one on the right). In fact they are 7 ¾" and 8" respectively. If we do a little math, we come up with an area of 50 ¼ square inches for the upper seat. Take that figure and multiply it by the 200 pounds per square inch of steam pressure that #21 operates at and we can see that there is over 5 tons of pressure forcing the spool onto the seat. To unseat the spool and open the throttle, you would have to overcome this 5 tons of pressure. This would not only require some rather stout and bulky mechanical linkages, but some mighty hefty biceps on the engineer. To overcome this pressure they put a slightly smaller seat below (on the bottom of the spool) with steam pushing up in the opposite direction. If we do the math on the bottom seat, we find we have about 9500 pounds of opposing pressure trying to lift the spool. The 10,053 pounds pushing down and the 9435 pounds pushing up nearly balance out and leave a difference of a mere 618 pounds to overcome to open the throttle. Hence the meaning of a "balanced" throttle.
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