Page:A Treatise on the Steam Engine (1847).djvu/104

Slide Valve. 91

TABLE III

Cover[1]	Steam cut off at 1-3d from the end of the stroke.		Steam cut off at 7-24ths from the end of the stroke.		Steam cut off at 1-4th from the end of the stroke.		Steam cut off at 5-24ths from the end of the stroke.		Steam cut off at 1-6th from the end of the stroke.		Steam cut off at 1-8th from the end of the stroke.		Steam cut off at 1-12th from the end of the stroke.		Steam cut off at 1-24th from the end of the stroke.
	Before[2]	Behind[3]	Before	Behind	Before	Behind	Before	Behind	Before	Behind	Before	Behind	Before	Behind	Before	Behind
1-8th	.178	.033	.161	.026	.143	.019	.126	.012	.109	.008	.093	.004	.074	.001	.053	.001
1-16th	.130	.060	.118	.052	.100	.040	.085	.030	.071	.022	.058	.015	.043	.008	.027	.002
1-32d	.113	.073	.101	.066	.085	.051	.069	.042	.053	.033	.043	.023	.033	.013	.024	.004
0	.092	.092	.082	.082	.067	.067	.055	.055	.043	.043	.033	.033	.022	.022	.011	.011

1. Cover on the exhausting side of the valve in parts of the length of its stroke.
2. Distance of the piston from end of its stroke, when the exhausting-port before it is shut (in parts of the stroke).
3. Distance of the piston from end of its stroke, when the exhausting-port behind it is opened (in parts of the stroke).

stroke. Let the stroke of the piston be 6 feet, or 72 inches. We wish to know when the exhausting port before the piston will be closed, and when the one behind it will be opened. At the top of the left-hand double column, the given degree of expansion (1-3d) is marked, and in the extreme left column we have at the top the given amount of cover (1-8th). Opposite the 1-8th, in the first double column, we have .178 and .033, which decimals, multiplied respectively by 72, the length of the stroke, will give the required positions of the piston: thus 72 x .178 = 12.8 inches = distance of the piston from the end of the stroke when the exhausting port before the piston is shut; and 72 x .033 = 2.38 inches = distance of the piston from the end of its stroke when the exhausting-port behind it is opened.

To take another example. Let the stroke of the valve be 16 inches, the cover on the exhausting side 1/2 inch, the cover on the steam side 3¼ inches, the length of the stroke of the piston 60 inches. It is required to ascertain all the particulars of the working of this valve. The cover on tbe exhausting side is evidently 1/32 of the length of the valve stroke. Again, looking at 16 in the left-hand column of Table II, we find in the same horizontal line 3.26, or very nearly 3¼ under 1/6 at the head of the column, thus showing that the steam will be cut off at one-sixth from the end of the stroke. Again, under 1/8 at the head of the fifth double column from the left in Table III., and in a horizontal line with 1/32 in the left-hand column, we have .053 and .033. Hence, .053 x 60 = 3.18 inches = distance of the piston from the end of its stroke when the exhausting port before it is shut, and .033 x 60= 1.98 inches = distance of the piston from the end of its stroke when the exhausting-port behind it is opened. If in this valve the cover on the exhausting side were increased (say to 2 inches, or 1/8 of the stroke,) the effect would be to make the port before the valve be shut sooner in the proportion of .109 to .053, and the port behind it later in the proportion of .008 to .033 (see Table III). Whereas, if the cover on the exhausting side were removed entirely, the port before the piston would be shut and that behind it opened at the same time, and (see bottom of fifth double column, Table III.) the distance of the piston from the end of its stroke at that time would be .043 x 60 = 2.58 inches.


[Figure 162. Boilers of steam vessels, Her Majesty and Royal Consort, by Tod and MacGregor.]