Valve Friction. Of the many different parts of a locomotive which have been studied from the scientific standpoint, few parts have been given more attention than the main steam valve. When the valves were small and steam pressures were not high, the force necessary to move the valve when in operation was not very great. With the pressures employed today and the sizes of steam ports found on our modern locomotives, the reduction of valve friction becomes a very important matter. From an examination of Figs. 70 and 71, it is an easy matter to see that the more completely a valve is balanced, the less work will be required to move it back and forth when in service.Valve Tests to Determine Friction. The question was considered such an important one that the Master Mechanics' Association appointed a committee to investigate different types of valves under conditions of service. The committee conducted its experimental work, in 1896, upon the locomotive testing plant at Purdue University, Lafayette, Indiana. The Purdue locomotive, known as Schenectady No. 1, was used, having cylinders 17 inches in diameter by 24 inches stroke. The ports were 16 inches long, the steam port being 1-1/4 and the exhaust port 2-1/2 inches wide. The bridges were 1-1/8 inches wide. The valve had a maximum travel of 5-1/2 inches, steam lap, 3/4-inch, exhaust lap, 1/32-inch, and was set with a 1/16-inch lead, with the reverse lever in its full forward position, and a 7/32-inch negative lead, with the reserve lever in its full backward position.
Four different slide valves were tested as follows: unbalanced D-valve, Richardson balanced valve, American balanced valve with single balance ring, and American balanced valve with two balance rings. A fluid dynamometer was placed in position between the valve stem and rocker arm in such a manner as to measure the force necessary to overcome the friction of the valve when operated under different conditions. The valves weighed 78, 85-1/2, 79-1/4, and 84 pounds, respectively. The weight of the dynamometer was 105 pounds and that of the valve yoke 37 pounds. The Richardson valve had 56 per cent of the area of the valve face balanced by the use of flat strips held against the balance plate by springs. The American valves had 61-1/2 ,and 66 per cent of their areas balanced by using single and double balancing rings, respectively.
The power required to operate the different valves was determined by means of the fluid dynamometer to which was attached a steam engine indicator. The arrangement was such that pressure diagrams could be taken in which the length corresponded to the stroke of the valve and the height to the pressure of the fluid on the piston of the dynamometer. Tests were conducted at different cut-offs and speeds. A few of the results secured are presented in Tables IX and X.
TABLE IX
Valve Tests Showing Mean Pull in Pounds for Different Valves
(Steam Chest Pressure, 100 Pounds per Square Inch)
Cut-Off in Inches 22 9-1/2 Speed in M.P.H. 10 20 40 10 20 40 Richardson 382 396 772 361 442 468 American single ........ 522 872 394 535 591 American double ........ 488 762 412 500 568 Unbalanced 1118 1062 1207 1322 1240 1180
TABLE X
Valve Tests Showing Per Cent of I.H.P. of One Cylinder Required to Move Valve
Cut-Off in Inches 22 9-1/2 Speed in M.P.H. 10 20 40 10 20 40 Richardson 0.43 0.49 1.54 0.32 0.34 0.61 American single 0.48 0.65 1.91 0.27 0.40 0.67 American double ........ 0.61 1.66 0.27 0.45 0.63 Unbalanced 1.20 1.30 2.42 0.82 ........ 1.62 The committee in their report to the society stated that the friction or resistance of unbalanced valves was about twice as great as that of balanced valves and recommended that the area of balance should equal the area of the exhaust port plus the area of the two bridges plus the area of one steam port. As a result of the work done by the committee and by some of the railway companies, it soon became evident that the D-valve for locomotive work was very inefficient. For this reason, in recent years the piston type of valve, which in itself is balanced, is being almost universally used.
Table of Contents; Page 95; Page 100; Index
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