never be altered in length; one reason being that such alteration would change the angularity of the radius rod; and, further, the upper end of the combination lever is so extremely short in comparison with the lower end, that to make the modification there the calculations would indicate fractions so small as to be impossible for the mechanic to work with; while a seemingly slight error in the length of the short end would introduce a noticeable irregularity in the valve's motion, yet an error of the same distance in the length of the long end of the lever would probably have no appreciable influence on the valve.

The "long travel" of the valve is the motion imparted by the eccentric; and the "short travel" is that derived from the main crosshead and imparted to the valve by the angularity of the combination lever. Place the reverse lever in the centre notch of the quadrant, link block in the exact centre of the link; now rotate the main drivers a full revolution and the valve will travel between its limits in each direction, a distance equal to twice the lap plus the lead.

With outside-admission valves the valve stem is connected with the upper end of the combination lever, while with inside-admission valves the radius rod is connected at the top, and the valve-stem connection is the intermediate one.

For Outside-Admission Valves.-Divide the piston

stroke by twice the steam lap plus lead, and multiply the quotient by the short end of the combination lever; the result is the proper length of long end of the combination lever (from centre to centre of pins connecting radius rod and crosshead link with combination lever).

-inch lead.

3

Example.-An engine having 30-inch piston stroke, outside-admission valves with 3-inch steam lap, having-inch lead. While in the back shop, it is desired to change this to -inch lap and The short end of the combination lever is 3 inches between centres of pins. We wish to find the required length of long end of combination lever to suit the readjustment. Lap (1") plus lead ("), multiplied by 2, equals 2 inches. Piston stroke (30") divided by 2 equals 131; 13 multiplied by 3 (length of short end of lever) equals 40 inches, which is the correct length of the long end of the combination lever.

For Inside-Admission Valves.-Divide the piston stroke by twice the steam lap plus lead, and multiply the quotient by the short end of the combination lever, the result equalling the whole length of the lever; subtract from this the length of the short end of the lever, and the remainder will equal the proper length of the long end of the lever-the distance between the centres of pin holes connecting it with valve stem and crosshead link.

Such readjustments of the proportions of the com

bination lever result in a slight difference in the long travel of the valve, but really so slight as to be of no moment.

If it should be desired to increase the lead by an equal reduction of the lap of the valve, or to reduce the lead by an equal extension of the lap, it should not be necessary, of course, to make any change in the proportions of the combination lever.

Lastly, the reverse lever should be set in mid-gear, bringing the link block into position at the exact centre of the link, in which the revolution of the driving wheel and the oscillation of the link will impart no movement to the radius rod; then revolve the main drivers, and see that the short travel of the valve, secured from the piston and crosshead, gives an equal lead opening at each end of the valve. On page 112, one of the locomotive-builders is quoted in "Notes for adjusting Walschaert gear," as follows: "The tram marks of the opening moments at both ends of the valve should be marked on the valve stem, and the latter lengthened or shortened until equal leads at both ends are obtained." To easily accomplish this equalization of the position of the valve, the valve rod is usually fitted with adjustable threaded nuts.

A model of the Walschaert valve gear with all parts made so that they may be adjusted in length, is one of the greatest helps to the student of this motion. There

are several such models made for sale, one of which is illustrated in Fig. 39.

OTHER VALVE GEARS

A great number of steam-valve actuating mechanisms are in use, of which only a very limited number are applicable to locomotives; and those which are may be divided into three general types, comprising, first, the "Stephenson" double eccentric gear with which every one is familiar, and which has been generally used on American locomotives until recently; and this type also includes the Allan and the Gooch valve gears. Secondly, there is the Walschaert type which is the principal subject in this book. Third, the Hackworth gear; this motion was invented by John Wesley Hackworth and patented by him in 1859; the principle is illustrated in Fig. 40, and is the mechanical movement upon which is based the Joy and other similar valve gears; in 1879, Marshall patented an arrangement of the Hackworth idea, in which a shifting radial lever was used instead of the straight link and slide of the eccentric rod.

Referring to Fig. 40, the Hackworth valve gear is explained as follows: the link, L, is suspended from a fixed fulcrum or trunnion pin, at its centre, upon which it can be oscillated and fixed at different angles by the reversing arm, R, which may be considered as an or