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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).
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 1-inch lap and 16-inch lead. The short end of the combination lever is
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 (i&"), multiplied by 2, equals 21 inches. Piston stroke (30") divided by 21 equals 131; 13multiplied 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
dinary reverse lever; the link fulcrum is on the vertical line through the centre of the main axle. E is the eccentric (the terminus of a "return crank” from the main pin). The eccentric rod D is connected at its extreme upper end A with the valve rod, and connected intermediately with the sliding link-block B. The eccentric's advance is zero (in this case with outside-admission valve), located on the line from axle centre to centre of crank pin. Now, if the valve should not have the advance that is always given it to overcome the lap and secure the lead, there would be no necessity for the extension of the eccentric rod above the link-block B, it would terminate at the link-block pin with which the valve rod would also connect, and in that event the vertical centre line P of the valve would register evenly with the centre C of the valve seat; as it is, the angle formed by the short upper section of the eccentric rod has advanced the valve far enough slightly to uncover the forward admission port to the cylinder --the distance between lines C and P equalling the lap plus the lead.
When the engine is started forward, the eccentric moving downward draws block B down also, but on account of the angle of the link, the block moves backward as well, and the valve is drawn its regular traverse to the right to open more fully the left admission