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from the up-and-down glide of pin P when yoke Y is inclined at an angle with the vertical; and to reverse the engine, it is only necessary to reverse the direction of inclination of yoke Y; or to shorten the cut-off, to raise the yoke toward the mid position as it is shown in the cut.
As to the necessary advance of the valve to equal the lap and lead: with the engine standing as in Fig. 48, it will be observed that the eccentric is at its extreme backward position, and if the reverse lever should be placed in full gear-either forward or back—the valve would occupy the position of its farthest traverse, either front or back, that would not be affected by combination lever E at all during the reversion, for there is a straight line of motion at this time from eccentric to pin L; also, if the crosshead link should be disconnected and combination lever E rigidly secured in the vertical position shown in the cut, as the wheels revolved the eccentric would give a motion to the valve scarcely affected by the connection at pin C, which would merely act as a swinging support to the eccentric rod and transmission bar. The motion received from the crosshead affects the valve, then, only when the piston and main pin are at the beginning of a stroke; the eccentric and pin C are then at the upper or lower middle of their paths of revolution, and with radius bar G paralleling the yoke Y, the valve would be in
a centred position with both admission ports covered if it were not for the distort effected at that time by the raised or lowered position of pin C, which, in raising or lowering the transmission bar R, shifts the angles of the bell crank X, and the latter—through connection with the valve rod at pin Q-moves the valve a distance equal to its steam lap plus the desired lead.
When this style of gear is used with inside-admission valves, the eccentric follows the main crank-pin—that is, it is located 180° from the position shown in Fig. 48; and the bell crank E is reversed in position, with the short, upper arm extending forward, link D connecting with the eccentric rod and transmission bar ahead of the combination lever.
The Improved Baker-Pilliod Valve Gear.-In the improved style of the Baker-Pilliod Valve Gear the reverse yoke with its radius bar, and the bell crank through which the motion of the eccentric rod is transformed to give the valve its “long” travel, is used just the same as in the original design, but there the similarity ends. Fig. 49 represents an arrangement of the improved gear for outside-admission valves, in which it is seen that the motion of the eccentric rod is transmitted to the gear-connecting rod which, after receiving the directing influence of the swinging radius bar, exerts an up-and-down motion to the upper arm of the bell crank and a forth-and-back motion to pin G at the extremity of the lower arm. If the valve rod was connected with pin G, the valve would travel at exactly one-fourth of a cycle of motion from the piston; so, to give the valve the advance motion required to overcome the steam lap and open the admission port for lead, the combination lever is connected with the bell crank at pin G, and the valve rod is connected with pin H at the upper end of the combination lever. The combination lever receives its motion from the crosshead, and its distorting effect on the motion received from the eccentric is quite the same as in the Walschaert gear.
To any one who has followed the explanation of the original Baker-Pilliod gear, it will be unnecessary to further describe the improved motion. Fig. 50 represents its adaptation to inside-admission valves; the upper arm of the bell crank is in reverse to the position shown in the preceding cut, and the valve rod joins the combination lever at a point beneath the connection of the combination lever with the bell crank.
In the improved gear no loose oil cups are employed; each bearing has an oil reservoir which is integral with the part. All bearings, pins, and bolts are exposed to view and easily gotten at; three pins, or two pins and a bolt, remove the hardest piece to take down. The heaviest piece, the bell crank, weighs but 86 pounds.
Except in extraordinary cases, all equipments of this gear are exactly alike for outside admission, no matter what the class or type of engine; and the same is true for inside admission. Further, the yokes and radius bars are the same for both admissions. The combination lever would have to suit the requirements as to lap, lead, and the stroke of the piston. All parts are interchangeable, and the castings—including the gear frame—are the same for both sides of the engine, excepting the gear-connecting rods; the combination levers are rights and lefts, but they are forged. The gear frame is one piece of cast steel and can be used on either side of the engine without refitting; one type for inside admission, and another for outside admission. The gear frame is designed with an extension, so that it can be applied to a variety of engines, thus reducing the number of kinds of frame that a road must carry in stock. Hence, if an engine with this gear meets with an accident it is a short job to repair the gear and put the engine in service again.
Valve Events in General.-The Baker-Pilliod improved valve gear gives a constant lead of the valve the distance of the steam lap, but a variable preadmission; at full gear we get practically no preadmission, but as the cut-off is shortened preadmission begins and increases until there is ample for high speeds, with no bad effects for the slow-speed, full gear position. The man