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connection from eccentric to valve is through a direct line of motion, as shown, without any levers interposed or “rocker-arms” to reverse the direction of movement, then, with the main-pin in the position as shown in Fig. 2, the eccentric must be located at E, one-fourth of the wheel's circle, or 90 degrees, ahead of the crank-pin--assuming the engine to be running forward-to the right.
But before going further it is best to explain how to take the expression, in hearing it said that an eccentric is “ahead” or “behind” the main crank-pin:
Fig. 3.—Location of Stephenson Eccentrics on Axle, in
Relation to the Crank-Pin.
Fig. 3 is intended to represent the Stephenson link and eccentrics; any movement of the link-block A, gives a corresponding movement to the valve, so, if the link EE should be lowered, eccentric B would actuate the valve, and as this represents an "indirect motion” engine—that is, whichever way the linkblock is moved it will cause the valve to travel in an opposite direction, on account of the rocker-armsnot shown—the engine would run forward. As the wheel revolved to the right eccentric B would follow in the same direction and one-fourth of a turn behind the main-pin, and be said to be “a quarter behind the main pin,” which it always is with indirect motion and outside admission valves, while with direct motion, such as is shown in Fig. 2, the eccentric is alwayswith the same kind of valve—a “quarter ahead of the pin.”
Referring again to Fig. 3, if the link EE was raised, eccentric C would control the motion of the link-block A, and, through a reverse action of the rocker-arms, the valve; the engine would run backward—to the left-and, through the changes in conditions, the “back-up” eccentric C would now be following the main-pin; so that whichever way the wheel is being turned the eccentric that actuates the valve is a "quarter” behind the pin, in common locomotive design.
On locomotives having but one eccentric, as with those of the Walschaert type, the eccentric follows, or precedes, the main-pin according to the direction in which the wheel is turning: it may do either; so, during the course of this article when the location of the eccentric is mentioned as being ahead of, or following, the main-pin, we will assume for the sake of clearer understanding that the engine is in all cases running forward.
It is understood that in Fig. 2 the valve is of outside admission; the eccentric E is just 90 degrees from the main-pin, and the valve is in an exactly central position on its seat-therefore without lead. If the steam throttle should now be opened, this engine could not move of its own volition, for two reasons: the valve being centred, all ports are covered and no steam would be admitted to the cylinder; working singly, the engine would have to be "pinched,” or moved off the centre by hand, but in the case of double power, as in the locomotive, the engine on the other side would be in a position to start the wheels turning; they would turn to the right, the eccentric on the visible side would push the valve forward, and the back, or left, port would begin to open and admit steam to the cylinder against the back of the piston.
In starting the engine, if we should pinch the wheel backward, the eccentric would pull the valve backward also, and steam would be admitted to the forward end of the cylinder, pushing the piston back in spite of our efforts, and it would hold the main-pin on the back dead-centre point, as it was originally, as shown in Fig. 2. So that to make the engine run backward the turning of the wheel to the left must be made to push the valve forward-the direction in which the piston will have to travel: and to accomplish this, the position of the eccentric must be changed to a point on the wheel exactly its opposite in location-90 degrees on the other side of the main-pin—a quarterturn ahead of the crank with the wheel turning backward. This would now give the proper motion to the valve for running the engine backward, and of course the direction is fixed: it can only run that way.
Besides having no power to turn the wheel when the main-pin is on the dead-centre regardless of how much pressure is in the cylinder, the other reason for the engine not starting promptly, even when assistance should be given by pinching the wheel, is that the valve has no lead; the edge of the valve and edge of the admission port are not even close to lineand-line, and the assisted rotation of the wheel would have to push the valve a distance equal to the amount of outside, or steam, lap, before the cylinder could begin to receive steam through the admission port. So that valve-advance, or lead, will be the first requirement in remodelling the engine of Fig. 2.
In these reference plates we are using a “return crank" eccentric, which shows the exact point at which the valve gear receives its initial motion, relative to hub centre and main-pin, much plainer thạn an eccentric of the Stephenson type would with its disk and strap around the axle; and in order to give this kind of an eccentric the advance necessary to secure lead, or overcome the outside lap, with outside admission valves we must lengthen the return crank slightly, but still keeping eccentric pin E the same distance from the hub centre.
Now, we find that the eccentric is more than 90 degrees from the main-pin, and that its advance that we have given has pushed the valve forward far enough so that the back-left port—is opened a very littlesay, one-thirty-second (32) of an inch; steam is thereby admitted against the piston at the prime starting point of its stroke-while the main-pin is on the deadcentre, and the engine standing as in Fig. 2, with the exception, only, that we have given it lead.
Lead is not expected to help move an engine past the dead centre; it has somewhat the opposite effectthat of cramping, from the main-pin to the main shaft, or axle,-a braking effect, but this is not generally admitted. Lead not only cushions the piston at the end of the stroke, but also gets the power of the steam against the piston earlier at the beginning of the stroke, and the full opening of the admission port takes place sooner: The amount of advance of the eccentric necessary to secure the decided port opening for lead depends on the amount of outside lap of the