« НазадПродовжити »
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 than 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 little-say, 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 valve, because the advance of the valve must be exactly the distance of lap plus lead.
In Fig. 4 we have taken the engine illustrated in Fig. 2, and interposed the double rocker-arms be
Indirect Valve Motion
Fig. 4.—Simple Form of Steam Engine with Indirect Valve
tween the eccentric and valve, so that as the eccentric moves in one direction the valve travels in the opposite one; to make the engine still run forward the position of the eccentric is changed to a directly opposite point on the wheel-right straight across, through the centre of the hub, to the “lower quarter”; and as the main-pin moves up-forward, the backward motion of the eccentric will be reversed by the rocker-arms and the valve will, as required, still have a forward motion, admitting live steam to the back end of the cylinder and exhausting the contents of the front end, either the dead steam that had been used in the preceding backward stroke, or the atmospheric contents of the cylinder if this forward movement of the piston represented the first starting of the engine. These changes taken together have not changed the direction in which the engine will run.
But notice: We had lengthened the return crank of the eccentric, placing it more than 90 degrees from the main-pin, in order to secure lead and overcome the outside lap of the valve; if we now use the same eccentric the lead will be negative: instead of pushing the valve ahead, as it would do in Fig. 2, it would pull the valve back, to the left, in an engine like Fig. 4, giving pre-admission to the front end of the cylinder and holding the piston back—with the crank on the back dead-centre. To obtain lead with engines having a double rocker-arm and outside admission valves, as in Fig. 4, the eccentric must be drawn nearer than 90 degrees to the main-pin: be less than a
quarter” from it. The return crank must be shortened to secure lead.
And this proves that with any kind of valve motion where but one eccentric is used, lead cannot be satisfactorily derived from advancing or receding the position of that eccentric on the main-shaft or its relation toward the main crank-pin, unless the engine is to run in but one constant direction; if there is to be any method of reversion, the lead must be gotten otherwise. The Walschaert valve gear is actuated by oné eccentric only, and it is set, theoretically, at exactly right angles to the main-pin-90 degrees from it-and the lead is derived from the action of the piston, through suitable connections with the crosshead, the established lead remaining unchanged throughout the different points of cut-off in both forward and backward gear. The engine is of direct valve motion when the reverse lever is in the goahead position, and of indirect motion when the lever in the back-up notch, the reasons for this becoming apparent as the method for reversing the gear is understood.
Now, let's revert our attention to the crude type of engine shown in Fig. 2, again, for we are ready to begin the reconstruction of its valve gear into the Walschaert motion; we will take out the pin at the connection between the valve-stem and the eccentric rod and dropping the end of the latter slightly, connect it to the "combination lever,"'-see Fig. 5.
With the combination lever in a plumb vertical position we will also connect the valve-stem to its extreme upper end, and while the combination lever remains at right angles to the valve-stem, the distance from eccentric to valve is not changed and its attachment has had no effect on the valve, which is central on its seat with both admission ports covered; but if we move the lower end of the combination lever