Page:The New International Encyclopædia 1st ed. v. 18.djvu/613

* STEAM ENGINE. STEAM ENGINE. of valve gear designed pirtieularly to meet one or the other of these conditions. Of these the link motion invented by Howe in 1843, and first used on Stephenson's engines, is perhaps the most familiar, it being the gear very generally used, either as originally designed or in some modiiied form, on locomotives and marine engines. Fig. 7 shows the Howe link, or, as it is more com- monly called, the Stephenson reversing link valve gear. The two eccentrics E and E, are set on the crank shaft, S, at right angles to the crank, C. These eccentrics carry two rods r and i diverging from each other at a slight angle and with their forward ends connected by the slotted, curved link, L. The valve stem, V, has a block or pin which connects it to the link by means of a sliding fit in the slot. If the link be lifted so that the rod r lies nearly in line with V, then the valve receives its motion merely as if driven directly by the single eccentric E ; if the link be lowered so that rod Ci lies nearly in line with V, then the valve acts practicallj- as if driven by the single eccentric E^. Now eccentric Ei being set for forward motion of the engine and eccen- tric E being set for backward motion, it will be seen that the movement of the link gives a ready means for reversing the engine. It does more than this, however, since by putting the link in intermediate positions between full forward and full backward gear the cut-off of the valve can be adjusted for expansive working of the steam. The train of levers v, w, x, y, and s in Fig. 7 shows the means of operating this link in an ordinary locomotive engine. The governor of a steam engine is a device whose function is to regulate the energy de- veloped in accordance with the load propelled. If the admission and pressure of the steam be constant, the speed will vary as the load varies; with a light load the speed will be high and with a heavy load it will be slow. These fluctuations in speed are always undesirable and may easily become dangerous, as, for example, wlien the engine 'races' or 'runs away,' causing the fly wheel to burst from centrifugal force. Gov- ernors act to regulate the energy in two ways, first by cutting oiT steam from the boiler as the speed increases, which amounts to a reduction in boiler pressure, and second by re- ducing the quantity of steam admitted into the cylinder. The first are called throttling govern- ors and the second are called cut-off gorernors. Govern- ors are also classi- fied according to their method of ac- tion and according to their form. The most common form of governor is the fly-ball or pendulum governor, which may operate either by throttling or by cut-off action. The diagram ( Fig. 8 ) shows a form of pendulum governor. The stem or shaft, S, is given rotary motion by means of a belt embracing the pulley, P. Keyed to the top of the shaft is the plate a, carrying, by means of the arms b and 6,, the heavy balls c and c„ and hung to the arms 6 and 6^ are the arms d and di which connect with the plate e, which is free to slide up and down the shaft S. The balU and their connecting arms revolve with the shaft, and because of the familiar law of centrifugal force they tend to fly apart as the speetl of ro- tation increases and to come closer together as this speed decreases. As the balls fly apart, due to increased speed, they lift the plate e, and this operates the train of levers x, y, and z in such a way as either to shut off steam from the boiler or from the cj'linders. That is, as the speed in- creases the energy is decreased, and conversely as the speed decreases the energy is increased. In this way the energj' is proportioned to the load at every instant, the exactness with which this proportion is maintained depending upon the sensitiveness of the gtnernor. The transmission mcclianism of a steam en- gine consists of the piston rod, the crosshead, and the connecting rod. The duty of the piston rod is to convey the energj- developed bj- the piston outside of the cylinder so that it may be transmitted to the crank .shaft or ily wheel. At its inner end the piston rod is attached rigidly to the piston at its centre and at its outer end it is rigidly attached to the crosshead. The rod passes out of the cylinder through an orifice in its front end, this orifice being so constructed that it is steam-tight. It is due largely to the fact that a circular orifice is more easily made steam-tight than any other form that the piston rod is universally cj'lindrical in form. The cross- head is the connecting link which permits the rectilinear motion of the piston rod to be trans- formed into the swaying motion of the connect- ing rod. The manner in which it accomplishes this is clearly shown by Fig. 9, which is a section through a familiar make of crosshead. The pis- ton rod coming from the right is rigidly at- tached to the crosshead by a threaded connec- tion, while the connecting rod is connected by means of a horizontal pin, so that it can swing up and down. The top and bottom of the cross- Fl9. 9. SECTION- OF CROSBOEAD. head are planed smooth and fit a groove or track in the upper and lower guides. These guides are a part of the engine frame, and serve to jjrevent any vibration of the outer end of the piston rod due to the swaying motion of the connecting rod. The connecting rod is com- monly a rectangular or cylindrical bar having at each end a circular bearing, one to embrace the crosshead pin and the other to embrace the crank pin. Fig. 10 shows the construction de- scribed. The articles Crank and Fly Wheei, de- scribe the construction and functions of these parts of the engine's mechanism, and they will not be mentioned further. In concluding this section reference may be made to Fig. 11 as sliowing in a very plain manner the character and relation of the several structural details which have been described individually. In this