The peculiar advantage of the eccentric-and-link valve-gear is not only its reversibility while the engine is running, but also its affording a wider range of valve travel and consequently of expansion ratio. The block may either slide in a parallel slot in the link or embrace it and slide over it, the different constructions producing, other things being equal, the same valve-motion. In nearly all reciprocating engines the valve-movement is effected by means of an eccentric or its equivalent upon the driving shaft. This introduces irregularities of motion, which increase with the compara tive shortness of the eccentric-rod, and are of the same class as those caused by the angularity of the connecting-rod. There is, moreover, considerable friction between the eccentric-sheave and its encircling strap.
An excellent class of valve-motion, and one which has already been largely introduced in marine engines and has made a good record in loco motive work, is that in which the valves are moved by lever-connections with the connecting-rod. Of this class the Joy valve-motion appears to be the best known and to have the best record.
The walking-beam was a great convenience in working the valves and the air-pump, as well as in making connections with the water-pumping machinery which constituted the only loads that were at first applied to steam-engines, and it was also a great convenience in paddle-wheel steam boats, where the high position of the shaft above the water-level demanded that the engines should sit low; but the necessity of having the shaft below the water-level, as in propeller engines, led in most cases to its abandonment, although it is still employed in certain classes of marine work (ferryboats, etc.), and, notwithstanding the large amount of room which it takes up, it is still used on large stationary engines in Europe. But the air-pinup and feed-pump are worked by eccentrics quite as well as they formerly were by the beam, and now, in many cases, the air-pump is operated by an independent motor.
The Cut-of—It is now thoroughly understood that, as discovered by Watt, steam does the most possible work (other things being equal) when it leaves the engine at the lowest pressure—that is, when the terminal pressure is least compared with the initial; it is therefore. desirable to " cut-off" after the piston has been started, so that its expansion from a high tension to a low one shall impel the piston, the release of the expand ing steam being effected at the end of the stroke, or slightly before it in order to permit a free exhaust.
The point at which cut-off is effected may be varied in two ways: by the hand of the engineer or by the governor of the engine. The first
is practicable for marine and locomotive but not for stationary engines. Cutting off the steam has the double advantage of working it (in most cases) more economically and of proportioning the force exerted and the steam expended to the load. The fly-wheel of course. aids greatly in a regular motion, but, leaving out of consideration the question of economy of steam-consumption, it would be an absurdity to keep on storing up power until there was more than the fly-wheel could absorb, or to let the amount thus stored run down to near that point at which there would not be enough left to supply the demand for power. Besides this, the fly-wheel can take care only of temporary and short-lived irregularities. Variations of load may be provided for by choking off the steam-supply either by hand, as in marine and locomotive engines, or by a governor, as in those which are stationary or so-called "portable." But this choking or throt tling, while an efficient means of taking care of load variations, uses the steam very wastefully (without proper expansion) under heavy loads, when there is the most steam needed and used.
The Drop Cut-off (also known as the Sickels cut-offi and claimed by both Sickels and Hogg) was introduced about 1841. It consisted of a set of steam-valves, each raised by a catch which could be thrown out at the proper moment by a wedge so adjusted that it would drop the valve at any desired point in the earlier half-stroke. Later, this was improved by the addition of a " wiper " having a motion at right angles to that of the valve and its catch, by giving to this wiper a motion in the direction coin cident with the piston. This enabled the cut-off to be effected at any point in the stroke. For stationary engines this detaching was made automatic, depending upon the action and position of the governor. The action of the governor to determine the cut-off had been made in 1834 by Allen, who had a cut-off valve separate from the main valve. In 1849, Corliss attached the governor to a drop cut-off, and in 1855, Greene produced an engine which had the advantages of plain slide-valves at all ports, a range of cut off from zero to full stroke, and automatic action of the governor to effect ent-off. In Wright's engine the governor operates cams which hold the valves open a longer or a shorter time, according to the speed, cutting off the steam earlier when the speed tends to grow too high, and vice versa.