Piston-valve.-The piston-valve is a modified form of the slide-valva The lap, lead, etc., are calculated in the same manner as for the common slide-valve. The diameter of valve and amount of port-opening are calculated on the basis that the most contracted portion of the steam-passage between the valve and the cylinder should have an area such that the velocity of steam through it will not exceed 6000 ft. per minute. The area of the opening around the circumference of the valve should be about double the area of the steam-passage, since that portion of the opening that is opposite from the steam-passage is of little effect. Setting the Valves of an Engine.-The principles discussed above are applicable not only to the designing of valves, but also to adjustment of valves that have been improperly set; but the final adjustment of the eccentric and of the length of the rod depend upon the amount of lost motion, temperature, etc., and can be effected only after trial. After the valve has been set as accurately as possible when cold, the lead and lap for the forward and return strokes being equalized, indicator diagrams should be taken and the length of the eccentric-rod adjusted, if necessary, to cor rect slight irregularities. To Put an Engine on its Centre.-Place the engine in a position where the piston will have nearly completed its outward stroke, and opposite some point on the cross-head, such as a corner, make a mark upon the guide. Against the rim of the pulley or crank-disk place a pointer and mark a line with it on the pulley. Then turn the engine over the centre until the cross-head is again in the same position on its inward stroke. This will bring the crank as much below the centre as it was above it before. With the pointer in the same position as before make a second mark on the pulleyrim. Divide the distance between the marks in two and mark the middle point. Turn the engine until the pointer is opposite this middle point, and it will then be on its centre. To avoid the error that may arise from the looseness of crank-pin and wrist-pin bearings, the engine should be turned a little above the centre and then be brought up to it, so that the crank pin will press against the same brass that it does when the first two marks are made. Link-motion.-Link-motions, of which the Stephenson link is the most commonly used, are designed for two purposes: first, for reversing the motion of the engine, and second, for varying the point of cut-off by varying the travel of the valve. The Stephenson link-motion is a combination of two eccentrics, called forward and back eccentrics, with a link connecting the extremities of the eccentric-rods; so that by varying the position of the link the valve rod may be put in direct connection with either eccentric, or may be given a movement controlled in part by one and in part by the other eccentric. When the link is moved by the reversing lever into a position such that the block to which the valve-rod is attached is at either end of the link, the valve receives its maximum travel, and when the link is in mid-gear the travel is the least and cut-off takes place early in the stroke. In the ordinary shifting-link with open rods, that is, not crossed, the lead of the valve increases as the link is moved from full to mid-gear, that is, as the period of steam admission is shortened. The variation of lead is equalized for the front and back strokes by curving the link to the radius of the eccentric-rods concavely to the axles. With crossed eccentric-rods the lead decreases as the link is moved from full to mid-gear. In a valve-motion with stationary link the lead is constant. (For illustration see Clark's Steamengine, vol. ii. p. 22.) The linear advance of each eccentric is equal to that of the valve in full gear, that is, to lap+ lead of the valve, when the eccentric-rods are attached to the link in such position as to cause the half-travel of the valve to equal the eccentricity of the eccentric. The angle between the two eccentric radii, that is, between lines drawn from the centre of the eccentric disks to the centre of the shaft equals 180° less twice the angular advance. Buel, in Appleton's Cyclopedia of Mechanics, vol. ii. p. 316, discusses the Stephenson link as follows: The Stephenson link does not give a perfectly correct distribution of steam; the lead varies for different points of cut-off The period of admission and the beginning of exhaust are not alike for both ends of the cylinder, and the forward motion varies from the backward. "The correctness of the distribution of steam by Stephenson's link-motion depends upon conditions which, as much as the circumstances will permit, ought to be fulfilled, namely: 1. The link should be curved in the arc of a circle whose radius is equal to the length of the eccentric-rod. 2. The eccentric-rods ought to be long; the longer they are in proportion to the eccentricity the more symmetrical will the travel of the valve be on both sides of the centre of motion. 3. The link ought to be short. Each of its points describes a curve in a vertical plane, whose ordinates grow larger the farther the considered point is from the centre of the link; and as the horizontal motion only is transmitted to the valve, vertical oscillation will cause irregularities, 4. The link-hanger ought to be long. The longer it is the nearer will be the arc in which the link swings to a straight line, and thus the less its vertical oscillation. If the link is suspended in its centre, the curves that are described by points equidistant on both sides from the centre are not alike, and hence results the variation between the forward and backward gear. If the link is suspended at its lower end, its lower half will have less vertical oscillation and the upper half more. 5. The centre from which the link-hanger swings changes its position as the link is lowered or raised, and also causes irregularities. To reduce them to the smallest amount the arm of the lifting-shaft should be made as long as the eccentric-rod, and the centre of the lifting-shaft should be placed at the height corresponding to the central position of the centre on which the link-hanger swings." All these conditions can never be fulfilled in practice, and the variations in the lead and the period of admission can be somewhat regulated in an artificial way, but for one gear only. This is accomplished by giving different lead to the two eccentrics, which difference will be smaller the longer the eccentric-rods are and the shorter the link, and by suspending the link not exactly on its centre line but at a certain distance from it, giving what is called the offset." For application of the Zeuner diagram to link-motion, see Holmes on the Steam-engine, p. 290. See also Clark's Railway Machinery (1855), Clark's Steam-engine, Zeuner's and Auchincloss's Treatises on Slide-valve Gears, and Halsey's Locomotive Link Motion. (See Appendix, p. 1077.) The following rules are given by the American Machinist for laying out a link for an upright slide-valve engine. By the term radius of link is meant the radius of the link-arc ab, Fig. 150, drawn through the centre of the slot; this radius is generally made equal to the distance from the centre of shaft to centre of the link-block pin when the latter stands midway of its travel. The distance between the centres of the eccentric-rod pins e, ea should not be less than 2% times, and, when space will permit, three times the throw of the eccentric. By the throw we mean twice the eccentricity of the eccentric. The slot link is generally suspended from the end next to the forward eccentric at a point in the link-arc prolonged. This will give comparatively a small amount of slip to the link-block when the link is in forward gear; but this slip will be increased when the link is in backward gear, This increase of slip is, however, considered of little importance, because marine engines, as a rule, work but very little in the backward gear. When it is necessary that the motion shall be as efficient in backward gear as in forward gear. then the link should be suspended from a point midway between the two eccentric-rod pins; in marine engine practice this point is generally located on the link-arc; for equal cut-offs it is better to move the point of suspension a small amount towards the eccentrics. For obtaining the dimensions of the link in inches: Let L denote the length of the valve, B the breadth, p the absolute steam-pressure per sq. in., and R a factor of computation used as below; then R= .01 √ L × B × p. Breadth of the link.. ............................. = RX 1.6 Thickness T of the bar............................................................................... = RX .8 Length of sliding-block.. ............. Diameter of eccentric-rod pins Diameter of suspension-rod pin when overhung.. = =RX 2.5 Diameter of block-pin when secured at both ends = (RX .8) + 1⁄4 The length of the link, that is, the distance from a to b, measured on a straight line joining the ends of the link-arc in the slot, should be such as to allow the centre of the link-block pin P to be placed in a line with the eccentric-rod pins, leaving sufficient room for the slip of the block. Another type of link frequently used in marine engines is the double-bar link, and this type is again divided into two classes: one class embraces those links which have the eccentric-rod ends as well as the valve-spindle end between the bars, as shown at B (with these links the travel of the valve is less than the throw of the eccentric); the other class embraces those links, shown at C, for which the eccentric-rods are made with fork-ends, so as to connect to studs on the outside of the bars, allowing the block to slide to the end of the ink, so that the centres of the eccentric-rod ends and the block-pin are in line when in full gear, making the travel of the valve equal to the throw of the eccentric. The dimensions of these links when the distance between the eccentric-rod pins is 2% to 234 times the throw of eccentrics can be found as follows: When the distance between the eccentric-rod pins is equal to 3 or 4 times the throw of the eccentrics, then Depth of bars. = (RX 1.25)+3/4" Thickness of bars............. ....................................................... = (RX .5)+ 1⁄44′′ All the other dimensions may be found by the first table. These are empirical rules, and the results may have to be slightly changed to suit given conditions. In marine engines the eccentric-rod ends for all classes of links have adjustable brasses. In locomotives the slot-link is usually employed, and in these the pin-holes have case-hardened bushes driven into the pinholes, and have no adjustable brasses in the ends of the eccentric-rods. The link in B is generally suspended by one of the eccentric-rod pins; and the link in C is suspended by one of the pins in the end of the link, or by one of the eccentric-rod pins. (See note on Locomotive Link Motion in Appendix. p. 1077.) Other Forms of Valve-Gear, as the Joy, Marshall, Hackworth, Bremme, Walschaert, Corliss, etc., are described in Clark's Steam-engine, vol. ii. The design of the Reynolds-Corliss valve-gear is discussed by A. H. Eldridge in Power, Sep. 1893. See also Henthorn on the Corliss engine. Rules for laying down the centre lines of the Joy valve-gear are given ia American Machinist, Nov. 13, 1890. For Joy's "Fluid-pressure Reversingvalve," see Eng'g, May 25, 1894. GOVERNORS. Pendulum or Fly-ball Governor.-The inclination of the arms of a revolving pendulum to a vertical axis is such that the height of the point of suspension h above the horizontal plane in which the centre of gravity of the balls revolve (assuming the weight of the rods to be small compared with the weight of the balls) bears to the radius r of the circle described by the centres of the balls the ratio which ratio is independent of the weight of the balls, v being the velocity of the centres of the balls in feet per second. If T number of revolutions of the balls in 1 second, v = 2′′rТ = ar, in which a the angular velocity, or 2r T, and gr2 g being taken at 32.16. If N = number of revs. per minute, h = inches. ... 35190 N2 40 45 50 60 75 21.99 17.38 14.08 9.775 6.256 For revolutions per minute...... The height in inches will be..... Number of turns per minute required to cause the arms to take a given angle with the vertical axis: Let = length of the arm in inches from the centre of suspension to the centre of gyration, and a the required angle; then The simple governor is not isochronous; that is, it does not revolve at a uniform speed in all positions, the speed changing as the angle of the arms changes. To remedy this defect loaded governors, such as Porter's, are used. From the balls of a common governor whose collective weight is 4 let there be hung by a pair of links of lengths equal to the pendulum arms a load B capable of sliding on the spindle, having its centre of gravity in the axis of rotation. Then the centrifugal force is that due to A alone, and the effect of gravity is that due to A+2B; consequently the altitude for a given speed is increased in the ratio (A + 2B): A, as compared with that of a simple revolving pendulum, and a given absolute variation in altitude produces a smaller proportionate variation in speed than in the common governor. (Rankine, S. E., p. 551.) For the weighted governor let the length of the arm from the point of suspension to the centre of gravity of the ball, and let the length of the suspending-link, l1 = the length of the portion of the arm from the point of suspension of the arm to the point of attachment of the link; G = the weight of one ball, Q = half the weight of the sliding weight, h the height of the governor from the point of snspension to the plane of revolution of the balls, a the angular velocity 2T, Tbeing the number of revolutions per in feet, or second; then a = 35190 N2 minute. 32.161 211 h ; in inches, N being the number of revolutions per For various forms of governor see App. Cycl. Mech., vol. ii. 61, and Clark's Steam-engine, vol. ii. p. 65. To Change the Speed of an Engine Having a Fly-ball Governor.-A slight difference in the speed of a governor changes the position of its weights from that required for full load to that required for no load. It is evident therefore that, whatever the speed of the engine, the normal speed of the governor must be that for which the governor was designed; i.e., the speed of the governor must be kept the same. To change the speed of the engine the problem is to so adjust the pulleys which drive the governor that the engine at its new speed shall drive it just as fast as it was driven at its original speed. In order to increase the engine-speed we must decrease the pulley upon the shaft of the engine, i.e., the driver, or increase that on the governor, i.e., the driver, in the proportion that the speed of the engine is to be increased." |