1902 Encyclopedia > Steam Engine > The Distribution of Steam -- Valves and Valve Motions

Steam Engine
(Part 8)

The Distribution of Steam -- Valves and Valve Motions

144. In early steam-engines the distribution of steam was effected by means of conical valves, worked by tappets from a rod which hung from the beam. The slide-valve, the invention of which in the form now know as the long D-slide is credited to Murdoch, an assistant of Watt, came into general use with the introduction of locomotives, and is now employed, in one or other of many forms, in the great majority of engines.

The common or locomotive slide-valve is illustrated in fig.

57. The seat, or surface on which the valve slides, is a plane surface formed on or fixed to the side of the cylinder, with three ports or openings which extend across the greater part of the cylinder’s width. The central opening is the exhaust-port through which the steam escapes; the others, of steam ports, which are narrower, lead to the two ends of the cylinder respectively. The valve is a box-shaped cover which slides over the seat, and the whole is enclosed in a chamber called the valve-chest, to which steam from the boiler is admitted. When the valve moves a sufficient distance to either side of the central position, steam enters one end of the cylinder from the valve-chest and escapes from the other end of the cylinder through the capacity of the valve into the exhaust-port. The valve is generally moved by an eccentric on the engine-shaft (fig. 58), which is mechanically equivalent to a crank whose radius is equal to the eccentricity, or distance of O, the centre of the shaft, from P, the center of the eccentric sheave. The sheave is encircled by a strap forming the end of the eccentric rod, and the rod is connected by a pin-point to the valve-rod, which come out of the valve-chest through a steam-tight stuffing box. The eccentric rod is generally so long that the motion of the valve is sensibly the same as that which it would receive were the rod infinitely long. Thus if a circle (fig. 59) be drawn to represent the path of the eccentric centre during a revolution of the engine, and a perpendicular PM be drawn from any point P on a diameter AB, the distance CM is the displacement of the valve from its middle position at the same time when the eccentric centre is at P. AB is the whole travel of the valve.

145. If the valve in its middle position did not overlap the steam ports (fig. 60), any movement to the right of the left would admit steam, and the admission would continue until the valve had returned to its middle position, or, in other words, for half a revolution of the engine. Such a valve would not serve for expansive working, and as regards the relative position of the crank and eccentric it would have to be set so its middle position coincided with the extreme position of the piston, in other words, the eccentric radius would make a right angle with the crank. Expansive working, however, becomes possible when we give the valve what is called "lap," by making it the "outside lap." Admission of steam (to either side) then begins only when the displacement of the valve from its middle position exceeds the amount of the outside lap, and continues only until the valve has returned to the same distance from its middle position. Further, exhaust begins only when the valve has moved past the middle by a distance equal to i, and continues until the valve has again returned to a distance i from its middle position. Thus on the diagram of the eccentric’s travel (fig. 62) we find, by setting off o and i on the two sides of the centre, the positions a, b, c, and d of the eccentric radius at which the four events of admission, cut-off, release, and compression occur for one side of the piston. As to the other side of the piston, it is only necessary to set off o to the right and i to the left of the centre, but for the sake of clearness we may confine our attention to one of the two sides. Of the whole revolution, the part from a to b is the arc of steam admission, from b to c is the arc of expansion, c to d the arc of exhaust, and form d to a the arc of compression. The relation of these, however, to the piston’s motion is still undefined. If the eccentric were set in advantage of the crank by an angle equal to ACa, the opening of the valve would be coincident with the beginning of the piston’s stroke. It is, however, desirable, in order to allow the steam free entry, that the valve be already some way open the piston stroke begins, and thus the eccentric may be set to have a position Ca’ at the beginning of the stroke. In that case the valve is open at the beginning of the stroke to the extent mm’, which is called the "lead." The amount of which the angle between Ca’ (the eccentric) and CA (the crank) exceeds a right angle is called the angular advance, this being the angle by which the eccentric is set in advance of the position it would occupy if the primitive arrangement without lap were adopted. The quantities lap, lead, and angular advance (O) are connected by the equation

outside lap + lead = half travel x cos O.

An effect of lead is to case preadmission, that is to say, admission before the end of the back stroke, which, together with the compression of steam left in the cylinder when the exhaust port of closes, produces the mechanical effect of "cushioning," to which reference has already been made. To examine the distribution of steam throughout the piston’s stroke, we may now draw a circle to represent the path of the crank pin (fig. 63, where the dotted lines have been added to show the assumed configuration of piston, connecting-rod, and crank) and transfer to it from the former diagram angular positions a, b, c, and d at which the four events occur. To facilitate this transfer the diagrams of eccentric path and of crank-pin path may by a suitable choice of scale be drawn of the same actual size. Then by projecting these points on a diameter which presents the piston’s path, by circular arcs drawn with a radius equal to the length of the connecting-rod, we find p, the position of the piston at which admission occurs during the back stroke, also q and r, the position at cut-off and release, during the stroke which take places in the direction of the arrow, and s, the point at which compression begins. It is obviously unnecessary to draw the two circles of figs. 62 and 63 separately; the single diagram (fig. 64) contains the solution of the steam distribution with a slide-valve whose laps, travel, and angular advance are known, the same circle serving, on two scales, to show the motion of the crank and of the eccentric.

165. A method of representing graphically the relations of valve and piston motion, sometimes convenient in dealing with valve-gears of a more complex character than the single eccentric, is to set off the valve’s and the piston’s simultaneous displacement at right angles to each other, as in fig. 65, the valve’s motion being exaggerated by using a coarser scale for it than for that of the piston. The result is an oval curve, from which the events in the steam distribution are determined by drawing lines AB and CD parallel to the piston’s path and distant from it by the amount of the outside and inside lap respectively. Then a, b, c, and d, and the corresponding points p, q, r, and s determine the four events as in former diagrams. Fig. 65 shows at a glance the amount of steam-opening at any part of the period of admission. AE is the lead. The events for the other side of the piston are determined by drawing AB above and CD below the middle line.

147. The graphic construction most usually employed in side-valve investigations is the ingenious diagram published by Dr. G. Zeuner in the Civilingenieur in 1856.1 On the line AB (fig.66), which represents the travel of the valve, let a pair of circles (called valve-circles) be drawn, each with diameter equal to the half travel. A radius vector CP, drawn in the direction of eccentric at any instant, is cut by one of the circles at Q, so that CQ represents the corresponding displacement of the valve from its middle position. That this is so will be seen by drawing PM (as in fig. 59) and joining QB, when it is obvious that QC=CM, which s the displacement of the valve. The line AB with the circles on it may now be turned back through an angle of 90&Mac251;+O (O being the angular advance), so that the valve-circles take and position shown to a large scale in fig. 67. This makes the direction of CQ (the eccentric) coincide on the paper with simultaneous direction of the crank, and hence to fine the displacement of the valve at any position of the crank we have only to draw CQ in fig. 67 parallel to the crank, when CQ represents the displacement of the valve to the scale on which the diameter of each valve circle represents the half-travel of the valve. CQo is the valve displacement at the beginning of the stroke shown by the arrow. Draw circular arcs ab and cd with C as centre and with radii equal to the outside lap o and the inside lap i respectively. Ca is the position of the crank at which preadmission occurs. The lead is aoQo. The greatest steam opening is a1B. The cut-off occurs when the crank has the direction Cb. Cc is the position of the crank at release, and Cd marks the end of the exhaust.

148. In this diagram radii drawn from C mark the angular position of the crank, and their intercepts by the valve circle determine the corresponding displacement of the valve. It remains to find the corresponding displacement of the piston. For this Zeuner employs a supplementary graphic construction, shown in fig. 68. Here ab or a’b’ represents the connecting rod, and bc or b’c the crank. With centre c and radius ac a circle ap is drawn, and with centre b and radius aq. Then for any position of the crank, as cb’, the intercept pq between the circles is easily seen to be equal aa’, and is therefore the distance by which the piston has moved form its extreme position at the beginning of the stroke. In practice this diagram is combined with that of fig.67, by drawing both about the same centre and using different scales for valve and piston travel. A radius vector drawn from the centre parallel to the crank in any position then shows the valve’s displacement form the valve’s middle position by the intercept CQ of fig. 67, and the piston’s displacement from the beginning of the piston’s motion by the intercept pq of fig. 68.

149. In all the figures which have been sketched the events refer to the front end of the cylinder that is the end nearest to the crank (see fig. 63). To determine the events of steam distribution at the back end, the lap circles shown by dotted lines in fig. 67 must also be drawn, Ca’ being the outside lap for the back end, and Cc’ the inside lap. These laps are not necessarily equal to those at the other end of the valve. From fig. 65 it is obvious that especially with a short connecting-rod, the cut-off and release occur earlier and the compression later at the front than at the back end if the laps are equal, and a more symmetrical steam distribution can be produced by making the inside lap greater and the outside lap less on the side which leads to the front end of the cylinder. On the other hand, and unsymmetrical distribution may be desirable, as in a vertical engine, where the weight of the piston assists the steam during the down-stroke and resists it during the up-stroke, and this may be secured by a suitable inequality on the laps.

150. By varying the ratio of the laps o and I to the travel of the valve, we produce effects on the steam distribution which are readily traced in the oval diagram of fig. 65 or in the other figure. Reduction of travel (which is equivalent to increase of both o and i) gives later preadmission, earlier cut-off, later release, and earlier compression; the ratios of expansion and of compression are both increased. The effect of a change in the angular advance is more easily seen by reference to Zeuner’s diagram, which shows that to increase O accelerates all the events and causes a slight increase in the ratio of expansion.

151. In designing a slide-valve the breadth of the steam ports in the direction of the valve’s motion is determined with reference to the volume of the exhaust steam to be discharged in a given time, the area of the ports being generally such that the mean velocity of the steam during discharge is less than 100 feet per second. The travel is made great enough to keep the cylinder port fully open during the greater part of the exhaust; for this purpose it is 2_ or 3 times the breadth of the steam port. To facilitate the exit of steam the inside lap is always small, and is often wanting or even negative. During admission the steam port is rarely quite uncovered, especially if the outside lap is large and the travel moderate. Large travel has the advantage of giving freer ingress and egress of steam, with more sharply-defined cut-off, compression, and release, but this advantage is secured at the cost of more work spent in moving the valve and more wear of the faces. To lessen the necessary travel without reducing the area of steam ports, double-and even treble-ported valves are often used. An example of a double-ported valve is shown in fig. 85. Fig. 69 shows the Trick valve, an ingenious device for the same purpose.

152. The eccentric must stand in advance of the crank by the angle 90&Mac251;+O,as in fig. 70, where CK is the crank, and CE the corresponding position of the eccentric when the engine is running in the direction of the arrow a. To set the engine in gear to run in the opposite direction (b) it is only necessary to shift the eccentric into the position CE’, when it will still be 90&Mac251;+O in advance of the crank. In the older engines this reversal was effected by temporally disengaging the eccentric-rod from the valve-rod, working the valve by hand until the crank turned back through an angle equal to ECE’, the eccentric meanwhile remaining at rest, and then re-engaging the gear. The eccentric sheave, instead of being keyed to the shaft, was driven by a stop fixed to the shaft, which abutted on one or other of two shoulders projecting from the sheave. In some modern forms of reversing gear means are provided for turning the eccentric round on the shaft, but the arrangement known as the link-motion is now the most usual gear in locomotive, marine, winding, and other engines which require to be often and easily reversed.

153. In the link-motion two eccentric are use, and the ends of their rods are connected by a link. In Stephenson’s link-motion—the earliest and still the most usual form—the link is a slotted bar or pair of bars curved to the same radius are as the eccentric rods (fig. 71), and capable of being shifted up or down by a suspension rod. The valve-rod ends in a block which slide within the link, and when the link is placed so that this block is nearly in line with the forward eccentric rod (E, fig. 71) the valve moves in nearly the same way as if it were driven directly by a single eccentric. This is the position of "full forward gear." In "full backward gear," on the other hand, the link is pulled up until the block is in nearly a line with the backward eccentric rod R’. The link-motion thus gives a ready means of reversing the engine, — but it does more than this. By setting the link in an intermediate position the valve receives a motion nearly the same as which would be given by an eccentric of shorter radius and of greater angular advance, and the effect is to give a distribution of steam in which the cut-off is earlier than in full gear, and the expansion and compression are greater. In mid gear the steam distribution is such that scarcely any work is done in the cylinder. The movement of the link is effected by a hand lever, or by a screw, or (in large engines) by an auxiliary steam-engine. A usual arrangement of hand lever, sketched in fig. 71, has given rise to the phase "notching up," to describe the setting of the link to give a greater degree of expansion.

154. In Gooch’s link-motions (fig. 72) the link is not moved up in shifting from forward to backward gear, but a radius rod between the valve-rod and the link (which is curved to suit this radius rod) is raised or lowered—a plan which has the advantage that the lead is the same in all gears. In Allan’s motion (fig. 73 ) the change of gear is effected partly by shifting the link and partly by shifting a radius rod, and the link is straight.

155. The movement of a valve driven by a link-motion may be very fully and exactly analysed by drawing with the aid of template the positions of the centre line of the link corresponding to a number of successive positions of the crank. Thus, in fig. 74, two circular arcs passing through e and e’ are drawn with E and E’ as centres and the eccentric rods are radii. These are loci of two known points of the link, and a third locus is the circle a in which the point of suspension must lie. By placing on the paper a template of the link, with these points marked on it, the position of the link is readily found, and by repeating the process for other positions of the eccentrics a diagram of positions (fig. 24) is drawn for the assigned state of the gear. A line AB drawn across this diagram in the path of the valve’s travel determines the displacements of the valve, and enables the oval diagram to be drawn (as in fig. 65), which is shown to a larger scale in another part of fig. 74. The example refers to Stephenson’s link-motion in nearly full forward gear; with obvious modification the same method may be used in the analysis of Gooch’s or Allan’s motion. The same diagram determines the amount of slotting or sliding motion of the block in the link. In a well-designed gear this sliding is reduced to a minimum for that position of the gear in which the engine runs the most usually. In marine engines the suspension-rod is generally connected to the link at the end of the link next the forward eccentric, to reduce this sliding when in the engine is in forward gear. A less laborious, but less accurate, solution of link-motion problems is reached by the used of what is called the equivalent eccentric—an imaginary eccentric, which would give the valve nearly the same motion as it gets from the joint action of the actual eccentrics. The following rule of finding the equivalent eccentric, in any state of gear, is due to Mr. ‘Farlane Gray:—

Connect the eccentric centres E and E’ (fig. 75) by a circular arc

whose radius = EE’ x length of eccentric rod .

2 x ee’

Then, if the block is at any point B, take EF such that EF: EE’:: eB: ee. CF then represents the equivalent eccentric both in radius and in angular position. If the rods of the link-motion used, — an arrangement seldom used, —the arc EFE’ is to be drawn convex towards C.

156. Many forms of gear for reversing and for varying expansion have been devised with the object of escaping the use of two eccentrics, and of obtaining a more perfect distribution of steam than the link-motion can often be made to give. Hackworth’s gear, the parent of several others, has a single eccentric E (fig. 76) opposite the crank, with an eccentric-rod EQ, whose mean position is perpendicular to the travel of the valve. The rod ends in a block Q, which slides on a fixed inclined guide-bar or link, and the valve-rod receives its motion through a connecting rod from an intermediate point P of the eccentric-rod, the locus of which is an ellipse. To reverse the gear the guide bar is titled over to the position shown by the dotted lines, and intermediate inclinations give various degrees of expansion without altering the lead. The steam distribution is excellent, and the cut-off is sharper than in the usual link-motion, but an objection to the gear is the wear of the sliding-block and guide. In Bremme’s or Marshall’s form this objection is obviated with some loss of symmetry in the valve’s motion by constraining the motion of the point Q, not by a sliding-guide, but a suspension-link, which makes the path of Q a circular arc instead of a straight line; to reverse the gear the centre of suspension R of this link is thrown over to the position R’ (fig. 77). In the example sketched P is beyond Q, but P may be between Q and the crank (as in fig. 76), in which case the eccentric is set at 180&Mac251; from the crank. This gear has been applied in a number of marine engines. In Joy’s gear, which is extensively used in locomotives, no eccentric is required; and the rod corresponding to the eccentric rod in Hackworth’s gear receives its motion from a point in the connecting rod by the linkage shown in fig. 78, and is either suspended, as in Marshall’s form, by a rod whose suspension centre R is thrown over to reverse the motion, or constrained, as in Hackworth’s by a slot=guide whose inclination is reversed. Fig 79 shows Joy’s gear as applied to a locomotive. A slot-guide E is used, and it is curved to allow for the obliquity of the valve connecting-rod AE. C is the crank-pin, B the piston path, and D a fixed centre. The reversing gears of Walschaert, Brown, and Kitson also dispense with eccentrics, and are closely related to the invention of Hackworth.1 A method of reversing with a common slide-valve, which is used in steam steering engines2 and some others, is to supply steam to what was (before reversal) the exhaust side of the valve and connect the exhaust to what was the steam side. This is done by means of separate reversing valve through which the steam and exhaust pipes pass.

157. When the distribution of steam is effected by the slide-valve alone the arc of the crank’s motion during which compression occurs is equal to the arc during which expansion occurs, and for this reason the slide-valve would give an excessive amount of compression if it were made to cut off the supply of steam earlier than about half-stroke. Hence, where an early cut-off is wanted it is necessary either to use an entirely different means of regulating the distribution of steam, or the supplement the slide-valve by another valve, — called an expansion-valve, usually driven by a separate eccentric, —whose function is to effect the cut-off, the other events being determined as usual by the slide-valve. Such expansion-valves belong generally to one or other of two types. In one the expansion-valve cuts off the supply of steam to the chest in which the main valve works. This may be done by a disk or double-beat valve (§ 163), as in the Proëll gear mentioned in § 175 below, or by a slide-valve working on a fixed set (furnished with one or more ports), which forms the back of side of the main valve-chest. Valves of this last type are usually made in the "gridiron" or many-ported form to combine large steam-opening with small travel. Expansion-valves working in a fixed seat may be arranged so that the ports are either fully open (fig. 80) or closed (fig. 81) when the valve is in its middle position. In the latter case the expansion-valve eccentric is set in line with or opposite to the crank, if the engine is to run in either direction with the same grade of expansion. Cut-off then occurs at P, fig. 82, when the shaft has turned through an angle Ø from the beginning of the stroke. The expansion valve reopens at Q, and the slide-valve must therefore have enough laps to cut off earlier than 180&Mac251;- Ø from the beginning of the stroke, in order to prevent a second admission of steam to the cylinder. In the valve of fig. 80 the expansion eccentric is set at right angles to the crank, if the action is to be the same both directions. If not, these angles may be deviated from, and in this way a more rapid travel at the instant of cut-off may be secured for one direction of running.

158. The other and much commoner type of expansion-valve is one sliding on the back of the main slide-valve, which is provided with through ports which the expansion-valve opens and closes. Fig. 83 shows one form of this type. Here the resultant relative motion of the expansion-valve and main-valve has to be considered. If ra and rb (fig. 84) are the eccentrics working the main and expansion valves respectively, then the CR drawn equal and parallel to ME is the resultant eccentric which determines the motion of the expansion-valve relatively to the main-valve. Cut-off occurs at Q, when the shaft has turned through an angle Ø which brings the resultant eccentric into the direction CQ and makes the relative displacement of the two valves equal to the distance l. Another form of this valve (corresponding to fig. 81) cuts off steam at the inside edges of the expansion-slides.

159. Expansion-valves furnish a convenient means of varying the expansion, which may be done by altering their lap, travel, or angular advance. Alteration of lap, or rather of the distance l in the figures, is often effected by having the expansion-valve in two parts (as in fig. 83) and holding them on the rod by right-and left-handed screws respectively; by turning the valve-rod the parts are made to approach or recede from each other. In large valves the adjustment is more conveniently made by varying the travel of the valve, which is one by connecting it to its eccentric through a link which serves as a lever of variable length.

160. To relieve the pressure of the valve on the seat, large slide-valves are generally fitted with a steam-tight ring, which excludes steam from the greater part of the back of the valve. The ring fits steam-tight into a recess in the cover of the steam-chest, and is pressed by springs against the back of the valve, which is planed smooth to slide under the ring. Fig. 86 shows a relief ring of this kind fitted on the back of a large double-ported slide-valve for a marine engine. Another plan is to fit the ring into a recess on the back of the valve, and let it slide on the inside of the steam-chest cover. Steam is thus excluded from the space within the ring, any steam that leaks in being allowed to escape to the condenser (or to the intermediate receiver when the arrangement is fitted to the high-pressure cylinder of a compound engine). A flexible diaphragm has also been used, instead of a recess, to hold the ring.

161. The pressure of valves on cylinder faces is still more completely obviated by making the back of the valve similar to its face, and causing the back to slide in contact with the valve-chest cover, which has recesses corresponding to the cylinder ports. This arrangement is most perfectly carried out in the piston slide-valves now very largely used in the high-pressure cylinder of marine engines. The piston slide-valve may be described as a slide-valve in which the valve face is curved to form a complete cylinder, round whose circumference the ports extend. The pistons are packed like ordinary cylinder pistons by metallic rings, and the ports are crossed here and there by diagonal bars to keep the rings from springing out as the valve moves over them. Figs. 86 and 87 show tow forms of piston valve designed by Mr. Kirk for the supply of high-pressure steam to large marine engines. P, P are the cylinder ports in each.

Fig. 85 illustrates an arrangement common in all heavy slide-valves whose travel is vertical —the balanced-piston, which is pressed up by steam on its lower side and so equilibrates the weight of the valve, valve-rod, and connected parts of the mechanism.

162. The slide-valve sometimes takes the form of a disk revolving of oscillating on a fixed seat, and sometimes of a rocking cylinder (fig. 88). This last kind of sliding motion is very usual in stationary engines fitted with the Corliss gear, which will be described in the next chapter, in which case four distinct rocking slides are commonly employed to effect the steam distribution, one giving admission and one giving exhaust at each end of the cylinder (see fig. 127).

163. In many stationary engines lift or disk valves are used, worked by tappets, cams or eccentrics. Lift valves are generally of the Cornish or double-beat type (fig. 89), in which equilibrium is secured by the use of two conical faces which open or close together. In Cornish pumping engines, which retain the single action of Watt’s early engine, three double-beat valves are used, as steam-valve, equilibrium-valve, and exhaust-valve respectively. These are closed by tappets on a rod moving with the beam, but are opened by means of a device called a cataract, which acts as follows. The cataract is a small pump with a weighed plunger, discharging fluid through a stop-cock which can be adjusted by hand when it is desired to alter the speed of the engine. The weighed plunger is raised by a rod form the beam, but is free in its descent so that it comes down at a rate depending on the extent to which the stop-cock is opened. When it comes down a certain way it opens the steam and exhaust valves, by liberating catches which hold them closed; the "out-door" stoke then begins and admission continues until the steam-valve s closed: this is done directly by the motion of the beam, which also, at a later point in the stroke, closes the exhaust. Then the equilibrium-valve is opened, and the "in-door" stroke takes place, during which the plunger of the cataract is raised. When it is completed, the piston pauses until the cataract causes the steam-valve to open and the next "out-door" stoke begins. By applying a cataract to the equilibrium-valve also, a pause is introduced at the end of the "out-door" stroke. Pauses have the advantage of giving the pump time to fill and of allowing the pump-valves to settle in their seats without shock.

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