Fig. 3296 represents a cylinder, piston and rod, cross head, connecting rod, and crank.
The piston b is shown in the middle of the cylinder, the cross head at e, and the crank pin at b, instead of being at g′, as it would but for the connecting rod, or if the connecting rod was infinitely long.
Now take a pair of compasses and set it from b to e, and then try it from a to d, and from c to f, and it will be seen that the three cross head positions d, e, and f correspond correctly to the three piston positions a, b, c. Then take a pair of compasses and set them to the length of the connecting rod (from e to b) and try them from d to a, from b to e, and from c to f, and it will be seen that crank pin positions a, b, and c correspond to cross head positions d, e and f, and therefore that the crank is not at half stroke when the piston is in the middle of the cylinder. Take these same compasses, and resting one point at (g′) mark the arc h, and that is where the cross head would be when the crank was at (g′). Now then we see that the connecting rod causes the piston to move slower while running from a to b than it does while running from b to c.
THE D SLIDE VALVE.
The various events which are governed by the D slide valve of a steam engine are as follows:
The live steam period is that during which the steam is admitted from the steam chest into the cylinder and the steam admitted during this period is termed live steam.
The point of cut off is that at which the valve closes the steam port, and the admission of steam into the cylinder is stopped, hence the point of cut off is at the end of the live steam period.
The period of expansion is that during which the steam is allowed to expand in the cylinder, and therefore begins at the point of cut off, and ends at the point of release.
The point of release is that at which the valve opens the port and permits the steam to escape.
The point of compression is that at which the exhaust port is closed, which occurs before the piston has reached the end of its stroke; the steam that has not passed out of the cylinder is therefore compressed, the compression continuing until the valve opens for the lead.
The lead of the valve is the amount the port is open to the live steam when the crank is on the dead centre.
The point of admission is that at which the port opens for the live steam to enter, and it follows that the lead and compression both act as a cushion, arresting the motion of the piston when it reaches the end of the stroke.
Cushioning begins, however, at the time the exhaust port is closed enough to arrest the escape of the steam, while compression begins when the valve has closed the exhaust port.
The construction of a common slide valve is shown in Fig. 3297, in which the valve is shown in its mid-position. p p are the cylinder steam ports (as the openings through which the steam passes from the steam chest to the cylinder are termed), and at x is the cylinder exhaust port, through which the steam escapes from the cylinder. z is the valve exhaust port or exhaust cavity.
The lip of a valve is the width of its flange face, or the distance l, which is measured from the steam edge a to the exhaust cavity z. At the other end of the valve, h is the lip extending from the steam edge b to the exhaust cavity.
Steam lap is the distance the steam ends (or the steam edges as they are called) a, b overlap the steam ports, this distance being shown on the ends of the valve at a c. If the valve had no steam lap, its steam edges would just cover the ports, as denoted by the dimension w.
Exhaust lap is the amount the exhaust cavity z overlaps the bridges q q′, as at p, r.
Unequal steam lap is given to cause the point of cut off to occur at equal points in the piston stroke; thus in the figure there is more steam lap at the head end than at the crank end of the valve. But unequal lap could also be given in order to greatly vary the points of cut off for the two piston strokes, if such was desired.
Unequal exhaust lap may be given to equalize the point of release, or to equalize the points of compression.
The head end of the valve (or of the cylinder) is that which is furthest from the crank shaft, the other end, or that nearest to the crank shaft, being termed the crank end.
THE ACTION OF A COMMON SLIDE VALVE.
The action of a common slide valve may be traced as follows:
Port a, open to the amount of the lead.
Port a, full open for the admission.
Port a, closed off for cut.
Valve opening port a, for the exhaust.
Port a, full open for the exhaust.
Suppose the port a to be at the head end of the cylinder and open to the amount of the lead with the crank on the corresponding dead centre, and if the valve travel be made equal to twice the lap and the lead, the various positions of the valve will be as marked in Figs. from 3298 to 3302; the event corresponding to each valve position being stated in the figures.
DOUBLE PORTED VALVES.
The term port applies strictly to the area of opening of the steam passage where it emerges upon the valve seat. The term steam passage includes the full length of the opening from the cylinder bore to the face upon which the valve is seated.
A double ported steam port is one in which there are two openings or steam ports, leading into one steam passage.
A double ported valve is one in which there are two ports at each end of the valve. These two ports in some cases admit steam to a single cylinder port, and in others to two steam ports, terminating in one steam passage.
A griddle valve is one that has two or more ports at each end upon a seat that has two or more ports for each steam passage.
Double ported valves are employed in some cases to increase the admission of live steam to the cylinder, and in others to increase the exhaust openings also. The effectiveness of a double ported valve is mainly valuable at the beginning of the stroke, and is especially valuable in cases when the travel of the valve is diminished to hasten the point of cut off, because in such cases the outer edges of the valve do not open the steam port to its full width, and a single port is apt to wire draw the steam. By the employment of more than one port, or several ports, a sufficient admission may be obtained with less valve travel.
The Allen double ported valve is one in which the second port increases the port opening for the admission only, as shown in Fig. 3303, in which the valve is moving in the direction of the arrow; the port k will receive steam through the opening at g, and from a port passing through the valve, the steam entering it as shown by the arrow. The second port forms part of the lap of the valve, and enables the travel to be short enough to be cut off at early points in the stroke, without employing so much steam lap as to widely distort the points of cut off, this latter being a defect of the D valve.
Webb’s patent slide valve is circular, and is so arranged as to be free to revolve in the hoop of the valve rod, the effect being that the valve moves around, or to and fro in the hoop, without any special mechanism to produce such movement, and the result is, that the valve and port facings wear smooth and even without any tendency to become grooved.
BALANCED VALVES.
A balanced valve is one in which means are employed to relieve the back of the valve of the steam pressure, and thus prevent its being forced to its seat with unnecessary pressure.
In some of the most successful balanced valves this is accomplished by providing a cover plate, which may be set up to exclude the steam from the back of the valve which works (a sliding fit) between the valve face and the face of the cover plate. Such a method of balancing is sufficiently effective for all practical purposes, if the following conditions are observed: The valve rod must be accurately guided so as to avoid side strains; the valve must fit accurately to its seat and to the cover plate, and the adjustment so made that the valve slides freely at first, being steam tight, and yet allowing room for lubrication to enter. When the travel of a valve, balanced by a cover plate, is varied to alter the point of cut off, the construction must be such that the ends of the valve at the shortest stroke pass over the ends of the seat and cover plate faces, or otherwise the middle of the seat and cover plate faces will wear hollow.
The Buckeye, Porter-Allen, and Straight-Line Engines are examples of practically balanced valves. The first of these has a balancing device that follows up the wear; the second has an adjustment whereby the cover plate may be set up to take up the wear; and in the third the wear is reduced to a minimum, by accurately fitting and guiding the parts.
The construction of the valve in the Straight-Line Engine is shown in Fig. 3304, in which b represents the cylinder bore; the valve v rests on a parallel strip n, and on its top rests the parallel strip m, the pressure relieving plate p is set up firmly against the pieces m n, whose thicknesses are such as to leave the valve a working fit between the faces of r r and of p.
Instead of the valve sliding on a flat face, it may work upon a shaft or spindle as a centre, its face moving in an arc of a circle, and its action will be the same as a flat valve having the same proportions. Fig. 3304a represents a valve v of this construction, whose shaft is at s, a being an arm fast on s, and driven by the eccentric rod r. To find the necessary amount of travel for such a valve, we draw lines, as f, g, from the inner edges of the steam ports, through the centre of the shaft s, and also draw an arc through the centre of the eye of arm a, and where lines f g cut the arc, as at d and e, are the extremes of motion of a.
PISTON VALVES.
A piston valve acts the same as a flat or plain (D) valve, having the same amount of lap lead and travel. In Fig. 3305 we have a cylinder with a flat valve on one side and a piston valve on the other, the head end ports being about to take steam, and it is seen that the eccentrics occupy the same positions for the two valves. The steam ports are, for the piston valve, annular grooves provided in the bore in which the valve fits. The piston valve is balanced because it receives its steam pressure on the ends, but it will not follow up its wear as the flat valve does, hence it is liable to leak.
SEPARATE CUT OFF VALVES.
Meyer’s cut off valve is constructed as shown in Fig. 3306, m being the main valve, and v v the two cut off valves, whose sole duty is to cut off the steam at an earlier point than the main valve would do. If the engine is to have a fixed point of cut off, or, in other words, if the cut off is always to occur at some one particular point in the stroke, the valves may be set to do so, and equalize the points of cut off.
Variable points of cut off with the Meyer’s valve may be obtained by shifting the position of the eccentric that operates the cut off valve, but it is usually done by means of moving the valve by a right and left hand screw, such as shown in Fig. 3306. The cut off eccentric is set ahead of the main eccentric, so that the cut off valve will close the ports before the main valve would do so; thus, in the figure the cut off valve is shown to have effected the cut off for port a by the time the main valve has fully opened port a, and is reversing its motion. If the engine requires to reverse its motion, the cut off eccentric is set exactly opposite to the crank, but otherwise, it may be set 8 or 10 degrees either ahead of or behind the crank, but if set too little ahead of the crank, the port may reopen after the cut off has been effected.
Gonzenback’s cut off valve is constructed as in Fig. 3307, the steam chest having two compartments. a, a are the cylinder steam ports, c the main valve, and e the cut off valve, whose ports (as g) are made wider than the ports f.
Reducing the travel delays the point of cut off in the Gonzenback valve, whereas in the common slide valve it gives an earlier cut off.
THE ECCENTRIC.
When a single eccentric is used, it is simply termed the eccentric. If a cut off valve (or two cut off valves) are used upon the engine, then the eccentric that works the main valve is called the main eccentric, while that which works the cut off valve or valves is called the cut off eccentric. The main valve is that which works on the cylinder face; the cut off valve is that which effects the cut off.
A shifting eccentric is one that is moved across the shaft so as to alter its amount of throw, and, therefore, the amount of valve travel, the effect being to vary the point of cut off.
In engines where a constant amount of lead is given, or in other words, when the eccentric position is intended to be fixed, the eccentric should be secured to the crank shaft by a feather or key sunk into the crank shaft so as to prevent the eccentric from moving, while enabling it to be taken off and replaced without requiring any operations to adjust its position with relation to the crank.
The feather should fit tight on the sides, as well as on the top and bottom, and may have a slight taper on the sides, which will make it easier to fit the featherway or keyway to the feather, and easier to put the eccentric on or take it off.
By this means the eccentric cannot shift, and may be replaced after being taken off without having to set the whole valve motion over again.
When the amount of valve lead or of compression is varied to suit the speed at which the engine is to run, or to aid the counterbalancing of the engine, a feather cannot be used because it will not permit the eccentric to be moved to effect the adjustment.
Set screws possess disadvantages, inasmuch as that the point of the set screw may leave an indentation, which, if the eccentric is moved a trifle, may cause the set screw point to fall back into the old indentation, thus rendering it difficult to make a small adjustment of eccentric position.
An eccentric is the exact equivalent of a crank having the same amount of throw, as may be seen from Fig. 3308, in which the outer dotted circle represents the path of the crank and the inner one the path of the centre of the eccentric. A small crank is marked in, having the same throw as the eccentric has, and the motion given by this small crank is precisely the same as that given by the eccentric whose outer circumference is denoted by the full circle.
Considering the motion of both the crank and the eccentric, therefore, we may treat them precisely the same as two levers, placed a certain distance apart, revolving upon the same centre (the centre of the crank shaft), and represented by their throw-lines.
In Fig. 3309, let the full circle e e represent an eccentric upon a shaft whose centre is at c, and let the centre of the eccentric be at e. The path of revolution of the eccentric centre will be that of the dotted circle whose diameter is b, d. As the eccentric is in mid-position (e being equidistant from b and d), the valve will be in mid-position as denoted by the full lines at the bottom of the figure. Now suppose the eccentric to be revolved on the centre c, until its centre moves from e to v, its circumference being denoted by the dotted circle a a, and if we draw from v a vertical line cutting the line b, d at f, then from c to f will be the distance the eccentric would move the valve, which would then be in the position denoted by the dotted lines at the bottom of the figure. It becomes clear then that if we suppose the eccentric to have moved from mid-position to any other position, we may find how much it will have moved the valve by first drawing a circle representing the path of the centre of the eccentric, next drawing a line (as b d) through its centre, and then drawing a vertical line as (c e) through its mid-position and also a vertical line from the eccentric centre in its new position, the distance between these two vertical lines (as distance c f in the figure) being the amount the eccentric will have moved the valve.
It may have been noticed that the diameter of the eccentric does not affect the case, the distance b d, or the diameter of the circle described by the centre of the eccentric, being that which determines the amount of valve motion in all cases. This being the case, we may use the circle representing the path of the eccentric centre for tracing out the valve movement without drawing the full eccentric, and the diameter of that circle will of course equal the full travel of the valve.
The position of an eccentric upon a shaft is often given in degrees of angle, which is very convenient in some cases. If a valve has no lap or lead, the eccentric will stand at a right angle or angle of 90 degrees when the crank is on the dead centre.
The division of a circle into degrees may be explained as follows:
Suppose we take a circle of any diameter whatever and divide its circumference into 360 equal divisions, then each of these divisions will be one degree. The number 360 has been taken as the standard, and this being the case, there are 360 degrees in a circle, in a quarter of a circle there will therefore be 90 degrees, because 90 is one quarter of 360. By means of dividing a circle in degrees therefore we have a means of measuring or defining any required portion of it.
In Fig. 3310 the degrees of a circle are applied for defining the relative positions of a crank and an eccentric. As the zero position of the crank is on a dead centre, it is so placed in the figure, while as the zero position of the eccentric (which is for a valve having no steam lap) is at 90 degrees from the crank, therefore the dotted circle representing the path of the eccentric centre has its o or zero point at 90 degrees from the crank. Now suppose the eccentric centre stood at v and the eccentric throw line at c v, and it will stand at 30 degrees from o, hence the angular advance of the eccentric is in this case 30 degrees, or in other words, it is 30 degrees in advance of its zero position, or the position it would occupy when the crank is on the dead centre and the valve has no lap and no lead.
If we measure the distance apart of the crank and the eccentric in degrees, we find it is 120 degrees, hence place the crank where we may, we can find the corresponding eccentric position because it is 120 degrees ahead of the crank. The sign for degrees is a small ° placed at the right hand of the figures and slightly above them; thus, thirty degrees would be written 30°.
FINDING THE WORKING RESULTS GIVEN BY A D SLIDE VALVE.
Although not strictly within the line of duty of an engineer or engine driver, he is nevertheless sometimes called upon to find out how a valve of given proportions will dispose of the steam, or what proportions to give to a valve to accomplish certain results.
This is easy enough when either the travel of the valve or the amount of the lap and the width of the port are given, but if the width of the port alone is given, and all the other elements are to be found, it becomes a more difficult problem.
An engineer, however, is rarely called upon to solve the question from this stand-point, which properly belongs to the draughtsman or engine designer.
If the amount of valve travel is given, however, all the other elements may readily be found by the following construction:
Suppose that in Fig. 3311 a D valve is to be designed to cut off the steam when the piston has travelled from position b′ to r′, or at three-quarters of its stroke. Then to find the position the crank pin will be in when the cut off occurs, we draw a circle, b d, representing the path of the crank on the same scale that the length of the piston stroke is represented. The straight line from b to d will, therefore, represent the piston stroke without drawing the piston or cylinder at all (this being done in the figure to make the explanation clear). When the crank is on its dead centre, b, the piston, will be at b′, and the valve in the position shown (supposing it to have no lead). As soon as the crank and valves begin to move, the steam will enter steam port a, and to find where the crank will be when the piston is at three-quarters stroke, and is, therefore, in position r′, we mark a point at r three-quarters of the distance from b to d. Then, taking no account of the length of the connecting rod, we draw a vertical line y from r to the circle, and this line gives at h the position the crank will be in when the piston is at r. We have so far, therefore, that while the piston travels from b′ to r′, the crank will travel from b to h. Now, it will be found that if we set a pair of compasses from b to f, which is half-way from b to h, and then rest the compasses at d, and mark an arc v, then a line from v to the centre of the crank will give us the proper position of the eccentric. As the centre of the crank pin and also the centre of the eccentric both travel in a circle, we may, therefore, take a circle having a diameter equal to twice the throw of the eccentric, (or, what is the same thing, equal to the full travel of the valve), and let it represent the paths of both the eccentric centre and the crank pin centre, the latter being drawn to a scale that is found by dividing the length of the piston stroke by the travel of the valve; thus, if the travel is 3 inches and the stroke 30 inches, the diameter of a 3 inch circle will represent the valve travel full size, and the piston stroke one-tenth full size, because 30 ÷ 3 = 10. It has been shown on page 376 that the length of the connecting rod affects the motion of the piston by distorting it, and it is necessary to take this into account in constructing the actual diagram, which may be done as follows:
The valve travel and point of cut off being given, to find the required amount of lap, there being no lead, draw a circle equal in diameter to the travel of the valve, and draw the line of centres b d, Fig. 3312; mark on the line of centres a point r, representing the position the piston is to be in at the time the cut off is to take place.
Set a pair of compasses to represent the length of the connecting rod on the same scale as the circle b d represents the path of the crank; thus, if the connecting rod is three times the length of the stroke, the compasses would be set to three times the diameter of the circle b d.
A straight line from b to d and passing through the centre c of the crank will represent the line of centres of the engine, which must be prolonged to the right sufficiently to rest the compasses on it and draw the arc y, which will give at h the position of the crank when the piston is at r, and the cut off is to occur.
We have thus found that the amount of circular path the crank will move through from the dead centre to the point of cut off is from b to h, and as the eccentric is fast upon the same shaft, it will, in the same time, of course, move through the same part of a circle.
One half of its motion will be to open and one half to close the port, so that we may by means of the arcs at f get the point f, which is midway between b and h, and with the compasses set from b to f, mark from d the two arcs v and v′ whose distance apart will obviously be the same as from b to h.
Then from v to v′ draw the line p, and from this line to the centre c of the crank shaft is the amount of steam lap necessary for the valve, while from this line (p) to d is the width of the steam port.
The proof of the diagram is as follows:
When the crank is on the dead centre, the centre of the eccentric is at v, its throw line being represented by the line from v to c, and the valve is about to open the port as shown in the figure.
While the eccentric is moving from v to d, the valve will move in the direction of the arrow and will fully open the port, while the crank pin will move from b to f.
Then, while the crank moves from f to h, the eccentric will have moved the valve to the position it occupies in the figure, having closed the port and effected the cut off.
We have here found the amount of lap and the position of the eccentric necessary for a given point of cut off when the latter is given in terms of the piston stroke. If, however, the point of cut off had been given in terms of the crank pin position, we might find the required amount of lap at once, by simply drawing a line from the centre b, the point to h where the crank pin is to be when the cut off occurs.
From this line we could then draw the dotted circle g, and just meeting the line p, which would give the eccentric position.
To find the piston position, the arc y would require to be drawn by the same means as before.
If the valve is to have lead, the diagram may be constructed as in Fig. 3313, in which the circle has a diameter equal to the travel of the valve and the cut off is to occur when the piston is at r and the crank at h.
When the valve is at the end of its travel and has fully opened the port, the eccentric will be at d, hence from d we mark an arc g distant from d to an amount equal to the width of the steam port, drop the vertical m from g, and at its lower end v′ is the position of the eccentric centre at the point of cut off. Then draw a line p, distant from m equal to the lead, which will give at v the position of the eccentric when the crank is on the dead centre, and the valve is open to the amount of the lead. The lap is obviously the distance from the centre c of the crank shaft to the arc g.
We have here found all the points necessary except the point at which the valve will open the port for the lead, and this we may find by setting a pair of compasses to the radius b h (or to radius v v′, as both these radii are equal), and from v as a centre, mark at a an arc, which will give the crank pin position at the time the port first opens for the lead, or in other words it will give the position. The proof of the construction is, that if we set the compasses to the distance between the crank pin position on the dead centre and the point of cut off (or from b to h), we may apply the compasses to the points v, v′, which represent the eccentric position when the port is opened to the amount of the lead, and when the cut off occurs.
If the point of cut off only is to be found, we mark from c, Fig. 3314, an arc g representing the amount of valve lap and arc s representing the lead. A vertical p gives the eccentric position v when the crank is on the dead centre at b, and a vertical m from g gives at v′ the eccentric position at the point of cut off. Then with the compasses set to the points v v′, we may mark from b an arc, locating at h the position of the crank at the point of cut off, and from this with compasses set to represent the length of the connecting rod on the same scale as the circle represents the path of the crank, we may, from a point on the line of centres, mark an arc y giving at r the piston position at the point of cut off.
When, therefore, the lap is given, we mark it from the center c of the crank shaft, and find the other elements from it, whereas, when the lap is to be found, we mark the width of the port from the end d of the valve travel, and find the other elements from that.
A proof of all the constructions is given in Fig. 3314, in which the letters of reference correspond to those in the previous figures, and the positions of the parts are marked in degrees of angle.
To find the piston position at the point of cut off, measured in inches, of the piston stroke it must be borne in mind that as the circle b d represents the full travel of the valve, the diagram gives all the positions of the eccentric and valve full size, but that as it represents the crank path on a reduced scale, therefore we must multiply the measurement on the diagram by that scale.
Suppose, for example, that the piston stroke is 10 inches, and the valve travel 21⁄2 inches, and the circle being 21⁄2 inches in diameter, is, when considered with relation to the eccentric motion, full size, but when considered with relation to the piston or crank motion, it is only 1⁄4 the size, hence to find the piston position at the time of cut off, we must multiply the distance from b to r by 4.
LINK MOTION FOR STATIONARY ENGINES.
The ordinary mechanism employed to enable a stationary engine to be reversed or run in either direction is the Stephenson link motion. Other forms of link motion have been devised, but the Stephenson form has become almost universal.
Fig. 3315 represents this link motion or reversing gear with the parts in position for the full gear of the forward motion, and Fig. 3316 represents it in full gear for the backward motion.
The meaning of the term full gear is that the parts are in the position in which the steam follows the piston throughout the longest or greatest part of the stroke. When in full gear the link motion operates the valve almost precisely the same as if the eccentric rod was attached direct to the valve spindle and no link motion was used.
Besides enabling the engine to run in both directions, however, the link motion provides a means of reducing the amount of valve travel and thus causes the live steam to be cut off earlier in the piston stroke, thus using the steam more expansively. This is done by moving the reversing lever more upright, the earliest point of cut off being obtained when it is upright and the latch is in the notch marked o on the sector in Fig. 3315. If with the engine standing still we move the link motion from full gear forward to full gear backward and watch the valve, we shall find that the valve lead increases as the reversing lever approaches the upright position, or mid gear as it is termed, and that after passing that point it gradually diminishes again, the valve being so set that the lead is the same for full gear forward as it is for full gear backward.
The reversing lever is used to move the link into the required position and to hold it there (the end of the latch fitting into the notches in the sector being the detaining or locking device); as the link is suspended by its saddle pin s and the link hanger, therefore its motion is to swing or partly rotate on the pin s, and at the same time ending in the arc of a circle whose centre of motion is in the pin at the upper end of the link hanger which is pivoted to the lower arm of the lifting shaft (which is sometimes termed the tumbling shaft). It will clearly be seen that with the position the parts occupy in Fig. 3315, and the crank motion being in the direction of the arrow, the forward eccentric will move the top of the link to the right and therefore the valve will move to the right, while the backward eccentric will move the bottom end of the link to the left.
In full gear, however, the bottom eccentric rod has but a very slight effect indeed on the motion of the valve because both the link hanger and the link block will permit the link to swing on centre of the link block pin as a pivot. If now we turn to Fig. 3316 for the full gear backward, we shall see that these conditions are reversed and the backward eccentric becomes the effective one, being in line with the valve spindle. By shifting the link from one gear to the other, therefore, we have merely changed the direction in which the link will move the valve, and, therefore, the direction in which the engine would run.
In Fig. 3315 for the full gear the parts are shown in position, with the piston at the crank end of the cylinder, and the crank pin on the dead centre, and the eccentrics must be set as shown in the cut, the eccentric rods being open and not crossed. When, however, the crank is on the other dead centre and the piston at the head end of the cylinder, the rods will cross each other, and it is necessary to remember that the rods should be open when the piston is at the crank end of the cylinder. If, however, the running gear contains a rock shaft, or rocker (as is the case in American locomotives), then these conditions are reversed, and the eccentric rods will cross when the piston is at the crank end of the cylinder.
In setting the slide valve of an engine having a link motion, there are two distinct operations. First, to put the crank on the respective dead centres, which will be fully described on page 394 and need not be repeated; and second, to set the eccentrics in their proper positions on the shaft, and correct, if necessary, the lengths of the eccentric rods. The crank being on the dead centre, with the piston at crank end of the cylinder, the eccentric should be moved around on the shaft by hand until there is the desired amount of lead at the crank end port, and temporarily fastened there, a set screw usually being provided (in the eccentric) for this purpose. The lead is best measured with a wedge, w, Fig. 3315. The crank is then put on its other dead centre, and the lead for the head end port is measured. If the lead is to be made equal for the two ports (as is usually the case in horizontal engines) and it is found to come so, the valve setting for the forward gear is complete. If the lead is not equal, the forward eccentric rod or else the valve spindle must be altered so as to make the lead equal. In some engines adjusting screws are provided for the purpose of regulating the length of either the eccentric rod or else of the slide spindle; it does not matter which is altered. The link motion is then put in full gear for the backward motion, and, with the crank on the dead centre (it does not matter which dead centre), the eccentric is moved by hand upon the crank shaft until there is the required amount of valve lead. The eccentric is then fastened on the shaft and the crank put on the other dead centre, and the lead tried for the other port, and made equal by lengthening or shortening the backward eccentric rod. It is to be noted that altering the length of the eccentric rod or of the valve spindle makes it necessary to reset the eccentric, as it affects the amount of lead at both ports; hence, if any alteration of rod length is made, the whole process here described must be repeated after each alteration of rod length.
FLY BALL OR THROTTLING GOVERNORS.
An isochronal governor is one in which the two opposing forces are equal throughout the whole range of governor action, or, in other words, equal, let the vertical height of the plane in which the balls revolve or swing be what it may.
A dancing governor is one that acts spasmodically. Such an action may occur from undue friction in the parts of the governor or of its throttle valve.
The friction offers a greater resistance to starting the parts in motion than it does to keep them in motion after being started; hence, the parts are apt to remain at rest too long, and to move too far after being put in motion.
Rule to find the number of revolutions a governor should make. Divide the constant number 375.36 by twice the square root of the height of the cone in inches. The quotient is the proper number of revolutions per minute.
Example.—A governor with arms 301⁄2 inches long, measuring from the centre of suspension to the centre of the ball, revolves, in the mean position of the arms, at an angle of about thirty degrees with a vertical spindle forming a cone of about 261⁄2 inches high. At what number of revolutions per minute should this governor be driven? Here the height of the cone being 26.5 inches, the square root of which is 5.14 and twice the square root 10.28, we divide 375.36 by 10.28, which give us 36.5 as the proper number of revolutions per minute at which the governor should be driven.
The construction of the Pickering governor is as follows: