CHAPTER XXII.

_THE JOY VALVE-GEAR._

DESCRIPTION OF MOTION.

In this form of valve-gear, eccentrics and their equivalents are entirely dispensed with. The motion for the valve is taken direct from the connecting rod; and by utilizing independently the backward and forward action of the rod, due to the reciprocation of the piston, and combining this with the vibrating action of the rod up and down, a movement results which is employed to actuate the valves of engines using any combination of lap and lead desired, and giving an almost mathematically correct cut-off for both sides of the piston for forward and backward motion, and for all points of expansion intermediately. The general outline of the motion can be understood by an examination of Fig. 28, which gives three views of the gear. The sub-figures 1, 2, and 3 show the motion in elevation, plan, and transverse section respectively.

HOW TO APPLY THIS GEAR TO AMERICAN LOCOMOTIVES.

To apply this gear, the valve and steam-chest are placed, as is usually the case in American locomotives, over the cylinders; the valve-stem center being, however, in the same vertical plane as the cylinder center line, immediately over it. All the valve-motion is thus arranged on this plane,--perfectly central,--and there are no crooked or side strains. To a point _A_ on diagram, about two-fifths along the connecting rod from the piston end, a small hook-link is pinned. From the lower end of this at _D_ is jointed a lever _E_, which, of course, moves with the connecting rod both backward and forward, and up and down; the point _D_, in fact, moving in a very irregular ellipse, hereafter explained. The fulcrum _F_ of the lever _E_ is carried in a block which slides in a slot-link _JK_, and from the upper end of this lever _E_ the valve-stem rod _G_ is carried to the valve. This slot-link _JK_ is centered so as to be able to be inclined from the vertical either way. When standing in the vertical position, the engine is in mid-gear, and the valve will only be opened for lead; but this will be exact for each end, and exact also on either side of the center line of the connecting rod. When the slot-link is inclined forward to the front of the engine, the engine is in full gear forward; and, when inclined backwards, it is in full gear for going back. Placed in any intermediate position, the cut-off is regulated for any required degree of expansion; the front and back of the piston receiving equal cut-off, or equal amounts of steam.

CONSTRUCTION DIRECTIONS.

In laying out this valve-motion, it is necessary to make the center or fulcrum _F_ of the lever _E_ coincide, when the crank is at either extreme stroke, with the center on which the slot-link _JK_ vibrates: then these two centers coinciding when the engine stands at lead (that is, at either end of the stroke), the slot-link _JK_, which is, in fact, the reversing agent, may be put forward or backward through all degrees, without moving the valve, which thus has constant lead at all grades of expansion.

If, however, it is desired to give increasing or decreasing lead in the grades of expansion, this may be given by varying the coincidence of these two centers accordingly, or by under-correcting or over-correcting the action of the lever _E_, by varying the position of _D_ on the link _B_.

HOW LAP AND LEAD ARE REGULATED.

The lap and lead together are dependent on the distance between the center _F_ of lever _E_, and the center from which the motion is taken for the valve.

ADVANTAGES CLAIMED FOR THE MOTION.

The first advantage claimed for Joy’s valve-gear over the link-motion is, that it is considerably less costly, and that the working-parts are lighter. The saving is not only in weight, however, but also in the greater simplicity of parts, allowing increased facility for tooling and fitting.

It may be needless to point out how readily this valve-motion is adaptable to the requirements of the typical American locomotive, considerably simplifying the present form of valve-motion, by dispensing with the eccentrics and other working-parts, and bringing the gear with all its strains into a direct line, and also leaving the whole of the under part of the engine clear, and permitting a considerable extension of the fire-box,--a most valuable feature.

All the parts are such that they can be readily made by machinery, dispensing with hand labor except in putting them together.

ACTION OF THE MOTION.

The action transmitted to the valve is altogether different from that transmitted by the link-gear, as by this gear it results from the two distinct motions of the connecting rod imparted to the lever _E_; and these two motions work variously, with and against each other, thus giving to the valve a resultant motion somewhat partaking of the nature of that produced by cams, it really being an accelerated and retarded motion; these accelerations and retardations being so arranged as to come at the right time on the revolution to give the desired result.

Thus, as the crank is passing over the center, the lever _E_ is not affecting the valve at all as a lever, but the motion imparted to the valve is due to the center of _E_ slipping down the curve of the slot-link _J_; and this gives a smart opening of the lead with a continued smart opening of the port. When the valve is fully open, as the crank travels on, the lever _E_ has commenced acting on the valve; but its center is still sliding down the slot-link _JK_; and the two movements are opposite and destroying, or partially destroying, each other, producing a retardation of the valve-motion almost amounting to an arrest; and thus is given a nearly straight line on a valve-path diagram where the port is kept fully open, the crank still traveling on, and approaching the time for the cut-off to be effected. The lever _E_ is in the best position to act as a lever at the same time that its center is completing its movement in the slot _JK_, and is commencing to return, being practically still: thus the lever action of _E_, having full effect, rapidly closes the port. And so on for the revolution, the time of release also getting its benefit.

RULES FOR LAYING DOWN THE CENTER LINES OF THE MOTION.

Lay down the center line _aa_ (Fig. 29) of the cylinder, and that of the valve-stem _bb_, at the relative distances required for the engine to which the application is to be made; the valve-stem center line being, however, in the plane of the vibration of the connecting rod. Draw the path of the crank-pin, and the center lines of the connecting rod _cc_^1_c_^1 for both upper and lower positions when the piston is at half stroke. Take a point _d_ on the center line of the connecting rod, where its vibration between _d_^1 and _d_^2 is equal to about double the length of the full stroke of the valve (it is better to allow rather more than less). It may, however, be chosen very much to suit the other arrangements of the engine, such as the position of the guides, brackets, etc., getting, however, if possible, a vibration of the connecting rod fully equal to double the stroke of the valve, to avoid too great an angle of the slide-link when put over for full forward or backward gear.

Having chosen the point _d_, draw a vertical line _zz_ through it and at right angles to _aa_, and mark off the two points _e_, _e_, on each side, these being the extreme positions of the point _d_ on the connecting rod for front and back stroke; from these points draw lines to a point _f_ on the vertical, so far down that the angle between them shall not be more than 90°,--less is better, if there is room to allow of it (these will represent the center lines of the first link pinned to the connecting rod). The point _f_, which will rise and fall with the vibration of the connecting rod, is to be controlled as nearly as may be on the vertical line by a link pinned forward near the cylinder at _f_^1, or, if more convenient, it can be pinned backward.

Next, on the valve-stem center line _bb_, mark off on each side of the vertical _zz_ the amount required for lap and lead together, at _g-g_^1 and _g-g_^2; _g-g_^1 being lap and lead for the front end of the cylinder, and _g-g_^2 being lap and lead for the back end of the cylinder. Then, assuming the piston to be at the front of the cylinder, and the centers of the connecting rod to be at _hh_^1 (_h_ being the crank-pin), the point _d_, which we have chosen to take motion from, will be at _e_^1, and the link pinned to the connecting rod, for transmitting motion to the valve, will be at _e_^1 _f_. From a point on this link, which has at first to be assumed, say at _j_ (which will be about the half vibration of the connecting rod; that is, _d_^1 to _d_), draw the center line of the lever actuating the valve, that is joining _j_ and _g_; the point where this line crosses the vertical _zz_, will be the center or fulcrum of the lever, and will also be the center of oscillation of the curved links in which the blocks carrying the center of the lever slide; this center is marked _m_. The function of the link _e_^1 _f_, and the attachment of the valve-lever to it at _j_, is to eliminate the error in vibration of the lever, center _m_, which would otherwise arise from the arc passed through by the lower end of that lever. Although the position of the point _j_ may be found by calculation, it is much more quickly found by a tentative process; and, to test it the assumed point _j_ be the correct one, we mark off on each side of _m_, vertically, the correct vibration required, _n_^1 _n_^2, which will be the same as the vibration of the connecting rod on the vertical line _zz_. Then apply the distance _e_^1--_j_ to _d_^1--_j_^1 and _d_^2 _j_^2. Then, if the length _jm_ be applied to _j_^1 _n_^1 (measuring from _j_^1), and to _j_^2 _n_^2 (measuring from _j_^2), and the point _m_ fall below _n_^1 _n_^2 in each case, it will be necessary to take a point on _e_^1 _f_ higher than _j_; or if, on the other hand, _m_ falls above _n_^1 _n_^2, then a point must be taken on _e_^1 _f_ lower than _j_. This point will generally be found on a second trial.

The point _m_, as said, now represents the center of oscillation for the links and the center or fulcrum of the lever. And these must coincide, when the piston is at each end of the stroke, the lead being then fixed; and the links can be pulled over from forward to backward, or any point of expansion, without altering the lead. This may be taken as a test of the gear being set out correctly.

The point _g_ will be the point of attachment for the valve-stem link, which may be made any convenient length; but, from that length as a radius, the curve of the links must be drawn from a center _m_^1 on the parallel line _m_-_m_^1; the angle at which this curve is set from the vertical (which is mid-gear), will give forward or backward gear,--the angle leaning forward _s_^1, or to the front of the engine, being forward gear, and the reverse _s_^2 being backward gear. The amount of the angle, marked on the curve of extreme vibration at _s_-_s_^1 or _s_-_s_^2, will be equal to one-quarter more than the full opening of the port at that angle (that is, if 1″ opening of port is required, then the amount of angle _s_ to _s_^1 must be 1¼″), and the point of cut-off will be about 75 per cent. Laid out in this form, the leads and cut-offs for both ends of the cylinder, and for backward and forward going, will be practically perfect and equal, and the opening of ports also as near as possible equal. If a longer cut-off than 75 per cent is required, it is only necessary to increase the angle of the curve _oo_ beyond _s^1_ for forward gear, or beyond _s^2_ for backward gear. It will be noticed, that, in this gear, the lap and lead are entirely dependent on the action of the lever _j_, _m_, _g_, as a lever, and may be varied according to the length of _mg_. And the opening of the port (beyond the amount given as lead) is dependent on the amount of angle imparted to the curved link _oo_, and will be, as above said, about four-fifths of the amount of that angle from the vertical measured on the line of extreme vibration.

Deviations from the above positions and proportions may be made without materially altering the correctness of the results.

Thus, if it is found necessary to raise or lower the center _m_, to clear wheels, frames, or other gear, this may be done till the angle of _mm^1_ is out of the parallel of the cylinder center line up or down by one in thirteen (1 in 13); it is not well to go beyond this; but the lines _mm^1_ and _bb_ will be parallel, and the position of the curve _oo_ for mid-gear will be at right angles to _mm^1_.

Again, the point _e^1_ may be taken either above or below the center line of the connecting rod, if it be wished to avoid piercing the rod; the pin at _e^1_ being carried in a small bush or block attached above or below the connecting rod.

Again, for locomotives, if the wheels are so small that the link _e^1f_ would come too low, it may be cut short at the point _j_, and this point connected by a link _ll_ to a small return-crank _p_, on the crank-pin; the movement of the counter-crank being equal to that from to _j^4_.

The diagram is drawn for an engine where the center of the crank-axle is on the center line of the cylinder; but if this be below, as is usual in American locomotives, then the base line on which to construct the diagram of the valve-gear itself will be the average center line assumed by the connecting rod for such lowering of the crank-axle center, drawn from _c_, the middle position, to a point, say _r_, representing the lowered center of the axle. The vertical _zz_ will be at right angles to this new base line _cr_, all the other processes following.

While the proportions shown on the diagram give the best average results, these proportions may be varied within very wide limits, according to the requirements of the design of the engine. Thus, when the distance between the center of the cylinder and center of valve-stem is small, as with a small cylinder and a long stroke, the link _e^1f_ may be considerably lengthened: the point _j_ will thus be dropped, and convenient angles for all the links, etc., will be maintained, the room for the various movements being got below the center line of the cylinder when it can not be had above; the reverse proportioning of the parts being made when the conditions are reversed, as with a large diameter of cylinder and a short stroke.

This system of laying out the motion, applies, with a few modifications, to the Strong motion, and to all others where the ellipse formed by the horizontal and vertical motion of the main rod is used to actuate the valves.