With the model referred to in the earlier chapters, the action of a Meyer or similar gear can be readily investigated if an extra eccentric-arm be added to the crank-pin disc and an extra travel-scale marked upon the baseplate.

In fig. 39 is shown the “Buckeye” valve used in America on the Buckeye engine; a is the main valve, which determines the admission, release and compression of the steam; and b is the small auxiliary valve which rides on the top of the main valve a, and which effects the cut-off. This valve is constructed so as to have all the advantages of a perfectly balanced valve as well as those of the Meyer valve. There are two faces between which the main valve slides steam tight, and the small valve works on a face inside of the main valve, as shown in the figure. Steam is exhausted past the outer edges of the main valve. In order that steam may enter the cylinder, the port in the main valve must be in communication with the steam port and at the same time it must not be covered by the auxiliary valve b. In the figure the valves are shown in such a position as to admit steam to the left-hand end of the cylinder and to allow it to exhaust from the right-hand end, as indicated by the arrows.

Each valve is moved by its own eccentric, quite independently of the other. By changing the advance or eccentricity, or both, of the auxiliary valve, the cut-off may be changed as desired, without in any way affecting the motion of the main valve. And as the main valve determines the admission, release, and compression of the steam, it is possible, with this valve, to change the cut-off as desired without affecting these at all.

The Trick or Allen Valve.—In connection with the subject of early cut-offs the Trick or Allen valve may be mentioned. It is made with a passage-way through the back of the valve, as shown in fig. 40, adapted to deliver an extra supply of steam into the port by rendering available an additional proportion of the area thereof, when the narrow port-opening given by the lap-edge alone, during linked-up working, is found to wire-draw the steam. Whilst the lap-edge a, for instance, uncovers the edge b of the right-hand port, the left-hand end of the Trick passage becomes uncovered by moving beyond the edge c of the port-face, so that a current of steam flows from left to right through that passage to join the stream passing the edge a. The total area of the two supply-streams flowing into the right-hand port being practically twice that which would be available were the Trick passage absent.

The occurrence and duration of the operations of steam-distribution are exactly the same with this valve as with an ordinary slide-valve of corresponding proportions, the distinctive feature of the valve having no effect upon them so far as their timing is concerned.

Reversal.—It may be of interest to consider the question—“Why do we need reversing gears?” The gears in common use are nearly all variable expansion gears, as well as being reversing gears, but it is desired to consider now only the question of reversing. Are reversing gears really needed? Suppose that one could have a transparent cylinder and valve-chest, so that movement of the piston and valve could be clearly seen. Suppose also that the connecting-rod, crank, valve gear, and fly-wheel could not be seen. We look into the cylinder and see valve and piston reciprocating regularly, and in harmony with one another. We turn away, and before we look again, the engine, let us assume, has been reversed without our knowledge; yet when we turn again to the engine there are the piston and valve reciprocating just as they were at first (although they must have “changed step,” as we shall presently find), and, look as intently as we may, with nothing else visible we shall be unable to tell that the engine has been reversed. The valve is doing now exactly what it did before, performing its operations in the same order. Why have we had to employ reversing gear? Solely because—when the engine ceases to run ahead for instance, stopping at half-stroke the piston and valve will be in a certain position, and will have been moving in certain directions relatively to one another. And to reverse the piston’s motion, we want the valve

Firstly—To take up a new position on the port-face, so as, for example, to give steam where it had previously given exhaust, and thus we get reversal of the engine; and

Secondly—To move away from that new position, after the engine has started astern, in the same direction as that in which it was moving when the engine was running ahead. And seeing that the valve gear derives motion from the crank-shaft directly or indirectly, we cannot give the valve motion in this same direction from an engine now moving reversely, unless we also reverse the gear. And thus we may conclude that a reversing gear is needed simply for the purpose of regulating the direction of the first motion of the valve after stopping (i.e., for “changing the step”), for after it next reaches the end of its travel its motion will be in all respects just as it was when the engine ran ahead,—so that, if we had a single-cylinder engine and never stopped it except on the dead centres, we could drive its valve by a separate valve-engine always running one way, and our main engine would, if pushed off by hand, run equally well either ahead or astern, whilst served by a valve having lap, lead, and travel as is customary. It is hoped that this little speculation may not appear unprofitable, for it has a bearing on the action of the reversing slides in radial valve gears such as those of Hackworth, Walschaert, and Joy.

FIG. 41 SHOWS ENLARGED KEY-VIEW OF CARDBOARD CRANK-DISC, WHICH SHOULD TURN WITHIN THE CIRCLE DRAWN IN THE CENTRE OF FIG. 1.

The arm marked C is the crank-arm, and serves as an index finger in the recording and reading of results.

The mark 1 is the index-mark for the forward eccentric to operate a valve without lap.^[5]

The mark 2 is the index for a forward eccentric set for lead, using a valve without lap.^[5]

The mark 3 is for a forward eccentric for a valve with lap and lead run in “full gear.”^[6]

The mark 4 is for a forward eccentric for a valve with lap and lead run with a link motion partially “linked up.”^[6]

The mark 5 is for a valve with lap and lead run with the link motion in mid-gear.^[5]

The mark 6 is for a backward eccentric for a valve with lap and lead, run with a link motion partially linked-up.^[6]

The mark 7 is a backward eccentric for a valve with lap and lead run in “full gear.”^[6]

The mark 8 is for a backward eccentric set for lead, using a valve without any lap.^[5]

The mark 9 is for a backward eccentric for a valve without lap.^[5]

The radial lines between the centre of the disc and the small circles represent the actual throw assumed for the various eccentrics and their angular position in relation to the crank; where the actual throw is not sufficient to enable the arms to reach the edge of the disc, the throw-lines are produced to the edge.