A.--Yes, very frequently; and the slide valve is sometimes on the side or back of the valve casing, and sometimes on the back of the main or distributing valve, and moving with it.
204. Q.--Are cams used in locomotive engines?
A.--In locomotive engines the use of cams is inadmissible, and other expedients are employed, of which those contrived by Stephenson and by Cabrey operate on the principle of accomplishing the requisite variations of expansion by altering the throw of the slide valve.
205. Q.--What is Stephenson's arrangement?
A.--Stephenson connects the ends of the forward and backward eccentric rods by a link with a curved slot in which a pin upon the end of the valve rod works. By moving this link so as to bring the forward eccentric rod in the same line with the valve rod, the valve receives the motion due to that eccentric; whereas if the backward eccentric rod is brought in a line with the valve rod, the valve gets the motion proper for reversing, and if the link be so placed that the valve rod is midway between the two eccentric rods, the valve will remain nearly stationary. This arrangement, which is now employed extensively, is what is termed "the link motion." It is represented in the annexed figure, fig. 35, where e is the valve rod, which is attached by a pin to an open curved link susceptible of being moved up and down by the bell-crank lever f'' f'', supported on the centre g, and acting on the links f, while the valve rod e remains in the same horizontal plane; d d' are the eccentric rods, and the link is represented in its lowest position. The dotted lines h' h'' show the position of the eccentric rods when the link is in its highest position, and l l' when in mid position.
206. Q.--What is Cabrey's arrangement?
A.--Mr. Cabrey makes his eccentric rod terminate in a pin which works into a straight slotted lever, furnished with jaws similar to the jaws on the eccentric rods of locomotives. By raising the pin of the eccentric rod in this slot, the travel of the valve will be varied, and expansive action will be the result.
207. Q.--What other forms of apparatus are there for working steam expansively?
A.--They are too numerous for description here, but a few of them may be enumerated. Fenton seeks to accomplish the desired object by introducing a spiral feather on the crank axle, by moving the eccentric laterally against which the eccentric is partially turned round so as to cut off the steam at a different part of the stroke. Dodds seeks to attain the same end by corresponding mechanical arrangements. Farcot, Edwards, and Lavagrian cut off the steam by the application of a supplementary valve at the back of the ordinary valve, which supplementary valve is moved by tappets fixed to the valve casing. Bodmer, in 1841, and Meyer, in 1842, employed two slides or blocks fitted over apertures in the ordinary slide valve, and which blocks were approximated or set apart by a right and left handed screw passing through both. [1] Hawthorn, in 1843, employed as an expansion valve a species of frame lying on the ordinary cylinder face upon the outside of the valve, and working up against the steam side of the valve at each end so as to cut off the steam. In the same year Gonzenbach patented an arrangement which consists of an additional slide valve and valve casing placed on the back of the ordinary slide valve casing, and through this supplementary valve the steam must first pass. This supplementary valve is worked by a double ended lever, slotted at one end for the reception of a pin on the valve link, the position of which in the slot determines the throw of the supplementary valve, and the consequent degree of expansion.