The Locomotive Of Today

88 The Engine : Fork and Link Motions. engage pins upon the valve spindles so that they continued to keep up the movement of the valve. It follows that the valve levers upon the footplate were constantly moving backwards and forwards while the engine was running and the eccentrics were in gear. I he next advance was to fit the engine with two eccentrics for each cylinder, and have forks at the ends of the eccentric rods with means by which they could be raised or lowered so as to engage or disengage with pins upon the valve spindles, either fork could then at will be made to communicate its motion to the valve; by having the lever in mid position neither acted, and the valves remained stationary. These forks were found to be a continual source of trouble and expense, as they broke off frequently, and to improve the gear the two rod ends were connected by a curved link, so that the valve spindle was always coupled to the link, and the raising- and lowering of the rods and links brought the valve under the influence of either eccentric as required. This with modifica- tion is the “link motion” most commonly used to this clay for locomotives. It is called the “shifting link” or “Stephenson” motion, because it was introduced by the firm of Messrs. R. Stephenson & Co. in 1843, and fitted to the engines built by them. Another valve gear is called the “stationary link” or “ Gooch’s ” motion, being inventecl by Daniel Gooch also in 1843, and fitted to all engines designed by him. It has a link hung with its curvature the opposite way to that described above, and to reverse the engine the valve rod is moved up or down, the link remaining hung in one position vertically, but free to vibrate backwards or forwards about the centre of a suspension link. A compromise between the two is known as the “straight link” or “Allan” motion from its inventor’s name. In this gear the link is made straight, and has a limited vertical movement, the valve rod is also provided with a lift, so that to reverne the engine the link rises, and the valve rod fails, or vice versa, as the case may be. These three are the most common forms of link motion applied to locomotives. Another gear, a favourite one on the Continent, is Walschaert’s valve gear, which has one eccentric and a slot link, and takes its valve movements mainly from the crosshead. To entirely dispense with eccentrics many gears have been devised. The best known of these, perhaps, is that of Joy, in which the valve receives its travel from the connecting rod, and a curved slot link. In Morton’s gear the motion is