The Locomotive Of Today

92 The Engine : Intermediate Valve Spindles. engine is moved from fore or mid gear into back gear, and to assist in lifting this it is usual to either put a spring upon the weigh shaft or place a weight upon an arm on the opposite side to the suspension arm ; the spring is recommended as it reduces weight and is not likely to become detached. The intermediate valve spindle is passed through a guide fixed to the motion plate, and, within a forked end which encloses the. link, it carries a quadrant block fitting in the slot of the link; a pin passes through this and the forked end, and is secured against coming out by small taper pins, as mentioned above. The other end of this spindle is formed into a socket to take the spindle of the valve buckle. As the v alves, when situated between the cylinders, are necessarily very close together, and the centres of the links have to be wider apart, this difference is got over by making the spindle large at the link end, then carrying- it forward on one side of the longitudinal centre line, finishing with a socket larger and yet further from the centre, if necessary. This is plainly shown on the plan. Owing to the faet that the valve is greater in width than the over-all breadth of the ports by twice the lap, the maximum point of cut-off is seldom more than 75 per cent, of the stroke, whilst the minimum cut-off is similarly restricted, although slightly greater in fore than in back gear; further, there is a difference in the cut-off on the front and back strokes, the angularity of the connecting rods making it later at the front end. These results, however, largely depend upon the amount of lap and the travel of the valve, less lap giving later cut-off. As the quadrant block in the fork of the intermediate valve spindle is prevented from having any vertical move- ment, and the suspension link is hung from a centre above, the slot link necessarily rises and falis, as the ends and centre of itS; stroke are reached, and causes a certain amount of “ slip ” upon the block, the faces here are worn if the engine runs for a long time notehed up at one particular point of cut-off. It is found that the slip is least at the point of suspension of the link, and for this reason the links are hung from the centre, for slip and wear combined would cause the distribution of steam to be irregular. A short link with long eccentric rods will allow more slip than a long link with short rods. Where the valves are placecl above the cylinders, either the faces may be inelined downwards to the axle and the gear work direct, or the faces may be made parallel with the pistons and a rocking shaft employed; in the latter case a stout shaft is placed across between the frames with arms below to take the