The Locomotive Of Today

The Engine : Joy s Valve Gear. 105 Next, upon the centre line of the valve spindle, which must be in the plane of vibration of the connecting rod, upon each side of the vertical line above mentioned, mark oft the amount requireel for lap and. lead, that for the front port being* drawn upon the crank side of the vertical line, and that for the back port upon the cylinder side of it. Now, assuming the crank to be upon its front dead centre, and the correcting link coupled to the anchor link, choose a point in the length of the correcting link, which has to be assumed, and draw a line through it to the lap and lead mark for the front centre, the crank being upon that centre; the point where this line crosses the vertical line will be the centre line ot the reversing shaft, and the centre of oscillation of the curved guides, which must be concentric at each end of the stroke of the piston. The exact point in the correcting link for the attachment of the valve levers must be found so that the quadrant blocks vibrate equally on each side of the centre of the quadrant block guides. It will now be seen that as the centre of oscill- ation of the quadrant block, and the centre of the reversing shaft exaetly coincide with each other when the piston is. at each end of its stroke, it will be possible to reverse the motion from full forward to full backward gear without communica- ting any movement to the valve rod, the reversing- shaft simply making a partial revolution, the lead being constant in all gears. This point is important, and is made use of when the valves are being set in the shop. The valve spindle may be of any convenient length, and is coupled up to the top of the valve levers at one end, and to a crosshead upon the valve buckle spindle at the other. Whatever length is chosen for the valve spindle must be also the length of the radius of the quadrant block guides. It will be noticed that the lap and lead are entirely dependent upon the distance apart of the two centres of the valve levers, which are connected to the valve spindle and the quadrant blocks respectively, and can only be altered by varying this length. ' The path described by the point in the connectmg rod where the correcting link is connected is a horizontal ellipse when the engine is running, and the end of the valve lever to which the valve spindle is coupled describes a vertical ellipse at the same time; therefore, this particular style of motion is known as “elliptical” gear. It would be possible to couple the end of the valve levers direct to the connecting rods, and dispense with the correcting link altogether, but this would give an irregular motion to the valve, and give a different distribution of steam in the front and back strokes.