Motion
The valve gear operation combines two motions; one is the primary lead motion which is imparted at the bottom of the combination lever (12). The secondary is the directional/amplitude motion which is imparted at the top. Consider that the driver has adjusted the reversing lever such that the die block is at mid-gear. In this position the secondary motion is eliminated and the piston valve travel is shortest, giving minimal injection and exhaust of steam. The travel of the piston valve is twice the total of lap plus lead.
Contrast this to when the die block is at the bottom of the expansion link (7), giving maximum steam injection and exhaust. This is the most powerful forward setting and is used in accelerating forward from rest. Conversely when the die block is at the top of the expansion link (7), maximal power in reverse is obtained. (On some engines the die block was in the top of the link in forward gear. This type was generally used on tank engines, which worked in forward and reverse equally.)
Once the locomotive has accelerated the driver can adjust the reverser toward the mid-gear position, decreasing cut-off to give a more economical use of steam. The engine's tractive effort is then less than it was at starting, but its power is greater.
Read more about this topic: Walschaerts Valve Gear
Famous quotes containing the word motion:
“It may be possible to do without dancing entirely. Instances have been known of young people passing many, many months successively, without being at any ball of any description, and no material injury accrue either to body or mind; Mbut when a beginning is made—when felicities of rapid motion have once been, though slightly, felt—it must be a very heavy set that does not ask for more.”
—Jane Austen (1775–1817)
“Speech belongs half to the speaker, half to the listener. The latter must prepare to receive it according to the motion it takes.”
—Michel de Montaigne (1533–1592)
“The moments of the past do not remain still; they retain in our memory the motion which drew them towards the future, towards a future which has itself become the past, and draw us on in their train.”
—Marcel Proust (1871–1922)