« НазадПродовжити »
DESIGNING AND ERECTING THE WAL
SCHAERT VALVE GEAR.
WHEN a newly equipped engine with Walschaert valve motion is received by a railroad and after the gear has been set up complete at the roundhouse, everything should be carefully inspected, with the view of securing a perfect initial adjustment of the entire valve motion, and to eliminate, if found, any slight error whatever while the other parts of the motion work of the engine are new and of perfect fit and properly adjusted all around; especially in regard to the driving-boxes, rod brasses, and other bearings. It must always be remembered in setting the Walschaert valve gear that two otherwise separate and distinct motions are now brought into a working combination with each other in order to produce the desired movements of the valve, and these two motions are due to the piston's travel, delivered through the crosshead connection, and the throw, or turning movement, of the eccentric.
The piston's motion really governs the lead by offsetting the valve from its central position far enough to overcome its steam lap and enough further to give the amount of port opening desired for lead, and it will do this in both front and back positions of the piston, the lead being the same at the beginning of each stroke, both forth and back. The piston must move the valve in a direction opposite to its own position in the cylinder to secure lead with outside admission valves, but the piston must draw the valve further in the direction the piston lies in-toward the end of the cylinder-to give the advance for lead with a valve of inside admission; and this principle is made plain by the diagram in Fig. 28, in which it is seen that the piston is connected to one end of common lever, and the connection to the valve-stem is either at the extreme opposite end, or at an intermediate point on the lever, according to whether the valve is of outside or inside admission. There are three points to a true lever, however, and the third point on the combination lever is its connection with the radius rod—the point where the valve, through the combination lever, receives the greater part of its motion due to the eccentric's action.
In Fig. 28, two combination levers are represented as being actuated by the piston and two radius rods, and are operating two valves,-one with inside admission and the other with outside admission; the latter, and upper, valve has its stem and the radius rod attached to the combination lever in such a manner that the ang'e given to the lever by the piston being at the extreme end of the cylinder has moved the valve from an otherwise central position a slight distance in an opposite direction from that occupied by the piston, as is required in order to open the proper admission port for lead.
Fig. 28.—Comparative Diagram of Combination Levers Used
with Outside Admission and Inside Admission Valves.
To give lead to the valve with inside admission the valve-stem and radius rod are connected to the lower combination lever in an exactly opposite manner to the way in which the same connections are made on the other combination lever just described. The angle of the combination lever in this case pushes the valve in the opposite direction, and the reason for this is plainly seen by reference to the plate: steam admission past the valve takes place externally with the upper valve and internally with the lower one, yet in each case the steam is admitted to the same end of the cylinder, and this requires that the two kinds of valves be advanced in opposite directions.
The Mason Machine Company undoubtedly were the pioneers in the attempt to introduce the Walschaert valve gear into this country, and if that company was still in the field, their experience would help greatly in the present-day application of the gear. The American Locomotive Company and the Baldwin Locomotive Works are, however, making a specialty of supplying the Walschaert motion to their heavier engines, and the Baldwin Company gives the following instructions for erecting the gear and setting the valves:
1. Check carefully the dimensions of the following parts, rejecting any that are not exactly to drawing: