Final Setting of the Eccentric Crank, Dial Indicator Method, Walschaert Valve Gear.

Following the principles illustrated in the previous figures, a dial indicator is attached to the locomotive in such a manner so as to indicate the position of the link when the main pin is placed on either dead center.

The radius rod should be elevated to such a position in the link that when the link is swung forward or backward there is no forward or backward motion imparted to the radius rod.

If the two indicator readings are the same, then the eccentric crank is set correctly. If there is a difference in the two indicator readings, then the eccentric crank is carefully rotated in the appropriate direction on the main pin, and the test repeated. The procedure is continued until the dial indicator readings are the same when the main pin is placed on either dead center.

It is good practice to repeat the procedure a number of times to insure that the eccentric crank setting has been established correctly.

If the valve gear has been accurately designed, constructed and adjusted, when the main pin is placed on either dead center the link will be in such a position that when the radius rod is raised and lowered through the link there will be no forward or backward motion of the radius rod. If this is not the case, then the length of the eccentric rod, or other design elements, may not be correct.

Effects of Lost Motion.

The dial indicator method requires that the direction of rotation of the main wheel be reversed each time the main pin is moved from one dead center to the other. This causes the eccentric rod and link to move in the same direction each time the link is moved to contact the dial indicator. This action neutralizes the effect of lost motion present in the eccentric rod and link bearings. When rotating the main wheel care must be taken to avoid going beyond the dead centers and thereby moving the link beyond the range of the dial indicator and spoiling its setting. When making adjustments, and it is necessary to move the eccentric crank away from the link, consideration must be given to the presence of lost motion and its effect on the dial indicator reading.

Timing Relationship between the Baker Valve Gear Connecting Rod and the Main Pin.

The Baker valve gear shares many of the same operating principles as that of the Walschaert valve gear. The same eccentric crank, eccentric crank pin and eccentric rod mechanism, as used on the Walschaert valve gear, is also used to drive the forward and backward swing of the gear connecting rod of the Baker valve gear.

The most common design of Baker valve gear is that having constant lead. In such a design the timing relationship between the gear connecting rod and the main pin is correct when the gear connecting rod is moved to the identical point of the swing of the gear connecting rod when the main pin is on either the front or back dead centers.

Initial Setting of the Eccentric Crank.

The location of the eccentric crank pin as illustrated on the locomotive construction drawings is used in making the initial setting of the eccentric crank. Due to the previously described possibility of variations in the finished dimensions of the components of the valve gear, a method must then be used which locates, or sets, the eccentric crank pin truly 90 degrees to the center line of motion.

Final Setting of the Eccentric Crank, Dial Indicator Method, Baker Valve Gear.

The dial indicator method of setting the eccentric crank may also be used for the Baker valve gear. In doing so the dial indicator is attached to the locomotive frame so as to contact a point near the lower end of the gear connecting rod when the main pin is on either the front or back dead center.

The reverse yoke should be placed in such a position that when the gear connecting rod is swung a few degrees forward and backward from the central position of its total swing there is no detectable motion imparted to the bell crank or valve rod. This will minimize the up and down motion of the gear connecting rod as it is swung forward and backward.

The final setting of the eccentric crank is then carried out in a similar manner as previously described for the Walschaert valve gear.

If the valve gear has been accurately designed, constructed and adjusted, when the main pin is placed on either dead center the gear connecting rod will be in such a position that when the reverse yoke is moved forward and backward a few degrees from its central position there will be no detectable forward or backward motion of the valve rod. If this is not the case, then the length of the eccentric rod, or other design elements, may not be correct.