Setting the Eccentric Crank
For Constant Lead Walschaert or Constant Lead Baker Valve Gears
Page 1 of 2
By Jeffrey G. Hook
(Scroll down for all illustrations. Captions above illustrations.)
Introduction.
The link of a Walschaert valve gear, or the gear connecting rod of a Baker valve gear, receive their reciprocating motion from the eccentric rod which is driven by the rotary motion of the eccentric crank pin. The eccentric crank pin is attached to the eccentric crank which is in turn attached to the end of the main pin. The eccentric crank may also referred to as the return crank.
Image 1. - View of the right side of Deerfield and Roundabout Railway engine number 284 equipped with Walschaert constant lead valve gear with principle parts named.
Fig. 1. - View of the right side of a locomotive equipped with Baker constant lead valve gear with principle parts named.
In order for either of these valve gears to provide correct steam distribution to the cylinder, the reciprocating motion, or forward and backward swing, of the Walschaert link or the Baker gear connecting rod must occur in specific relation, or timing, to the rotation of the main pin. The dimensions and position of the eccentric crank that will place the eccentric crank pin in the proper position relative to the main pin in order to provide the correct timing is determined by the engineering department and is specified on the locomotive construction drawings.
During the manufacture of a locomotive unavoidable discrepancies occur between the intended design dimensions and the actual finished dimensions of the various components and their locations relative to one another when assembled on the frame. If the eccentric crank is installed exactly as specified on the construction drawings, without taking into consideration any of the dimensional discrepancies that may exist, then the correct relationship, or timing, between the Walschaert link and the main pin, or the Baker gear connecting rod and the main pin, may not be achieved. This may result in incorrect steam distribution to the cylinder and a locomotive that sounds "lame."
After completing the construction and assembly of the valve gear, adjustments are made to the various components in an effort to compensate for any dimensional discrepancies that may exist. These adjustment procedures comprise what is generally referred to as valve setting. The procedure which pertains to the adjustment of the angular position of the eccentric crank on the end of the main pin is referred to as setting the eccentric crank.
Image 2. - Close up view of the eccentric crank. The red arrows indicate the directions of movement that may be required when setting the eccentric crank.
Timing Relationship between the Walschaert Link and the Main Pin.
The most common design of Walschaert valve gear is that having constant lead. For such a design the timing relationship between the link and the main pin is correct when the reciprocating motion, or swinging motion, of the link is 90 degrees out of phase in relation to the dead centers of the main pin.
The dead centers of the main pin are used as the primary reference points for setting the eccentric crank, therefore it is of utmost importance that it is known that they have been accurately established. For further instructions go to Finding Locomotive Dead Centers.
The following figures illustrate a theoretical Walschaert valve gear having no lost motion and correct timing relationship between the link and the main pin.
Fig. 2. - Main pin on front dead center and link at position (X) of the swing of the link.
Fig. 3. - Main pin on bottom quarter and link at position (Y) which is one of two points of maximum swing of the link.
Fig. 4. - Main pin on back dead center and link again at position (X) of the swing of the link.
Fig. 5. - Main pin on top quarter and link at position (Z) the other point of maximum swing of the link.
Center Line of Motion.
In Figure 6 below, a blue line is drawn through the center of the axle and extends through the center of the link foot pin. This line is referred to as the center line of motion. The positions of the eccentric rod, eccentric crank pin and eccentric crank when the main pin is on the front dead center is illustrated by solid lines and circles. The positions of these same components when the main pin is on the back dead center is illustrated by dashed lines and circles. Note that when the main pin is on either the the front or back dead centers the eccentric crank pin is 90 degrees from the the center line of motion, and that the link is at position (X). These facts confirm that the reciprocating motion of the link will indeed be 90 degrees out of phase in relation to the main pin.
In Figure 6 above the eccentric crank pin is located 98.6 degrees counter clockwise from the main pin. This is due to the fact that the center line of motion is inclined above the line of dead center. A common misunderstanding is that the eccentric crank pin is always located 90 degrees from the main pin. This would only be true if the center line of motion coincided with the line of dead center. Generally the center line of motion of a Walschaert valve gear is inclined some amount above the line of dead center.