Finding Locomotive Dead Centers
Wep Site Page 5 of 5
By Jeffrey G. Hook
(Scroll down for all illustrations. Captions above illustrations.)
Image 7. - Center punch mark (C) is located by following the instructions previously described for Figures 3 and 4. The wheel tram is shown engaged in both center punch marks (C) and (F). The main driving wheel on this side of the locomotive is on front dead center.
Image 8. - The locomotive on right front dead center. Note the distances between the center punch marks (red areas) on the rim of the 8 5/8 inch diameter main driving wheel. These distances are controlled by the position selected for the dial indicator.
The wheel tram should be stored in a safe location so that its dimensions will not be disturbed and that it may be readily found in case it is again necessary to place the locomotive on dead centers.
Image 9. - For finding the back dead center the dial indicator and associated support rods are clamped to the gear frame and positioned so as not to foul moving parts such as the crosshead wristpin grease fitting or the lubricator reach rod.
Image 10. - The dial indicator is positioned to make contact with the rear flat of the crosshead wrist pin nut. The previous procedures are used in a similar manner to find the back dead center.
Conclusion.
Accurately finding the dead centers is the first step in setting the valve gear of a steam locomotive.
Some readers may have questioned why the dial indicator is not used to directly indicate the dead center by marking the dead center at the instant when the indicating hand stops moving. This is generally not possible due to the fact that an extremely sensitive dial indicator would be required. As the crank pin and crosshead approach the dead center, the movement of the crosshead becomes less and less for the same amount of rotation of the main driving wheel. In model locomotive practice, if a main driving wheel of 9.875 inches in diameter, having a crank pin stroke of 4.000 inches, is coupled to a main rod having a length of 15.000 inches, and theoretically no lost motion is present, the crosshead will move 0.0003 inches if the main driving wheel is turned 1 degree to either side of the dead center. 1 degree of rotation at the rim of the 9.875 inch diameter main driving wheel will amount to 0.086 inches of travel. The method described in the article will establish the dead centers accurately using a commonly available type of dial indicator.
In full size locomotive practice a crosshead tram is fabricated similar to the wheel tram. Instead of the dial indicator, the crosshead tram is used in a similar manner as the wheel tram to indicate when
the crosshead is at the selected reference location.
Suggested Readings.
Williams, Frank, "Locomotive Valve Setting," International Text Book Co., Scranton, Pa., 1930.
Harding, J. W., "Locomotive Valves and Valve Gears," International Text Book Co., Scranton, Pa., 1937
Credits.
Thanks to Karl Kobel, Robert Kurth and Jim O' Connor for reviewing and making suggestions regarding content and indicating where corrections were required in this article.
Special thanks to Edward "Bud" Buker for his excellent workmanship used in the construction of D&R Engine 284, which has permitted setting the valves so the engine sounds square whether in the corner or company notch.
About the Author.
Jeffrey G. Hook has had a life long interest in steam locomotives and steam era railroad engineering and operating practices. Exposure in early childhood to his father's Lionel and "HO" electric trains is undoubtedly what began this interest. A number of years ago the author completed the machining and construction of an Allen Models "Chloe" 7.5 inch gage steam locomotive modified to be an 0-4-0 with tender. In 1985 the author was introduced to Edward "Bud" Buker, owner and builder of the Deerfield and Roundabout Railway. From that time and to the present, the author has been intimately involved with all aspects of the continued engineering, operation, construction and maintenance of the Deerfield and Roundabout Railway.