The On-Line Magazine of Rideable Model Railroading
NUMBER EIGHT

WWW.DISCOVERLIVESTEAM.COM 

AUGUST  27, 2001

 


Finding Locomotive "Dead Centers" 
(part 2)

By Jeffrey Hook




continued from part 1
 

 


Fig. 3

Fig. 3. - Using a hermaphrodite caliper arc (C) is scribed on the rim of the main driving wheel at a constant distance from the tread and intersecting arcs (A) and (B). Center punch marks (A) and (B) are then made at the two intersections.
 



Fig. 4

Fig. 4. - A divider is adjusted until, with the same setting, it will produce arc (AA) from center punch mark (A) and arc (BB) from center punch mark (B) that intersect each other at the same point on arc (C). Center punch mark (C) is made at the intersection of the three arcs and represents the dead center. Care must be taken to label center punch mark (C) in order to prevent future confusion as to which center punch mark represents the dead center.


Fig. 5

Fig. 5. - In order to place the engine on the front dead center the long pointed end of the wheel tram is placed in center punch mark (F) and the main driving wheel is carefully turned until the bent pointed end of the wheel tram will enter center punch mark (C). 

To find the opposite dead center the same procedure is used except the dial indicator is mounted so as to contact the opposite end of the crosshead.


Photo 1

Photo 1. - Deerfield and Roundabout Railway engine number 284 over the "drop pit" at the Deerfield shops. The side rods will not be taken down which will require all driving wheels to be turned when finding the dead centers. The locomotive is equipped with roller bearing driving boxes and pedestal binders of sufficient strength that allow shim blocks to be placed between the driving boxes and pedestal binders which raises the axle center lines to their normal working height relative to the cylinder center lines. If a "drop pit" was not available, or the bottom half of the driving boxes or the pedestal binders could not support the driving wheels, then the locomotive would be moved forward and backward as needed on a section of straight level track. In full size practice, to eliminate the necessity of moving the locomotive, a power operated roller device is placed under the main driving wheels. The side rods are removed, the main rods are installed, and the drive rollers of the device are adjusted to raise the main driving wheels slightly off the rails. If necessary, jacks are applied to the frame to bring the center line of the cylinders into proper relation to the center line of the main driving axle.


Photo 2 

Photo 2. - For finding the front dead center the dial indicator and associated support rods are clamped to the valve-stem crosshead guide and positioned so as not to foul moving parts such as the combination lever.

Photo 3

Photo 3. - The dial indicator is positioned to make contact with the front flat of the crosshead wrist pin nut and adjusted to read zero when the crosshead is near to the apparent front dead center. The crosshead wrist pins on this locomotive do not rotate which makes it feasible to use the crosshead wrist pin nuts as contact points.

Photo 4

Photo 4. - Center punch mark (F) is made on the gear frame rear support bar.

Photo 5 

Photo 5. - The driving wheels have been turned clockwise until the dial indicator reads zero. The long end of the wheel tram has been placed in center punch mark (F). The wheel tram is shown in position to scribe arc (A) on the rim of the main driving wheel. Use of the surface gage to hold the wheel tram is for photographic proposes only. Normally the valve setter uses their hand to hold the wheel tram in center punch mark (F) while simultaneously scribing either arcs (A) or (B) on the rim of the main driving wheel.


Photo 6 

Photo 6. - The driving wheels have been turned counterclockwise until again the dial indicator reads zero. The wheel tram is shown in position to scribe arc (B) on the rim of the main driving wheel.

 
Photo 7

Photo 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.


Photo 8

Photo 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.


Photo 9

Photo 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 wrist pin grease fitting or the lubricator reach rod.


Photo 10 (click to enlarge)

Photo 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 motion of a steam locomotive.
Future articles will explain how the dead centers are used as reference points for setting the eccentrics of the Stephenson valve gear, or for setting the eccentric crank used on the Walschaert or Baker valve gear. Other details of valve setting will also be explained.

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 E. "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.

the end

About the author

photo by Karl Kobel 


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 1986 the author was introduced to E. "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. Jeffrey Hook is also the official web master of the D&R web site.

AUGUST  27, 2001 Reprinted by permission. Originally published at www.jghtech.com  All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.