Finding Locomotive Dead Centers
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By Jeffrey G. Hook
(Scroll down for all illustrations. Captions above illustrations.)
Introduction.
The valve gear of a steam locomotive, also referred to as the valve motion, is designed and adjusted so the valve will admit approximately equal quantities of steam, at the appropriate time, to the cylinder on the forward and backward stroke of the piston. By doing so the thrust exerted on the crank pin during the forward and backward stroke will be approximately equal. On a conventional two cylinder steam locomotive, the crank pins on one side of the locomotive are positioned ninety degrees from the crank pins on the opposite side relative to the center line of the axles. Therefore the two pistons of the locomotive will provide a total of four thrust impulses to the cranks pins per revolution of the driving wheels. One thrust impulse will occur for each quarter revolution of the driving wheels.
The sound of the exhaust steam from the cylinders escaping from the smoke stack of a locomotive may be used as an audible indicator of the relative intensity, or pressure, of each thrust exerted by the pistons. If a locomotive is in good condition and the valve gear been designed and adjusted correctly, each exhaust sound will be of equal intensity and duration, and at a constant frequency for a given speed. A locomotive in this condition is referred to as being "in square." If the locomotive is in a condition where there are leaks in the valves or piston rings, the valve gear is poorly designed, or the valve gear is not adjusted correctly, then the exhaust sounds may be of unequal intensity, unequal duration or varying in frequency. A locomotive in this condition is referred to as being "lame."
During the manufacture of a locomotive discrepancies may occur between the intended design dimensions and the actual finished dimensions of the various components of the valve gear, and their locations relative to one another when assembled on the frame. 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.
The procedures of adjusting the valve and valve gear to obtain correct steam distribution and to compensate for any dimensional discrepancies is generally referred to as "valve setting." The valve for each cylinder of a locomotive is usually driven by an independent valve gear. In what may be called the first phase of valve setting, adjustments are made to the valve and valve gear so that the valve is moved to specific points of its stroke when the crank pin is located at specific reference points of its rotation. The dead centers of the locomotive serve as the primary reference points for setting the valve and valve gear. Accurately establishing the dead centers is the foundation of accurate valve setting.
The final phase of valve setting uses a device called an "indicator" which plots a drawing of steam pressure in the cylinder vs. the position of the piston. By comparing the "indicator diagrams" drawn by the devise when attached to either end of the cylinder, the valve setter can make final adjustments to the valve to provide for optimal steam distribution under actual operating conditions. Completing only the mechanical alignment, or first phase, is usually sufficient in order to obtain satisfactory steam distribution during actual operation. Further discussion of the use of the indicator is beyond the scope of this article.
Dead Centers.
A reciprocating steam engine is defined as being on dead center when the crank shaft is in such a position that an imaginary straight line may be drawn through the center line of the crank shaft, the center line of the crank pin and the center line of the crosshead wrist pin. This occurs at two positions of the crank shaft therefore the engine has two dead centers. In locomotive practice, the the main driving axle is in effect the crank shaft.
On a locomotive it is usually not possible to directly ascertain when the center line of the main driving axle, the center line of the main crank pin and the center line of the crosshead wrist pin are on the same line. Instead an indirect method is used where marks are made on the rim of the main driving wheel which indicate when the main crank pin is on either of the dead centers.
The following describes an indirect method of finding the front dead center of the right side of a conventional two cylinder steam locomotive. The principles described would also be used for finding the back dead center, the dead centers for the left side of the locomotive, and for finding the dead centers of other types of reciprocating steam engines or similar mechanical devices.
Fig. 1. - A wheel tram with sharp end points is fabricated from a round steel rod as illustrated. A reference center punch mark (F) is made on a cross member of the locomotive frame in such a position that when the long pointed end of the wheel tram is placed in center punch mark (F) the bent pointed end of the wheel tram may be used to scribe an arc on the rim of the main driving wheel. The dimensions of the wheel tram and the location of center punch mark (F) will vary depending on the design of the locomotive. The long end of the wheel tram should be of sufficient length so that the arc scribed on the rim of the main driving wheel is roughly perpendicular to the wheel tread.
The main driving wheel is turned clockwise to move the crosshead to a position that is near to the apparent front dead center. A dial indicator is mounted to a fixed point on the locomotive so as to contact a clean flat surface on the front of the crosshead. With the crosshead in this position the dial indicator is adjusted to read zero, or a selected reference number. If a typical short range dial indicator is used, extreme care must be taken to avoid moving the crosshead beyond the range of the dial indicator and spoiling the setting.
The main driving wheel is turned approximately one quarter of a revolution counterclockwise to bring the crosshead away from the dial indicator. The main driving wheel is then again carefully turned clockwise to bring the crosshead into contact with the dial indicator and stopped when the dial indicator reads zero or the selected reference number. The long pointed end of the wheel tram is placed in center punch mark (F) and with the bent pointed end of the wheel tram arc (A) is scribed on the rim of the main driving wheel.
Consideration must always be given to the lost motion that is present due to the clearances that are allowed between the crank pin, wrist pin and the associated main rod bushings. In order to neutralize the effect of this lost motion the crosshead is always brought to bear against the dial indicator from the same direction. If the main driving wheel is turned to move the crosshead beyond the dial indicator reference, then the main driving wheel must be turned back in the opposite direction a sufficient distance so that the crosshead will approach the dial indicator with the lost motion taken up as before.
Fig. 2. - The main driving wheel is turned counterclockwise past the back dead center. The counterclockwise rotation is continued, carefully bring the crosshead again into contact with the dial indicator and stopping when the dial indicator reads zero or the selected reference number. The long pointed end of the wheel tram is placed in center punch mark (F) and with the bent pointed end of the wheel tram arc (B) is scribed on the rim of the main driving wheel.
Consideration must be given to the location selected for the dial indicator in order to provide a sufficient, but not excessive, distance between arcs (A) and (B). In 1.5 inch scale model locomotive practice, approximately 3.5 inches between these arcs is adequate.
It is good practice to completely repeat the previous operations required to scribe arcs (A) and (B) in order to determine if they have been established correctly.
