Home  

1043

EXPANSION AND CONTRACTION.

1611. In laying track, provision must be made for expansion and contraction of the rails, due to changes of temperature. As the temperature rises the rail lengthens, and unless sufficient space is left between the ends of the rails to allow for the expansion, the ends of the rails abut one against another with such force as to cause the rails to kink or buckle, marring the appearance of the track and rendering it unsafe for trains, especially those running at high speeds. If, on the other hand, too much space is left between the rails, the contraction or shortening of the rails due to severe cold may do equally great harm by shearing off the bolts from the splice bars, leaving the joints loose and unprotected. The coefficient of expansion, i.e., the amount of the change in the length of an iron bar due to an increase or decrease of 1 degree F. is taken at .00000686 per degree per unit of length.

EXAMPLE.—If an iron rod measures 30.015 ft. at a temperature of 90 degrees, what is its normal length, assuming 60 degrees as the normal temperature? The temperature of the bar must be 90 degrees - 60 degrees = 30 degrees above the normal temperature.

SOLUTION.—As the increase in length is .00000686 ft. per degree for each foot in length of the bar, the total increase for 1 foot of the bar due to a rise of 30' in temperature is .00000686 x 30 = .0002058 ft., and for 30 ft. the increase in length above the normal is .0002058 x 30 = .006174 ft , or about one-sixteenth of an inch. As the rail at a temperature of 90 degrees measures 30.015 ft., of which length .00617 ft., say, .006 ft., is due to expansion, the normal length of the rail is 30.015 - .006 = 30.009 ft.

1044

To provide against the effect, of expansion, an opening is left between the ends of the rails, and to provide against contraction, the holes in both rail and splice bar are made oblong, allowing about ¼-inch for extreme movement. The following table of expansion is a safe guide to track-layers for most latitudes in the temperate zones:

To give to the track the proper opening at the joints, expansion shims are used. They are made of iron, and are of various forms. A simple and effective shim is made by bending a piece of one-eighth inch iron into the form of a right angle, as shown in Fig. 498. This gives a combination shim of two thicknesses, viz., one-sixteenth and one-eighth inches. After the angle is formed, the one-sixteenth-inch shim is obtained by hammering the one-eighth-inch bar to the required thickness. The thickness of each shim should be clearly stamped upon it. When put in place, the shim reaches the full depth of the head of the rail, and the bent portion lies flat on the top of the rail. The shims should not be removed until the joint is

1045

full bolted, and there should be a sufficient number of them on hand to keep the track-layers constantly employed, and not require them to wait until shims can be removed from bolted joints.


Track Page | Contents Page

Home
Do you have any information you'd like to share on this subject? Please email me!
The Catskill Archive website and all contents, unless otherwise specified,
are 1996-2010 Timothy J. Mallery