various defects are described in the next two
paragraphs, you will see how these four gages are
DEFECT AND WEAR LIMIT--CAST-
United States military railway practices
conform generally to the material and maintenance
specifications of the Association of American
Defects and wear limits of cast-steel
wheels are covered in the AAR Wheel and Axle
Manual; the AAR rule number and the symbol used
for marking condemning wheel defects and
excessively worn wheels are given along with the
name and description of the defects; no attempt is
made in this text to cover all possible wheel defects;
however, those that occur most frequently are
Figure 2.4. Steel-Wheel Gage.
discussed in the following subparagraphs.
a. Slid-flat (Rule 68--AAR symbol 68). When brakes are applied with so much force that the brakeshoe is
pressed against the wheel hard enough to prevent the wheel from turning, the wheel slides on the rail. The
wheel area in contact with the rail is small and heat is generated rapidly, because of the great amount of friction
between the wheel and the rail. The area of the wheel in contact with the rail is worn off quickly, leaving a flat
spot whose length depends on the distance the wheel slides. Once the brakes are released, the wheel is free to
move, and the slid-flat spot lengthens itself with each rotation of the wheel.
A wheel with a slid-flat defect 2 1/2 inches in length or with two-or more adjoining spots each 2 inches or
over is condemnable. Although a slid-flat defect of 2 1/2 inches is not considered dangerous, it causes the car to
ride roughly and damage the rails. For these reasons, slid-flat defects on wheels under passenger cars are
limited to 1 inch in length. To measure a slid-flat spot, a wheel-defect gage is used, as shown in figure 2.5.