Wheelset Gauging: Back to Back

Wheelset gauging for back-to-back is the measurement of the distance between the inside (back) faces of the two wheels mounted on a single wheelset axle. A wheelset is one rigid assembly: two wheels pressed onto a common axle so they rotate together. The back-to-back dimension fixes how the pair of wheels sits relative to the running rails and is one of the most fundamental checks in wheelset inspection.

Why It Matters

Because the two wheels turn as a unit, the back-to-back distance directly controls how the wheelset tracks through the gauge of the track and through turnouts and frogs. If the wheels are too close together or too far apart, the flanges no longer line up correctly with the rails. That can cause poor tracking, wheel climb, picked switch points, or in the worst case a derailment. The measured surfaces are the inside faces of the wheels, which are not wear surfaces, and the dimension is set very tightly when the wheels are pressed onto the axle. As a result it rarely changes on its own - a back-to-back reading out of range almost always means the wheels have been displaced or were mounted improperly, rather than worn (a bent axle, the other possible cause, is exceedingly rare).

Procedure

The inspector applies a back-to-back gauge or trammel across the inside faces of both wheels and compares the reading against the specified range. A wheelset that falls within the allowed range passes. A wheelset outside the range cannot simply be adjusted in place: the wheels must be pressed off the axle and the wheelset re-processed before it can return to service. The specific nominal value and tolerance band are defined in the AAR interchange rules and field manual.