Axial shaft expansion can be an issue in machines that generate or use heat in operation. In industrial hot gas fans, the heat is transmitted through the shaft. In paper machine drying cylinders, hot steam is passed through a hollow shaft directly through the bearing bore. There are a number of ways to allow bearings to handle the linear expansion of a shaft. The most common are:
1. Locate (fix) one bearing in the housing, and allow the other bearing axial freedom (non-locating).
2. Choose bearing types that allow one bearing to act as the locating bearing, and allow the other bearing to accommodate the shaft expansion internally. Prevent both bearing outer rings from moving axially.
CARB (SKF's Compact Aligning Roller Bearing) bearings are capable of accommodating linear expansion, misalignment and heavy loads simultaneously.
How much expansion can we expect from a paper machine drying cylinder? Approximately 6 microinches (0.000006") per 1 degree Fahrenheit, per inch of shaft length. (This expansion rate depends on the material in use; gray cast iron is typical.)
Using the expansion rate above, how much could the shaft grow if there was a temperature rise of 200 degrees F (111 degrees Celsius) in the machine, and the bearings are 400 inches (10.2 meters) from center-to-center?
Just multiply the numbers together: 0.000006" /°F / " x 200 " x 400" = 0.480", or just under 1/2" (~12mm)
There are other ways to accommodate thermal expansion. Contact your local SKF Applications Engineering Service for assistance.
