The most common reason for an overheated pillow block bearing is over-packing it with grease. The churning lubricant produces heat. As speed increases, so does the heat. Adding more grease only worsens the problem, and, you run the risk of blowing out a seal. Excess grease will usually purge through the seals and the bearing will run cooler.
In nasty environments (gravel pits, fertilizer operations and construction sites) bearing life can be significantly increased by using bearings that have flushing chambers. These units have three (3) grease fittings. A flushing chamber with an outboard labyrinth seal is on either side of the central bearing chamber, which is protected by lip seals. Each flushing chamber is periodically flushed with grease through the labyrinth seals to purge contaminants. Only the central grease fitting supplies lubricant to the bearing. This technique will extend bearing life to years when a proper maintenance program exists.