There are basically (3) classifications of conveyor rollers. Light-Duty, Medium-Duty, and Heavy-Duty rollers as described below:  

1.   “Light-Duty” conveyor rollers. These conveyor rollers are designed only for light use. The materials remain the same, either steel, aluminum or plastic for the body and its components, but they usually include non-precision roller bearings, and the roller diameter typically ranges from 1 3/8” to 2.5” and gauge thicknesses of 18 Ga. – 14 Ga. 

2.   “Medium-Duty” conveyor rollers. These usually include commercial, and precision bearings, and a roller diameter greater than that of a light conveyor roller: from 2.5” to 5” and gauge thicknesses of 11 Ga. – 7 Ga. 

3.   “Heavy-Duty” conveyor rollers. The roller diameter range is from 3.5” to 7.” Heavy duty rollers also have thicker tubes, with thicknesses of .300” to .500” wall. Generally these have more durable bearings and axle designs.