The vast majority of idler rolls used in cast-link belt furnaces today are eleven inches in diameter and are smooth on the contact surface. Because their primary function is to support the cast link belt on the charge end of the furnace and provide “take-up” pressure to the belt, a smooth surface is desired. If the belt is not stretched more on one side than the other, it should remain centered over the idler roll. If the belt is not uniform, if some hearth rolls are not aligned perpendicular to the belt, or some hearth rolls are not level, the belt will track to one side or another. This tracking can only be seen if the idler roll is smooth; thus, it acts as a warning alarm indicating something is not right and repair is necessary.

Some furnaces are fitted with a toothed idler roll to provide consistent uniform tracking. This idler roll style is acceptable if all other measures have been taken to ensure against the above problems. However, if problems exist, the stress produced by those errors will be applied to the belt, and damage will occur. The most prevalent issue caused by inconsistent tracking is broken belt pins due to fatigue stress.

Idler roll construction is much the same as drive drum construction with trunnions inserted to the depth of one diameter. It is preferable to keep trunnion diameters near a maximum of four inches to prevent metal fatigue due to take-up pressure.