Roll Pitch Diameter and Gearing Ratio

The roll “pitch” diameter is usually selected to engage a given profile at its widest and most nearly horizontal area.  This area is important as it provides the most effective and balanced traction.  The ratio between the pitch diameter of the top and bottom rolls must be the same as the ratio of the gearing connecting the top and bottom spindles.

Some manufacturers provide machines that are able to accommodate either equal ratio or unequal ratio rolls.  On the left of Figure #9 is a cross-section through a stage of equal pitch diameter rolls.  The drawing on the right shows the same roll stage in a machine fitted out for unequal ratio rolls.  The pitch diameter in the latter is well below the mid-point between spindles and as you see, gains enough space to allow forming a much deeper section with the same vertical centers.

In machines to which only equal ratio rolls can be applied, deep sections are formed by so called “floating the pitch line”.  This is not a good procedure but can be applied in specific instances, particularly where light gauge and perhaps narrow shapes are concerned.  In principle, it involves a different pitch diameter in successive roll passes which, as each bottom spindle is driven at the same RPM, can lead to “roll fight” between passes, excessive gear loads and excessive HP demands.