Vari-Roll Distance Calculator

A Vari-Roll — like most gear checking systems — relies on the precise spacing between a test gear and a master gear to evaluate errors such as spacing variation, profile deviation, or runout. The measurement principle is based on rolling the two gears together under controlled conditions and detecting differences in their motion.

One complication, however, is that test gears and master gears rarely mount on shafts of the same diameter. In addition, many gears under inspection are not part of a defined gear set, so there may be no specified design center distance.

To overcome this, the Vari-Roll uses a combination of interchangeable arbors, floating bushings, and adjustable slides. These features allow gears of different bore sizes to be mounted securely and positioned until they achieve the correct rolling engagement with the master. Instead of relying on a fixed center distance, the system automatically establishes the functional operating distance — the condition where the gears mesh with no backlash. Once this reference mesh is set, the Vari-Roll measures deviations in motion as the gears rotate, which directly reflects errors in the test gear.

This approach ensures that the accuracy of the inspection does not depend on the nominal shaft sizes or a pre-defined center distance. Instead, the master gear itself defines the standard, and the Vari-Roll measures how the test gear departs from it.

The calculator requires the two shaft diameters and, in addition, either the test center distance or the gear pair geometry. Using these inputs, it determines the measurable spacing between the shafts. If the calculated spacing falls between 0.3" and 8.0", the tool also generates a recommended gauge block stack-up based on standard available sizes.