Center-Distance Measuring Instrument

(or center gauge), a gear-measuring instrument designed for determining the deflections and variations of the standard center (interaxial) distance of gears. The first such measuring instruments were made in the 1920’s.

During testing on a center-distance measuring instrument, the gear being inspected is engaged with a measuring gear (see Figure 1). The gears are engaged radially—that is, without lateral clearance (side play)—and they come in contact with each other simultaneously along both the left and right tooth flanks (this is called the combination double-flank method). Thus, a special gear drive is created to determine the deviation from the nominal center distance set on the instrument and its variations during a revolution or fractions of a revolution of the gear being in-

Figure 1. Diagrams of center-distance measuring instruments: (a) with radial displacement of measuring carriage; (b) with angular displacement of measuring carriage; (c) with tangential displacement of two measuring racks, which engage the right and left flanks of the gear being inspected; (d) automatic device with radial displacement of the measuring carriage, on which the gear being inspected rolls along a rack under the measuring gear

Center-distance measuring instruments are most frequently used for the inspection of spur gears. The instruments are manufactured in three standard sizes, for center distances of 120-150 mm, 300-400 mm, and 500-1,000 mm. During inspection, the measuring instruments determine only radial errors of the gear-cutting process; tangential finishing errors are only partly revealed or remain completely undetected. However, because of the simplicity of their design, method of measurement, and circuit, such measuring instruments are widely used in series and mass production of gears.

Instruments for measuring the distances between hole centers or shaft axes—for example, center-distance sliding (vernier) calipers—are also called center-distance measuring instruments.