Backlash for any gear drive is usually defined as the circular shake, measured at the gear pitch radius of the final gear when the input shaft is held stationary. It is the distance measured in a specific direction between the non-working flanks of a pair of gears, mounted in a housing when the working flanks are in contact. Measurement is carried out by preventing the rotation of the worm, setting a dial indicator against a gear tooth on the wheel pitch radius, and moving the gear to and fro after the gear set has been properly adjusted. The total indicator reading is the backlash. Locking the gear and then rotating the worm does not measure the backlash.
The measurement is usually given under conditions of no load and should not be confused with a torque wind-up or twist that results when a significant torque load is applied to the output shaft with the input shaft held stationary. There are different conditions of backlash, and they are detailed in the German DIN standard #3975. ISO are, also, writing standards with the intention of defining such terms.
The Standard #3975 provides definitions for four categories of backlash:
(1) Circumferential backlash: Is the circumferential movement which the worm wheel, with the worm fixed, is capable of undergoing from the contact area of the working flanks up to the contact area of the nonworking flanks.
(2) Axial backlash: The distance measured along a line parallel to the worm axis between the non-working flanks of worm and worm wheel when their working flanks are in contact.
(3) Normal backlash: Described as the shortest distance between the nonworking flanks of worm and worm wheel, when the working flanks are in contact.
(4) Radial backlash: The radial backlash is the shortest distance measured along a line parallel to the line of centers within the depth of meshing of the teeth, between the non-working flanks of worm and worm wheel, when the working flanks are in contact.