Backlash in gears is best described as which of the following?

• A. Gear clearance due to frame flex
• B. The difference between tooth thickness and the width of the tooth space
• C. The ratio of pitch diameters
• D. The axial play at the base circle

Answer: (B)

Backlash is the clearance between meshing gear teeth—the small gap that lets teeth move without rubbing. It is defined by the difference between the thickness of a gear tooth at the pitch circle and the width of the space between the mating teeth. This difference creates the room needed for lubrication, manufacturing tolerances, and thermal expansion so gears can mesh smoothly. If backlash is too small, the gears can bind or seize under load; if it’s too large, the mesh becomes sloppy and reduces precision and stiffness. The other descriptions refer to different ideas: clearance caused by frame flex, the gear ratio from pitch diameters, and axial movement at the base circle, none of which describe the tooth-to-space clearance that defines backlash.