backlash
The looseness through which one part of connected machinery, such as a wheel, gear, piston, or screw, can be moved without moving the connected parts, or a measurement of the distance moved thereby; either intentional (as allowance) or unintentional (from error or wear).
backlash: the play that bites when precision matters
Backlash is the gap between meshing parts in a mechanical transmission. When a gear, screw, or piston moves, backlash is the distance it travels before the connected part moves with it. In a simple two-gear mesh, if you rotate the driving gear slightly, the driven gear stays still until the gap closes; that dead zone is backlash. It exists in nearly every assembled mechanism and directly limits precision and repeatability.
Some backlash is deliberate. Gear pairs need small gaps to allow oil film between teeth, prevent jamming from thermal expansion, and reduce noise and shock loading. A well-designed spur gear reduction might have 0.004 to 0.008 inches of backlash at the gear rim. Lead screws in machine tools are often cut with intentional backlash of 0.001 to 0.003 inches to prevent binding. Without this allowance, parts seize.
Unintentional backlash accumulates from poor manufacture and wear. Worn bearing journals enlarge the radial clearance. Gear teeth round at the flanks from fatigue. Screw threads lose height to corrosion or repeated loading. In a positioning system, small errors in each component stack: a worn spindle bearing adds play, a damaged gear mesh adds more, and suddenly a machine tool that once held 0.01 inch tolerances cannot hold 0.05 inch.
Measuring and managing backlash
Backlash is measured by locking one member of a pair and applying small rotational or linear motion to the other, then measuring the distance traveled before the driven member moves. For a gear pair, this is usually done with the gears under no load and measured at the pitch circle. For a screw, a feeler gauge or dial indicator shows the axial play. Specifications vary by application: precision grinding spindles may allow less than 0.0005 inches; heavy industrial conveyors may tolerate 0.1 inch or more.
Backlash is reduced through tighter tolerances in manufacture, preload (applying force to close gaps), and anti-backlash devices such as split gears or spring-loaded screw nuts that maintain constant contact. However, every attempt to eliminate backlash increases friction, heat, and cost. The term comes straightforwardly from the back (reverse) and lash (whip or movement), describing the loose movement that occurs when a part tries to reverse direction without resistance.