lash

Lash is the play or clearance between two mating parts in a machine. In a spindle bearing, lash appears as radial movement before the shaft actually rotates. In a leadscrew assembly, it shows up as axial backlash: the nut moves slightly before the screw turns. This looseness can stem from manufacturing tolerances, wear over time, or deliberate design choices.

Some lash is intentional. A designer may specify clearance between a gear and its mounting bore to allow for thermal expansion, or to ease assembly without interference fits. Radial lash in a spindle bearing can actually reduce friction and heat. However, most machinists view lash as a problem to minimize. Excessive play degrades repeatability, causes chatter in boring and milling, and reduces the lifespan of precision work.

Lash becomes critical in CNC machines and grinding operations. A few thousandths of an inch of axial lash in a ballscrew means the table moves without cutting when you reverse direction. This ruins surface finish and dimensional accuracy. Preloading, in which springs or shims squeeze bearing races together, eliminates radial lash at the cost of slightly higher running temperature and friction.

Measuring and managing lash

Lash is measured by applying a known force in one direction and noting the resulting movement before the mating part responds. A dial indicator mounted on a spindle housing will show radial lash as you push the shaft sideways by hand. Leadscrew lash is found by rotating the screw while monitoring axial nut movement.

The term appears across trades beyond machining: automotive technicians speak of lash in valve trains, and riggers deal with lash in crane hoist mechanisms. The word likely derives from the notion of something flopping loosely, similar to the lashing of a ship's cargo. In older machine shops, hand-scraped fit surfaces were a standard method of taking lash out of heavily loaded assemblies before the era of precision grinding and rolling.