⚙️ Backlash
The small amount of free play between meshing gear teeth, most often discussed for the ring and pinion inside a solid axle.
Too little and the gears bind and overheat; too much and they hammer each other and strip. Manufacturers specify a shim stack to set it correctly.
What it actually does
Backlash is the small amount of free play between two meshing gears before they make contact. It sounds like a flaw and it is essential: gears need clearance to run without binding, and to allow for the tiny amounts of flex and heat expansion in a running drivetrain.
In practice
Too little backlash and the gears bind, generating heat and eating current. Too much and the teeth hammer against each other under load, which is what strips them. The paper trick when setting gear mesh exists purely to produce a sensible amount of it.
Worth knowing
Check backlash at several points around the spur, not just one. A spur that is slightly warped or a bent motor mount will have tight and loose spots, and the tight one is what does the damage.