One of the first mistakes of a RC car modeler is getting wrong with gear backlash, which immediately compels in buying a new spur gear.
In fact, when the mesh is too loose or too tight, the spur gear is the gear that loses out first, since it is made of plastic material.
Neither too tight nor too loose
The clearance in the gear mesh must be properly ensured.
When it is too loose, the gears are too far apart to keep continuous engagement, and the teeth tend to bang and impinge on each other when meshing, wearing out at the ends or even breaking.
On the other hand, low or no clearance makes the gears work better, but just for a short time. Friction generates heat that expands the teeth and reduces the gap in between; heat production increases and the spur gear teeth yield by fusion, as you can see in the following image.
The right gear backlash
A rather popular and effective method of ensuring proper gear backlash consists in placing a small strip of paper documents in between the two gears before screwing the motor.
Once the motor screws are tightened, simply rotate the spur gear to make the paper strip getting out. The backlash left can be felt by swinging the gear in one direction and out the other. This is the correct gear backlash to prevent future damage.
Have a good job!