Why do single speed RC cars use a gearbox?

Question

Being a mechanical engineering novice (I stick closer to solid state electronics), when I look inside of any toy-grade RC car, I see a series of different-sized gears between the motor shaft and the drive axle.

• Why does a single-speed RC car require a gearbox?
• Instead, why can't a gear on the motor shaft connect to a gear on the axle?

So far, two of the plausible explanations I have come up with are that

1. The motor turns at a different RPM than desired by the axle, thus conversion is required.
2. To put protection between the spinning of the axle and the spinning of the motor.

I ask because I am setting out to make my own Lego car driven by a DC brush motor and have no idea what to do for a single speed gearbox other than hook one gear to the motor shaft and one gear to the axle and let it go.

Answer 1

Your first guess covers it fairly well.

Small DC motors work best at high speeds, the gearbox is needed to reduce the higher motor speed down to something more reasonable for the wheels.

Why use a series of gears rather than a single one? Because there are practical limitations on gear ratios, especially when you need to fit them into a small space.

Gears don't offer any protection to the motor unless they are designed to break before the motor does. Since little bits of broken gear flying around is not a nice thing to happen this isn't normally the case.

If you have a lego car that you want to protect against the motor getting overloaded in the event that it drives into a wall and tries to keep going then adding a belt drive stage is probably the easiest thing to do. If the load gets too high the belt will slip. This costs you a little performance and efficiency but is a simple way to protect the motor.

Answer 2

While it is certainly possible to connect input and output shafts with gears there are a number of reasons why having a separate gearbox might be desirable.

• For more complex compound gear systems the gearbox casing provides mounting points for shaft bearings for intermediate gears. For larger gear rations compound gears are generally more compact than a single pair of gears with a large difference in diameter.
• The gearbox casing allows lubricant (oil or grease) to be retained and helps prevent contamination by dust, debris, water etc from outside.
• Having a self contained modular gearbox can make assembly and maintenance easier and can allow for a standard gearbox to be used for a number of different applications.
• Gears and their shafts need to be accurately aligned to work effectively, a rigid casing helps with this. If one of the shafts needs to move or has large loads on it, as in the case of a drive shaft in a vehicle with suspension the gears can be separated from the loads on the shaft via constant velocity joints or other articulated couplings.