Process of gear mechanics drives countless inventions for consumers.

The word “gear” is embedded into the lexicon of our culture in many ways, often with metaphors related to its purpose as a mechanical element. When a football team has a miserable game, its coach may say the players never got out of first gear. When people change direction with their lives, they talk about switching gears.

In an episode of “Family Guy,” main character Peter Griffin says that life’s frustrations are “grinding his gears.” And then there’s this quote from Shakespeare’s “The Merchant of Venice”: “Well, if Fortune be a woman, she’s a good wench for this gear.” (Modern translation: If luck is a lady, she’s good at this business.)

The translation that links “gear” and “business” fits well today. Gears are found in a number of consumer products—from printers to paper towel dispensers—and are a useful mechanical element that can help us magnify the torque or speed of innovations. This Part 1 of a two-part series on gears will explain what they are, when to use them, and some basic calculations. Part 2 will discuss alternative types of gears and how to use gears in prototypes.

A gear is a mechanical element with teeth that interlocks with the teeth of another gear. The most common type is the spur gear, which is circular and has triangular teeth. Rack gears use the same shape of gear tooth, but the teeth are arranged in a straight line. When multiple gears are meshed together, it is called a gear train.

When do you need gears?

Anyone who has ridden a multi-speed bike knows the value of gears. When riding up a hill, it gets harder to pedal, so you shift down. It is easier to pedal, but you have to pedal more revolutions to get up the hill. On the way down the hill it is easier to pedal, so you shift up. This makes it harder to pedal, but you can get a much higher top speed.

The usual reason a product needs a gear or set of gears is when it needs more torque or rotational force. For example, a toy car may have a very small motor that can spin very fast, but the wheels may be too heavy for the motor to move it. In this case, adding a gear train multiplies the torque the motor can put out so that it can drive the wheels.

In some instances, gears can increase rotational speed. For example, hand-cranked battery chargers require the motor inside to turn very fast, to generate electricity and charge the battery. The hand crank is connected to a gear train run in reverse, which multiplies the cranking speed from the hand to the motor and generates more current from the motor.

Another reason to use gears is to multiply the number of rotational outputs. In a four-wheel-drive car, pistons fire to rotate a single crankshaft. Through a series of gears and axles, this single output can be expanded to rotate all four tires at once.

Gear basics

The easiest way to understand the effect a gear train can have is to start with the spur gear, which is characterized by its number of teeth and pitch. The pitch is the size of a gear’s teeth, specifically the distance from a point on one tooth to the corresponding point on the next tooth. The higher the pitch, the smaller the gear teeth, and the smaller the diameter of the gear. For example, a 46-tooth gear that is 48 pitch (48 teeth per inch) is 1 inch in diameter, but a 46-tooth 64 pitch gear is 3/4 inches in diameter. Gears that are meshed together must have the same pitch in order to mesh and move properly.

The fundamental characteristic of a gear train is the gear ratio. The gear ratio is the force multiplier that the gear train adds to a motor. For example, a 2:1 (said 2 to 1) gear ratio multiplies the torque of the motor by 2 while dividing the speed by 2.

The gear ratio is easy to calculate in a two-gear system. It is simply the number of teeth on the output gear divided by the number of teeth on the motor gear. Any number of gears can be put in between the motor and the output gear; as long as they are on different axles, they will not change the gear ratio of the system. These are called idler gears, and they do not contribute to the gear ratio.

Compound gears

When two or more gears are fixed onto a common axle, it is called compound gear. When compound gears are assembled together into a gear train, they can create more torque in a much smaller package than a two-gear train. Servos are a great example of the use of a compound gear train. They use a very small motor that can spin very fast and drives a compound gear train to create massive torque in a small footprint.

In a compound gear train, gear ratios between every step are multiplied together to get the final gear ratio. It is not uncommon to see compound gear trains that have ratios in the hundreds that utilize the same space of a simple gear train that has a ratio of 5 or 6 to 1.