protoTYPING: Time to Demystify Gears

All About Those Gears

The word “gear” is embedded into the lexicon of our culture, and has become a useful metaphor for our mental and physical state. When a football team has a miserable game, they often say they never got out of first gear. When people change direction with their life, they talk about switching gears.

Gears have even popped up in television and theatre. Family Guy had an episode where Peter Griffin talked about things “grinding his gears” for most of the episode. In the pre-television era, Shakespeare mentions gears in “The Merchant of Venice” saying that “Well, if Fortune be a woman, she’s a good wench for this gear.” Far be it from me to know what he was on about, but there is something about gears that transcends their purpose as a mechanical element.

Gears are found in a number of our consumer products from printers to paper towel dispensers, and are a useful mechanical element that can help us magnify the torque or speed of our innovations. The following is part one of a two-part series to demystify gears. The first part will explain what gears are, when to use them, and some basic calculations. Part two will talk about alternative types of gears and how to use gears in prototypes.

What is a Gear?

A gear is a mechanical element with teeth on it that can interlock with the teeth of another gear. The most common type of gear 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 you are 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 in it 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 will multiply the torque the motor can put out so that it can drive the wheels.

Gear train on an R/C truck to get enough torque to turn the wheels.

In some instances gears can increase the rotational speed. For example, hand crank battery chargers require the motor inside to turn very fast to generate electricity to 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 to generate more current from the motor.

Another reason to use gears is to multiply the number of rotational outputs. For example, in a 4WD car, the 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.

Basics

The easiest way to understand the effect a gear train can have is to start with the spur gear. A spur gear is characterized by its number of teeth and its pitch. The pitch is the number of teeth per unit length around the circumference. The bigger 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 .75" in diameter. Gears that are meshed together have to have the same pitch in order to mesh and move properly.

The 48 pitch gear on the left will not mesh with the 64 pitch gear on the right. Note how much taller the teeth are on the left gear and how they are spaced further apart.

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 at the same time divides the speed by 2. In a 2 gear system, the gear ratio is easy to calculate. 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 and 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.

16 tooth gear (blue) and 38 tooth gear (green). This gear ratio is 2:1

This gear train has 5 gears, but the three in the center do not contribute to the overall reduction. These are called idlers. The ratio of this gear train is also 2:1.

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 which drives a compound gear train to create massive torque in a small footprint.

In a compound gear train the 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.

Compound gear train inside of a servo.

Wrap up

Now that we know what a gear is and what a gear ratio means, we can build on this knowledge. In part two we will look at some different types of gears and how to deploy gears in a prototype.

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Originally published at Edison Nation Blog.