Reducer gear ratio

　　Almost anything with rotating parts has gears. A car’s transmission and engine contain multiple gears. If you open a VCR and take a look inside, you’ll find it packed with gears. Spring-driven clocks and pendulum clocks also have several gears, especially if they’re equipped with chimes or bells. And if the power meter outside your home has a transparent cover, you might see that it contains 10 to 15 gears. Gears are everywhere where engines and motors generate rotational motion.

　　This article will teach you about gear ratios and gear trains, enabling you to understand the function of each gear.

　　Get the gears turning.

　　Gears are typically used for one of the following four purposes:

　　Reverse the direction of rotation

　　Increase or decrease the rotational speed.

　　Transfer the rotational motion to a new axis.

　　Enable the two shafts to rotate in synchrony.

　　Understanding the concept of gear ratio

　　If you understand the concept of a circle’s circumference, then grasping the concept of gear ratio becomes quite straightforward. Remember that a circle’s circumference equals its diameter multiplied by Pi (where Pi is approximately 3.14159…). Therefore, a circle or gear with a diameter of 1 inch has a circumference of 3.14159 inches.

　　Suppose you have another circle with a diameter of 0.635 inches (1.27 cm), and you roll it in the same manner as shown in the figure below. Since its diameter is half that of the circle depicted, it must complete two full rotations to cover the same 4-inch distance. This explains why the gear ratio between two gears—one half the size of the other—is 2:1: the smaller gear must rotate twice to traverse the same distance that the larger gear covers in a single rotation.

　　In real life, most gears have teeth. Teeth offer three major advantages:

　　They prevent gear slippage. Consequently, the shafts connected by gears remain precisely synchronized at all times.

　　They can precisely determine the gear ratio. Simply calculate and divide the number of teeth on the two gears. Thus, if one gear has 60 teeth and the other has 20 teeth, the gear ratio between them is 3:1.

　　They render even slight deviations in the diameters and circumferences of two gears irrelevant. Even if the diameters vary slightly, the gear ratio remains determined by the number of teeth.

　　The function of gear ratios

　　The primary function of gear ratios is to reduce torque by increasing speed, and vice versa. Your vehicle won’t operate with a higher gear ratio because early acceleration requires more torque than horsepower. Conversely, a lower gear ratio is disadvantageous on the highway, where speed takes precedence over torque. Thus, gear ratios can be viewed as a trade-off between torque and speed.

　　Other gear applications

　　If a high gear ratio is desired, a worm gear is the optimal choice. The threaded shaft meshes with the teeth of the worm gear. With each complete rotation of the shaft, the gear advances by one tooth. If the gear has 40 teeth, the gear ratio is 40:1. They are highly compact, such as in windshield wipers.

　　Mechanical odometers are another area where worm gears are widely used.

　　The larger the gear, the greater the torque.

　　Torque is a measure of rotational force, calculated by multiplying the circumferential force by the gear’s radius. This means that larger gears generate much greater torque than smaller gears, owing to their larger radii.

　　Introduction to Planetary Gears

　　Gears have several other applications as well. A planetary gear set is a specialized type of gear train. Planetary gears address the problem described below: suppose you need a gear ratio of 6:1, with the input and output shafts rotating in the same direction.

　　Planetary gear sets are also quite interesting, as they can produce different gear ratios depending on which gear serves as the input, which acts as the output, and which remains stationary. For example, if the sun gear is the input and we connect the output shaft to the planet carrier while keeping the ring gear fixed, we obtain a distinct gear ratio. In this case, as the planet carrier and the planets rotate around the sun gear, the sun gear must turn seven times for the planet carrier to complete a single revolution. This is because the planet carrier rotates once in the same direction as the sun gear, thereby causing the sun gear to rotate one fewer revolution. In this example, the reduction ratio is 7:1.

　　You can rearrange everything once again, but this time keep the sun gear fixed, take the output from the planet carrier, and connect it to the ring gear input. This will result in a gear reduction ratio of 1.17:1. Automatic transmissions use clutches and brake bands to lock different components of the gear set and adjust the input and output, thereby generating various gear ratios through the planetary gear set.

　　Planetary gear operation

　　Consider the following scenario: You have two red gears that you want to keep synchronized, but there is a gap between them. As shown in the figure, if you want them to rotate in the same direction, you can insert a large gear between them.

　　Alternatively, if you want them to rotate in opposite directions, you can use two gears of the same size.

　　However, in both cases, the additional gear is expected to be quite heavy, necessitating the fabrication of a dedicated shaft. In such situations, chains or toothed belts are typically employed as a workaround.

　　Chains and belts have the advantages of being lightweight, capable of transmitting motion between two gears over a certain distance, and able to connect multiple gears on the same chain or belt.

　　In an automotive engine, a single‑ribbed belt may come into contact with the crankshaft, both camshafts, and the alternator. If gears must be used instead of a belt, the task becomes considerably more challenging.