In this blog post, we’ll explore how gears—which we encounter frequently in everyday life—transmit power and how they have evolved into devices capable of delivering greater force, changing direction in various ways, and achieving high precision.
You’re scrolling down, completely engrossed in a webtoon. You’re clearly swiping the screen with your finger, but the screen keeps moving downward. What time is it? I glance at the clock and see that the minute hand has already made several full rotations, and the hour hand has moved quite a bit as well. Next to me, my youngest sibling is busy turning the pencil sharpener—perhaps because they’ve broken a pencil. My new front-loading washing machine must have finished the cycle, as it’s gradually slowing down. Even at this very moment, we’re turning many things, and many things are turning. Yet, strangely enough, even though we’re just turning them, the pencil gets sharpened perfectly, and the car changes direction here and there. In order to perform the tasks we want where we want them, direct power transmission is necessary—and that’s exactly the role played by “gears.”
A gear is a device that transmits power and motion by having its teeth mesh with one another. Basically, when one gear is rotated while its shaft is fixed, the other gear meshed with it also rotates. You can think of it as pushing someone who is pressed against a wall—they will be pushed back. Here, we need to make one assumption: that the gear is a rigid body that does not deform under force. In reality, all objects deform when subjected to force, but the instantaneous deformation that occurs in gears made of steel or metal is small enough to be ignored in general analysis. Since they transmit power while rotating in mesh with one another, the input rotational force and motion are transferred to the next gear.
Starting from this simple principle, “gears” have evolved to meet specific needs depending on their application, such as transmitting large forces, changing direction in various ways, and achieving minimal error. When the force to be transmitted exceeds a certain threshold, the “stress”—calculated by dividing the force by the area bearing it—increases; the moment this stress exceeds the material’s strength, the gear may become damaged or break. Ultimately, to transmit large forces, a design was needed that increases the area of the teeth bearing the force and distributes the load across multiple teeth; this design has become increasingly sophisticated with advancements in mathematical modeling and mechanical design technology. In the case of helical gears, the teeth are cut at an angle, so that as the gears rotate, multiple teeth engage progressively; this helps distribute the force, making them well-suited for transmitting large forces. Changes in direction are achieved in various ways, such as by altering the tooth profile or by meshing the gears with the two shafts oriented at different angles. A rack-and-pinion system converts rotational motion into linear motion, while a ratchet mechanism is used to transmit or restrict rotational motion into intermittent motion in a single direction. Thus, gears go beyond simply transmitting rotational force to also serve the role of changing rotational speed and torque, as well as altering the direction and form of motion.
Gears have been studied not only for transmitting large forces and changing directions but also for achieving small tolerances. Due to their structural characteristics, gears inevitably have a certain degree of error, one of which is “backlash.” Backlash refers to the gap or clearance intentionally left between the teeth of two meshing gears. A certain amount of clearance between the teeth is necessary for the gears to rotate smoothly. Without backlash, friction and interference between the gear teeth would increase, potentially causing noise or heat generation. Conversely, if the backlash is too large, the play—where the gear does not move immediately in response to the input—increases, leading to shock, vibration, and a higher risk of tooth damage.
Ultimately, while a certain amount of backlash is necessary for gears to operate smoothly, it also reduces positional accuracy. This is because the gap means that when one gear is rotated, the next gear may not rotate immediately and precisely in response.
To reduce this error, an innovative gear system was devised: the harmonic drive, also known as strain wave gearing. Unlike conventional gears, which use materials and structures that are nearly rigid to minimize deformation while transmitting high forces, the harmonic drive actively utilizes the elasticity and elastic deformation of metal. The harmonic drive essentially consists of three main components: a wave generator, a flexspline, and a circular spline. The central wave generator features an elliptical structure; as it rotates, it causes the thin, elastic flexspline to deform into an elliptical shape. This causes the teeth of the flexspline to mesh with the teeth of the circular spline on both sides of the major axis of the ellipse. Because the number of teeth on the flex spline and the circular spline differ, the flex spline moves a small distance relative to the circular spline for each full rotation of the wave generator, corresponding to that difference in the number of teeth. In a typical configuration, the difference in the number of teeth between the two gears is two; therefore, when the wave generator rotates one full revolution, the flex spline moves two teeth in the opposite direction. Because the teeth remain in constant mesh over a wide area, backlash can be minimized to an extremely small degree—or virtually eliminated—allowing for both a high reduction ratio and high positional accuracy. In this way, the harmonic drive utilizes the elastic deformation of metal in reverse to significantly reduce the backlash issues associated with conventional gears, and is used in robot joints, automation equipment, and precision machinery that require precise positional control.
So far, we’ve explored the principles of gears—which transmit power to where it’s needed—and how they’ve evolved depending on their applications. Gears do not necessarily refer only to devices with particularly complex principles or enormous sizes. However, they are all the more valuable precisely because they are so fundamental. Cars contain countless parts, and the gears that transmit power and motion so these parts can function properly resemble us working quietly to keep a vast nation running smoothly. If even a single gear fails to function properly, a large machine cannot operate normally, and the development of new gears enables new forms of power transmission and motion. We look forward to the development of gears capable of transmitting power more precisely and in greater quantities through various means, thereby advancing industry—and through that industrial progress, we hope to be able to generate even greater power ourselves.
