In parts 1 and 2 of our Common Mechanisms Explained with Animation series, we showed you how mechanisms such as cams, the Geneva Wheel, and universal joints work. These posts can be found here and here, respectively.
For the last installment in this series, we will be looking at gears and some of their variations.
Gears are one of the most versatile and adaptable mechanisms invented. Gears are rotating disc-like machine elements that have teeth cut into their circumference. They interact with other similarly cut discs in order to transmit rotational movement. They can be modified to suit any problem that requires the transmission of a rotary input into a rotary output. The difference between the input and output could be the direction of rotation, the speed of rotation, or the amount of torque produced. The aim may be to increase or decrease any of these parameters, which is achieved by the ratio between the sizes of the gears that are interacting together. This is referred to as the mechanical advantage.
Typical Gear Mechanisms
No matter how much two gears vary from each other, one thing always remains exactly the same: the profile of the gear teeth at the exact moment of interaction between two gears. This means that the teeth of two gears, no matter how dissimilar they are, will merge perfectly every time they interact.
There are many types of gears and gear applications. A few of them will are described below:
Worm Gear Mechanisms
The worm gear mechanism includes a screw-like part – the worm – that has a thread that interacts with a regular gear. As the worm rotates, its thread impinges on the teeth of the other gear and drives it in a manner that can be likened to the way a drill elevates cut material as it drills into it. A worm gear is a very ingenious way of changing the axis of rotation as well as ensuring that the transmission of motion or movement can be limited to only one direction. It is always possible for the worm to drive the gear but not always possible for the gear to drive the worm. This relationship is governed by the angle of the teeth and the friction between the worm and the gear.
Sun and Planet Gear Mechanisms
Sun and planet gear mechanisms convert rotational motion into a reciprocating motion much like the way a slider crank mechanism does. The difference is in the way the crank or reciprocating arm is moved: in slider crank mechanism, one end of the crank is attached to a point on the surface of the rotating part and a reciprocating movement is brought about in the course of every rotation. In a sun and planet mechanism, however, the reciprocating arm is attached to a gear, which in turn interacts with another rotating gear as they mesh with each other. The gear connected to the reciprocating arm is called the planet because it revolves around the stationary arm, which is the sun as it rotates fixed in one position, with the connected arm translating the motion of the planet into a reciprocating motion.
Rack and Pinion Mechanisms
A rack and pinion mechanism involves a regular gear wheel rotating and impinging on what could be called a gear belt or rack. Although it appears as though rotation is being translated into linear motion, the gear mechanisms simply transmit rotational motion from one gear to another. In the rack and pinion mechanism, the rack can be thought of as a gear with an infinite radius, thereby making a small arc of it appear to be a straight line.
There are many more types of gear mechanisms, but the governing principle is the same: the impingement of teeth between two parts in order to transfer motion continuously and efficiently. All the different types of gear mechanisms could be combined to achieve very unique and complex motions and the limit of the possibilities is really just how far the imagination of a design team can reach.
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