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Gears and transmission! - Part 1

From a tiny watch to the largest ship, gears are moving things ahead!

Invention of the wheel has revolutionized the workings of the world. Considered as a breakthrough invention that has lead to other great things. Many centuries after the wheel was invented, another great invention turned the pages of industrialization and machineries - GEAR!

What is gear? Nothing but a wheel having some teeth along the circumference!


Gears are mechanical transmission elements which are used for the transmission of motion and power amongst the machine components. The gears have a circular geometry with even spaced teeth along the circumference. Two or more gears work in pairs to transmit the power. The teeth of the gears mesh with one another allowing permissible slippage for smooth rotation of gears.

Shafts are connected to the machines and center of the gears, when the gear rotates along with the shaft the power is translated into the machine component. Here we have two shaft elements, driving and driven connected to respective gears or mechanism. Depending on the design and construction of the gear pair, the transference of motion between the driving shaft and the driven shaft can result in a change of the direction of rotation or movement. Additionally, if the gears are not of equal sizes, the machine or system experiences a mechanical advantage which allows for a change in the output speed and torque (i.e., the force which causes an object to rotate).

Gears and their mechanical characteristics are widely employed throughout industry to transmit motion and power in a variety of mechanical devices, such as clocks, instrumentation, and equipment, and to reduce or increase speed and torque in a variety of motorized devices, including automobiles, motorcycles, and machines. Other design characteristics, including construction material, gear shape, tooth construction and design, and gear pair configuration, help to classify and categorize the various types of gears available. Each of these gears offers different behaviors and advantages, but the requirements and specifications demanded by a particular motion or power transmission application determine the type of gear most suitable for use.

Characteristics of Gear Design:

Gears have a wide range of application. Based on the application, there are a variety of gears depending on the design, construction and working configuration. Gears are classified into different categories depending on:

  • Gear Shape

  • Gear tooth design and construction

  • Gear axes configuration

Gear Shape

The most common type is circular. The gear teeth are arranged around a cylindrical disc also called as face. There are a few gears which are not circular, such as elliptical, triangular and square shaped face.

The gear ratios (i.e., the ratio of the output to the input) specified in circular gear devices and systems are consistent, both for rotational speed and torque. The term "consistency of gear ratio" refers to the device or system consistently producing the same output speed and torque when given the same input (either speed or torque).

Devices and systems that use non-circular gears, on the other hand, have changeable speed and torque ratios. Non-circular gears with variable speed and torque can meet unusual or irregular motion needs such alternatingly increasing and reducing output speed, multi-speed, and reversing motion. Linear gears, such as gear racks, can also transform the driving gear's rotating motion into the driven gear's translational motion (or a combination of translational and rotational motion).

Gear tooth design and construction

Cogs are also used to refer to gear teeth, which is why a gear is often referred to as a cogwheel. While gears were classified in the preceding part based on the general form of the gear body, this section discusses the design and construction of their teeth (i.e., cogs). Gear teeth are available in a variety of design and construction variations, including:

  • Teeth structure

  • Teeth placement

  • Tooth profile

Teeth structure

Gear teeth are either cut directly into the gear blank or placed as separate, formed components into the gear blank, depending on the gear construction. When a gear becomes fatigued in most situations, it may be completely replaced. However, using gears with distinct tooth components has the benefit of allowing you to replace individual teeth as they wear out rather than having to replace the entire gear component. Individual cogs are less expensive than a full gear, hence this feature may assist to minimize the overall cost of gear replacement over time. It also enables for the retention and preservation of unique, bespoke, or otherwise difficult-to-find gear bodies.

Teeth placement

The teeth of the gear are profiled on either the internal or external surface of the gear body. In the outer gears, the teeth are placed on the outer surface of the gear body, pointing outwards from the gear center. Similarly, inner gears have internal surface plied with teeth. In mated pairs, the placement of the gear teeth on each of the gear bodies largely determines the motion of the driven gear.

When both gears in a mated pair are of the external type, the driving gear and driven gear (and their respective shaft or base component) rotate or move in opposite directions. If an application requires the input and output to rotate or move in the same direction, an idler gear (i.e., a gear placed between the driving gear and driven gear) is typically employed to change the direction of rotation of the driven gear.

Tooth Profile

The cross-sectional shape of a gear's teeth determines a range of performance parameters, including the speed ratio and experienced friction. While there are many other tooth profiles to choose from when designing and building gears, the three most common tooth shapes are involute, trochoid, and cycloid.

The involute curve of a circle is a locus created by the end point of an imaginary line tangent to the base circle as the line rolls around the circumference of the circle. Involute gear teeth follow this shape. Because of its ease of manufacture and smooth functioning, the involute tooth profile is used in the majority of gears manufactured in industry.

Unlike an involute curve, which is formed by a point at a fixed distance (a) from the center of a circle with a given radius (r) as the circle rolls along a straight line, a trochoid curve is formed by a point at a fixed distance (a) from the center of a circle with a given radius (r) as the circle rolls along a straight line. Trochoids, which contain cycloids, are a generic group of curves.

  • If a<r, the resulting curve is called a curtate cycloid.

  • If a=r, the resulting curve is a cycloid.

  • If a>r, the resulting curve is a prolate cycloid.

Gear axes configuration

The orientation of the axes—along which the gear shafts lie and around which the gears rotate—in relation to each other is referred to as a gear's axes configuration. Gears use three different axis configurations:

  • Parallel

Parallel configurations feature gears coupled to rotating shafts on parallel axes inside the same plane, as the name implies. Because the driving shaft (and driving gear) rotates in the opposite direction as the driven shaft (and driven gear), power and motion transfer efficiency is often excellent. Spur gears, helical gears, internal gears, and several rack and pinion gear variations are examples of gears that use parallel designs.

  • Intersecting

The gear shafts in intersecting configurations are on intersecting axes inside the same plane. This design, like the parallel configuration, provides great transmission efficiency. Bevel gears, which include miter, straight bevel, and spiral bevel gears, are one type of gear that uses intersecting arrangements. Intersecting gear pairs are commonly used to change the direction of motion in power transmission systems.

  • Non-parallel, non-intersecting

Shafts occur on axes that cross (i.e., are not parallel) but are not in the same plane in gear pairs with a non-parallel, non-intersecting arrangement (i.e., do not intersect). This arrangement, unlike parallel and intersecting layouts, has low motion and power transmission efficiency. Screw gears, worm gears, and hypoid gears are examples of non-parallel, non-intersecting gears.


This is just an introduction to gears. Follow Mech n Flow to know more on Gears and Transmission!

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