What is the proper term for my toothed wheel

Sprockets, cogs, pulleys, and gears: How do I know which one I need?

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Over the river and through the woods to Grandmother’s house we go.

This snippet from a holiday classic details how we will travel any distance to be with our relatives during the holiday season. Although each member of the family is a descendant of the matriarch, we all have unique qualities that define our individuality. In the toothed wheel family, there are several descendants including cogs, pulleys, sprockets, and gears. Although similar, they are all distinctly unique in their design and use.

Figure 1: A cog is the simplest toothed wheel.

The simplest toothed wheel is a cog (Figure 1). It consists of a cylindrical disk with teeth that are geometrically shaped. The spacing of the teeth is set by the size of the tooth that the cog will mate with. As the simplest form of toothed wheel, cogs were used in many medieval mechanical designs. The advantage of a cog is that a fixed speed ratio can be developed. The disadvantages of cogs are that they do not permit smooth motion, they must be set at a fixed distance, and they do not operate as efficiently as other toothed wheels.

Figure 2: A sprocket.

A sprocket (Figure 2) is a toothed wheel that is specifically designed to work in tandem with a chain (Figure 3). The teeth on a sprocket are curved in order to mesh the pin diameter of the chain that it mates with. A sprocket can hold significantly more load than a cog of the same size and operates with a smooth motion. The material selection for the sprocket and chain will impact the maximum transmitted load. The disadvantages of sprockets are that there needs to be two sprockets and a connecting chain in order to transmit motion, and the possibility of slack in the chain leading to transmission losses. However, a chain can be sized to be any length which permits the sprockets to be fixed at any position. As with a cog drive, a chain drive will operate at a fixed speed ratio.

Figure 3: Sprocket and chain.
Figure 4: A pulley.

Pulleys (Figure 4) are a close relative to sprockets. Instead of operating with a chain like sprockets do, pulleys operate with belts (Figure 5). Pulleys with teeth are commonly known as timing pulleys. The tooth form of a timing pulley is the exact inverse of the tooth form on the belt. There are trapezoidal, curvilinear, and hybrid pulley tooth profiles. A pulley system load capacity is dependent only on the design strength of the belt. The material choice of the pulley has little impact on the load capacity of the design. Pulleys mesh with belts to create a smooth and quiet transfer of load. However, over time belts can stretch, resulting in the drive slipping. Like sprockets, pulleys have the disadvantage of needing two pulleys and a belt in order to transmit motion. Similar to chains, belts can be sized to be any length which permit the pulleys to be fixed at any position. As with a cog drive, a pulley drive will operate at a fixed speed ratio. 

Figure 5: A pulley and belt.

Gears (Figure 6) are the most refined version of a toothed wheel. The tooth form for most industrial gears is known as an involute. This tooth form allows for transmission of large loads and operates with a very smooth motion. As a direct drive system, gears are able to transmit the highest loads when compared to similar sized sprockets or pulleys. The disadvantages of gears are that, like cogs, they must operate at a fixed distance, and due to their ability to operate at high speeds, they must be lubricated. 

Figure 6: Gears are the most refined version of a toothed wheel.

Although each of these transmission components are toothed wheels, each style operates in a unique manner. A sprocket will not function with a belt, a gear will not work with a chain, and a pulley will not mate with another pulley. Each component is uniquely designed to transmit motion and will perform well when mated properly. 

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is general manager of KHK USA Inc, a subsidiary of Kohara Gear Industry with a 24-year history of working in the industrial automation industry. He is skilled in assisting engineers with the selection of power-transmission components for use in industrial equipment and automation. Dengel is a member of PTDA and designated as an intern engineer by the state of New York. He is a graduate of Hofstra University with a Bachelor’s of Science in Structural Engineering.