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# Gear and Belt Drive Systems Study Guide for McGraw-Hill's Firefighter Exams

McGraw-Hill Professional
Updated on Jun 26, 2011

Gears are toothed wheels that are meshed together to transmit a twisting force (torque) and motion. They are usually attached to a shaft and can be considered a rotating lever. Utilizing leverage principles, gears can enhance or inhibit effort (force) or change effort (force) direction. Gears are either turned by a shaft or they turn the shaft. A large gear can apply more twisting force on a shaft to which it is attached than a smaller gear.

Gears that have straight teeth (perpendicular to their facing) and that mesh together in the same plane with axles parallel are known as spur gears. Spur gears provide an important way of transmitting a positive motion between two shafts. They give a smooth and uniform drive.

In the diagram, one gear, labeled the driver (also known as the driving gear) is turned by a motor. As it turns, it turns the other gear, known as the driven gear. A basic rule concerning gears states that each gear in a series of gears reverses the direction of rotation of the previous gear.

Another basic rule of gears is that when you have a pair of meshing gears, and the smaller gear with less pitch diameter (number of teeth) is the driver, torque output will be enhanced, with the trade-off being a decrease in speed of rotation. Conversely, when a larger gear with greater pitch diameter is the driver, torque output will be inhibited, but the trade-off is the speed of rotation will be enhanced.

Example: The driving (driver) gear has 9 teeth while the driven gear has 36 teeth. Find the gear ratio and mechanical advantage (torque) of this gear system.

Note: When computing gear ratio, always compare the larger gear rotating once to the smaller gear regardless of whether it is a driver or driven gear.

Gear ratio formula:

Input movement (Driver): Output movement (Driven)

1: 4

OR

The driver gear rotates 4 times faster than the driven gear (decrease in speed).

Example: The driving (driver) gear has 90 teeth while the driven gear has 15 teeth. Find the gear ratio and mechanical advantage (torque) of this gear system. Gear ratio formula:

Input movement (Driver): Output movement (Driven)

1: 6

OR

The driver gear rotates 6 times slower than the driven gear (increase in speed).

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