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

By — 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).

Mechanical advantage formula:

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).

Mechanical advantage formula:

Revolutions Per Minute

As a general rule, if the number of revolutions per minute (rpm) of the driver gear is given and the driver gear is smaller than the driven gear, divide the gear ratio number of the driver gear into the rpm of the smaller gear (driver).

Example: In a two-gear system, the driver gear with 25 teeth revolves at 60 rpm; what is the rpm for the driven gear with 75 teeth?

Gear ratio formula:

Input movement (Driver): Output movement (Driven)

                                   3: 1

OR

The driver gear rotates 3 times faster than the driven gear (decrease in speed).

Divide the gear ratio number of the driver into the rpm for the driver gear.

(60 rpm/3) = 20 rpm (agrees with driver gear rotating 3 times faster than the driven gear)

This adheres to the general rule of gears, which states that when a smaller gear drives a larger gear the speed should decrease.

Another general rule is that if the driver rpm is given and the driver gear is larger than the driven gear, multiply the gear ratio number of the driven gear by the rpm of the larger gear (driver).

Example: In a two-gear system, the driver gear with 60 teeth revolves at 120 rpm; what is the rpm for the driven gear with 30 teeth?

Gear ratio formula:

Input movement (Driver): Output movement (Driven)

                                   2: 1

OR

The driver gear rotates 2 times slower than the driven gear (increase in speed).

Multiply the gear ratio number of the driven gear by the driver gear's rpm.

2 × 120 rpm = 240 rpm (agrees with driver gear 2 times slower than the driven gear)

This adheres to the general rule of gears, which states that when a larger gear drives a smaller gear the speed should increase.

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