Gears

Types of Gears

Spur Gears

Inside the Globe motor gearhead.

A closeup of the gearhead, clearly showing 5 stages of spur gear reduction (the gearhead output shaft is disconnected) resulting in a 187.68:1 gear ratio.

Rack and Pinion

Bevel Gears

Helical Gears

Worm Drives

Planetary Gears

Ball Screw/Lead Screw

Harmonic Drive Gears

Spur Gear Geometry

For external spur gears (most common), the gear geometry is as shown in the figure below. The line of action is the line that passes through the intersection of the pitch circles and also tangent to the base circles.

Spur Gear Meshing and Forces

Gear meshing results in contacts with normals along the line of action, so the resultant force, $W$ is along this line. This results in both tangential and radial forces, $W_t$ and $W_r$, on the gear pair. When these gears are mounted on a shaft, the radial force causes a bending moment while the tangent force causes both a bending moment and a torque.

References

• Stock Drive Products/Sterling Instrument, http://www.sdp-si.com
• McMaster-Carr, http://www.mcmaster.com
• Harmonic Drive, LLC, http://www.harmonic-drive.com