Types of Gears


This post is a reference for the different types of gears that might be relevant for robotics applications.

Mar 06, 2023
Table of Contents
Change Gearing Ratio
Internal Gear
Change Direction
Right Angle
Rotational to Linear

This post is a reference for the different types of gears that might be relevant for robotics applications.

Change Gearing Ratio

Spur

A cylindrical gear with straight teeth that run parallel to the axis of rotation.

A spur gear, used to change the gearing ratio between two axels.
Helical

A cylindrical gear with angled teeth that form a helix shape around the gear.

A helical gear. Like a spur gear but capable of handling higher torques.

Internal Gear

Planetary

A gear system in which multiple gears rotate around a central sun gear to transmit power and torque.

Gearing ratios for planetary gears typically max out around 10:1.

A planetary gear assembly.
Harmonic Drive / Strain Wave Generator

A harmonic drive is a type of gear system that uses an elliptical flexure to transfer motion between a wave generator and a flexible cup. It offers high gear ratios, high accuracy, and zero backlash, making it useful in applications where precision is important.

The gearing ratio for a harmonic drive can be expressed as r = E / (C - E) where E is the number of teeth on the (outer) circular ring gear and C is the number of teeth on the (inner) elliptical gear.

For example, a gear with 50 outer teeth and 52 inner teeth would have a gearing ratio of 50 / (52 - 50) = 50 / 2 = 25.

Note that the inner axel spins slower than the outer axel.

An example of a harmonic drive.
Cycloidal Drive

A cycloidal drive uses an eccentrically mounted cycloidal disc to transfer motion between a input shaft and a rotating output shaft. It offers high torque capacity and low noise, making it useful in industrial machinery.

The gearing ratio for a cycloidal drive can be expressed as r = L / (P - L) where P is the number of (outer) ring pins and L is the number of (inner) roller pins.

For example, a gear with 10 outer pins and 9 inner pins would have a gearing ratio of 9 / (10 - 9) = 9.

Note that the inner axel spins slower than the outer axel.

An example of a cycloidal drive.
Cluster

Two coaxial gears glued together. Simple enough to reason about.

An example of a cluster gear.

Change Direction

Bevel

A gear with angled teeth that intersect at a point and used to transfer motion between non-parallel shafts.

A bevel gear.

Right Angle

Worm

A gear with a screw-like shape that meshes with a helical gear and is used to transfer motion at a right angle. These are usually used for high torque applications because they have a big gearing ratio.

A worm gear.
Miter

A type of bevel gear that is designed to transfer power between two shafts that are at a right angle to each other.

A miter gear.
Hypoid

A bevel gear with non-intersecting axes and an offset between the shafts.

A hypoid gear.

Rotational to Linear

Rack and Pinion

A gear in which a flat bar (the rack) meshes with a gear (the pinion) to convert rotational motion into linear motion.

A rack and pinion gear.