Editing Coupling (section)

===Gear===
[[Image:Gear coupling.png|thumb|A gear coupling]]
A ''gear coupling'' is a mechanical device for transmitting [[torque]] between two shafts that are not [[Line (geometry)|collinear]]. It consists of a flexible joint fixed to each shaft. The two joints are connected by a third shaft, called the spindle.

Each joint consists of a 1:1 [[gear ratio]] internal–external [[gear]] pair. The tooth flanks and outer diameter of the external gear are crowned to allow for [[angular displacement]] between the two gears.  Mechanically, the gears are equivalent to [[rotating spline]]s with modified profiles. They are called gears because of the relatively large size of the teeth.

Gear couplings and [[universal joint]]s are used in similar applications. Gear couplings have higher [[torque density|torque densities]] than universal joints designed to fit a given space while universal joints induce lower [[vibration]]s. The limit on torque density in universal joints is due to the limited [[cross section (geometry)|cross sections]] of the cross and yoke. The gear teeth in a gear coupling have high [[backlash (gear)|backlash]] to allow for angular misalignment. The excess backlash can contribute to vibration.{{Citation needed|date=September 2020}}

Gear couplings are generally limited to angular misalignments, i.e., the angle of the spindle relative to the axes of the connected shafts, of 4°–5°.{{Citation needed|date=September 2020}}  Universal joints are capable of higher misalignments.

Single joint gear couplings are also used to connect two nominally coaxial shafts. In this application the device is called a gear-type flexible, or [[flexible coupling]]. The single joint allows for minor misalignments such as installation errors and changes in shaft alignment due to operating conditions.  These types of gear couplings are generally limited to angular misalignments of {{1/4}}°–{{1/2}}°.{{Citation needed|date=September 2020}}