Though one may not think of gears as being flexible, gear couplings are very much considered to be a flexible coupling. A gear coupling is a mechanical device designed to transmit torque between two shafts that are not collinear. The coupling typically consists of two flexible joints, one fixed to each shaft. These joints are often connected by a third shaft called the spindle.
Each joint generally 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 splines with modified profiles. They are called gears because of the relatively large size of the teeth. Gear couplings are generally limited to angular misalignments of 4 to 5°.
Gear couplings ordinarily come in two variations, flanged sleeve and continuous sleeve. Flanged gear couplings consist of short sleeves surrounded by a perpendicular flange. One sleeve is placed on each shaft so the two flanges line up face to face. A series of screws or bolts in the flanges hold them together. Continuous sleeve gear couplings feature shaft ends coupled together and abutted against each other, which are then enveloped by a sleeve. Generally, these sleeves are made of metal, but they can also be made of Nylon.
Single joint gear couplings are 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 to 1/2°.