Gear Couplings
gear coupling is a mechanical device for transmitting torque between two shafts that are not coaxial. 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 splines with modified profiles. They are called gears because of the relatively large size of the teeth.

Gear couplings and universal joints are used in similar applications. Gear couplings have higher torque densities than universal joints designed to fit a given space while universal joints induce lower vibrations. The limit on torque density in universal joints is due to the limited cross sections of the cross and yoke. The gear teeth in a gear coupling have high backlash to allow for angular misalignment. The excess backlash can contribute to vibration.

Single joint gear couplings are also used to connected 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.

Types of Couplings

  • Flexible Coupling:They couple or connect rotating members such as motors and drive shafts while allowing misalignment in either angular or parallel offset orientation. Coupling types can be sliding block, roller or silent chain. When searching for flexible couplings some mechanical properties are to be considered. These include rated torque, rated speed, torsional stiffness, and backlash. Rated torque is the maximum service torque for which coupling is rated. The rated speed is the maximum rated rotational speed of coupling. Stiffness is expressed in torque per unit angular deformation. Backlash is the rotational position loss due to a direction change.
  • Rigid Coupling: Connect or couple rotating members such as shafts. They secure onto both members and provide transmission of torque and motion. They do not allow for angular or parallel misalignment.

Mounting
There are many methods to mount couplings on a shaft. For motors with access to the rear end of the shaft, couplings can be pressed directly onto the shaft. Another method is to secure couplings to the shaft with a setscrew. The presence of a flat on the shaft helps prevent slipping. It can also be mounted with the help of adhesives. Adhesive should be applied to the couplings and not to the shaft directly. Couplings can be pinned by drilling a cross-hole across the shaft.