Flexible couplings couple or connect rotating members such as motors and drive shafts while allowing misalignment in either angular or parallel offset orientation.

Flexible coupling types include:

• Sliding block
• Metal contoured diaphragm
• Roller or silent chain
• Steel grid
• Coil spring
• Wafer or flexible disc
• Schmidt
• Beam or flexured (helical)
• Gear or multi-jaw
• Metal bellows
• Elastomeric tire
• Magnetic
• Flexible link

Types of Flexible Couplings

Sliding blocks are slotted or jawed flanges on shafts that transmit torque through a captured sliding disk or block. Metal contoured diaphragms transfer torque through contoured, metallic diaphragms and provide high horsepower and revolutions per minute (RPM).

Roller or silent chain couplings have sprockets on each shaft coupled by a chain or chains. They are useful for heavy-duty, high torque applications. Steel grids have slotted flanges on each shaft coupled by a flexible steel band that is laced through the slots. They provide good shock absorption.

Coiled spring flexible couplings couple shafts with a coiled spring and are suitable for instrument and low horsepower applications. Wafer or flexible disc couplings apply torque through sheet-metal plates mounted to shafts and intermediate plates by two-point supports. They are suitable for light- to moderate-duty applications.

Schmidt couplings transmit torque through a series of plates connected by links and allow large lateral misalignment. Beam, flexured or helical couplings prevent backlash and provide constant velocity with angular misalignment. With gear or multi-jaw couplings, gears with external teeth are mounted on shafts. A sleeve with internal teeth fits over the gears. These couplings provide high torque and allow axial misalignment, but usually needs lubrication and good shaft alignment.

Metal bellows are high speed couplings that prevent backlash and do not provide cyclic speed variation. They are designed for light-duty applications. Elastomeric tire-shaped flexible couplings are shaft hubs attached to either side of an elastomeric "tire.” They accommodate angular and parallel misalignment and end-float.

Magnetic couplings are permanent magnets mounted on shafts separated by nonmagnetic barrier. Torque ratings to 500 ft.-lb. can eliminate the need for seals. Flexible links are three cantilevered beams or links arranged in a triangle and are attached to hubs and torque sleeves. These flexible couplings can handle angular, parallel, axial misalignment. 

Mechanical Properties

• Rated torque is the maximum service torque for which the coupling is rated.
• Rated speed is the maximum rated rotational speed of the coupling.
• Torsional stiffness is expressed in torque per unit angular deformation (e.g., required torque to deform the coupling one degree, etc.).
• Backlash is the rotational position loss due to a direction change. It is measured in angular units such as degrees.

Dimensions

• Diameter - The bore diameter is the internal diameter for mating to the motor or shaft-end.
• Coupling diameter - The coupling diameter is the outside diameter (OD) of the coupling and includes the housing, etc. 
• Coupling length - The coupling length refers to the overall length of the flexible coupling.
• Design units - Design units can be specified in English or metric. Some suppliers may offer both.  

Selecting the Right Coupling for Your Application.

Video Credit: Stock Drive Products / Sterling Instrument

Alignment and Motion Parameters

• Angular misalignment tolerance is the maximum angular misalignment between coupled shafts that flexible couplings can accommodate.
• Parallel misalignment tolerance is the maximum parallel offset between shafts that couplings can accommodate.
• Axial motion allowed refers to the relative axial motion allowed by the coupler between shafts.
• Operating temperature is an important environmental parameter to consider when searching for flexible couplings.

Standards 

AGMA 9000 - Flexible couplings - potential unbalance classification.

BS 3170 - Specification for flexible couplings for power transmission.

DIN 740-2 - Power transmission engineering; flexible shaft couplings; parameters and design principles.

Image credits:

Lika Electronic | Zero-Max