Angular Misalignment:

A type of shaft misalignment normally associated with a lack of flatness in the mounting surfaces, causing one shaft axis to be tilted relative to the other and creating a bend in the flexible coupling.

Axial Misalignment:

A type of shaft misalignment normally caused by end play or thermal expansion. In either case the coupled shafts are free to move relative to one another along their axes, creating tensile or compressive forces which are normally absorbed by the flexible coupling.


Free play, or clearance, between two interlocking mechanical parts, such as gear teeth, allowing for one component in the driveline to rotate freely without affecting the other. This results in "lost motion" in power transmission applications and can be problematic in positioning, reversing, or indexing drive systems, as well as in servo controlled applications including position feedback. Not to be mistaken with torsional deflection, backlash refers only to the permanent clearance and not to momentary twisting.

Bellows Coupling:

A type of shaft coupling basically comprised of two hubs or flanges with a flexible metal bellows joined between them. As with all flexible shaft couplings their intent is to transmit rotary power while absorbing misalignment loads between the coupled shafts. Often referred to as "servo couplings" they are specifically used for their high level of torsional stiffness and low moment of inertia, facilitating rapid and precise transmission of angle and torque.

Elastomer Coupling:

A type of flexible shaft coupling involving the use of elastic segments, usually rubber or plastic, to isolate the driving and driven hubs. In addition to transmitting rotary power and absorbing shaft misalignment loads, the elastic segments are intended to damp vibration between the coupled shafts. Many different layouts and materials exist to suit a wide variety of specific purposes.


In mechanical drive technology a flange normally refers to a bolted connection including some overlapping feature between the two adjoined surfaces, assisting with the proper centering of the two items being connected.

Flexible Shaft Coupling:

A device which is used to link two sections of shafting, facilitating the transmission of rotary power while absorbing misalignment loads between them.

Gear Coupling:

A type of flexible shaft coupling involving two hubs with gear teeth machined around the outside, which fit inside an internally geared connecting element. They compensate for shaft misalignment through the gear mesh of each hub.

Jaw Coupling:

A type of flexible shaft coupling involving interlocking jaws which are normally buffered by an elastic element made from rubber or plastic. They are known primarily for their durability and low cost.


A square or rectangular notch cut into a shaft hub in order to create a positive drive connection, whereby a square profile bar is used to transmit torque from the key seat of the shaft into the keyway of the hub.

Lateral Misalignment:

Also referred to as parallel shaft misalignment, this form of misalignment is usually the most stressful on couplings and bearings. It results from an offset between the shafts of the coupled equipment.

Line Shaft:

This term often refers to older styles of machinery which depend upon a single motor or prime mover, from which rotary power is distributed to individual machine axes via belts, gearboxes, shafts, and couplings. One of many examples is web handling systems, often found in printing and converting machinery, where right angle gearboxes are lined up parallel to the machine and linked to one another with line shafts, consisting of two flexible couplings and an intermediate drive shaft. The perpendicular shaft of each gearbox is then coupled to its respective roller shaft, while the entire system is driven by a single motor.

Magnetic Coupling:

A type of flexible shaft coupling wherein two magnetically attracted hubs are mounted to their respective drive components with an air gap between them. Torque is transmitted via the magnetic field.

Moment of Inertia:

A unit of measure used to describe the resistance of a rotating body to changes in its angular velocity. The value depends not only on the mass of the body, but also the distance the mass is concentrated from the central rotating axis, also known as the radius of oscillation. More energy is required to accelerate and decelerate objects with a larger moment of inertia.

Newton Meter (Nm):

A unit of measure for torque, using Newtons as the force value and meters as the lever arm length value.

Oldham Coupling:

A type of elastomer coupling involving a single tooth on each hub which freely slides inside a groove in the intermediate piece to compensate for misalignment during rotation.

Rigid Coupling:

A type of simple shaft coupling which is completely solid and is not flexible. They are often used in situations where a drive shaft is not supported by multiple bearing sets or when shafting is aligned with a very high degree of accuracy. Restoring forces can be very high with rigid couplings.

Safety Coupling:

A torque overload safety device wherein an input hub and output hub are able to transmit torque up to a specific maximum level, and either disconnect or slip beyond that level.

Servo Coupling:

A term used to describe flexible shaft couplings with special features to accommodate the high speed, rapid acceleration, and high precision requirements typical of many servo drive systems. They normally have zero backlash and a low moment of inertia.

Shaft Misalignment:

Differences in the angular, lateral, or axial positioning of two connected shafts, resulting from equipment manufacturing tolerances and error in the installation of rotating equipment. Shaft misalignment is inherent to many machinery designs and is normally kept to a minimum in the interest of durability and efficiency. Flexible shaft couplings are designed to compensate for varying levels of residual shaft misalignment while transmitting torque.


A measure of rotational force or moment about an axis, involving the combination of a force and a radius (e.g. Newton meters).

Torque Limiter:

A torque overload safety device wherein an input hub and output hub are able to transmit torque up to a specific maximum level, and either disconnect or slip beyond that level.

Torsional Deflection:

The twisting of a shaft, coupling, or other component under an applied torque load, typically measured in arc-seconds, arc-minutes, degrees, or radians.

Torsional Stiffness:

A unit of measure for the rate of torsional deflection of an object, expressed as a torque value and an angle (e.g. Nm/rad).