A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power.In machinery, a device for providing a connection, readily broken and restored, between two adjacent rotating shafts.A coupling may provide either a rigid or a flexible connection; the flexibility may permit misalignment of the connected shafts or provide a torsionally flexible (yielding) connection, mitigating effects of shock

Couplings do not normally allow disconnection of shafts during operation, though there do exist torque limiting couplings which can slip or disconnect when some torque limit is exceeded.

A common type of rigid coupling consists of two mating radial flanges (disks) that are attached by key-driven hubs to the ends of the shafts and bolted together through the flanges. Alignment of the shafts is usually achieved by means of a short cylindrical projection (rabbet joint) on the face of one flange that fits snugly into a circular recess on the face of the other flange.

The chain coupling consists of two hardened-steel sprockets, one on each shaft, with a nylon or metal roller chain wrapped around the closely aligned sprockets and connected at the ends. Clearances between the sprocket teeth and the chain allow for a small amount of shaft misalignment.

For connecting shafts whose axes intersect but are inclined to one another at a larger angle than a flexible coupling can accommodate, universal joints are used. The most common of these is the Hooke, or Cardan, joint, which consists of two yokes attached to the shaft ends and a cross-shaped connecting member.

Uses

Shaft couplings are used in machinery for several purposes, the most common of which are the following.

To provide for the connection of shafts of units that are manufactured separatelty such as a motor and generator and to provide for disconnection for repairs or alternations.

To provide for misalignment of the shafts or to introduce mechanical flexibility.

To reduce the transmission of shock loads from one shaft to another.

To introduce protection against overloads.

To alter the vibration characteristics of rotating units.

Types of shaft couplings

Drive couplings

A coupling is used to connect two in-line shafts to allow one shaft (driver) to drive the second shaft(driven) at the same speed. A coupling can be rigid or, more normally, it can be flexible allowing relative radial, axial or angular movement of the two shafts. Unlike the clutch the coupling transmission is not designed to engage-disengage as a normal operation

Coupling Type

Coupling type

Description

Rigid

Flange locked onto each shaft. One flange with recess and the other with matching spigot. Flanges bolted together to form rigid coupling with no tolerance for relative radial, angular or axial movement of the shafts.

Muff Coupling

Long cylindrical coupling bored and keyed to fit over both shafts. Split axially and clamped over both shafts with recessed bolts. Rigid coupling for transmitting high torques at high speeds

Beam Coupling

Single piece cylindrical coupling with a hole bored through its entire length. Each end bored to suite the relevant shaft. The helical slot is machined in the coupling in the central region. The reduces the coupling stiffness. The coupling is positive with some flexibility.

Pin

As rigid coupling but with no recess and spigot and the Bolts replaced by pins with rubber bushes. Design allows certain flexibility.

Flexible Rubber disc

As rigid coupling except that a thick rubber disc bonded between steel plates is located between the flanges. The plates are bolted to the adjacent coupling flanges.

Spider

Both half of the couplings have three shaped lugs . When the coupling halves are fitted together the lugs on one half fit inside the spaces between the lugs on the other side. A Rubber insert with six legs fits within the spaces between the lugs. The drive is by the lugs transmitting the torque through the rubber spider spacer… This coupling is only used for low power drives.

Bibby Coupling

The outer flanges of the two half couplings are serrated. A spring fits into the serrations connecting the two halves.

Chain Coupling

Flanges replaced a sprocket on each shaft. The coupling is by a duplex chain wrapped over both adjacent cnoupling.

Gear Coupling

Both coupling halves have a raised rim machined as an external gear. The sleeve which couples the two shafts comprises two halves bolted together, each half having a machine internal gear. This coupling requires lubrication. The coupling is capable of high speeds and high power capacity.

Metastream Coupling

Coupling halves connected via stainless steel diaphragms. High speed high torque capability with good dynamic balance. Single coupling will accommodate angular and radial misalignment and fitted in pairs also allows lateral misalignment.

Fluid Coupling

Based on both coupling halves having vanes within a housing containing viscous fluid. The rotation is transmitted from one side to the other via the viscous fluid. the coupling provides a soft start.

Universal Coupling

Coupling which allows large angle between drive halves(20-30o). Generally based on a yoke mounted on each shaft . Between to yokes is mounted a trunnion cross. Needle bearings are used at the bearing points between the cross and the yokes. These type or units are used in pairs on carden shafts. Uses widely on rear wheel drive vehicle propshafts

Universal Coupling-

Uni-Joint

Simplest type of coupling which allows large angle between drive halves. Each side of coupling includes protruding pins. The halves of the coupling are fastened in a pivotting assembly. At all angles up to about 40othe pins interlock with each other and rotation on one half forces the other half to rotate. Low power use only . Not smooth. Not reliable. Really only suitable for remote manual operations.

Rigid coupling

Rigid couplings are used when precise shaft alignment is required; shaft misalignment will affect the coupling’s performance as well as its life. Examples:

Sleeve or muff coupling

Clamp or split-muff or compression coupling

Flange coupling

Flexible coupling

Flexible couplings are designed to transmit torque while permitting some radial and axial and angular misalignment. Flexible couplings can accommodate angular misalignment up to a few degrees and some parallel misalignment. Examples:

Bushed pin type coupling

Universal coupling

Oldham coupling

Bellows coupling — low backlash.

Spider or jaw coupling — elastomeric inserts for flexibility, vibration reduction.

Thompson coupling

Resilient coupling

Disc coupling

Some applications like printing machines, roll forming machines, laminating machines, corrugated paper machines and paper making machines require an extreme and variable parallel offset. For these machines the coupling Schmidt-Kupplung is a solution.

Torque limiting coupling

Torque limiting couplings, or torque limiters, protect systems against overtorque conditions.

Requirements of good shaft alignment / good coupling setup

it should be easy to connect or disconnect the coupling.

it should transmit the full power from one shaft to other without losses.

it does allow some misalignment between the two adjacent shaft roation axis.

it is the goal to minimise the remaining misalignment in running operation to maximise power transmission and to maximise machine runtime (coupling and bearing and sealings lifetime).

it should have no projecting parts.

it is recommended to use manufacturer’s alignment target values to set up the machine train to a defined non-zero alignment, due to the fact that later when the machine is at operation temperature the alignment condition is perfect

Tools to measure shaft axis alignment condition

it is possible to measure the alignment with dial gages or feeler gages using various mechanical setups.

it is recommended to take care of bracket sag, parallaxe error while reading the values.

it is very convenient to use laser shaft alignment technique to perform the alignment task within highest accuracy.

it is required to align the machine better, the laser shaft alignment tool can help to show the required moves at the feet positions.

Coupling maintenance and failure

Coupling maintenance is generally a simple matter, requiring a regularly scheduled inspection of each coupling. It consists of:

Performing visual inspections, checking for signs of wear or fatigue, and cleaning couplings regularly.

Checking and changing lubricant regularly if the coupling is lubricated. This maintenance is required annually for most couplings and more frequently for couplings in adverse environments or in demanding operating conditions.

Documenting the maintenance performed on each coupling, along with the date.

Even with proper maintenance, however, couplings can fail. Underlying reasons for failure, other than maintenance, include:

Improper installation

Poor coupling selection

Operation beyond design capabilities.

The only way to improve coupling life is to understand what caused the failure and to correct it prior to installing a new coupling. Some external signs that indicate potential coupling failure include:

Abnormal noise, such as screeching, squealing or chattering

Excessive vibration or wobble

Failed seals indicated by lubricant leakage or contamination.

Checking the coupling balance

Couplings are normally balanced at the factory prior to being shipped, but they occasionally go out of balance in operation. Balancing can be difficult and expensive, and is normally done only when operating tolerances are such that the effort and the expense are justified. The amount of coupling unbalance that can be tolerated by any system is dictated by the characteristics of the specific connected machines and can be determined by detailed analysis or experience.