Coupling

This article describes a mechanical connection between two things. For other meanings, see Coupling (disambiguation).

Rotating coupling

A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. Couplings do not normally allow disconnection of shafts during operation, however there are torque limiting couplings which can slip or disconnect when some torque limit is exceeded.

The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end movement or both. By careful selection, installation and maintenance of couplings, substantial savings can be made in reduced maintenance costs and downtime.

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 separately 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

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, axial, and angular misalignment. Flexible couplings can accommodate angular misalignment up to a few degrees and some parallel misalignment. Examples:

• Universal joint or Cardan joint
• Oldham coupling
• Constant-velocity joint
  • Rzeppa joint
  • Double cardan joint
  • Thompson coupling
• Bellows coupling — low backlash.
• Elastomeric coupling
  • Bushed pin coupling
  • Donut coupling
  • Spider or jaw coupling (or lovejoy coupling)
• Resilient coupling
• Disc coupling, by definition, transmits torque from a driving to a driven bolt tangentially on a common bolt circle. Torque is transmitted between the bolts through a series of thin, stainless steel discs assembled in a pack. Misalignment is accomplished by deforming of the material between the bolts.
  • Rag joint
  • Giubo
• Diaphragm coupling transmits torque from the outside diameter of a flexible plate to the inside diameter, across the spool or spacer piece, and then from inside to outside diameter. The deforming of a plate or series of plates from I.D. to O.D accomplishes the misalignment.
• Gear coupling
• Hirth joint
• Roller chain and sprocket coupling
• Fluid coupling

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 rotation 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 gauges or feeler gauges 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.

See also

• Clutch
• Laser shaft alignment
• Compression fitting
• Piping and plumbing fittings

External links

• Shaft Coupling Glossary
• List of coupling types
• Laser Measurement Systems for Shaft & Machine Alignment
• Different types of flexible/shaft couplings