2. 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.
2.1 PURPOSE OF COUPLING: 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.
2.2 TYPES OF COUPLING:
Rigid Couplings
Flexible Or Compensating Couplings
2.2.1 RIGID COUPLING: A rigid coupling is a unit of hardware used to join two shafts within a motor or mechanical system. It may be used to connect two separate systems, such as a motor and a generator, or to repair a connection within a single system. A rigid coupling may also be added between shafts to reduce shock and wear at the point where the shafts meet.
2.2.2 FLEXIBLE COUPLING: . Flexible couplings are used to transmit torque transmit torque from one shaft to another when the two shafts are slightly misaligned. Flexible couplings can accommodate varying degrees of misalignment up to 3° and some parallel misalignment. In addition, they can also be used for vibration damping or noise reduction.
Fig 2.1
The aim of the flexible coupling is to transfer motion between two shafts on the same axis whilst accounting for possible misalignments.
2.2.2a FUNCTIONS OF FLEXIBLE COUPLING: The two basic functions of flexible coupling are,
To transmit power. To accommodate misalignment.
TRANSMIT POWER
Couplings are used primarily transfer mechanical power from one machine to another so that useful work can be done. To the coupling, the power is in the form of mechanical torque at operating speed, or work per unit of time, so the coupling is being “twisted” as it is spin at some high rpm. Torque is transferred into and out of the coupling at the equipment connection, usually by shaft fits or flange fits. MISALIGNMENT:
There are three types of misalignment and which the coupling must accommodate: angular, offset and axial. This is commonly caused by thermal growth of the machine casing and rotors as they change between ambient and operating temperatures. Angular misalignment is produced when the centre lines of the two equipment shafts the coupling connects are not parallel and they intersect at an angle. Offset misalignment commonly called parallel offset, is when the two shaft centerlines are parallel and they do not intersect, they are offset by some distance. Axial misalignment is caused by changes in the axial position of the shafts which moves the close to each other or further apart. 2.2.2b Types of Flexible Coupling:
The different types of flexible couplings are,
Flanged Pin Bush Couplings
Bibbly Coupling
Gear Tooth Coupling
Tyre couplings
Elastomeric Couplings – This consists of jaw type and S-flex couplings.
Oldham’s Coupling
Universal Coupling or Hooke’s Coupling
Bellows Coupling.
Flexible coupling are categorized as one of four types. They are
Mechanically flexible
Elastomeric
Flexible element
Miscellaneous
The general operating principles of the four b asic categories of couplings are as follows:
Mechanically flexible couplings: in general, these couplings obtain their
flexibility from loose fitting parts and /or rolling or sliding of mating parts.
Elastomeric couplings: In general, these couplings obtain their flexibility from
stretching or compressing a resilient material. Some sliding or rolling may takes place, but it is usually minimal.
Flexible element couplings: In general, the flexibility of these coupling is
obtained from the flexing of thin discs or diaph ragms.
Miscellaneous couplings: These couplings obtain their flexibility from a
combination of the mechanisms described above or through a unique mechanism. 2.2.2c BEAM COUPLING:
A beam coupling, also known as helical coupling, is a flexible coupling for transmitting torque between two shafts while allowing for angular misalignment, parallel offset and even axial motion, of one shaft relative to the other. This design utilizes a single piece of material and becomes flexible by removal of material along a spiral path resulting in a curved flexible beam of helical shape.
Fig 2.2
Increasing number of coils allows for greater angular misalignment
2.2.2d GEAR COUPLING:
A gear coupling is a mechanical device for transmitting torque between two shafts that are not collinear. 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 rotating splines with modified profiles. They are called gears because of the relatively large size of the teeth.
Fig 2.3 2.2.2d OLDHAM:
An Oldham coupling has three discs, one coupled to the input, one coupled to the output, and a middle disc that is joined to the first two by tongue and groove. The tongue and groove on one side is perpendicular to the tongue and groove on the other. The middle disc rotates around its center at the same speed as the input and output shafts. Its center traces a circular orbit, twice per rotation, around the midpoint between input and output shafts.
Fig 2.4a Oldham coupler, assembled
Fig 2.4b Oldham coupler, disassembled
