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Type of Couplings

Flexible Couplings

It is nearly impossible completely to eliminate all chance misalignment between the shafts of separately built machines, and such misalignment of shafts always leads to eventual failure of bearings or fatigue failure of shafts. The relative position of the connected shafts, inaccurate at the outset due to inevitable errors of manufacturing, is in course of time aggravated by deformations caused by the working load, temperature fluctuations, the uneven sinking of foundations or supports, etc. In such cases ineffective rigid couplings are replaced by flexible couplings. Therefore the purpose of a flexible coupling is to allow for imperfect alignment of two joining shafts, or to absorb impact from the fluctuation of torsional moment or of angular speed.

These couplings are rigid under torsion, but, allow the correction of errors in the alignment of drive shafts. They are widely used in the fields of robotics or automatisms, because they are excellent for very accurate system piloting. They are also known as "precision couplings"

Flexible Couplings may be split into two categories from the stand point of design.

1. Rigid Slip Coupling
2. Couplings With Incorporated Flexible Members

1. Rigid Slip Couplings :- Couplings whose flexibility is obtained kinematically by the use of rigid members in which constraint is absent in certain directions. This category of coupling can be used for any velocities and loads and cause comparatively small additional loads on shafts and bearings.

Oldham
Bellows Coupling
Universal Joint Couplings

1.1 Oldham Coupling

Oldham Couplings Oldham Coupling

Oldham can be used for any velocities and loads and cause small additional loads and bearings.
The special advantage of this coupling is that it can be used for shafts which are not in alignment but whose axes are parallel up to the extent of 0.05 times the shaft diameter. By using this type of coupling minor axial alignment of shafts can be compensated.

The coupling consists of two flanges keyed or secrewed to the respective shafts and the flanges have the slots on the face surface. A cylindrical piece called disc with a tongue running across each flat is present The tounges are at right angles to each other.

The Oldham joints allow rotational movement between two parallel shafts, placed near to each other.
(Parallel mis-alignment). It is made up of two hubs, and a disk, which has two grooves, at right angles to each other.
During rotation, the grooves in the hubs slide into the grooves in the disk, thus correcting the mis-alignment.

The result of this is that the drive shaft and driven shaft turn at the same speed. The transmission is then said to be 'homokinetic'
The attachment to the shafts is done in one of two ways with fixing screws at right angles to each other, or, by a clamp fitting.

In the case of fixing screws, a flat section must be machined into the shaft, when using a clamp fixing, the max. torque must be checked, (possible slipping of the shaft if the torque is too high in comparison to the shaft diameter.

1.2 Bellows Coupling

Bellows_Coupling

Bellows Couplings Bellows Coupling

These couplings link a torsional rigidity, high torque, mis-alignment and high rotational speeds. They are light in weight, they have a large range of applications and they are very good value for money.

They are made up of two hubs which are mounted on the shafts, and a bellows, which whilst being rigid under torsion, has the flexibility in a longitudinal sense to correct any mis-alignments. The bellows is welded onto the hubs. These couplings are capable of very high rotational speeds. At high speed, the least eccentricity would lead very high levels of vibration which could be detrimental to the system as a whole.

1.3 Universal Flexible Couplings/ Hooke's Couplings:- In cases where the angular or offset misalignment of the shaft reaches values which cannot be accommodated by flexible couplings, a different type, known as Hooke’s coupling or universal Flexible coupling can be applied to good advantage. It is named after Robert Hook , who developed the device. The universal coupling is used for two shafts whose center lines form a large angle , as 5 to 15 degrees, or even 30 degrees. The joint itself has no torsional flexibility nor can it accommodate any parallel misalignment.

This coupling is widely used in agricultural machinery, machine tools and automobiles. It consists of two fork mounted on the shaft end, either with the help keys or screwed on the shaft ends. There is a center piece through which pass two pins whose axes are at right angles to each other.

2. Couplings With Incorporated Flexible Member:- This type of coupling anticipates both misalignment and impact.

Slip Type
Jaw Type
Bushed Pin Type

2.1 Slip Type Flexible Coupling:- The object of slip coupling is to permit relative rotation, or slip , between the driving shaft and the driven shaft. A slip coupling is a safety device that prevents damage to rotating parts because of overloading . The slip coupling is adjusted so that it will begin to slip if the transmitted torsional moment exceeds a predetermined value. Usually the slip begins if the load exceeds by 10 to 20 percent the maximum load the shafts and other parts are designed.

2.2 Jaw Type Coupling

Three Jaw Inset Coupling
Three Jaw Insert Coupling

Multi Jaw Coupling

Three-Jaw Insert couplings are used to provide quieter running and to minimize vibration.

Multi Jaw Couplings, light duty couplings have spider-ring design with a special elastomer insert.

2.3 Bushed Pin Type:- The simplest and commonest type of coupling is the flexible rubber – bushed coupling. In the pin type coupling motion form one half flange is transmitted to the other half by mans of pins or bolts. The pins are rigidly bolted to one flange and loosely fitted in the corresponding holes of the other flange.

In the various designs this type of flexible coupling is extensively used, especially where the driving and driven units are mounted on a common base plate, where excessive misalignment is not likely, for example, a prime mover connected to a generator, a compressor connected to an electric motor, an electric motor connected to a centrifugal blower etc.

This couplings makes up for the parallel misalignment of shafts upto 0.5 mm, angular misalignment upto 1.5 degrees and axial displacement. This coupling can drive in either direction , absorb impulses, shocks and vibrations. The pins are covered by rubber, leather or some other suitable material. Since the driving pins are easily removable, either shaft may be removed without having to be shifted endwise. These types of couplings are made with capacities 12000 KW at 2,500 rev/mi. The coupling halves are mounted with axial clearance.

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