Couplings used to connect two shafts for torque transmission in varied applications. It may connect two units such as a motor and a generator or it may be to form a long line shaft by connecting shafts of standard lengths say 6-8m by couplings. Coupling may be rigid or they may provide flexibility and compensate for misalignment. They may also reduce shock loading and vibration. A wide variety of commercial shaft couplings are available ranging from a simple keyed coupling to one which requires a complex design procedure using gears or fluid drives etc. However there are two main types of couplings:
1) Rigid couplings
2) Flexible couplings
Rigid couplings are used for shafts having no misalignment while the flexible couplings can absorb some amount of misalignment in the shafts to be connected. In the next section we shall discuss different types of couplings and their uses under these two broad headings.
Types and uses of shaft couplings: -
Rigid couplings: - Since these couplings cannot absorb any misalignment the shafts to be connected by a rigid coupling must have good lateral and angular alignment.
Sleeve coupling: - One of the simple type of rigid coupling is a sleeve coupling which consists of a cylindrical sleeve keyed to the shafts to be connected. Normally sunk keys are used and in order to transmit the torque safely it is important to design the sleeve and the key properly. The key design is usually based on shear and bearing stresses. If the torque transmitted is T, the shaft radius is r and a rectangular sunk key of dimension b and length L is used then the induced shear stress.
Flange coupling: - It is a very widely used rigid coupling and consists of two flanges keyed to the shafts and bolted. Design details of such couplings will be discussed in the next lesson. The main features of the design are essentially.
(a) Design of bolts
(b) Design of hub
(c) Overall design and dimensions.
Oldham coupling: - These couplings can accommodate both lateral and angular misalignment to some extent. An Oldham coupling consists of two flanges with slots on the faces and the flanges are keyed or screwed to the shafts. A cylindrical piece, called the disc, has a narrow rectangular raised portion running across each face but at right angle to each other. The disc is placed between the flanges such that the raised portions fit into the slots in the flanges. The disc may be made of flexible materials and this absorbs some misalignment.
Universal joints :- These joints are capable of handling relatively large angular misalignment and they are widely used in agricultural machinery, machine tools and automobiles. There are many forms of these couplings, available commercially but they essentially consist of two forks keyed or screwed to the shaft. There is a center piece through which pass two pins with mutually perpendicular axes and they connect the two fork ends such that a large angular misalignment can be accommodated. The coupling, often known as, Hooke’s coupling has no torsional rigidity nor can it accommodate any parallel offset.
Pin type flexible coupling:- One of the most commonly used flexible coupling is a pin type flexible flange coupling in which torque is transmitted from one flange to the other through a flexible bush put around the bolt. These are used when excessive misalignment is not expected such as a coupling between a motor and a generator or a pump mounted on a common base plate. Detail design procedure for these couplings will be discussed in the next lesson.