Introduction, Types & Uses: Couplings | Design of Machine Elements - Mechanical Engineering PDF Download

Introduction 

Couplings are used to connect two shafts for torque transmission in varied applications. It may be to 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:

Rigid couplings
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
  5.1.2.1 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. The types of misalignments are shown schematically in figure5.1.2.1.1.

5.1.2.1.1 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. A typical sleeve coupling is shown in figure- 5.1.2.1.1.1.

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 τ ( figure- 5.1.2.1.1.2) in the key is given by

and for safety

where τ_y is the yield stress in shear of the key material. A suitable factor of safety must be used. The induced crushing stress in the key is given as

and for a safe design

where σ_c is the crushing strength of the key material.

The sleeve transmits the torque from one shaft to the other. Therefore if di is the inside diameter of the sleeve which is also close to the shaft diameter d (say) and d₀ is outside diameter of the sleeve, the shear stress developed in the sleeve is   and the shear stress in the shaft is given bySubstituting yield shear stresses of the sleeve and shaft materials for τ_sleeve and τ_shaft both di and d0 may be evaluated.

However from the empirical proportions we have:
d₀ = 2d_i + 12.5 mm and L=3.5d.
These may be used as checks.

Sleeve coupling with taper pins

Torque transmission from one shaft to another may also be done using pins as shown in figure-5.1.2.1.2.1.

The usual proportions in terms of shaft diameter d for these couplings are: d₀ = 1.5d, L = 3d and a = 0.75d. The mean pin diameter d_mean = 0.2 to 0.25 d. For small couplings d_mean is taken as 0.25d and for large couplings d_mean is taken as 0.2d. Once the dimensions are fixed we may check the pin for shear failure using the relation

Here T is the torque and the shear stress τ must not exceed the shear yield stress of the pin material. A suitable factor of safety may be used for the shear yield stress.

Clamp coupling

A typical clamp coupling is shown in figure-5.1.2.1.3.1. It essentially consists of two half cylinders which are placed over the ends of the shafts to be coupled and are held together by through bolt.

The length of these couplings ‘L’ usually vary between 3.5 to 5 times the and the outside diameter ‘d₀’ of the coupling sleeve between 2 to 4 times the shaft diameter d. It is assumed that even with a key the torque is transmitted due to the friction grip. If now the number of bolt on each half is n, its core diameter is d_c and the coefficient of friction between the shaft and sleeve material is μ we may find the torque transmitted T as follows: The clamping pressure between the shaft and the sleeve is given by

where n is the total number of bolts, the number of effective bolts for each shaft is n/2 and σ_t is the allowable tensile stress in the bolt. The tangential force per unit area in the shaft periphery is F = μ p. The torque transmitted can therefore be given by 

Ring compression type couplings

The coupling (figure-5.1.2.1.4.1) consists of two cones which are placed on the shafts to be coupled and a sleeve that fits over the cones. Three bolts are used to draw the cones towards each other and thus wedge them firmly between the shafts and the outer sleeve. The usual proportions for these couplings in terms of shaft diameter d are approximately as follows:

and the taper of the cone is approximately 1 in 4 on diameter.

Flange coupling 

It is a very widely used rigid coupling and consists of two flanges keyed to the shafts and bolted. This is illustrated in figure-5.1.2.1.4.2.

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.

Flexible coupling

As discussed earlier these couplings can accommodate some misalignment and impact. A large variety of flexible couplings are available commercially and principal features of only a few will be discussed here.

 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. A schematic representation is shown in figure5.1.2.2.1.1.

Universal joints 

These joints are capable of handling relatively large angular misalignment and they are widely used in agricultural machinery, machine tools and automobiles.A typical universal joint is shown in figure- 5.1.2.2.2.1.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. This is shown in the next lesson and is shown in figure-5.2.2.1. 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.