INTRODUCTION OF COUPLING

COUPLINGS

In engineering applications there arise several cases where two shafts have to be connected so that power from driving shaft is transmitted to driven shaft without any change of speed. Such shafts are normally coaxial with slight or no misalignment and can be connected through devices known as couplings. Permanent couplings, often referred to as couplings, are the connectors of coaxial shafts and cannot be disengaged when shafts are running. On the other hand, those couplings which can be readily engaged or disengaged when driving shaft is running are termed as clutches. The power is transmitted when a clutch is engaged and not transmitted when clutch is disengaged. In this unit only permanent couplings will be considered. Figure 6.10 shows one such coupling connecting the shaft of an electric motor with the shaft of a worm and worm wheel reducer.

Several types of couplings are used in practice. A few are described here. 

Muff or sleeve coupling is shown in Figure 6.11. It is the simplest form of a permanent coupling, consisting of a steel or cast iron sleeve fitted on the ends of shaft to be connected. The Keys and Couplings sleeve is connected to the shaft by means of keys. The length of sleeve can be taken as (3.5 to 4) diameter of the shaft while the outer diameter of the muff or sleeve, D, is given by

D = 2 d + 13mm

L = 3.5 d

where d is the diameter of shaft in mm, , the thickness of the muff . However, the shear stress in the muss must be checked by treating it as a hollow shat of internal diameter d and external diameter D. The muff or sleeve coupling has the advantage of simple design and easy manufacture. However, need of perfect alignment of shafts is apparent and if not present the connection through a sleeve will induce bending stresses in the shafts. Yet another disadvantage is that while removing the sleeve must move on one of the shafts at least over a distance equal to half its length. This requires the shaft to be longer by this much amount.

The outer diameter of the muff, D, the length of the muff, L, and the bolt diameter db are the dimensions required to be determined for split muff coupling.

The dimensions of the key can be calculated by strength consideration or selected from standards. Such standards will be described later in this unit. it will be worthwhile to check compression force and consequent frictional torque which results from tightening of these bolts.

6.6.1 Flange Coupling

Flange coupling, as was mentioned earlier is used to connect two strictly coaxial shafts. One such coupling is shown in Figure. The two flanges are usually made in cast iron. These flanges are separately keyed to driving and driven shafts.

Image - Flange Coupling

Fig - Flange Coupling

Image - Protected Type Flange Coupling

Fig - Protected Type Flange Coupling

Image - Unprotected Type Flange Coupling

Fig - Unprotected Type Flange Coupling

The two flanges are identical in all respects except that one has a circular projection and other has a corresponding recess to make a register. When the two faces of flanges are brought in contact the projection fits into recess ensuring condition of coaxiality. The flanges are further connected through bolts placed near the periphery of the flanges. The faces of flanges are machine finished true right angled to the axis of shafts. The power may be transmitted by friction between the flange faces or by bolts in which case bolts will be subjected to shearing stress.

Flange couplings are often employed to transmit great torque and are largely dependable connections for shafts ranging in diameter between 18 mm to 200 mm. They are easily designed and manufactured.

Flange coupling normally refers to unprotected types as shown in Figure. The bolt head and nut, in this case are fully exposed and may present risk to operators. The bolt heads and nuts are often protected by providing cover in the flange on them as shown in Figure . This coupling is known as protected flange coupling.

While designing, the shaft diameter is calculated for transmission of torque, designated as d. The hub diameter of the flange may be calculated by treating the hub as hollow shaft but hub diameter D = 2d is often adopted and is found safe. The thickness of the flange may be calculated by considering it to be in shear along the circumference where it joins the hub. However, this thickness, t, is often taken as slightly greater than diameter of the bolt.