Chapter 11: Shaft Coupling Clutches – Machine Drawing with AutoCAD

Chapter 11

Shaft Coupling, Clutches

Chapter Outline

Couplings are necessary for power transmission from one shaft to another or from the drive shaft to the driven shaft. For domestic purposes, the pump and motor set is extensively used to deliver water from underground reservoirs to overhead tanks. It may be observed that there are two separate shafts, one emerging from a motor and the other from a pump, connected by means of some shaft coupling to transmit torque from the motor shaft to the pump shaft. The advantage of this kind of arrangement is that if there is a problem with any component (pump or motor), it is possible to separate the problem part from the coupling and repair it without bothering about the other part. Sometimes it is inconvenient to use a long shaft due to space limitation or mobilisation difficulties. A long shaft could be composed of a number of small shafts put together end to end by couplings and supported on bearings at the appropriate places.

In industry, various types of shaft couplings are used for different purposes. They can be broadly classified according to the kind of alignment and the deviation of the center lines of the shafts that are connected by them. There are two basic classes of coupling, namely i) rigid coupling and ii) flexible coupling.

 

FIG. 11.1   Alignment of shaft centerline

 

FIG. 11.2   Solid model of a flange attached to a shaft by means of key

RIGID COUPLING

As the name suggests, this coupling connects shafts in rigid alignment without allowing any relative movement between the two shafts. There could be three possible types of misalignment between the axes of the connecting shafts–lateral, angular, or their combinations. Lateral and angular shaft misalignments are shown in Fig. 11.2. Since rigid couplings do not accommodate misalignment of the shaft axes, they should not be used indiscriminately.

There are a variety of rigid couplings in use. Some common types are described below.

Flange Coupling

A flange coupling essentially consists of two cast iron flanges, each being keyed to the end of two shafts. A 3-D solid model view, drawn in AutoCAD, of one of the flanges is presented in Fig. 11.2. For a better under-standing of the com-ponent, a cut section of the component is shown. The flanges are connected by means of nuts and bolts as can be seen from the orthographic projection drawing (Fig. 11.3). The flanges are mounted on each end of the shafts to be coupled and are secured to the respective shafts by means of keys. In order to secure proper alignment, one of the flanges has a protruding portion (spigot) that fits into the recess of the other flange (socket). During manufacturing, care has to be taken to see that both the flange faces are at right angles to the axis of the shaft. Since the dimensions of the various parts of the coupling are standardised, for the purpose of drawing, the proportions as specified in Table 11.1 may be adopted.

 

FIG. 11.3   Flange coupling

 

A flange coupling may have some variations–straight or unprotected flange coupling, protected type flange coupling, and so on.

 

Table 11.1

Shaft diameter = d
Boss or hub diameter = 1.8 d to 2 d
Pitch circle diameter of the bolt = 2.6 d to 3
Bolt diameter = (0.432 d/n + 8) mm
Thickness of the flange = 0.5d
Length of the hub = 0.75 d to 1.25 d
No. of bolts, n = 3 for shaft diameter up to 40 mm
No. of bolts, n = 4 for shaft diameter above 40 mm to 100 mm
No. of bolts, n = 6 for shaft diameter above 100 mm to 180 mm
Fitting boss or spigot diameter = d + D/4 mm
Height, h = 4 to 10 mm
Clearance t c = 2 to 3 mm
Thickness of the protective flange = 0 .25 d (required for protective flange coupling)

Protected Flange Coupling

In this coupling, protective flanges, or shrouds, are provided to each flange to cover the bolt head and nut so that they do not pose any threat to the fingers and clothes of workers. The annular projection or shrouding of the flanges are clearly visible in the 3-D solid model of the one flange shown in Fig. 11.4a. The projection view, in half section, of the coupling is also illustrated in Fig. 11.4b. The correct alignment of the shafts are achieved through spigot and socket arrangement, as described in the previous section. This can also be achieved by protruding one shaft through its flange and inserting it partially into the bore of the other flange.

 

FIG. 11.4   Protected flange coupling

 

 

All the bolts should fit well into the reamed holes, without much clearance between them. This ensures more or less identical distribution of loads on each bolt. The proportions of different parts of the coupling may also be assumed from Table 11.1. Inspite of having many disadvantages of flange coupling, a protected flange coupling fulfils the condition of accurate and rigid connection between the shafts. Hence, this coupling is generally used for important machinery, such as, steam turbine generator sets, hydroelectric turbines, marine propeller shaft, and so on.

FLEXIBLE COUPLING

It is not possible to completely eliminate misalignment between the connecting shafts. If necessary precaution is not taken, this misalignment of shafts could lead to increased vibration and eventual failure of the bearings or the shaft. It is the function of flexible coupling to absorb the impact arising out of misalignment by incorporating flexible members. This type of coupling is employed where power transmission between shafts is moderate to heavy. Flexible couplings are made in a large variety of styles and principles of construction, some of which are described below.

Pintype Flexible Coupling

In this type of coupling, motion from one flange is transmitted to the other flange by means of pins or bolts. One end of the pin is rigidly fitted to one flange while the other end of the pin is connected to the other flange with flexible leather/rubber bushes shown in Fig. 11.5. The rubber bush can be designed to provide appropriate elasticity and damping for control of torsional vibration and to accommodate slight misalignment between the two shafts. The other parts of the coupling along with materials are mentioned in the part list included in Table 11.2. Pintype flexible coupling is extensively used where the driving and driven units are mounted on a common base plate, for example, electric motor connected to machines, prime mover connected to a generator, and so on. Thereby chances of excessive misalignment are not present. This kind of coupling allows a parallel misalignment of shafts upto 0.5 mm and angular misalignment upto 1.5 degree. The cross-section of the pins are designed in such a fashion that they will fail first during any overloading, thus acting as a mechanical fuse.

 

FIG. 11.5   Pintype flexible coupling

 

Table 11.2

Oldham Flexible Coupling

This type of coupling is particularly suitable for connecting shafts whose axes are parallel but not aligned. The extent of misalignment may be .06 times that of the shaft diameter. Fig. 11.6a illustrates two views of an Oldham coupling in an assembled condition. Two flanges (Fig. 11.6b) having grooves across their inner faces are keyed to the respective shafts. A circular disc (Fig. 11.6a) with two rectangular projecting parts (known as the tongue) on the opposite sides of the disc and at right angles to one another, is placed in between the two flanges so that the projecting parts fit into the corresponding recesses of the flanges. The circular disc is allowed to slide along the grooves of the flanges. The greater the shaft misalignment, the greater is the amount of sliding. Since frictional loss and wear and tear between the flanges is high, this type of coupling is rarely used now.

 

FIG. 11.6a   Orthographic views of Oldham flexible coupling

 

FIG. 11.6b   Flange and disc of Oldham coupling

UNIVERSAL COUPLING OR HOOKE'S JOINT

This coupling is useful to transmit power from one shaft to another when their axes intersect at an angle less than 30°. This coupling cannot allow any parallel misalignment. In other words, the shafts must intersect. One of the major advantages of this type of coupling is that the angle between the shafts may be varied within a certain range even when it transmits motion. The different components of the coupling are shown in Fig. 11.7. They are developed using the 3-D solid modelling feature of AutoCAD to help have a better understanding of its operating principle. In Fig. 11.8, a sectional view in assembled condition is also shown. It consists of a center cross-piece with holes to accommodate two pins having axes at right angles to each other. The two forks, attached to the pins, are keyed to the respective shafts to make a positive connection.

 

FIG. 11.7   Universal coupling

 

FIG. 11.8   Universal coupling

 

The cross-type Hooke's joint is extensively used at the ends of the driving shaft of rear wheel-drive automobiles and is shown in Fig. 11.9a. Another application of Hooke's joint, to transmit motion from one position to another in a machine tool, is shown in Fig. 11.9b.

 

FIG. 11.9   Universal coupling or Hooke's joint used in automobiles

CLUTCHES

The characteristic use of a clutch is to couple or disengage two shafts rotating at different speeds and bring the output shaft, up to the speed of the input shaft, smoothly and gradually. The major function of a clutch is to start and stop a machine or any rotating element without starting and stopping the prime mover that supplies the power. It is also employed for automatically disconnecting the machine or limiting the torsional moment transmitting to it. Based on the principle of engagement, clutches can be broadly classified into two categories: 1) positive contact clutches and 2) gradual engagement clutches or friction clutches.

Positive Contact Clutches

These clutches have interlocking engaging surfaces in the form of meshing jaws or teeth for a rigid mechanical junction. These clutches operate without any slip and heat generation. For comparable torque capacity, this clutch is smaller, lighter and less expensive than a friction clutch. However, they cannot be engaged at high speeds since gradual engagement between the driving and driven shaft is not possible.

There are various types of positive contact clutches depending on the design of the jaws; for example, ratchet type, spiral shaped, gear tooth shaped, and so on. A claw clutch with two flanges is shown in Fig. 11.10. One of the flanges is rigidly connected to the driving shaft through a taper key whereas the other is keyed to the driven shaft by a feather key so that it can slide on the shaft for easy engaging and disengaging.

 

FIG. 11.10   Claw coupling or clutch

Gradual Engagement Clutches

These clutches, also called frictional clutches, are extensively used in industry. Two opposing surfaces are forced into firm contact to transmit power from one shaft to another. During engagement, these clutches allow slip between the friction surfaces, thus enabling the drive to pick up and accelerate the load with minimum shock. Since they do not use teeth or jaws as a medium for power transmission, friction clutches can be used at high speed. They can be categorised into different classes depending on the type of friction surfaces being used. We shall discuss two of the friction clutches commonly used in the industry.

Cone friction clutch This type of clutch consists of a cup keyed to one of the shafts (the driving shaft) and a cone on the other shaft (driven shaft). The cone may slide axially on the key to make contact with the cup when necessary. A spring mounted on the driven shaft holds the clutch in engagement. Fig. 11.11 shows different components of a cone clutch.

 

FIG. 11.11   Schematic diagram of a cone clutch

 

Axial frictional clutch In this clutch, mating frictional members are moved in a direction parallel to the shaft.

Although the cone clutch was very popular for its design simplicity, it has been largely replaced by the disk clutch employing one or more disks as the operating members. The design of the disk clutch is compact, heat-dissipation is good, and it is free from centrifugal effects. As a result, disk clutches are widely used in the automobile industry from a two-wheeler to a truck.

 

FIG. 11.12   Automotive plate clutch

 

Fig. 11.12 shows an automotive type clutch. The flywheel, clutch cover, and pressure plate rotate with the engine crankshaft. There are a number of disk type plates that are placed alternately and attached to the driving member and the driven member of the clutch respectively. A series of circumferentially distributed springs are provided to force the pressure plate towards the flywheel, thus clamping the clutch plates of the driven shaft between the plates of the driving member. Motion is transmitted from the crankshaft to the driven shaft by means of friction. The clutch plates are mounted on the driven shaft through a spline. The engagement/disengagement of the clutch is controlled by depressing and releasing the clutch pedal. The release mechanism is also shown in the figure. This clutch has three driving surfaces and three driven surfaces. The disk clutch has the advantage of transmitting different amount of torsional moments by varying the number of disks of identical size. This helps in standardising the product for a range of operations with minimum design changes.

EXERCISE
  1. Draw two views with all necessary dimensions of a rigid shaft coupling to connect shafts, each having 50 mm diameter.
  2. Develop solid models of one of the flanges of a rigid coupling described in Question 1.
  3. Draw two views of a rigid coupling with protective flanges to connect two shafts of 60 mm diameter.
  4. How are shafts kept in correct alignment?
  5. What is the function of a shaft coupling? Why are different types of shaft couplings used in practice?
  6. Draw two views of a pintype flexible shaft coupling where the shaft diameter is 50 mm. One of the views should be in section.
  7. Develop the solid model of one of the flanges of a coupling described in Question 6. Generate the orthographic views of the flange from its solid model. Use the Layout option.
  8. What are the differences between a cotter joint and shaft coupling?
  9. Develop the solid model of the Oldham coupling shown in Fig. 11.6a.
  10. One of the views of universal coupling is shown in Fig. 11.8. Draw this view and add the top view.
  11. Two components of universal coupling are shown in Fig. 11.7. Develop the solid models and hence complete the 3-D assembly drawing of the coupling. Take the dimensions of the pin and other components from Fig. 11.8.
  12. What is a clutch? How many different types of clutches are available?
  13. Draw two views of a claw clutch shown in Fig. 11.10.
  14. Develop the solid models of the individual components of the claw clutch described in Question 13.
  15. Describe, with the aid of sketches, the working principle of a cone friction clutch.
  16. Draw a sketch of a multiple plate automotive clutch indicating various parts.
  17. Develop the solid model of the movable sleeve of the friction clutch shown in Fig. 11.11. Any dimension missing may be assumed proportionately.