Fundamentals of machine design considers the concepts of design for each element separately such as shaft, bearing, pulley and gears. Since the number of parts is very large, the book is divided into two volumes. The language is direct and simple, so that every student can understand easily. Volume 1 of the book describes the basic knowledge needed for designing a part. Various types of stresses that appear due to load and the analysis to calculate the size of a part, which will work satisfactorily, are given. Designs of various types of permanent and temporary joints like riveted, welded, threaded, cotter etc. and some important parts such as shafts, keys, couplings, and springs are described.

Volume 2 has four units; covering design of drives, bearings, I.C. engine parts, and miscellaneous parts such as clutches, brakes, and pressure vessels.

Unit 1 describes different types of drives. Chapter 1 gives the design of belts and pulleys of various types. Chapter 2 is on the design of ropes used for large power transmission and hoisting applications. Chapter 3 describes the design of chain drives. One of the important methods of transmitting power is using gears and is discussed in detail. Chapter 4 describes the fundamentals of the gear design. Various types of gears such as spur, helical, bevel, and worm are described separately in chapters 5 to 8. Design of gear boxes using gear trains and epicyclic gears is explained in Chapter 9.

Unit 2 gives the design of two important types of bearings used in many applications. Design of slide bearings is given in Chapter 10 and rolling element bearings in Chapter 11.

Unit 3 is on the design of I.C. engine parts. Each important part is described as a separate chapter. Chapter 12 is on the design of cylinder, Chapter 13 on piston, Chapter 14 on connecting rod, and Chapter 15 on crank shaft. These engines require valves to control flow of air and exhaust. All parts in valve gear mechanism such as cam, its follower, push rod, rocker arm, and valves are described in Chapter 16. There is large variation in torque in these engines and hence to reduce the fluctuations in speed, flywheels are required, which are described in Chapter 17.

Couplings

Couplings are used to make a semi-permanent joint to transmit torque. These may be used for any of the following functions:

• a. Shafts are available in a maximum length of 7 meters due to transportation limitations. Hence, to increase the length of a shaft, couplings are used to join two shafts (Plate 20.1).

• b. The shafts may be aligned co-axially or could be misaligned also. Typical couplings are used for misaligned shafts.

• c. Transmission of angular shocks can be reduced by using rubber bushings in the couplings.

• d. These can be used as an overload protection device also. If the power transmitted exceeds beyond a certain limit, the key breaks to stop power.

In the design of couplings, generally the sizes are fixed by empirical relations and then these sizes are checked for strength in torsion, shear, crushing, etc.

Types of Couplings

Couplings can be classified as under:

1. Rigid couplings These are used to connect the shafts, which are in exact axial alignment. Following are the rigid type of couplings:

• a. Marine coupling

• b. Sleeve or Muff coupling

• c. Split muff coupling

• d. Half lap muff coupling

• e. Rigid flange coupling:

• • Unprotected rigid coupling (See Plate 20.1)

• • Protected rigid coupling

2. Flexible couplings These couplings can tolerate some amount of misalignment or offset without hampering power transmission. Following are the flexible types of couplings:

• a. Rubber bush coupling can tolerate some misalignment).

• b. Oldham's coupling can tolerate some amount of offset between the axes of the shafts.

• c. Universal coupling can transmit power at any angle.

Eccentric Loading

If the axis of the load does not pass through the centroid of the fasteners, the joint is called eccentrically loaded (Figure 17.1). A riveted joint is shown in the figure, whose centroid is at C. A load P passes at a distance e from the centroid of the rivets. Eccentric load can be for any type of joint and each is described in the following sections.

There can be two types of eccentricities as shown in Figure 17.1:

• a. Load is eccentric in plane of fasteners Figure 17.1(a). Plane of fasteners is shaded.

• b. Load is eccentric parallel and offset to plane of fasteners. See Figure 17.1(b).

Eccentrically Loaded Riveted Joints

Eccentric loads are analysed by first considering the load at the rivet section causing primary shear. Then the bending moment or the torque caused by the eccentric load is superimposed on the primary shear as secondary shear or tensile, as the case may be.

Eccentricity in plane of rivets

When the load is in plane of rivets it causes bending moment, which causes the primary shear stress. Figure 17.2 shows a bracket riveted to a column using six rivets. A load P acts at a distance e from the centroid C of the rivets. The design procedure of eccentric load is as follows: Step 1 Calculate direct primary shear stress. First consider the load at rivets, assuming each rivet shares load equally. The direction of this primary force is taken same as the direction of the load.

Variable Loads

Many machine parts are subjected to variable loads, that is, load varies in magnitude, as well as in its type, that is, tensile or compressive. Figure 10.1(a) shows a railway wheel rolling over the rail, which is supported on slippers. When the wheel is in between the slippers at Point C, the rail undergoes bending causing tension at its bottom, and when the wheel crosses and takes the position on the wooden slipper, the load on the rail at Point C is reduced. Thus, the load is changing with time.

Figure 10.1(b) shows a rotating shaft supported on two bearings at its ends with a load W in its middle. Points A and B are marked on the shaft in its position. Due to load, fibres of Point A will be in tension, while of Point B in compression. When the shaft turns by half revolution, Point A will take position on top and, hence, will be in compression, while Point B will be down and, hence, in tension. Thus, the stress varies from compression to tension and again to compression. Variable loads can be grouped as under:

a. Fluctuating loads Stress varies from some maximum value σmax to a minimum value σmin. There is some mean value of stress σm, which could be positive or negative, and some variable stress σa , as shown in Figure 10.2(a).

b. Repeated loads Stress varies from some maximum value σmax to zero and there is some mean stress σm, as shown in Figure 10.2(b).

Introduction

Screws are not only used for joining components, but also for transmission of power. Where these are used for power transmission, these are called as power screws. Their use can be seen in some of the following applications:

• • Lifting loads such as screw jack, toggle jack.

• • Transmission of power such as lead screw of a lathe.

• • Converting rotary motion to linear motion such as ball screw and nut.

• • Applying loads such as power press, vice, clamp, etc.

Types of Power Screws

Out of all the different types of threads described in Chapter 16, only square and acme threads are used for such applications due to their more strength than other screws. Square threads have square cross section of size half the pitch. Due to parallel surface of threads, it is difficult for the nut to engage and disengage. Acme threads become superior to square threads due to trapezoidal cross section with inclined surfaces forming an included angle of 29°. Owing to a bigger pitch than V threads, these threads are not cut by taps and dies and are generally cut on a lathe machine. Buttress threads are used for carpentry vice. The force is applied only in one direction while tightening. Diameters and pitch of square threads are given in Table 18.1.

Torque for Raising Load

When a screw is used to raise load, it can be considered analogues to a load being pushed on an inclined plane at helix angle ϕ The Helix angle of a screw is tan–1 (Pitch / mean circumference).