Theory of simple mechanisms

Topics: Internal combustion engine, Connecting rod, Reciprocating engine Pages: 14 (4210 words) Published: October 23, 2013
Definitions and Terminology
1. Introduction
The subject Theory of Machines may be defined as that branch of Engineering-science, which deals with the study of relative motion between the various parts of a machine, and forces which act on them. The knowledge of this subject is very essential for an engineer in designing the various parts of a machine. 1.1 Sub-divisions of Theory of Machines

The Theory of Machines may be sub-divided into the following four branches:
1. Kinematics. It is that branch of Theory of Machines which deals with the relative motion between the various parts of the machines. 2. Dynamics. It is that branch of Theory of Machines which deals with the forces and their effects, while acting upon the machine parts in motion. 3. Kinetics. It is that branch of Theory of Machines which deals with the inertia forces which arise from the com­bined effect of the mass and motion of the machine parts 4. Statics. It is that branch of Theory of Machines which deals with the forces and their effects while the ma­chine parts are at rest. The mass of the parts is assumed to be negligible.

1.2 A machine is a de­vice which receives energy and transforms it into some use­ful work. A machine consists of a number of parts or bodies. In this chapter, we shall study the mechanisms of the various parts or bodies from which the machine is assembled. This is done by making one of the parts as fixed, and the relative motion of other parts is determined with respect to the fixed part.

2. Kinematic link or Element

Any part of a machine which moves relative to some other part is known as a kinematic link (or simply link) or an element. A link may consist of several parts, which are rig­idly fastened together, so that they do not move relative to one another. For example, in a reciprocating steam engine, as shown above, crank, connecting rod, piston, engine frame and main bearings are the four links.

A link or an element need not to be a rigid body, but it must be a resistant body. A body is said to be a resistant body if it is capable of transmitting the required forces with negligible deformation. Thus a link should have the following two characteristics:

1. It should have relative motion, and
2. It must be a resistant body
2.1 Types of Links

In order to transmit motion, the driver and the follower may be connected by the following three types of links:

2.1.1. Rigid link- A rigid link is one, which does not undergo any deformation while transmitting motion. Strictly speaking, rigid links do not exist. However, as the deformation of a connecting rod, crank etc. of a reciprocating steam engine is not appreciable, they can be considered as rigid links.

2.1.2. Flexible link -A flexible link is one which is partly deformed in a manner not to affect the transmission of motion. For example, belts, ropes, chains and wires are flexible links and transmit tensile forces only.

2.1.3. Fluid link- A fluid link is one which is formed by having a fluid in a receptacle and the motion is transmitted through the fluid by pressure or compression only, as in the case of hydraulic presses, jacks and brakes.

2.2 Structure
It is an assemblage of a number of resistant bodies (known as members) having no relative motion between them and meant for carrying loads having straining action. A railway bridge, a roof truss, machine frames etc., are the examples of a structure.

2.3 Difference between a Machine and a Structure (frame)

The following are the main differences between a machine and a structure

1. The parts of a machine move relative to one another, whereas the members of a structure do not move relative to one another. 2. A machine transforms the available energy into some useful work, whereas in a structure no energy is transformed into useful work. 3. The links of a machine may transmit both power and motion, while the members of a structure transmit forces only.

3. Kinematic pair...
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