Dynamics of Machinery Sample Notes
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Relation between motion and forces causing is a fascinating subject. This study is a generally referred as dynamic. Modern Engineering aims at analysing and predicting dynamics behavior of physical systems
Theory of Mechanisms & Machines is used to understand the relationships between the geometry and motions of the parts of a machine or mechanism and forces which produce motion.
TOM (M&M theory) is divided into two parts:-
1) Kinematics of Machinery: Study of motion of the components and basic geometry of the mechanism and is not concerned with the forces which cause or affect motion. Study includes the determination of velocity and acceleration of the machine members
2) Dynamics of Machinery: Analyses the forces and couples on the members of the machine due to external forces (static force analysis) also analyses the forces and couples due to accelerations of machine members ( Dynamic force analysis)
Deflections of the machine members are neglected in general by treating machine members as rigidbodies (also called rigid body dynamics). In other words the link must be properly designed to withstand the forces without undue deformation to facilitate proper functioning of the system.
In order to design the parts of a machine or mechanism for strength, it is necessary to determine the forces and torques acting on individual links. Each component however small, should be carefully analysed for its role in transmitting force.
The forces associated with the principal function of the machine are usually known or assumed.
a) Piston type of engine: gas force on the piston is known or assumed
b) QRM – Resistance of the cutting tool is assumed.
a & b are called static forces.
Example of other static forces are:
i. Energy transmitted
ii. Forces due to assembly
iii. Forces due to applied loads
iv. Forces due to changes in temperature
v. Impact forces
vi. Spring forces
vii. Belt and pulley
viii. Weights of different parts
Apart from static forces, mechanism also experiences inertia forces when subjected to acceleration, called dynamic forces.
Static forces are predominant at lower speeds and dynamic forces are predominant at higher speeds.
The analysis is aimed at determining the forces transmitted from one point to another, essentially from input point to out put point. This would be the starting point for strength design of a component/ system, basically to decide the dimensions of the components
Force analysis is essential to avoid either overestimation or under estimation of forces on machine member.
Under estimation: leads to design of insufficient strength and to early failure.
Overestimation: machine component would have more strength than required. Over design leads to heavier machines, costlier and becomes not competitive
Graphical analysis of machine forces will be used here because of the simplification it offers to a problem, especially in cases of complex machines. Moreover, the graphical analysis of forces is a direct application of the equations of equilibrium.
General Principle of force analysis:
A machine / mechanism is a three dimensional object, with forces acting in three dimensions. For a complete force analysis, all the forces are projected on to three mutually perpendicular planes. Then, for each reference plane, it is necessary that, the vector sum of the applied forces in zero and that, the moment of the forces about any axis perpendicular to the reference plane or about any point in the plane is zero for equilibrium..
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Unit 6 Governors Sample Notes
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Flywheel which minimizes fluctuations of speed within the cycle but it cannot minimize fluctuations due to load variation. This means flywheel does not exercise any control over mean speed of the engine. To minimize fluctuations in the mean speed which may occur due to load variation, governor is used. The governor has no influence over cyclic speed fluctuations but it controls the mean speed over a long period during which load on the engine may vary.
The function of governor is to increase the supply of working fluid going to the primemover when the load on the prime-mover increases and to decrease the supply when the load decreases so as to keep the speed of the prime-mover almost constant at different loads.
Example: when the load on an engine increases, its speed decreases, therefore it becomes necessary to increase the supply of working fluid. On the other hand, when the load on the engine decreases, its speed increases and hence less working fluid is required.
When there is change in load, variation in speed also takes place then governor operates a regulatory control and adjusts the fuel supply to maintain the mean speed nearly constant. Therefore, the governor automatically regulates through linkages, the energy supply to the engine as demanded by variation of load so that the engine speed is maintained nearly constant.
After studying this unit, you should be able to
• classify governors,
• analyse different type of governors,
• know characteristics of governors,
• know stability of spring controlled governors, and
• compare different type of governors.
Types of Governors
The governors may, broadly, be classified as
1. Centrifugal governors, and
2. Inertia governors.
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