16-11-2012, 05:17 PM
KINEMATICS OF MACHINERY
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BASICS OF MECHANISMS
Introduction:
Definitions : Link or Element, Pairing of Elements with degrees of freedom, Grubler‟s criterion (without derivation), Kinematic chain, Mechanism, Mobility of Mechanism, Inversions, Machine.
Kinematic Chains and Inversions :
Kinematic chain with three lower pairs, Four bar chain, Single slider crank chain and Double slider crank chain and
their inversions.
Mechanisms:
i) Quick return motion mechanisms – Drag link mechanism, Whitworth mechanism and Crank and slotted lever mechanism
ii) Straight line motion mechanisms – Peacelier‟s mechanism and Robert‟s mechanism.
iii) Intermittent motion mechanisms – Geneva mechanism and Ratchet & Pawl mechanism.
iv) Toggle mechanism, Pantograph, Hooke‟s joint and Ackerman Steering gear mechanism.
1. Terminology and Definitions-Degree of Freedom, Mobility Kinematics: The study of motion (position, velocity, acceleration). A major goal of understanding kinematics is to develop the ability to design a system that will satisfy specified motion requirements. This will be the emphasis of this class. Kinetics: The effect of forces on moving bodies. Good kinematic design should produce good kinetics. Mechanism: A system design to transmit motion. (low forces) Machine: A system designed to transmit motion and energy. (forces involved) Basic Mechanisms: Includes geared systems, cam-follower systems and linkages (rigid links connected by sliding or rotating joints). A mechanism has multiple moving parts (for example, a simple hinged door does not qualify as a mechanism). Examples of mechanisms: Tin snips, vise grips, car suspension, backhoe, piston engine, folding chair, windshield wiper drive system, etc.
Key concepts: Degrees of freedom: The number of inputs required to completely control a system. Examples: A simple rotating link. A two link system. A four-bar linkage. A five-bar linkage. Types of motion: Mechanisms may produce motions that are pure rotation, pure translation, or a combination of the two. We reduce the degrees of freedom of a mechanism by restraining the ability of the mechanism to move in translation (x-y directions for a 2D mechanism) or in rotation (about the z-axis for a 2-D mechanism). Link: A rigid body with two or more nodes (joints) that are used to connect to other rigid bodies. (WM examples: binary link, ternary link (3 joints), quaternary link (4 joints)) Joint: A connection between two links that allows motion between the links. The motion allowed may be rotational (revolute joint), translational (sliding or prismatic joint), or a combination of the two (roll-slide joint). Kinematic chain: An assembly of links and joints used to coordinate an output motion with an input motion. Link or element:
Linkage, Mechanism and structure:
A linkage is obtained if one of the links of kinematic chain is fixed to the ground. If motion of each link results in definite motion of the others, the linkage is known as mechanism. If one of the links of a redundant chain is fixed, it is known as a structure.
To obtain constrained or definite motions of some of the links of a linkage, it is necessary to know how many inputs are needed. In some mechanisms, only one input is necessary that determines the motion of other links and are said to have one degree of freedom. In other mechanisms, two inputs may be necessary to get a constrained motion of the other links and are said to have two degrees of freedom and so on.
The degree of freedom of a structure is zero or less. A structure with negative degrees of freedom is known as a Superstructure.