Fundamentals and Types of Mechanism

Definitions:

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.

5. Machine:

   It is defined as a combination of a number of links having relative motion between them so as to do some useful work by consuming some energy as input.

6. 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 examples of a structure.

7. Mechanism:

   When one of the links of a kinematic chain is fixed, the chain is known as a mechanism. It may be used for transmitting or transforming motion, e.g., engine indicators, typewriters, etc.

8. Inversion of Mechanism:

   The method of obtaining different mechanisms by fixing different links in a kinematic chain is known as inversion of the mechanism.

9. Kinematic Link:

   Each part of a machine, which moves relative to some other part, is known as a kinematic link.

10. Kinematic Chain:

    A kinematic chain may be defined as a combination of kinematic pairs, joined in such a way that each link forms a part of two pairs and the relative motion between the links or elements is completely or successfully constrained.

a) Machine and Structure

	Machine	Structure
1	Relative motion exists between its parts.	No relative motion exists between its members.
2	Links are meant to transmit motion and forces which are dynamic (both static and kinetic).	Members are meant for carrying loads or subjected to forces having straining actions .
3	Machines serve to modify and transmit mechanical work.	Structures serve to modify and transmit forces only.
4	Examples: shaper, lathe, screw jack, etc.	Examples: roof trusses, bridges, buildings, machine frames, etc.

a) Machine and Mechanism

	Machine	Mechanism
1	It transmits or modifies the available energy into some kind of desired work.	It only transmits or modify motion.
2	The machine is constructed or built on the skeleton of mechanism considering strength requirement for desired application.	It is a skeleton for any machine which is designed for getting the required motion.
3	The dimensions and shape are very important parameter.	The dimensions of link are not so important parameter while constructing mechanism.
4	e.g. Steam engine, reciprocating pump.	e.g. slider crank mechanism, pump mechanism.

List down types of kinematic chain.

Type of Kinematic Chain	Description
Four Bar Chain	It consists of four turning pairs.
Single Slider Crank Chain	It consists of three turning pairs and one sliding pair.
Double Slider Crank Chain	It consists of two turning pairs and two sliding pairs.

Kinematic Pair Definition and Types

Kinematic pair Definition	The two links or elements of a machine, when in contact with each other, are said to form a pair. If the relative motion between them is completely or successfully constrained (i.e., in a definite direction), the pair is known as a kinematic pair.
Types of Kinematic Pairs
1. According to the type of relative motion between the elements:	Sliding pair: When the two elements of a pair are connected in such a way that one can only slide relative to the other, the pair is known as a sliding pair. Turning pair: When the two elements of a pair are connected in such a way that one can only turn or revolve about a fixed axis of another link, the pair is known as turning pair. Rolling pair: When the two elements of a pair are connected in such a way that one rollover another fixed link, the pair is known as rolling pair/li> Screw pair: When the two elements of a pair are connected in such a way that one element can turn about the other by screw threads, the pair is known as screw pair. Spherical pair: When the two elements of a pair are connected in such a way that one element (with spherical shape) turns or swivels about the other fixed element, the pair formed is called a spherical pair
2. According to the type of contact between the elements:	Lower pair: When the two elements of a pair have a surface contact when relative motion takes place and the surface of one element slides over the surface of the other, the pair formed is known as lower pair. Higher pair: When the two elements of a pair have a line or point contact when relative motion takes place and the motion between the two elements is partly turning and partly sliding, then the pair is known as higher pair.
3. According to the type of closure:	Self-closed pair: When the two elements of a pair are connected together mechanically in such a way that only required kind of relative motion occurs, it is then known as self closed pair. The lower pairs are self closed pair. Force-closed pair: When the two elements of a pair are not connected mechanically but are kept in contact by the action of external forces, the pair is said to be a force-closed pair. The cam and follower is an example of force closed pair, as it is kept in contact by the forces exerted by spring and gravity.

Types of Inversion

Four bar chain	Single slider crank chain	Double slider crank chain
Coupled wheel of locomotive Pantograph Beam engine Watt straight line indicator mechanism	Rotary internal combustion engine Whitworth quick return mechanism Oscillating cylinder engine Crank and slotted lever quick return mechanism Pendulum pump	Scotch yoke mechanism Elliptical trammel Oldham's coupling

Types of Constrained Motions

Motion Type	Description
Completely constrained motion	When the motion between a pair is limited to a definite direction irrespective of the direction of force applied, then the motion is said to be a completely constrained motion.
Incompletely constrained motion	When the motion between a pair can take place in more than one direction, then the motion is called an incompletely constrained motion.
Successfully constrained motion	When the motion between the elements, forming a pair, is such that the constrained motion is not completed by itself, but by some other means, then the motion is said to be successfully constrained motion.

Working of Scotch Yoke Mechanism

This mechanism is used for converting rotary motion into a reciprocating motion. The inversion is obtained by fixing either link 1 or link 3. In the diagram, link 1 is fixed. When link 2 (which corresponds to the crank) rotates about point B as the center, link 4 (which corresponds to the frame) reciprocates. The fixed link 1 guides the frame.

Image credit: Wikipedia

Quick return mechanism of shaper and explain its working

This mechanism is mostly used in shaping machines, slotting machines and in rotary internal combustion engines. In this mechanism, the link AC (i.e. link 3) forming the turning pair is fixed, asshown in fig. The link 3 corresponds to the connecting rod of a reciprocating steam engine. The driving crank CB revolves with uniform angular speed about the fixed centre C. A sliding block attached to the crank pin at B slides along the slotted bar AP and thus causes AP to oscillate about the pivoted point A. A short link PR transmits the motion from AP to the ram which carries the tool and reciprocates along the line of stroke R1R2. The line of stroke of the ram (i.e. R1R2) is perpendicular to AC produced In the extreme positions, AP1 and AP2 are tangential to the circle and the cutting tool is at the end of the stroke. The forward or cutting stroke occurs when the crank rotates from the position CB1 to CB2 (or through an angle β) in the clockwise direction. The return stroke occurs when the crank rotates from the position CB2 to CB1 (or through angle α) in the clockwise direction. Since the crank has uniform angular speed,

Image credit: Wikipedia

Working Principle of Oldham's Coupling

An Oldham's coupling is used for connecting two parallel shafts whose axes are at a small distance apart. The shafts are coupled in such a way that if one shaft rotates, the other shaft also rotates at the same speed. This inversion is obtained by fixing the link 2, as shown in the figure. The shafts to be connected have two flanges (link 1 and link 3) rigidly fastened at their ends by forging. The link 1 and link 3 form turning pairs with link 2. These flanges have diametrical slots cut in their inner faces. The intermediate piece (link 4) is a circular disc with two tongues (diametrical projections) T1 and T2 on each face at right angles to each other. The tongues on the link 4 closely fit into the slots in the two flanges (link 1 and link 3). The link 4 can slide or reciprocate in the slots in the flanges. When the driving shaft A is rotated, the flange C (link 1) causes the intermediate piece (link 4) to rotate at the same angle through which the flange has rotated, and it further rotates the flange D (link 3) at the same angle, thus rotating the shaft B. Hence, links 1, 3, and 4 have the same angular velocity at every instant. There is a sliding motion between the link 4 and each of the other links 1 and 3. Applications: An Oldham's coupling is used for connecting two parallel shafts whose axes are at a small distance apart. Used to transmit motion and power.

Image credit: Occupational Safety and Health Administration part of the U.S. Department of Labor, Public domain, via Wikimedia Commons