Machine - Elements - Structure of Materials

Machine

A machine is a combination of resistant bodies, with successfully constrained relative motions, which is used for transforming other forms of energy into mechanical energy or transmitting and modifying available energy to do some particular kind of work. 

A heat engine is a machine. It receives heat energy and transforms it into mechanical energy. Some machines may receive electrical energy and convert it into mechanical energy. A motor is an example.

The majority of machines receive mechanical energy, and modify it so that the energy can be used for doing some a useful  task. Machine are designed for specific tasks. Common examples of such machines being hoist, lathe, screw Jack, etc. 

The transmission and modification of energy within the machine require the inclusion of a number of elements. They are to be designed to produce the desired motion so that the machine can perform its task successfully and carry with safety the forces to which they arc subjected. 

The analysis of forces involved and the design of machine parts, so that they can perform their duties without failure or undue distortion, lie within the province of machine design. In study of this subject  knowledge of mathematics, classical mechanics, strength of materials, mechanics of machines (theory of machines), metallography and technical drawing is to be used. Hence a review of the knowledge of these subjects is recommended for effective understanding of the further use of these subjects in machine design. The knowledge acquired in study of basic physics and chemistry also comes in handy. 

1-2. General considerations regarding material for the machine element:

Material choice for the machine elements is one of the first points to be decided at the start of the design process.  The choice of the material is governed by the following considerations:

(i) Suitability of the material of the component for working conditions during service

(ii) Amenability of the material to the fabrication processes required in making the component

(iii) Cost of the material in relation to selling price of the component.

The quantity required, delivery date, material availability and scrap utilisation are the other factors which determine the choice of the material in case of custom designs or design to satisfy a specific order. In the case of standard products, the most ideal materials are specified. Alternative materials and designs may be created so that in case that materials is not available, the other material can be used. This will give flexibility to the producing organization. 

Materials of construction are classified as metallic or non- metallic. Non-metallic materials include ceramics, glass, rubber, plastics, etc. 

The metals used more frequently in engineering are  classified under two main headings: (a) ferrous metals (b) non-ferrous metals. There are several sub-divisions in these main groups, which depend primarily upon the base metal and the alloying elements. The sub-divisions may be arranged in the order as given below: 

Ferrous Metals: 

Ferrous metals may be classified as follows:

Cast irons comprising grey cast iron, malleable cast iron, alloy cast iron, and chilled iron.

Wrought Iron & Steel comprising of wrought iron, carbon steel, alloy steel for engineering construction, high alloy steel and tool steel.

Non-ferrous Metals: 

Non-ferrous group may be divided into three main sections ;

Light metal group comprising aluminium and aluminium alloys, and manganese and magnesium alloys.

Copper-based alloys comprising copper, copper-zinc alloys or brasses and gilding metals, copper-zinc-nickel alloys or nickel- silvers, bronzes, copper- aluminium group or aluminium bronzes,
copper-lead- tin group giving leaded bronzes, copper- zinc or copper-silver-zinc group giving hard solders.

White-metal group comprising nickel silver, white bearing metals, nickel alloys, tin, white metal solders (soft solders), lead, type metal, zinc.

1-3. Structure of Materials:

To study the properties of materials and use them appropriately in selecting materials for machine elements,  it is necessary to understand the basic structures of them.

Structure of Metallic Materials:

The atoms of metals when they are brought together tend to arrange themselves in infinitesimal cubes, prisms and other symmetrical shapes. These geometrical units join to each other like perfectly fitted blocks and are embryos of the larger structure known as crystals. Crystals start forming when molten metal begins to solidify. As cooling continues, each tiny crystal grows by adding to itself other crystals in a pine-tree or dendritic fashion until each group of crystals touches every other group and the metal becomes solid. These groups of crystals are called grains. After abrasive polishing and etching the metal with an acid, these grains can be examined with a high powered microscope. 

Each grain consists of millions of tiny unit cells made up of atoms arranged in a definite geometric pattern (Refer Space Lattice,  Unit Cell and Close Packing in Crystalline Solids from Class XII Chemistry). 

Each unit cell may take the form of an imaginary cube, with an atom in each corner and one in the centre. This is called a body-centred cubic space lattice and is the structure of iron at normal temperature. 

If, however, the centre of the cube is vacant, and a single atom is contained in the centre of each face,  it is called a face-centred cubic structure. This is the structure of copper, aluminium and nickel. It is *also the structure of iron at elevated temperatures. 

When the unit cell takes the form of an imaginary hexagonal prism, having an atom in each corner, another at each of the top and bottom hexagons, and three atoms equally spaced in the centre of the prism  it is known as a close-packed hexagonal structure. This is the structure of magnesium, zinc and titanium. 

The distance between the atoms is extremely small. These closely spaced atoms have a tremendous attraction for each other. This attraction constitutes the force that resists any attempt to tear the metal apart. 

The metals used in practice are subjected to heavy stresses and strains. When the metal is deformed or cut certain rows of crystals slip or flow in fixed direction and in one or more parallel planes. Slippage occurs in those planes that have the greatest number of atoms. The ability of the slipping crystal to hold together makes the metal ductile. 

Body centred crystals have no planes of dense atomic concentration and so pure iron which has a body centered cubic structure is somewhat less ductile then pure aluminium, copper or nickel, which have face centred cubic structure. 

In pure metals the force that must be exerted to cause slippage is much less than the force that holds the crystals together and this is the reason for providing ductility to the material. The crystal structure of metals could be deformed by only a fraction of the stress needed to overcome the binding force between atoms in a crystal lattice. As such the crystal structures could be very strong under perfect condition but tiny imperfections would cause some misalignment of the atoms called dislocations which tend to weaken the crystaline structure which results into deformation of metals. 

There are two main types of dislocations — edge and screw. The edge dislocation occurs at the end of an extra half plane of atoms, while the screw dislocation corresponds to a partial tearing of the crystal planes. Most dislocations are combination of both types. 

The effect of alloying in metals and heat treatment of metals can be more thoroughly understood if the crystaline structure of the metal is understood in detail. The addition of alloying element in the parent metal changes the properties by forming new phases and by heat treating the alloy the dispersion of the phases or new phases formed at the boundaries of crystals or at the grains will give further changes in properties of the alloy. 

Non-metallic materials are usually characterized by ionic, covalent and intermediate bonding. They may exist as crystals, glasses or gels (a colloidal suspension) e.g. silica may be found in either of these three forms. Non-metallic materials are mostly brittle. The effects of impurity, locking of dislocations and the limited number of independent slip systems cause the randomly oriented polycrystalline forms of these materials to be brittle. They may be formed from the melt or they may be fabricated by sintering or by cementing powder particles. Although non-metallic materials are generally weak in tension, their strength in compression is often appreciable. 

The mechanical behaviour of polymers (plastics) is markedly influenced by their molecular structure. The degree of polymerization, branching and cross linking affect their strength. The strength and density of polymers can be increased by increasing their crystallinity. As polymers are heated they pass through five general states i.e. glassy, leathery, rubbery, viscous-rubbery and liquid. Polymeric materials are sometimes classified as thermoplastic or thermosetting, depending on their behaviour at elevated temperatures.

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