In designing a machine component, there is no rigid rule. The
problem may be attempted in several ways. However, the general procedure to
solve a design problem is as follows :
1. Recognition of need.
First of all, make a complete statement of the problem, indicating the need,
aim or purpose for which the machine is to be designed.
2. Synthesis (Mechanisms). Select the possible mechanism or group
of mechanisms which will give the desired motion.
3. Analysis of forces. Find the forces acting on each member of
the machine and the energy transmitted by each member.
4. Material selection. Select the material best suited for each
member of the machine.
5. Design of elements (Size and Stresses). Find the size of each
member of the machine by considering the force acting on the member and the
permissible stresses for the material used. It should be kept in mind that each
member should not deflect or deform than the permissible limit.
6. Modification. Modify the size of the member to agree with the
past experience and judgment to facilitate manufacture. The modification may
also be necessary by consideration of manufacturing to reduce overall cost.
7. Detailed drawing. Draw the detailed drawing of each component
and the assembly of the machine with complete specification for the
manufacturing processes suggested.
8. Production. The component, as per the drawing, is manufactured
in the workshop.
Engineering Materials
A) The
knowledge of materials and their properties is of great significance for a
design engineer. The machine elements should be made of such a material which
has properties suitable for the conditions of operation. In addition to this, a
design engineer must be familiar with the effects which the manufacturing
processes and heat treatment have on the properties of the materials.
Classification
of Engineering Materials
The engineering materials are mainly classified as :
1. Metals and their alloys, such as iron, steel, copper,
aluminium, etc.
2. Non-metals, such as glass, rubber, plastic, etc.
The metals may be further classified as :
(a) Ferrous metals, and (b) Non-ferrous metals.
The ferrous metals are those which have the iron as their main
constituent, such as cast iron, wrought iron and steel.
The non-ferrous metals are those which have a metal other than
iron as their main constituent, such as copper, aluminium, brass, tin, zinc,
etc.
property of material for designing a component?
Selection of a proper material for the machine component is one of
the most important steps in the process of machine design. The best material is
one which will serve the designed objective at minimum cost .Selection of
material by trial and error method. While selecting the material to follow the
factors
(1) Availability (2) Cost (3). Mechanical properties (4) Manufacturing
considerations.
Availability: The material should be readily available in
the market, in large enough quantities to meet the requirement.
Cost: For every application, there is a limiting cost beyond which the
designer cannot go. When this limit is exceeded the designee has to consider
other alternative materials. In cost analysis there are two factors, namely
cost of material and the cost of processing the material into finished goods.
It is likely that the cost of material might be low, but the processing may
involve costly machining operations.
Mechanical properties: These properties govern the selection of
materials. Depending upon the service conditions and the functional
requirement, different mechanical properties are considered and a suitable
material is selected.
Ex: connecting rod of I .C-Withstand fluctuating stresses due to
combination of fuel- Endurance strength criterion of design.
Piston rings resist wear- Surface hardness criterion of design.
Bearing material – have low coefficient of friction.
Clutch or brake lining -has high coefficient of friction.
Manufacturing considerations: The manufacturing processes
such as casting, rolling, forging, extrusion, welding and machining govern the
selection of the material.
Machine ability of material is an important consideration in
selection. Sometimes, an expensive material is more economical than a low
priced one. Which is difficult to machine. Where the product is of a complex
shape, casting properties are important.
Past experience is a good guide for the selection of material.
Various Properties of materials?
A)
1. Physical properties:
They include density, porosity, structure, Fusibility, Shape and size.
2. Mechanical properties:
They include strength, stiffness, elasticity, plasticity, ductility,
brittleness, creep, fatigue, hardness etc…
3. Magnetic properties:
They include thermal permeability and hysterics.
4. Thermal properties:
They include thermal conductivity, specific heat, latent heat and thermal
stresses.
5. Electrical properties:
They include dielectric strength, conductivity and resistively.
6. Chemical properties:
They include chemical composition, corrosion resistance, acidity and
alkalinity.
selection of materials for engineering purposes
The selection of a proper material, for engineering purposes, is
one of the most difficult problem for the designer. The best material is one
which serve the desired objective at the minimum cost. The following factors
should be considered while selecting the material :
1. Availability of the materials,
2. Suitability of the materials for the working conditions in
service, and
3. The cost of the materials.
Mechanical
properties of materials
A) The
mechanical properties of the metals are those which are associated with the
ability of the material to resist mechanical forces and load. Which undergo any
changes in shape and structure during the application of force on these
elements.
1. Strength: It is the ability of a material to resist the
externally applied forces without breaking or yielding.
2. Stiffness. It is the ability of a material to resist
deformation under stress. The modulus of elasticity is the measure of
stiffness.
3. Elasticity: It is the property of a material to regain its
original shape after deformation when the external forces are removed. This
property is disinable for materials used in tools and machines. It may be noted
that steel is more elastic than rubber.
4. Plasticity: It is the property of a material which retain the
deformation produced under load permanently. This property of the material is
necessary for forging, in stamping images on coins and in or non-metal work.
5. Ductility: It is the property of a material enabling it to be
drawn into wire with the application of a tensile force. A ductility is usually
measured by the terms percentage elongation and percentage reduction in area.
The ductile material commonly used in engineering practice are mild steel, copper,
aluminum, nickel, zinc, tin and lead.
6. Brittleness: It is the property of a material opposite to
ductility it is the property of breaking of a material with little permanent
distortion. Brittle materials when subjected to tensile loads snap off without
giving any sensible elongation. Cast iron is a brittle material.
7. Toughness: It is the property o a material to resist fracture
due to high impact loads like hammer blows. The toughness of the material
decreases when it is heated. This property is desirable in parts subjected to
shock and impact loads.
8. Malleability: It is a special case of ductility which permits
materials to be rolled or hammered into thin sheets due to compressive force. A
malleable material should be plastic but it is not essential to be so strong.
The malleable materials commonly used in engineering practice are lead, soft
steel, Wrought iron, copper and aluminum.
9. Creep: When a part is subjected to a constant stress at high
temp for a long period of time, it will undergo a slow and permanent
deformation called creep. This property is considered in designing I.C engine,
boilers and turbines.
10. Fatigue: When a material is subjected to repeated stresses it
fails at stresses below the yield point stresses. Such type of failure of a
material is known as fatigue. The failure is caused by means of progressive
crack formations which are usually fine and microscopic size. This property is
considered in designing shafts, connecting rod, springs, gears etc.
11. Machinability. It is the property of a material which refers to
a relative case with which a material can be cut. The machinability of a
material can be measured in a number of ways such as comparing the tool life
for cutting different materials or thrust required to remove the material at
some given rate or the energy required to remove a unit volume of the material.
It may be noted that brass can be easily machined than steel.
12. Resilience. It is the property of a material to absorb energy
and to resist shock and impact loads. It is measured by the amount of energy
absorbed per unit volume within elastic limit. This property is essential for
spring materials.
13. Hardness. It is a very important property of the metals and has
a wide variety of meanings. It embraces many different properties such as
resistance to wear, scratching, deformation and machinability etc. It also
means the ability of a metal to cut another metal. The hardness is usually
expressed in numbers which are dependent on the method of making the test. The
hardness of a metal may be determined by the following tests :
(a) Brinell hardness test,
(b) Rockwell hardness test,
(c) Vickers hardness (also called Diamond Pyramid) test, and
(d) Shore scleroscope.
The General Procedure explained for Machine Design is very helpful content, you can Learn Analysis of Mechanical Elements with Our Adaptable Online Videos Course Materials Video Lectures on Elemental Analysis from Superior Faculty.
ReplyDeleteU can short paragraph
ReplyDelete