Machine Design is the creation of new and better machines and improving the existing ones. A new or better machine is one which is more economical in the overall cost of production and operation. The process of design is a long and time consuming one. From the study of existing ideas, a new idea has to be conceived. The idea is then studied keeping in mind its commercial success and given shape and form in the form of drawings. In the preparation of these drawings, care must be taken of the availability of resources in money, in men and in materials required for the successful completion of the new idea into an actual reality. In designing a machine component, it is necessary to have a good knowledge of many subjects such as Mathematics, Engineering Mechanics, Strength of Materials, Theory of Machines, Workshop Processes and Engineering Drawing.
Classifications of Machine Design
The machine design may be classified as follows :
1. Adaptive design. In most cases, the designer’s work is concerned with adaptation of existing designs. This type of design needs no special knowledge or skill and can be attempted by designers of ordinary technical training. The designer only makes minor alternation or modification in the existing designs of the product.
2. Development design. This type of design needs considerable scientific training and design ability in order to modify the existing designs into a new idea by adopting a new material or different method of manufacture. In this case, though the designer starts from the existing design, but the final product may differ quite markedly from the original product.
3. New design. This type of design needs lot of research, technical ability and creative thinking. Only those designers who have personal qualities of a sufficiently high order can take up the work of a new design.
The designs, depending upon the methods used, may be classified as follows :
(a) Rational design. This type of design depends upon mathematical formulae of principle of mechanics.
(b) Empirical design. This type of design depends upon empirical formulae based on the practice and past experience.
(c) Industrial design. This type of design depends upon the production aspects to manufacture any machine component in the industry.
(d) Optimum design. It is the best design for the given objective function under the specified constraints. It may be achieved by minimising the undesirable effects.
(e) System design. It is the design of any complex mechanical system like a motor car.
(f) Element design. It is the design of any element of the mechanical system like piston,
crankshaft, connecting rod, etc.
(g) Computer aided design. This type of design depends upon the use of computer systems to
assist in the creation, modification, analysis and optimisation of a design.
General Considerations in Machine Design
Following are the general considerations in designing a machine component:
1. Type of load and stresses caused by the load. The load, on a machine component, may act in several ways due to which the internal stresses are set up.
2. Motion of the parts or kinematics of the machine. The successful operation of any machine depends largely upon the simplest arrangement of the parts which will give the motion required.
The motion of the parts may be :
(a) Rectilinear motion which includes unidirectional and reciprocating motions.
(b) Curvilinear motion which includes rotary, oscillatory and simple harmonic.
(c) Constant velocity.
(d) Constant or variable acceleration.
3. Selection of materials. It is essential that a designer should have a thorough knowledge of the properties of the materials and their behaviour under working conditions. Some of the important characteristics of materials are : strength, durability, flexibility, weight, resistance to heat and corrosion, ability to cast, welded or hardened, machinability, electrical conductivity, etc.
4. Form and size of the parts. The form and size are based on judgement. The smallest practicable cross-section may be used, but it may be checked that the stresses induced in the designed cross-section are reasonably safe. In order to design any machine part for form and size, it is necessary to know the forces which the part must sustain. It is also important to anticipate any suddenly applied or impact load which may cause failure.
5. Frictional resistance and lubrication. There is always a loss of power due to frictional resistance and it should be noted that the friction of starting is higher than that of running friction. It is, therefore, essential that a careful attention must be given to the matter of lubrication of all surfaces which move in contact with others, whether in rotating, sliding, or rolling bearings.
6. Convenient and economical features. In designing, the operating features of the machine should be carefully studied. The starting, controlling and stopping levers should be located on the basis of convenient handling. The adjustment for wear must be provided employing the various takeup devices and arranging them so that the alignment of parts is preserved. If parts are to be changed for different products or replaced on account of wear or breakage, easy access should be provided and the necessity of removing other parts to accomplish this should be avoided if possible. The economical operation of a machine which is to be used for production, or for the processing of material should be studied, in order to learn whether it has the maximum capacity consistent with the production of good work.
7. Use of standard parts. The use of standard parts is closely related to cost, because the cost of standard
or stock parts is only a fraction of the cost of similar parts made to order. The standard or stock parts should be used whenever possible ; parts for which patterns are already in existence such as gears, pulleys and bearings and parts which may be selected from regular shop stock such as screws, nuts and pins. Bolts and studs should be as few as possible to avoid the delay caused by changing drills, reamers and taps and also to
decrease the number of wrenches required.
8. Safety of operation. Some machines are dangerous to operate, especially those which are speeded up to insure production at a maximum rate. Therefore, any moving part of a machine which is within the zone of a worker is considered an accident hazard and may be the cause of an injury. It is, therefore, necessary that a designer should always provide safety devices for the safety of the operator. The safety appliances should in no way interfere with operation of the machine.
9. Workshop facilities. A design engineer should be familiar with the limitations of his employer’s workshop, in order to avoid the necessity of having work done in some other workshop. It is sometimes necessary to plan and supervise the workshop operations and to draft methods for casting, handling and machining special parts.
10. Number of machines to be manufactured. The number of articles or machines to be manufactured affects the design in a number of ways. The engineering and shop costs which are called fixed charges or overhead expenses are distributed over the number of articles to be manufactured. If only a few articles are to be made, extra expenses are not justified unless the machine is large or of some special design. An order calling for small number of the product will not permit any undue expense in the workshop processes, so that the designer should restrict his specification to standard parts as much as possible.
11. Cost of construction. The cost of construction of an article is the most important consideration involved in design. In some cases, it is quite possible that the high cost of an article may immediately bar it from further considerations. If an article has been invented and tests of hand made samples have shown that it has commercial value, it is then possible to justify the expenditure of a considerable sum of money in the design and development of automatic machines to produce the article, especially if it can be sold in large numbers. The aim of design engineer under all conditions, should be to reduce the manufacturing cost to the minimum.
12. Assembling. Every machine or structure must be assembled as a unit before it can function. Large units must often be assembled in the shop, tested and then taken to be transported to their place of service. The final location of any machine is important and the design engineer must anticipate the exact location and the local facilities for erection.
General Procedure in Machine Design
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.
The flow chart for the general procedure in machine design is shown
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.
The important properties, which determine the utility of the material are physical, chemical and mechanical properties. We shall now discuss the physical and mechanical properties of the material in the following articles.
Physical Properties of Metals
The physical properties of the metals include luster, colour, size and shape, density, electric and thermal conductivity, and melting point. The following table shows the important physical properties of some pure metals
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Physical properties of materials
Mechanical Properties of Metals
The mechanical properties of the metals are those which are associated with the ability of the material to resist mechanical forces and load. These mechanical properties of the metal include strength, stiffness, elasticity, plasticity, ductility, brittleness, malleability, toughness, resilience, creep and hardness. We shall now discuss these properties as follows:
1. Strength. It is the ability of a material to resist the externally applied forces without breaking or yielding. The internal resistance offered by a part to an externally applied force is called stress.
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 desirable for materials used in tools and machines. It may be noted that steel is more elastic than rubber.
4. Plasticity. It is property of a material which retains the deformation produced under load permanently. This property of the material is necessary for forgings, in stamping images on coins and in ornamental 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 ductile material must be both strong and plastic. The ductility is usually measured by the terms, percentage elongation and percentage reduction in area. The ductile material commonly used in engineering practice (in order of diminishing ductility) are mild steel, copper, aluminium, 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. Malleability. It is a special
case of ductility which permits materials to be rolled or hammered into thin
sheets. A malleable material should be plastic but it is not essential to be so
strong. The malleable materials commonly used in engineering practice (in order
of diminishing malleability) are lead, soft steel, wrought iron, copper and
aluminium.
8. Toughness. It is the property
of a material to resist fracture due to high impact loads like hammer blows.
The toughness of the material decreases when it is heated. It is measured by
the amount of energy that a unit volume of the material has absorbed after
being stressed upto the point of fracture. This property is desirable in parts
subjected to shock and impact loads.
9. 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.
10. 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.
11. Creep. When a part is
subjected to a constant stress at high temperature for a long period of time,
it will undergo a slow and permanent deformation called creep. This property is
considered in designing internal combustion engines, boilers and turbines.
12. 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 a progressive crack formation which are usually fine and of microscopic
size. This property is considered in designing shafts, connecting rods,
springs, gears, etc.
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.
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