DME 1.ppt

MACHINE DESIGN

ASHISH AGARWAL R.V.C.E, BANGALOREDr P R Venkatesh, Mech Dept, RVCE, Bangalore

Dr P R Venkatesh, Mech Dept, RVCE, Bangalore

Engineering MaterialsAn engineering material is a material in the solid state whose properties are technologically useful.They can be metals or non metals.Metals: Iron, Steel, Brass, Copper, Lead, Tin, Aluminum, Titanium, Chromium, Nickel, etc.Non Metals:Plastics, Rubber, Metallic carbides and oxides, etc.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Mechanical PropertiesThe mechanical properties to be considered while selecting a material for a machine element are;Strength: Ability to resist failure or fracture Stiffness: Ability to resist deformationDuctility: Ability to undergo considerable plastic deformation before failure.Resilience: Energy absorbed in the elastic range.Toughness: Energy absorbed in the plastic range.Hardness: Resistance to indentation or abrasion.Corrosion resistance: Ability to resist atmospheric oxidation.Wear resistance: Ability to resist loss of material while in relative motion with other components.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Factors affecting selection of materialThe major factors considered are;Availability: The material should be readily available in the market, in large enough quantities to meet the requirement.Cost: The cost of material and the cost of manufacturing will be a limiting factor for the designer. Mechanical Properties: Different mechanical properties will be important for different types of loading & service conditions.Ex: Connecting rod should have high endurance strength Piston rings should have high wear resistanceClutch or brake lining should have high coefficient of friction



Factors affecting selection of material (contd)(iv) Manufacturing Considerations: The manufacturing processes such as Casting, Forging, Extrusion, Welding & Machining govern the selection of material.Ex: For high strength bolts, axles & shafts, free cutting steels (with small % of sulphur) are suitable as they have excellent machinability.For complex shapes, castability or ability of the molten metal to flow into intricate passages is the criterion.In fabricated assemblies of plates & rods, weldability becomes the governing factor.



Machine DesignMachine design is defined as the use of Scientific principles, Technical information and imagination in the description of a machine or mechanical system to perform specific functions with maximum economy & efficiency. Scientific Principles: Knowledge of physics, mathematics, statics & dynamics, vibrations & heat transfer, etc.Technical information: Information about fastening devices, chain, belt & gear drives, Springs, bearings.Imagination: Use of creative skills to produce a configuration.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Six phases of machine design (Shigleys Model)According to Shigley, the process of designing consists of six phases as follows;Recognition of needDefinition of the problemSynthesisAnalysis & OptimizationEvaluation Presentation



Recognition of need: It involves the realization by someone that a problem exists for which some corrective action should be taken. Definition of the problem: It involves a thorough specification of the item to be designed. Ex: Physical and functional characteristics, Cost, quality, & operating performance.Six phases of machine design (Shigleys Model)Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Synthesis It is the conceptualization of the component by the designer. Analysis: It is the iterative process of Improvisation of design and redesign till the optimized design is achieved within the constraints imposed by the designer. Six phases of machine design (Shigleys Model)Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Evaluation: It is concerned with measuring the design against the specifications established at the problem definition phase. This often requires fabrication and testing of a prototype to assess operating performance, quality, reliability, etc.Presentation: It includes documentation of the design by means of drawings, material specifications, assembly lists, etc.Six phases of machine design (Shigleys Model)Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Types of DesignThere may be several types of design such as;Adaptive design: This is based on existing design, for example, standard products or systems adopted for a new application. Conveyor belts, control system of machines and mechanisms or haulage systems are some of the examples where existing design systems are adapted for a particular use.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Types of DesignDevelopmental design:Here we start with an existing design but finally a modified design is obtained. A new model of a car is a typical example of a developmental design .New design: This type of design is an entirely new one and requires creative thinking to solve a problem. Examples of this type of design may include designing a small vehicle for transportation of men and material on board a ship or in a desert. Some research activity may be necessary.



Types of Design based on methodsRational design:This is based on determining the stresses and strains of components using mathematical formulae of principle of mechanics and thereby deciding their dimensions.Empirical design: This is based on empirical formulae which in turn is based on practice & past experience. Industrial design: These are based on industrial considerations and norms viz. market survey, external look, production facilities, low cost, use of existing standard products.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Factors to be considered in machine designThere are many factors to be considered while tackling a design problem. What device or mechanism to be used? This would decide the relative arrangement of the constituent elements.MaterialForces on the elementsSize, shape and space requirements. The final weight of the product is also a major concern.The method of manufacturing the components and their assembly.How will it operate?Reliability and safety aspectsInspectabilityMaintenance, cost and aesthetics of the designed product.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Standards and Codes A standard is a set of specifications for parts, materials or processes intended to achieve uniformity, efficiency and a specified quality.One of the important purposes of a standard is to limit the variety, sizes & shapes.A code is a set of specifications for analysis, design, manufacture and construction of something.The purpose of a code is to achieve a specified degree of safety, efficiency, and performance.



Standards and Codes The organizations listed below have established standards and design codes;American Gear Manufacturers Association (AGMA)American National Standards Institute (ANSI)American Society of Mechanical Engineers (ASME)American Society of Testing & Materials (ASTM)British Standards Institution(BSI)Deutsches Institut fur Normung(DIN)Indian Standards(IS)International Standards Organization(ISO)Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Design against Static LoadA static load is defined as a force that is gradually applied to a mechanical component and which does not change in magnitude & direction w.r.t time.A ductile material will undergo a considerable amount of plastic deformation before necking. (cup & cone)Ex: Steel, Aluminum, Copper, etc.A brittle material undergoes little amount of plastic deformation prior to fracture.Ex: Cast Iron, Glass, Concrete.



Selection of value for factor of safety The common range for factor of safety is 1.25 to 4 for metals & 5 to 10 for concrete, wood, etc.The selection of an appropriate value of FOS is based primarily on the following factors;Degree of uncertainty about the loading.Degree of uncertainty about the material strength.Imperfect workmanship which leads to variations in dimensions of components.Human safety & economics.



STRESSES IN MACHINE ELEMENTSStress in a machine member is defined as the intensity of internally distributed forces that resist the external forces acting on that element.The unit commonly used is Mega Pascal (MN/m2) or 106N/m2 which is numerically equal to N/mm2.1 Pascal = 1N/m21 Kpa (Kilo Pascal) = 1000 N/m21 Bar = 100 Kpa=105 N/m21MPa (Mega Pascal) = 106 N/m2=1 N/mm21GPa (Giga Pascal)=109 N/m2=1000 N/mm2



Mechanical properties of metalsCAST IRON: (2 to 4% carbon): It is classified on the basis of distribution of carbon content in the microstructure. For example, FG 300 indicates it is cast iron with carbon in the form of Free Graphite flakes and its ultimate tensile strength if 300 MPa. SG 450 (Spheroidal graphite cast iron) has tensile strength of 450 MPa.For properties of Cast iron, refer Table 1.2 & 1.3, page 413. (Design Data book by Mahadevan)



Mechanical properties of metalsSTEELS:Low Carbon Steels (Or Mild Steel): < 0.3 % carbonMedium carbon Steel : 0.3 to 0.5% carbonHigh carbon Steels: (0.5 to 1% Carbon)It is designated by, 100 times the percentage of carbon, the letter C, followed by 10 times the percentage of manganese. For example, 40C8 or C 40 indicates the average carbon percentage is 0.4% and average percentage of manganese is 0.8%. For properties of carbon steels, refer Table 1.8, page 418 & 419. (Design Data book by Mahadevan)



Rectangular cross section member under axial tensionDr P R Venkatesh, Mech Dept, RVCE, Bangalore


Prob 1:A link shown in fig is required to transmit a tensile load of 60 KN. Determine the stresses induces at sections 1-1 & 2-2.



Prob 2:A steel rod of circular cross section and length 200 mm is subjected to a compressive load of 40 KN. If the safe stress in rod material is 80 MPa, determine;Diameter of the rodContraction of the rodTake the modulus of elasticity of the material as 207 GPa.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Machine Elements subjected to lateral bendingWhen a beam is subjected to a load in the transverse direction, it is subjected to pure bending moment.It results in tensile stresses on the convex side and compressive stresses on the concave side and zero stress on the neutral layer.



Prob 1:A short cantilever beam is of width 20 mm and depth 40 mm supports a transverse load of 2 KN at a distance of 200 mm from its fixed end. Determine the maximum bending stress in the beam.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Prob 2:A circular cross section beam of 50 mm diameter and 1m length is supported between bearings. Determine the bending stress induced in the beam when it is subjected to a 5 KN transverse load at its center.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Prob 3:A steel saw blade 1 mm thick is bent into an arc of a circle of 500 mm radius. Determine the flexural stress induced and the bending moment required to bend the blade if it is 15 mm wide. Take E=210 Gpa.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Machine Elements subjected to TorsionWhen a rod or shaft is subjected to a twisting moment about its axis, it is subjected to pure Torsion.It results in torsional shear stresses which vary linearly with the radius of the shaft.



Prob 1:A circular rod of 50 mm diameter is subjected to a twisting moment of 1 KN-m. Determine the shear stress induced in the shaft & the angular twist per unit length of the shaft. Take rigidity modulus as 84 Gpa.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Machine Elements subjected transverse shearWhen a component is subjected to a force perpendicular to its axis, so as to shear it at one or more cross sections, it is said to be under transverse shear.The shear stress in case of single shear is given by

In case of double shear,

Where F is the shear load and A is the area of resisting cross section.



Prob 1:A knuckle joint is used to connect shafts of diameter D to transmit a tensile load of 5 KN. Determine the diameter of the shaft and the diameter of the pin required using permissible tensile stress in the shaft as 60 MPa and the shear stress in the pin as 40 MPa.A knuckle joint is a mechanical joint used to connect two rods which are under a tensile load, when there is a requirement of small amount of flexibility, or angular moment is necessary. Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Prob 2: Determine the diameter of the rivet subjected to(i) Single shear and (ii) Double shear due to a load of 5 KN. The permissible shear stress in the rivet is 40 MPa.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Machine Elements subjected to eccentric loadingMany structural and machine members are subjected to loads that are offset from their centroidal axes.Such members may be considered as elements subjected to an axial load coupled with a bending moment. The stresses may then be added algebraically to evaluate the total stresses at the critical sections.

Fig 1.4, Sl no 5 page 13 DDHB MahadevanDr P R Venkatesh, Mech Dept, RVCE, Bangalore


Prob 1:A steel bracket of rectangular cross section is loaded as shown in fig. Determine the width b at section A-A by limiting the tensile stress to 80 N/mm2.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Prob 2:Determine the cross sectional dimensions for the wall bracket shown in fig if the maximum stress in the material of the bracket is limited to 120 MPa. Take the depth of the section equal to three times its width.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Prob 3: A C frame is subjected to a force of 15 KN as shown in fig. It is made of grey cast iron FG 300 and the factor of safety is 2.5. Determine the dimensions of the cross section of the frame.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Prob 4: Determine the normal stresses at the extreme fibers of the cross section of a C-clamp loaded as shown in fig.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Prob 5: For the member shown in fig, determine the maximum load F that can be applied if the allowable compressive stress at cross section A-A is 100 MPa.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Machine Elements subjected to Combined LoadingMany structural and machine members are subjected to loads that are combinations of axial, bending, & torsional types.Such members may be modeled as elements subjected to uniaxial, biaxial & triaxial loadings with or without shear loads.The principal stresses and the maximum shear stress may be found and these values may be used in the various theories of failure for design of the machine members.



Also the expressions for maximum & minimum normal stresses (Principal stresses) and max shear stress Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Prob 1: A point in a structural member is subjected to plane stress as shown. Determine the following;Principal stressesMaximum shear stressDr P R Venkatesh, Mech Dept, RVCE, Bangalore


Prob 2: A circular rod of diameter 60 mm and length 200 mm is fixed at one end. The free end is subjected to a transverse load of 6 KN and a torque of 400 N-m. Determine the stresses at the critical points.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Prob 3: A circular rod of diameter 60 mm and length 200 mm is fixed at one end. The free end is subjected to an axial load of 10 KN, a transverse load of 6 KN and a torque of 400 N-m. Determine the stresses at the critical points.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Prob 3: Determine the maximum normal stress and the maximum shear stress at section A-A for the overhung crank shown in fig. when a load of 12 KN is assumed to be concentrated at the center of the crank pin.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Prob 4: Determine the maximum normal stress and the maximum shear stress at section A-A for the overhung crank shown in fig. Neglect the effect of transverse shear.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Here, the bending moment arm remains the same while the torque arm becomes cos component of the crank radius as the line of action of force is inclined to the horizontal.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Prob 5: A 50 mm diameter steel rod supports 9 KN load in addition to a torsional moment of 100 Nm as shown in fig. Determine the maximum tensile and maximum shear stress.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Theories of failureThe design of machine parts subjected to combined loads should be related to experimentally determined properties of material under similar conditions.However, it is not possible to conduct such tests for different combination of loads and obtain mechanical properties.In practice, the mechanical properties such as yield strength, ultimate strength and percentage elongation are obtained from a simple tension test.



Theories of failureTheories of failure provide a relationship between the strength of the machine component subjected to complex state of stress with the mechanical properties obtained from tension test.With the help of these theories, the data obtained in tension test can be used to determine the dimensions of the component, irrespective of the nature of stresses induced in the component due to complex loads.



Theories of failureSeveral theories of failure have been proposed, each assuming a different hypothesis of failure.The important theories are;Maximum Principal Stress theory (Rankines theory)Maximum strain theory (St. Venant's theory)Maximum shear Stress theory (Coulomb, Tresca & Guests theory)Distortion energy theory (or) Shear energy theory (Hencky Von Mises theory)Maximum total strain energy theory (Haighs theory)



Prob 1A machine member C 40 steel is loaded in the following ways;(a) sx = 40 MPa and sy = 20 MPa (b) sx = 60 MPa and txy = 40 MPa (c) sx = 60 MPa , sy = -20 MPa and txy = 30 MPa Determine the factor of safety in each type of loading by using;Maximum Principal Stress theory (Rankines theory)Maximum shear Stress theory (Coulomb, Tresca & Guests theory)Distortion energy theory (or) Shear energy theory (Hencky Von Mises theory)



From Table 1.8, Page 419, for C 40 steel, syt =324 MPa



Prob 2A steel shaft is subjected to a bending moment of 9 KNm & a twisting moment of 12 KNm. The yield strength of steel is 360 MPa & factor of safety is 2. Determine the permissible diameter of the shaft by using;Maximum Principal Stress theory (Rankines theory)Maximum shear Stress theory (Coulomb, Tresca & Guests theory)Distortion energy theory (or) Shear energy theory (Hencky Von Mises theory)



Prob 3A mild steel shaft of 60 mm diameter is subjected to a bending moment of 2.5 KNm & a twisting moment T. If The yield strength of shaft material in tension is 200 MPa, find the maximum value of torque at which the shaft just begins to yield, according to;Maximum Principal Stress theory Maximum shear Stress theory Distortion energy theory



Prob 4Determine the diameter of the shaft loaded as shown in fig based on maximum shear stress theory. Take yield point as 380 MPa and factor of safety as 2.



Stress ConcentrationIn the basic stress equations for tension, compression, bending & torsion, it is assumed that there are no discontinuities in the cross section of the machine component.But while designing a machine component it is necessary to have changes in cross sections such as hole, notch, keyway, etc. Such discontinuities will alter the stress distribution and are called stress raisers.Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Internal force lines are denser near the hole

Methods of reducing stress ConcentrationIt is rather impossible to avoid stress concentration, but care can be taken to reduce it.In a machine element, the force flow lines must be uniformly spaced and the number of flow lines at any cross section must be same.The same may be achieved by using fillets at steps, by drilling additional holes or making additional notches, by using symmetrical shapes, etc. Dr P R Venkatesh, Mech Dept, RVCE, Bangalore


Prob 1A steel plate of width 50 mm, thickness 10 mm, with a hole of diameter 10 mm drilled at its center is subjected to a tensile load F. Determine the load F the material can withstand by taking the stress concentration into account. The maximum stress in the member is 60 N/mm2.



Data: Width of plate w = 50 mm, hole dia a or d =10 mm, Thickness of plate t=10 mm, smax= 60 MPa



Dr P R Venkatesh, Mech Dept, RVCE, BangaloreBar in tension or compression with a transverse hole. A=(w-d)t where t= thickness of plate

Fig 2.8, Page 33, Mahadevan DDHB


Prob 2Determine the maximum stress induced in the notched plate as shown in fig.



Notched bar in tension or compression with a transverse hole. A=td where t= thickness of plate

Fig 2.4, Page 31, Mahadevan DDHB

Prob 3A rectangular plate as shown in fig is subjected to an axial pull of 200 KN. Determine the thickness of the plate if the plate material is made of C 60 steel and the factor of safety desired is 2.



Prob 4Determine the diameter of the hole shown in fig, if the stress concentration factor at the hole is to be same as that at the fillet.



Dr P R Venkatesh, Mech Dept, RVCE, BangaloreFig 2.6, Page 32, Mahadevan DDHBFig 2.8, Page 33, Mahadevan DDHB


Prob 5 A shaft is stepped down from 80 mm diameter with a fillet radius of 6 mm. Determine the maximum stress induced in the shaft when subjected to;Bending moment of 200 Nm.Twisting moment of 400 Nm.



Dr P R Venkatesh, Mech Dept, RVCE, BangaloreFig 2.16, Page 37, Mahadevan DDHB


Prob 6 A grooved shaft of larger diameter 60 mm has a semicircular groove of radius 5 mm. Determine the maximum stress induced in the shaft taking stress concentration into account when subjected to;An axial load of 40 KN.A bending moment of 400 Nm.A twisting moment of 500 Nm.



Prob 7 A shaft of diameter 1.5d is stepped down to diameter d with a fillet radius of d/8. It carries a transverse load of 60 KN as shown in fig. Find suitable diameter d if the shaft is made of SAE 1045 annealed steel. Assume factor of safety 2.5.



Prob 8 A shaft of diameter 60 mm has a semicircular groove of radius 5 mm & is made of steel having allowable shear stress of 60 MPa. Determine the safe power that can be transmitted at 900 rpm.



Prob 9 A stepped shaft shown in fig is subjected to a transverse load. The shaft is made of steel with an ultimate strength of 400 MPa. Determine the diameter d based on a factor of safety of 2.



Prob 10 An infinite plate with an elliptical cut out having major diameter 60 mm and minor diameter 30 mm is subjected to uniaxial tension. Determine the stress concentration factor when;Load is at right angles to the major axis Load is parallel to the major axis



Dr P R Venkatesh, Mech Dept, RVCE, BangaloreNote: From Table 2.1, Page 23, Sl No 4 & 5, Kt = 1+2b/c when load is perpendicular to major axis Kt = 1+2c/b when load is perpendicular to minor axiswhere b= semi major axis & c=semi minor axis.


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DME 1.ppt