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MT-253 MACHINE DESIGN

MT-253 MACHINE DESIGN. Lecture #2 Syed Ehtisham Gillani Lecturer Department of Mechanical Engineering Technology University of Technology Nowshera. Standards and Codes.

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MT-253 MACHINE DESIGN

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  1. MT-253 MACHINE DESIGN Lecture #2 Syed EhtishamGillani Lecturer Department of Mechanical Engineering Technology University of Technology Nowshera

  2. Standards and Codes • A standard is a set of specifications for parts, materials, or processes, which is applied to achieve uniformity, efficiency, and a specified quality. • One of the important purposes of a standard is to limit the variations that can arise from the random creation of a part, material, or process. • A designer will use the standard to design the product, and a manufacturer will use the standard for the manufacturing of the product. • Standard serves as a common language for defining quality and establishing safety criteria for the product. ASTM, ASME, ISO are some examples of the standard. • ASTM has more than 12000 standards, they cover all most everything.

  3. Standards and Codes • A code is a set of specifications for the analysis, design, manufacture, and construction of something. • The purpose of a code is to achieve a specified degree of safety, efficiency, and performance or quality. • ASME Boiler and Pressure Vessel Codes are the examples of the codes.

  4. Materials Selection • The designer of any product/device, other than software must get involved with material selection. • Only occasionally will the exact grade of material be specified by the customer. • Even then the designer must understand the material to be able to design the product.

  5. Materials Selection • So many materials, so much information. • How do we decide? • How do we start selection of materials? • First we need to look at the function of the product - Product Analysis

  6. Materials Selection Product Analysis • Every product is designed in a particular way - productanalysis enables us to understand the:important materials, processing, economic and aestheticdecisions which are required before any product can bemanufactured.

  7. Materials Selection Product Analysis Just what it says – analyse the product!• What does it do?• How does it do it?• Where does it do it?• Who uses it?• What should it cost?• What does it look like?

  8. Materials Selection Case Study – a bicycle

  9. Materials Selection Product Analysis Just what it says – analyze the product!• What does it do?• How does it do it?• Where does it do it?• Who uses it?• What should it cost?• What does it look like? • Provides travelling • Using Force, wheels, gears, pedals, etc. • Roads, Mountains, Racing Tracks, etc. • Normal users, race drivers, children, etc. • Cheap or expensive? • Aesthetics, color, etc.

  10. Materials Selection • After product analysis, we have answers of all our questions. • These answers will play a vital role in selection of materials Example: • If we have to design a racing/mountain bicycle, the frame could be of expensive but strong material like composite. • whereas, a normal bike could be manufactured with steel which is cheap and have less strength as compared to composites. • This shows that if we don’t know the function, usage and other factors of the product, we cannot select the proper material for it. • So product analysis is must before selection of materials.

  11. Design Factor & Factor of Safety • Factor of Safety (n or FOS or SF) is defined as “ the ratio of material strength to actual working stress.” n = n = • FOS should be always equal to 1 or greater than 1. • A component/structure with FOS of exactly 1 will support only the design load and not more than that. • A component/structure with FOS equals to 2 means it can actually withstand 2 times the design load.

  12. Design Factor & Factor of Safety • Design factor (nd) and Factor of Safety (n) are both used to indicate the safety of a product/device/structure. • The difference between design factor and Factor of Safety is that the design factor is what value of stress a structure/component is required to withstand without failure. nd = • The design factor for a component generally provided in advance and often set by regulatory code or policy. It is not based on an actual calculation. • Whereas, the factor of safety is a ratio of maximum strength to intended load for the actual item that was designed.

  13. Design Factor & Factor of Safety

  14. Design Factor & Factor of Safety

  15. Reliability • The statistical measure of the probability that a mechanical element will not fail inuse is called the reliability of that element. • The reliability R can be expressed by • where pfis the probability of failure, given by the number of instances of failures pertotal number of possible instances. • The value of R falls in the range • A reliability of R = 0.90 means that there is a 90 percent chance that the part will perform its proper function without failure.

  16. Reliability System reliability: • If a mechanical systems consists of series of components that it collectively forms a series system. • If the reliability of component iis Riin a series system of n components, then the reliability of the system is given by For example consider a shaft with two bearings having reliabilities of 95 percent and 98 percent. The overall reliability of the shaft system is then R = R1.R2 = 0.95 (0.98) = 0.93 or 93 %.

  17. Tolerances & Fits Tolerance • No component can be manufactured precisely to a given dimension; it can only be made to lie between two limits, upper (maximum) and lower (minimum). • The difference between maximum and minimum dimensions of a component, i.e. between the upper limit and lower limitis known as Tolerance. • E.g. If a Shaft Length is 5 + 0.02 mm, it means it could be 5.02mm, 5.01mm, 5.00mm, etc. but not less than 5mm and not more than 5.02mm • So 5.02 – 5 = 0.02mm is the tolerance in above example. • What would be the nominal length of shaft in above example?

  18. Tolerances & Fits Types of Tolerances Bilateral Tolerance: When tolerance is present on both sides of nominal size, it is termed as bilateral Tolerance. • For example, a shaft has to be manufactured to a diameter of 40± 0.02 mm. This means that the shaft, which has a basic size of 40 mm, will be acceptable if its diameter lies anywhere between the limits of sizes, that is, an upper limit of 40.02 mm and a lower limit of 39.98 mm. Then permissive tolerance is equal to 40.02 − 39.98 = 0.04. • Unilateral Tolerance: When tolerance is present only on the one side of nominal size, it is called Unilateral Tolerance.

  19. Tolerances & Fits (a) (b) Unilateral Bilateral What is Allowance? It is an intentional deviation from the nominal size. It is same as tolerance but the only difference is that allowance is planned or intentional deviation whereas tolerance is unplanned or unintentional.

  20. Tolerances & Fits FITS • Manufactured parts are required to mate with one another during assembly. • The relationship between the two cylindrical mating parts that are to be assembled, such as, a hole and the shaft, with respect to the difference in their dimensions before assembly is called a fit. • An ideal fit is required for proper functioning of the mating parts. • Three types of fits: • Clearance Fit • Interference Fit • Transition Fit

  21. Tolerances & Fits FITS • Clearance Fit • Interference Fit • Transition Fit

  22. Fits • Clearance Fit • The type of fit in which the largest permissible diameter of the shaft is smaller than the diameter of the smallest hole, is known as clearance fit. • This type of fit always provides clearance. • Small clearances are provided for a precise fit that can easily be assembled without the assistance of tools.

  23. Fits 2. Interference Fit • The type of fit in which the minimum permissible diameter of the shaft exceeds the maximum allowable diameter of the hole, is called Interference Fit. • This type of fit always provides interference. • Interference fit is a form of a tight fit. • Tools are required for the precise assembly of two parts with an interference fit. • When two mating parts are assembled with an interference fit, it will be an almost permanent assembly, that is, the parts will not come apart or move during use.

  24. Fits 3. Transition Fit • Transition fit may have either clearance or interference in the fit. • If the shaft dimension is minimum and hole dimension is also minimum, then there will be a clearance between the minimum dimension of the shaft and the minimum dimension of the hole, and it behaves as a clearance fit. • If we look at the figure carefully, then it is observed that if the shaft dimension is maximum and the hole dimension is minimum then an interference will result and this creates a certain amount of tightness in the fitting of the shaft inside the hole.

  25. Fits Hole max limit Shaft max limit Hole min limit Shaft min limit

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