JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III DESIGN OF MACHINE MEMBERS - I Previous Question Paper (September - 2021 - R16)
Answer any five questions. All questions carry equal marks.
Main
Explain the influence of preferred number and design of components.
Answer
Preferred numbers are a set of internationally standardized numbers used to select sizes, ratings, and dimensions in engineering design. They are based on geometric series, typically the Renard series (R5, R10, R20, R40, R80), which ensure a constant percentage increment between successive values.
The influence on the design of components includes:
- Standardization and Interchangeability: By limiting the number of sizes to those in the preferred series, manufacturers reduce inventory costs and ensure that components from different sources are interchangeable.
- Simplified Design Process: Designers can select from a pre-determined range of sizes rather than calculating custom dimensions for every application, which saves time and minimizes errors.
- Economic Production: Using preferred numbers allows for batch production and the use of standard tooling, significantly lowering manufacturing costs.
- Optimal Variety: It provides a logical progression of sizes that covers the required range with the minimum number of steps, avoiding unnecessary proliferation of parts.
State and illustrate two principal design rules for casting design.
Answer
Casting design requires specific considerations to ensure structural integrity and ease of manufacturing. Two principal rules are:
- Uniform Section Thickness: To prevent defects like hot tears, shrinkage cavities, and internal stresses, the cross-sectional area of a casting should be as uniform as possible. If thickness variations are unavoidable, transitions must be gradual (tapered) rather than abrupt. Abrupt changes cause uneven cooling rates, leading to localized stress concentrations.
- Avoidance of Sharp Corners: Sharp internal or external corners act as stress raisers and are prone to cracking during cooling. All corners should be rounded with fillets or radii. Fillets improve the flow of molten metal during pouring and promote more uniform cooling, which significantly enhances the fatigue strength of the cast component.
It is required to design a cotter joint to connect two steel rods of equal diameter. Each rod is subjected to an axial tensile force of 50 kN. Design the joint and specify its main dimensions.
Answer
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A cantilever beam made of cold drawn steel 40C8 (Sut = 600 N/mm2 and Syt = 380 N/mm2) is shown in Figure 1. The force P acting at the free end varies from –50 N to +150 N. The expected reliability is 90% and the factor of safety is 2. The notch sensitivity factor at the fillet is 0.9. Determine the diameter d of the beam at the fillet cross-section using Gerber curve as failure criterion.
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A bar of circular cross-section is subjected to alternating tensile forces varying from a minimum of 200 kN to a maximum of 500 kN. It is to be manufactured of a material with an ultimate tensile strength of 900 MPa and an endurance limit of 700 MPa. Determine the diameter of bar using safety factors of 3.5 related to ultimate tensile strength and 4 related to endurance limit and a stress concentration factor of 1.65 for fatigue load. Use Goodman straight line as basis for design.
Answer
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Two circular plates with (2d) and (d) as outer and inner diameters respectively, are clamped together by means of a bolt as shown in Figure 2. The bolt is made of plain carbon steel 45C8 (Syt = 380 N/mm2 and E = 207000 N/mm2), while the plates are made of aluminium (E = 71000 N/mm2). The initial pre-load in the bolt is 5 kN and the external force acting on the bolted joint is 10 kN. Determine the size of the bolt, if the factor of safety is 2.5.
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A welded connection of steel plates is shown in Figure 3. It is subjected to an eccentric force of 50 kN. Determine the size of the weld, if the permissible shear stress in the weld is not to exceed 70 N/mm2.
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The cross-section of a flat key for a 40 mm diameter shaft is 22 mm × 14 mm. The power transmitted by the shaft to the hub is 25 kW at 300 rpm. The key is made of steel (Syc = Syt = 300 N/mm2) and the factor of safety is 2.8. Determine the length of the key. Assume (Ssy = 0.577 Syt).
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A steel solid shaft transmitting 15 kW at 200 r.p.m. is supported on two bearings 750 mm apart and has two gears keyed to it. The pinion having 30 teeth of 5 mm module is located 100 mm to the left of the right hand bearing and delivers power horizontally to the right. The gear having 100 teeth of 5 mm module is located 150 mm to the right of the left hand bearing and receives power in a vertical direction from below. Using an allowable stress of 54 MPa in shear, determine the diameter of the shaft.
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