JNTU
JNTU B.Tech III DESIGN OF MACHINE MEMBERS - I Previous Question Paper (September - 2021 - R18)
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 standardized numbers used to select sizes, ratings, and specifications for machine components. They are based on geometric progressions, typically using Renard series (R5, R10, R20, R40, R80), which ensure that the ratio between consecutive sizes remains constant.
The influence of preferred numbers on design includes:
- Standardization: It limits the variety of sizes to a minimum, reducing inventory costs and simplifying manufacturing processes.
- Interchangeability: Using standardized dimensions ensures that components from different manufacturers can be replaced easily.
- Economic Efficiency: By selecting sizes from preferred series, designers avoid custom tooling and utilize readily available stock materials, significantly lowering production costs.
- Scaling: It allows for the systematic development of a product range, where different power or capacity ratings can be achieved by simply scaling dimensions according to the geometric series.
State and illustrate two principal design rules for casting design.
Answer
Casting design requires adherence to specific rules 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, sections should be kept as uniform as possible. If a change in cross-section is necessary, it must be gradual rather than abrupt to ensure even cooling rates throughout the mold.
- Avoidance of Sharp Corners: Sharp internal corners act as stress concentrators and are prone to cracking during cooling. All internal corners should be provided with fillets, and external corners should be rounded. This promotes smooth metal flow and prevents the formation of weak crystalline structures at the junctions.
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.
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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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