JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B. Tech III DESIGN OF MACHINE MEMBERS - II Previous Question Paper (January/February - 2021 - R16)
Answer any five questions
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Main
Explain the terms Bearing characteristic number and Bearing modulus as applied to the journal bearings.
Answer
The Bearing characteristic number is a dimensionless parameter used to predict the performance of a hydrodynamic journal bearing. It is defined as the ratio of the absolute viscosity of the lubricant to the bearing pressure, multiplied by the ratio of the journal speed to the bearing pressure. It is expressed as:
where Z is the absolute viscosity, n is the journal speed in revolutions per second, and p is the bearing pressure. It indicates the operating state of the bearing relative to the minimum friction point on the Stribeck curve.
The Bearing modulus is defined as the value of the bearing characteristic number at which the friction is minimum. It represents the transition point between hydrodynamic lubrication and boundary lubrication. For a given bearing design, if the operating bearing characteristic number falls below this modulus, the bearing enters the unstable region of thin-film lubrication, leading to increased wear and potential failure.
Derive Petroffs equation for the co-efficient of friction in a lightly loaded bearing.
Answer
Petroff's equation assumes a concentric journal and bearing, where the radial clearance is completely filled with a lubricant of constant viscosity. The frictional torque T is determined by the shear stress acting on the surface of the journal.
- The shear stress is given by Newton's law of viscosity: τ = μ(du/dy) = μ(v/c), where v = πDN and c is the radial clearance.
- The frictional force F is the product of shear stress and the surface area of the journal: F = τ(πDL) = (μπDN/c)(πDL).
- The frictional torque T is F multiplied by the radius of the journal (r = D/2): T = (μπ²D³NL) / (2c).
- The coefficient of friction μf is defined by the relation: F = μf W, where W is the load on the bearing (W = pDL).
- Equating the frictional force F = T/r = (μπ²D³NL) / (Dc) = (μπ²D²NL) / c to μf(pDL):
- Solving for μf: μf = (2π²μN/p) * (r/c).
This results in the final form of Petroff's equation:
where μ is the absolute viscosity, n is the rotational speed, p is the pressure, r is the journal radius, and c is the radial clearance.
Determine the dimension of the bearing and journal to support a load of 6 kN at 750 rpm using hardened steel journal and bronze backed babit bearing. An abundance of oil is provided which has a specific gravity of 0.95 at 15.5 C and viscosity of 9.5 centistokes at 82 C that may be taken to the limiting temperature of oil. Assume a clearance of 0.001 mm per mm of diameter is allowed.
Answer
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Design a helical coil spring for an engine valve to exert force of 700 N in the open and 450 N in the closed position. The lift of the valve is 16 mm. Keep the outside spring coil diameter between 4 cm and 5 cm. The spring ends are square and ground. The allowable shear stress of spring material is 450 MPa. For the material of the spring
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Design the connecting rod for a petrol engine, from the following data: Diameter of the piston = 110 mm; Mass of the reciprocating parts = 2 kg; Length of the connecting rod = 325 mm; Stroke length = 150 mm; Speed = 1500 r.p.m, with permissible over speed of 2500 r.p.m; Compression ratio = 4; Maximum explosion pressure = 2.5 N/mm2.
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Why is the cross-section of the pulley an elliptical arm? Why is the major axis of the cross-section in the plane of rotation?
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Design a Flat belt drive to transmit 25 kW at 720 rpm to an aluminium rolling machine the speed reduction being 3.0. The distance between the shaft is 3 m. Diameter of rolling machine pulley is 1.2 m.
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The following particulars of a single reduction spur gear are given: Gear ratio = 10 : 1; Distance between centres = 660 mm approximately; Pinion transmits 500 kW at 1800 r.p.m.; Involute teeth of standard proportions (addendum = m) with pressure angle of 22.5 deg; Permissible normal pressure between teeth = 175 N per mm of width. Find:
- The nearest standard module if no interference is to occur;
- The number of teeth on each wheel;
- The necessary width of the pinion; and
- The load on the bearings of the wheels due to power transmitted.
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Write the expressions for static, limiting wear load and dynamic load for spur gears and explain the various terms used there in.
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What is helical torsion spring? How does it differ from helical compression spring? Explain.
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