SESM6050 Tribology for Future Mobility
SEMESTER 2 FINAL ASSESSMENTS 2020/21
Question 1
(a) Explain the difference between static and kinetic coefficient of friction and what is the practical significance of these two terms [5 marks]
(b) Why are the tribo-junctions formed when two surfaces come into contact? [5 marks]
(c) Explain the transition between lubrication regimes on the basis of increasing load? [5 marks]
(d) Describe the similarities and differences between hydrodynamic thrust and journal bearings. [10 marks]
[Total: 25 marks]
Question 2
(a) What is the meaning of surface engineering of gear teeth? [3 marks]
(b) An EV powertrain with spur gears has a module of 4 mm, 45 teeth in the gear, 20 teeth in the pinion, a pressure angle of 20。, a pinion rotational speed of 5000 rpm, a face width of 10 mm, and gear and pinion material having modulus of elasticity of 207 GPa and Poisson’s ratio of 0.30. The Hertzian contact pressure is 1.2 GPa. At ambient conditions the advanced ester lubricant used has a dynamic viscosity of 0.0253 Pa s and pressure viscosity coefficient of 1.28×10−8 m2 N– 1 . The composite surface roughness is 0.424 µm. Determine the following parameters:
i. Transmitted load and power; [3 marks]
ii. Minimum film thickness; [5 marks]
iii. The film parameter and the RMS surface roughness, assuming both gear and pinion have identical surface roughness. Describe the surface finish and manufacturing method? [3 marks]
iv. Is the film parameter adequate? Justify your answer. If it is not
adequate, identify specific changes to improve the tribological situation (assuming the gear and pinion teeth, face width, pressure angle and modulus remain unchanged). [11 marks]
[Total: 25 marks]
Question 3
(a) Overheating and lubrication-type failures are being encountered in
sealed 6205 deep-groove ball bearings in a 4200 rpm shaft for an electric motor. The initial operation was satisfactory, but noise, overheating, and bearing failures were encountered within one month of operation. There is no evidence of external contamination, misalignment or incorrect mounting. Suggest a likely reason of the overheating and provide a mitigation. [6 marks]
(b) An NU211 hybrid cylindrical roller bearing design consists of a cylinder loaded against a flat-bottomed groove.
i. A 6 mm diameter Si3 N4 cylinder is loaded against a steel outer race with a bottom groove diameter of 87 mm. The uniaxial yield strength of the steel is 1.5 GPa. Stating any assumptions that you make, calculate the mean contact pressure and load per unit length (w′ ) at the onset of subsurface yield in the race, also calculate the depth below the surface at which yield initiates (for steel: E = 207 GPa and v = 0.30; for Si3 N4 : E = 314 GPa and v = 0.26). [15 marks]
ii. To limit the risk of yield, a judgement is made to reduce the load
such that the maximum stress is only half the yield stress. Calculate the maximum load the hybrid bearing can now carry. [4 marks]
[Total: 25 marks]
Question 4
(a) With illustrated notes describe the physical mechanisms that influence friction in a hydrodynamic thrust bearings, and explain how they are related to the operational performance and load-carry capacity. [12 marks]
(b) A steam turbine rotor for a maritime power plant has two plain journal bearings each carrying half the weight of a 20000 N rotor. The rotor speed is 3000 rpm, the bearing diameters are both 120 mm, and the bearings are 120 mm wide.
i. Using the design procedure (see lecture notes), define the journal
bearing operational parameters for a surface roughness Ra = 0.6 μm and minimum power loss. [6 marks]
ii. Calculate the coefficient of friction, the attitude angle, the oil flow rate and the side-leakage flow rate. Choose an eccentricity ratio of 0.82 to avoid instability. Identify a critical performance parameter that has been neglected and how it would improve the tribological bearing design. [7 marks]
[Total: 25 marks]