This set of Powder Metallurgy Questions and Answers for Entrance exams focuses on “Production of Porous Bearings – 2”.
1. Resin impregnation of porous bearings makes them _________
a) heat-treatable
b) non-heat treatable
c) reduces density
d) weak
View Answer
Explanation: Resin impregnation of porous bearings involves the use of polymers such as Teflon or polyacetal to fill up the interconnected pores. This treatment improves the density of the parts significantly but renders parts non-heat treatable.
2. Oil impregnation of porous bearings is done for _____ minutes.
a) 80
b) 50
c) 30
d) 70
View Answer
Explanation: After the sizing operation, the bearings are impregnated with the lubricant. Oil impregnation can be done using immersion in hot oil at 80 to 1000C for 30 minutes. Longer impregnation times, oxidation of oil at high temperature are the limitations of this technique.
3. Which of the following is an alternative to the immersion technique for impregnation of porous bearings?
a) Reducing impregnation
b) Grease impregnation
c) Oxidizing impregnation
d) Vacuum impregnation
View Answer
Explanation: The most widely used technique as an alternative to the immersion technique for impregnation of porous bearings is the vacuum impregnation. In this method, the sintered parts are placed in a basket and subjected to vacuum to remove all the moisture and air trapped within the pores. The oil is then introduced into the chamber and allowed to enter the pores.
4. ________ bearing is used where soft journal is employed.
a) Brass bearings
b) Aluminum bearing
c) Lead bronze
d) Straight tin bronze
View Answer
Explanation: Lead bronzes have lower strength compared to tin bronzes and thus used where soft journal is employed but their performance in dry and water lubrication is superior.
5. The life of aluminum bearings is ________
a) 35 hours
b) > 200 hours
c) 7 hours
d) < 160 hours
View Answer
Explanation: The life of aluminum bearings is > 200 hours whereas that of bronze (copper- lead bearings) and babbit bearings are 35 and 7 hours respectively.
6. Which of the following conditions are necessary for the development of fluid film in porous bearings?
a) Viscosity
b) High speed
c) Low speed
d) High speed and viscosity
View Answer
Explanation: For boundary film lubrication, the performance of a bearing depends upon the ability of fluid film to carry load. Thus, for the development of fluid film, higher speed and viscosity are essential in the porous bearings than that in solid sintered bearings.
7. The loss of oil occurs from the _____ in a cylindrical bearing.
a) top
b) cylindrical surface
c) ends
d) bottom
View Answer
Explanation: The pressure under which lubricant circulates around the shaft leads to expulsion of part of the oil (i.e. loss of oil or lubricant) from the free surface. In a cylindrical bearing, the loss of oil usually occurs from the ends.
8. Which of the following factors determine the rate of circulation of oil in oil-impregnated bearings?
a) Permeability of the porous bearing
b) Viscosity of the oil
c) Temperature
d) Permeability of the porous bearing, and viscosity of the oil
View Answer
Explanation: The rate of circulation of oil in oil-impregnated bearings is measured by the permeability of the porous bearing, and viscosity of the oil. The permeability is determined by the pore size and the quantity of the porosity while the viscosity depends on the choice of the correct oil.
9. The temperature of oil should not be _______ than the proper running temperature.
a) lower
b) higher
c) equal
d) very low
View Answer
Explanation: When the oil is kept at a proper running temperature, its performance will be optimum. If it is heated to a higher temperature above the normal, then physical and chemical changes will take place which will have an adverse effect on the properties of the oil.
10. The running temperature of the oil affects the ______
a) bearing performance
b) bearing life and performance
c) bearing life
d) bearing dimensions
View Answer
Explanation: The running temperature of the oil affects the bearing performance and life. The bearing life can be determined by the oil losses and renewing ability considering the oil within the bearing or reservoir.
Sanfoundry Global Education & Learning Series – Powder Metallurgy.
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