Powder Metallurgy Questions and Answers for Entrance exams 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
Answer: b
Clarification: 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
Answer: c
Clarification: 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
Answer: d
Clarification: 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
Answer: c
Clarification: 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
Answer: b
Clarification: 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
Answer: d
Clarification: 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
Answer: c
Clarification: 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