This set of Powder Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Products – Porous Bearings and Types – 1”.
1. What is the pore volume in bearing materials?
a) 14-20%
b) 17-30%
c) 11-50%
d) 28-43%
View Answer
Explanation: The bearing materials fall under the category of porous components produced by powder metallurgical route. The bearing material contain a pore volume of between 17% and 30%.
2. Which of the following substance is added to iron powder to improve the compressive strength of bearing?
a) Bismuth
b) Zinc
c) Copper
d) Lead
View Answer
Explanation: Copper (up to 10%) is added to the iron powders for improving the compressive strength of the P/M bearings. Iron is used for low-cost bearings, majorly used in automotive applications, farm equipment, and machine tools.
3. Which of the following is not a characteristic of the bearings produced from iron?
a) Good wear resistance
b) High-oil-volume capacity
c) High porosity
d) Low-oil-volume capacity
View Answer
Explanation: Low-oil-volume capacity is not a characteristic of the bearings produced from iron. The iron material used for production of P/M bearings have a relatively low-limiting value of V in the PV factor, but have high-oil-volume capacity due to the high porosity. They have good wear resistance and are used with hardened and ground steel shafts for heavy and shock loads.
4. Copper is added to iron-copper-graphite powder mix to suppress the formation of ________
a) Cementite
b) Iron oxide
c) Copper oxide
d) Ledeburite
View Answer
Explanation: The addition of free carbon in iron-copper mix makes it heat-treatable but in case of iron-graphite mix, formation of cementite occurs leading to poor bearing properties. Thus, addition of copper (2-8%) serves to suppress the formation of cementite and makes iron-copper-graphite amenable to heat treatment.
5. What should be added to the iron powder mix to improve the speed capability?
a) Bismuth
b) Lead
c) Vanadium
d) Molybdenum
View Answer
Explanation: The addition of lead to iron improves the speed capability and even reduces cost. This material so produced for production of P/M bearing is called leaded iron.
6. Which of the following is not a feature of oil-impregnated bearings?
a) Excellent self-lubrication properties
b) Wide range of operating temperature
c) High-thermal expansion
d) Low-thermal expansion
View Answer
Explanation: The important features of oil-impregnated bearings include: excellent self-lubrication properties, wide range of operating temperature (-200°C to 250°C), low-thermal expansion, no water absorption, extended bearing life in severe environments with no need for external manual lubrication, and silent operation.
7. Dry bearing is made up of ______ with metal backing of _______ is available.
a) teflon, aluminum
b) aluminum, teflon
c) leaded bronze, aluminum
d) teflon, bismuth
View Answer
Explanation: Dry bearings are made from metallic alloys or from fluoropolymer (Teflon) and with metal backing of aluminum and bronze-steel. This bearing material is soft enough to handle interference fits with minimal stick-slip and a low coefficient of friction.
8. Which kind of bearing material is suitable for oscillating and reciprocating motions?
a) Low-graphite content
b) High graphite content
c) High-oil content
d) High porosity
View Answer
Explanation: A bearing material with low-oil content, with high graphite content and low-porosity is suitable for oscillating and reciprocating motions.
9. What is the property of material for the application of fractional-horsepower motor bearings?
a) Moderate porosity
b) Low porosity
c) Minimum lubricating oil
d) High porosity
View Answer
Explanation: The material which has high porosity that is which has the more pore volume which also has a maximum amount of lubricating oil is used for fractional-horsepower motor bearings.
10. The porous-metal bearings are used where re-lubrication is difficult.
a) True
b) False
View Answer
Explanation: Most porous-metal bearings are made of either bronze or iron, with 10 to 35 vol% interconnected voids or porosity. In operation, lubricating oil stored in these pores feed the bearing surface through these interconnected pores, when the temperature increases during operation. Any oil, which is forced from the loaded zone of the bearing, is later reabsorbed by the pores through capillary action. Since these bearings can operate for long periods of time without additional supply of lubricant, they are convenient to be used where re-lubrication is difficult.
11. P/M bearings may be of the type ____________
a) dry type containing aluminum
b) porous type infiltrated with a lubricant
c) dry type containing teflon
d) porous type infiltrated with a lubricant, dry type containing teflon
View Answer
Explanation: P/M bearing materials are characterized by multiphase structures. These include two-phase alloys, immiscible components like plastics as well as lubricants. P/M bearings may be (1) porous type infiltrated with a lubricant or (2) dry type containing teflon or graphite.
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