5 Ivy-League University Standard Bearing Design Assignment Questions With Answers
Question 1: How are rolling-bearings designed and assembled?
Fig 17.12 Ball-bearing assembly
To ensure that a bearing operates properly it must be protected against foreign matter entering the housing, and at the same time the lubricant must be kept inside the bearing. This function is performed by the bearing seals or shields (Fig. 17.11), or by providing the end covers with contact or non-contact seals, as shown in Figs. 17.12 and 17.13.
The contact type of seal usually consists of a ring made of felt, leather, synthetic rubber, etc., fixed into the groove in the end cover and contacting the rotating shaft.
Fig 17.13 Bearing Seals
The non-contact type of seal is used when high temperatures and speeds are employed. A close clearance is provided by a series of grooves in the end cover and in the shaft or shaft-collar which are then filled with sealing grease. This method eliminates the friction and wear of the rubbing contact-type seals. When a shaft is supported by two bearings, only one of the bearings should be fixed axially. The other bearing must provide axial adjusting movement by the ring sliding to accommodate the tolerances of position and to allow for relative differences in axial dimensions between the shaft and the housing due to temperature changes, as shown in Fig. 17.14.
Fig 17.14 Shaft supported by two bearings
Fig 17.15 Live tailstock centre of a lathe
In Fig. 17.15, the thrust load due to the work holding is taken up by a thrust ball bearing. The feed-motion thrust is carried by a tapered- roller bearing, which also supports the cutting loads. Due to space limitation, a needle bearing is used which also provides the axial adjusting movement.
Question 2: Explain in detail the uses of duplex bearings
Answer : Duplex bearings are used to support thrust loads in either direction and have a split outer or inner ring. They should be used for thrust loads alone or for combined thrust and radial loads only when the thrust load is very much greater than the radial one.
Prelubricated bearings (Fig. 17.11) These bearings incorporate metal shields and/or seals, which are usually fastened to the outer ring. The close clearance between the seal and the inner ring retains the long-life grease within the bearing and prevents foreign matter entering from outside.
Fig 17.2 Comparison of different types of rolling bearing
Fig 17.11 Prelubricated sealed ball bearing
Question 3: How is the ductility and toughness of white metals restored?
Answer: These alloys, also known as Babbit metals, consist mainly of tin and/or lead with small amounts of antimony and copper. The addition of antimony improves the compressive strength and provides resistance to wear but introducers brittleness. By adding a small amount of copper the desirable ductility and toughness are restored. Lead is used mainly in the interests of cheapness.
White metals have low melting points and are therefore not suitable for high-temperature applications, but their embeddability is very good.
Tin-base alloys are preferable to lead-base alloys as:
- they flow more readily when molten
- they shrink less when solidifying
- they are more ductile
- they are more corrosion-resistant
However, they are more expensive to produce than lead-base alloys. A typical composition of a tin-base bearing alloy used for general work would be 86% tin, over 10% antimony, and over 3% copper.
Lead-base alloys are more liable to be corroded by products of oxidation of oil and have a tendency for abrasive wear. They are used for low speeds and light duties. A typical composition of a lead-base bearing alloy used for low- duty applications would be over 63% lead, 20% tin, 15% antimony, and over 1% copper.
Question 4: Discuss the properties and uses of Copper-based alloys
Answer : Copper-lead alloys are used where loads are too high for white metals and sizes of bearings are restricted, an alloy of about 70% copper and 30% lead may be used. About 2% of tin may be added to reduce hardening and brittleness. These types of bearing are used for high speeds, notably in diesel engines.
Lead bronzes These alloys have a bronze matrix consisting of copper and tin, in which the lead is distributed. The hardness and fatigue strength will depend upon the tin content, which may be from 1% to 15%. The lead content may be up to 25%. These bearings have excellent casting properties, are easily machined, and are used for high-duty applications.
Tin bronzes These copper-tin alloys are hard and strong. They have high fatigue and compressive strengths, good resistance to wear, but poor embeddability. They are used for low speeds and high-duty applications.
Phosphor bronzes These alloys have a small phosphorus content which has a deoxidising effect. They are corrosion-resistant, and stronger than simple tin bronzes.
Gun metals These are copper-tin-zinc alloys with occasional small amounts of lead. They are wear- and corrosion-resistant and are used for marine fittings, valves, etc.
Brasses These copper-zinc alloys with very small amounts of aluminium, iron, and manganese are often used as bearings. Bearing brasses have usually about 60% of copper and about 40% of zinc.
Question 5: Discuss the composition of aluminium-base alloys
Answer: These alloys may contain up to 20% tin, to provide embeddability, with about 1% of copper and nickel. Their fatigue strength is similar or somewhat higher than that of copper-lead and copper-tin bronzes.
They are mainly used as bearings for diesel engine and motor-car crankshafts and for connecting rods.
|Alloy||Hardness||Embedding ability||Corrosion resistance||Fatigue resistance||Thermal Conductivity|
|Tin-base Babbit||Extremely low||Very high||Very high||Low||Medium|
|Lead-base Babbit||Extremely low||Very high||Medium||Low||Low|
|Copper-lead alloy||Low||Medium||Low||Medium||Very high|
|Tin bronze||Low||Low||High||Vey high||Medium|
|Aluminium-base alloy||Very Low||Medium||Very high||Very high||High|