Top 4 Bearings Assignment Questions & Answers To Aid Your Revision
Question 1: Discuss the three main materials used to make bearings and their applications
- Cast-iron bearings
- Porous metal bearings
- Non-metallic bearings
A cast-iron bearing is usually simply a hole bored in a cast-iron part accommodate a mating part. For grey cast-iron, adequate lubrication free from dust is essential. These types of bearing are in general used for light duties.
Very fine metal powders are partially compressed to a required shape and then sintered at a high temperature. The metal sponges so produced may be filled with a low-friction thermoplastic for dry running or be impregnated with oil. Porous metals may absorb up to one third of their own volume of lubricant and when running they give off the lubricant, thus wetting the bearing surfaces. When stationary, most of the lubricant is reabsorbed again. Due to their low compressive strength, these bearings are used for
Thermoplastics Thermoplastics are suitable for injection moulding and extrusion as they can be softened and reshaped with the application of heat. These materials include PTFE (polytetrafluoroethylene), nylon, polystyrene, polypropylene, etc.
Applications include bushes for medical equipment, food preparation equipment, textile machinery, pumps, etc. PTFE, with its high thermal resistance and load-carrying capacity, is used for chemical pumps, railway-point pivot bushes, conveyor bushes, etc.
Thermosetting plastics Thermosets are chemically changed when heated - they become rigid, and this change cannot be reversed. For use as bearings these materials are always reinforced by the addition of asbestos, silicone resins, carbon, or metals.
These types of bearing are used for hot-duct supports, jet-pipe supports, cranes, vibratory rollers, pump bushes, etc.
Carbon (graphite) Graphite bearings have a very high resistance to devated temperatures, need no lubrication, have a polishing effect on the mating parts, and can run in fluids which attack other bearing materials.
Carbon bearings are used for food and textile machinery, furnace and boiler equipment, chemical agitators, bottle-washing plant, trolley wheels, etc.
Rubber Rubber bearings are easily deflected, thus reducing stresses and damping vibrations. They are usually used for pumping purposes with water as the lubricant. Rusting mating surfaces should be avoided, as rough surfaces will damage the rubber.
Wooden bearings Lignum vitae is one of the hardest and densest of all woods. It is very strong in compression, can resist the action of certain chemicals, and is used for bearings in food- and chemical- processing machinery, etc.
Hard maple when impregnated with oil is used for textile machinery, loose pulleys, etc.
Jewel bearings are used for instruments, clock mechanisms, etc.
Question 2: Why are ball and roller-rolling bearings used for devices which are subject to frequent starting and stopping?
Fig. 17.5 Rolling-bearing parts
(a) sectional view of ball bearing, (b) conventional representation of a bearing
Question 3: Name the parts of ball and roller bearings and the materials used
Answer: Ball and roller bearings consist of :-
- an inner ring, which fits on the shaft
- an outer ring, which fits inside the housing
- balls or rollers, which provide a rolling action between the rings
- a cage, which separates adjacent ball surfaces, which rotate in opposite relative directions, and prevents sliding friction between them
Note: Rings sometimes are called 'races'.
Bearing materials used
The materials used for rolling bearings are high-carbon chromium steels which are very hard and resistant to wear. Surfaces in contact must be highly polished to reduce wear and to provide smooth rolling movement without any sliding.
The cages are made of low-carbon steels, bronzes, or brasses, though for high-temperature applications case-hardened and stainless steels are used.
The inner ring of a journal bearing must have an interference fit on a revolving shaft, to prevent creep. (Creep is the slow rotary movement of the inner ring relative to the rotating shaft or, alternatively, the slow rotary movement of the outer ring relative to the rotating housing).
The inner ring is usually held axially between a shaft shoulder and a nut, as shown in Fig 17.6(a) and (b).
The outer ring is assembled inside the stationary housing, with a sliding fit to permit correct axial position without preload. Often the outer ring is not held axially- a clearance is introduced in order to allow for inaccurate machining and for relative expansion or contraction of the shaft and the housing due to temperature changes, as shown in Fig. 17.6(a).
For revolving parts on the shaft, such as wheels, pulleys, etc., the outer rings have an interference fit inside the housing of these wheels, whereas the inner rings have a sliding fit on the stationary shaft, as shown in Fig. 17.6(b).
General rule: Rotating rings require interference fits; stationary rings require sliding fits.
Question 4: Discuss the common types of rolling bearing
Answer: Single-row deep-groove ball bearings :-
These bearings incorporate a deep hardened raceway or track which makes them suitable for radial and axial loads in either direction, providing the radial loads are greater than the axial loads. These bearings are self-contained units - they can be handled and assembled as a single component. Also, they may be of the prelubricated types, having integral seals or shields which retain the grease within the bearing and prevent foreign matter entering it (Fig. 17.11).
Applications These bearings can be used as locating bearings for high-speed spindles, motorcycle engines, electric motors, circular saws, turbine shafts, wood cutters, gearboxes, pulleys, pumps, etc.
Fig 17.7 (a) Single- and double-row deep-groove ball bearings, (b) Single- and double-row angular-contact ball bearings
Single-row angular-contact ball bearing
Outer rings and sometimes inner rings are machined with high and low shoulders to take one-directional thrust or combined radial and axial loading. To support thrust loads in either direction, these bearings can be mounted in opposing pairs, suitably adjusted to prevent end play or preloading due to over-adjustment. Double-row angular-contact ball bearings can be used as alternatives to opposing pairs of single-row bearings, simplifying the mounting and saving space. Applications
These bearings are used for high-speed spindles for boring, milling and drilling machines and for motorcycle engines, worm gears, hot-gas fans, radar aerials, etc.
Double-row self-aligning ball bearings
These bearings are designed for the cases where alignment of inner and outer rings cannot be assured during assembly or in service. The inner ring has two deep-groove raceways, whereas the outer ring has a single spherical raceway. This allows the inner and outer rings to be misaligned relative to each other.
Applications These bearings should not be used for very heavy radial loads and they will not support any axial loads. Protective shields cannot be fitted. These bearings are used for gearboxes, air blowers, cutter shafts of planning machines, etc.
Fig 17.8 (a) Double- row self-aligning ball bearing, (b) single- and double-row roller bearings
Single-row roller bearings
Roller bearings are usually detachable units. They have a greater load-carrying capacity than ball bearings of equivalent size as they make line contact rather than point contact with their rings. These bearings are not suitable for axial loading. A slight axial displacement is permissible between the inner and outer rings. These types of bearing are cheaper to manufacture than equivalent ball bearings.
Applications These bearings are suitable for heavy and sudden loading, high speeds, and continuous service. They are used for vibrating motors, stone crushers, dredging machinery, ship propellers and rudder shafts, belt conveyors, locomotive axles, flywheels. crankshafts, presses, etc.
These bearings are so designed that, when projected, construction lines corresponding to the surfaces of contact of rollers and rings will meet at an apex point on the bearing axis.
Like angular-contact ball bearings, these bearings will carry a combination of radial and single-direction thrust loads. The inner ring, called the cone, with a tapered roller and cage, is assembled as a complete unit; whereas the outer ring is detachable.
Two bearings can be mounted on a shaft, but they must be accurately adjusted axially to ensure the proper running clearance between the roller and the outer ring, called the cup. Applications These bearings are suitable for lathe spindles, bevel gear transmissions, gearboxes for heavy trucks, car drives, car wheels, etc.
Fig 17.9 (a) Tapered roller bearings, (b) needle roller bearings
These bearings are fitted with small-diameter rollers and are used for radial loads at slow speeds and oscillating motion. They are especially suitable for restricted spaces and where the weight of components is critical, as in the aircraft industry. Sometimes, due to space limitations, the inner ring is not used; instead, the shaft is hardened and ground and is used directly as an inner ring.
Applications These bearings are used for aircraft applications, live tailstock centres, bench-drill spindles, light gearboxes, etc.
Thrust ball bearings
These bearings can only take thrust loads. They consist of two loose thrust rings grooved to accommodate the balls with their corresponding cage. One of the rings has a smaller bore than the other and engages on the shaft, while the other ring has a larger outer diameter which engages in the housing. A double-thrust bearing has three rings.
Applications Thrust ball bearings are used for heavy axial loads and low speeds, and are suitable for thrust spindles, tailstock centres for heavy work, vertical shafts, pillars supporting heavy vertical loads, etc.
Fig. 17.10 (a) Thrust ball bearings, (b) duplex bearing