Anti-friction or rolling element bearings
The commonest form of ball bearing is the deep-groove type as shown in section in Figure 10.11. These are the most popular of the rolling element types and can operate with both radial and axial loads and at high speed. For fans where quiet running is required, deep-groove ball bearings are the first choice with special “low noise” versions available for silent running. This only applies to small fans where other sources of noise generation can also be minimized or eliminated.
Outer nng
Inner nng
Figure 10.11 Deep-groove ball bearing
The only disadvantage of this type of bearing is its inability to accept misalignment of the inner and outer rings. At most a misalignment of 10 minutes of arc can be tolerated with some bearings only able to tolerate 2 minutes of arc. If the bearing rings are misaligned then the life is reduced and the noise level can increase appreciably.
The clearance is defined as the total distance that one ring can be moved relative to the other in either the radial direction (radial internal clearance) or axial direction (axial internal clearance). The interference fits with respect to the shaft and bearing housing, operating loads and thermal effects usually reduce
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Figure 10.12 Self-aligning ball bearing
The clearance (operational clearance), and ideally this should be virtually zero, otherwise some preload may develop. The initial clearances usually conform to ISO 5753 being designated as either C1, C2, C3, C4 or C5 (the lowest numeral being the lowest clearance) with C3 being the most widely used. Many suppliers designate normal clearance CN and this is likely to be between C2 and C3.
Bearings can be supplied with two rows of balls or as matched pairs for extra load carrying capacity but these arrangements can tolerate even less misalignment and usually run with an increased noise level.
Self-aligning bearings have two rows of balls with the outer ring having a spherical race as shown in Figure 10.12. The two rows of balls are staggered with respect to each other. This type of bearing can be used where the shaft may suffer misalignment, either because of errors that could occur due to the method of assembly or due to shaft deflections. They can be run at high speed, but not to the same extent as deep-groove ball bearings, and are reasonably quiet in operation. As with deep-groove ball bearings they are unsuitable if axial displacement takes place with the bearing performance and life suffering as a consequence. They cannot tolerate any axial load. The permitted misalignment is generally in the range 1° to 3° depending on design and size.
By displacing the ball races in the two rings the bearing can be optimized to withstand a combined axial and radial load. The bearing performance is similar to that of deep-groove ball bearings except they are not able to run at quite the same high speed and the noise level is slightly higher. A section through a typical angular-contact bearing is shown in Figure 10.13. The contact angle is as shown in the Figure and this is usually about 40°. Figure 10.14 shows typical bearings with the cage details.
Angular-contact ball bearings cannot tolerate misalignment and there must be at least a small load on the bearing for satisfactory operation. A bearing with a contact angle of 40° should have an axial load greater or equal to the radial load. As with
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Figure 10.14 Examples of angular-contact ball bearings Courtesy of ABB Drives |
Deep-groove ball bearings, angular-contact bearings can be supplied with two rows of balls to operate with the axial load in either direction or as matched pairs for increased load capacity.
A version of the angular-contact ball bearing is the four-point ball bearing which can operate well with axial loads in either direction. In this case both the outer and inner race is in the shape of a “V” as shown in Figure 10.15.
Figure 10.15 Four-point, angular-contact ball bearing
When the axial load is in excess of the radial load a modified version of the deep-groove ball bearing can be used as an angular-contact bearing. Known as a duplex bearing, either the outer or inner ring is split into two separate rings. Figure 10.16 shows an example with the outer ring split.
SpM outer ring
Figure 10.16 Duplex angular-contact ball bearing
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Contact angle |
For improved radial load-carrying capacity and greatest bearing stiffness, roller bearings can be used. Atypical cylindrical roller bearing is shown in Figure 10.17. This may have longer rollers for enhanced load carrying or long small-diameter rollers (needle bearings) if space is limited. As shown in the figure, the inner ring has flanges to retain the rollers in position but this may equally well be on the outer ring.
This type of bearing is ideal for non-location bearings because axial displacement is possible within set limits. However misalignment is limited to about 3 minutes of arc for most bearings and 4 minutes of arc for bearings with short length rollers. They
Figure 10.20 Double-row tapered roller bearing |
Figure 10.17 Atypical cylindrical roller bearing
Can be used at high speed and run reasonably quietly. The two bearing rings can be separated and this may make assembly easier in some cases.
As with ball bearings, if a spherical outer race is provided then self-aligning properties can be obtained. In this case the rollers are also required to be spherical and by using two rows — as with self-aligning ball bearings — a self-aligning bearing with good radial load carrying and some axial load carrying capability is obtained. The maximum running speeds are not quite as high as with cylindrical roller bearings and the noise level can be higher. Atypical arrangement is shown in Figure 10.18.
With two rows of rollers tapered in the opposite directions, as shown in Figure 10.20.
Thrust bearing versions of most of the journal bearing types described above are available. Figure 10.21 shows a typical thrust ball bearing orientated to withstand a vertical thrust such as the weight of a rotor — but this type of bearing, and indeed any journal or thrust bearing, can be used in any attitude.
To withstand thrust in either direction, two rows of balls are required as shown in Figure 10.22. This shows the outer rings held by housing washers with spherical seatings to compensate for misalignment during assembly. The inner ring is attached to the shaft, embodying a suitable shoulder and collar to withstand the thrust loads.
Ring attached to shaft
Sphercai utrtwcmon
Ring attached to bearing housing
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Rng attached to baanng housing
Figure 10.23 Cylindrical-roller thrust bearing
Cylindrical-roller thrust bearings can be used, as shown in Figure 10.23, but like the thrust ball bearing these cannot accommodate any radial forces and offer no location function in the radial direction. Tapered roller bearings can be used where thrust and radial loads are present, as shown in Figures 10.19 and 10.20, and high bearing stiffness is required.
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Figure 10.19 Tapered roller bearing |
For high thrust loads where radial loads are present and misalignment may be a problem, the spherical-rollerthrust bearing is necessary, as shown by Figure 10.24.
Ring attached to shaft
Spherical rollers Ring attached lo housing
With spherical seating
Figure 10.24 Spherical-roller thrust bearing
Other aspects of rolling element bearings
Rolling element bearings are available in versions with various features that are suitable for particular applications and the bearing supplier should be consulted for special applications and hazardous environments. Clearances may need to be non-standard in some applications (See Chapter 8, Section
8.6.4) and different materials are available for the ball or rollers, the rings or raceways and the bearing cage.
Carbon chromium through-hardening steel is a common material with manganese and molybdenum added on large bearings to improve the hardening. Equally common is chromium nickel and manganese-chromium alloys as case-hardening steels with little difference in performance. These materials are acceptable up to about 125°C but for higher running temperatures a special heat treatment and/or special material is required and advice should be sought from a bearing manufacturer. If corrosion resistance is required, stainless steel — typically chromium or chromium/molybdenum based — can be supplied but with a reduced bearing load capacity.
The rolling elements are held in place and with the correct spacing by means of a cage. The cage also serves to hold lubricant and, where bearing rings are separable, hold the rolling elements together. The cages must present a minimum friction, withstand the inertia forces and be acceptable in the environment (the external environment as well as the grease or oil used for lubrication). The cage must be centred on the rolling elements or one of the rings.
Cages are made of steel, brass or plastics and for a given type and size there will be a normal standard cage material. Plastic cages, for example fibre reinforced polyamide, have a temperature limit, depending upon the lubrication, of between about 80°C and 120°C and are unsuitable at very low temperatures, below about -40°C. Pressed steel cages can be used up to 300°C and are usually used on large size bearings whereas brass cages can be used up to the same temperature but are more common on medium and small size bearings. Brass cages in some environments can suffer from “season cracking” and steel cages can become corroded in the presence of water. Experience has shown that the cage design and material can affect the noise performance.
Other features which may be available include lubrication holes in the outer ring and circlip grooves in the outer ring to provide axial alignment.
Perhaps the most popular feature for fan manufacturers has been the provision of a tapered bore instead of a cylindrical bore. This is used with a tapered adaptor sleeve and locking nut. By this means the bearing may be clamped on to a parallel shaft without the need for shoulders or complicated fitting procedures, (see Figure 10.25).
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Figure 10.25 Bearings with taper sleeve adaptors fitted to parallel shaft Courtesy of SKF (UK) Ltd |
The main dimension of any rolling element bearing is the bore size but for a given bore there can be numerous outer diameters and bearing widths. The International Organization for Standardisation (ISO) has published several “Dimension Plans” to cover dimensions which are followed by most bearing suppliers. Publication ISO 15 covers radial bearings, except for tapered roller bearings which are covered by ISO 355, and thrust bearings which are covered by ISO 104.
The Dimension Plans are based on a series of outer diameters for each bore diameter and for each outer diameter there is a series of widths (or heights in the case of vertical thrust bearings). Each diameter and width series is designated by a numeral. In the case of tapered roller bearings the numerals are replaced by letters and a numeral is introduced to cover the contact angle. There are numerous additional numerals and/or letters to indicate the bearing type and its features and this complicates the final form of the bearing designation.
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