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10. PRELOAD
Rolling bearings usually retain some internal clearance 10.2.2 Constant-Pressure Preload 10.3.2 Constant-Pressure Preload and Rigidity In the case of a constant-pressure preload, it
while in operation. In some cases, however, it is A constant pressure preload is achieved using a coil is possible to minimize any change in the preload
desirable to provide a negative clearance to kee„p or leaf spring to impose a constant preload. Even if A preload diagram for duplex bearings under a because the variation of the spring load with shaft
them internally stressed. This is called “preloading . the relative position of the bearings changes during constant-pressure preload is shown in Fig. 10.5. The expansion and contraction is negligible. From the
A preload is usually applied to bearings in which the operation, the magnitude of the preload remains deflection curve of the spring is nearly parallel to the foregoing explanation, it is seen that position preloads
clearance can be adjusted during mounting, such as relatively constant (refer to Fig. 10.2) horizontal axis because the rigidity of springs is lower are generally preferred for increasing rigidity and
angular contact ball bearings or tapered roller bearings. than that of the bearing. As a result, the rigidity under constant-pressure preloads are more suitable for high
Usually, two bearings are mounted face-to-face or Fig. 10.1 Position Preload a constant-pressure preload is approximately equal to speed applications, for prevention of axial vibration, for
back-to-back to form a duplex set with a preload. that for a single bearing with a preload Fa 0 applied use with thrust bearings on horizontal shafts, etc.
to it. Fig. 10.6 presents a comparison of the rigidity
10.1 Purpose of Preload of a bearing with a position preload and one with a Axial Load Bearing A
constant-pressure preload. δa
The main purposes and some typical applications of
preloaded bearings are as follows: Fig. 10.2 Constant-Pressure Preload 10.4 Selection of Preloading Method and Fa
Amount of Preload
(1) To maintain the bearings in exact position both 10.3 Preload and Rigidity Fa 0
radially and axially and to maintain the running 10.4.1 Comparison of Preloading Methods
accuracy of the shaft. 10.3.1 Position Preload and Rigidity δa0 Axial Displacement
...Main shafts of machine tools, precision When the inner rings of the duplex bearings shown A comparison of the rigidity using both preloading δaA
instruments, etc. in Fig.10.3 are fixed axially, bearings A and B are methods is shown in Fig. 10.6. The position preload
and constant-pressure preload may be compared as
(2) To increase bearing rigidity displaced δ a0 and axial space 2δ a0 between the inner follows:
...Main shafts of machine tools, pinion shafts of
final drive gears of automobiles, etc. rings is eliminated. With this condition, a preload Fa0 is (1) When both of the preloads are equal, the position
imposed on each bearing. A preload diagram showing preload provides greater bearing rigidity, in other
(3) To minimize noise due to axial vibration and bearing rigidity, that is the relation between load and words, the deflection due to external loads is less
resonance displacement with a given axial load Fa imposed on a for bearings with a position preload.
...Small electric motors, etc. duplex set, is shown in Fig. 10.4.
(2) In the case of a position preload, the preload
(4) To prevent sliding between the rolling elements and varies depending on such factors as a difference
raceways due to gyroscopic moments in axial expansion due to a temperature difference
...High speed or high acceleration applications between the shaft and housing, a difference in
of angular contact ball bearings, and thrust ball radial expansion due to a temperature difference
bearings between the inner and outer rings, deflection due to
load, etc.
(5) To maintain the rolling elements in their proper
position with the bearing rings Fig. 10.5 Axial Displacement with Constant-Pressure Preload
...Thrust ball bearings and spherical thrust roller
bearings mounted on a horizontal shaft Bearing B Axial Load Bearing A
δa
10.2 Preloading Methods
Bearing Bearing Axial Load
10.2.1 Position Preload AB PBoesiatirionngPrwietlhoad
A position preload is achieved by fixing two axially Fa Fa0 Fa 0 Fa CoBBeneasatrriiannntgg-PwwirittehhsosuutrePrPerleolaodad
opposed bearings in such a way that a preload is Fa A
imposed on them. Their position, once fixed, remain
unchanged while in operation. Fa 0
In practice, the following three methods are generally
used to obtain a position preload. Fa B
(1) By installing a duplex bearing set with previously δ a0 δ a0 δ aA δ aB Axial Displacement δa
adjusted stand-out dimensions (see Page A7, Fig. Fig. 10.3 Back-to-Back Duplex Bearing Preload δa0 δa0 δa
1.1) and axial clearance.
Fa : Axial load applied from outside δ a : Displacement of duplex δa Fa
(2) By using a spacer or shim of proper size to obtain Fa A: Axial load imposed on Bearing A bearing set
the required spacing and preload. (Refer to Fig. FaB: Axial load imposed on Bearing B
10.1) δa A : Displacement of Bearing A
δ aB : Displacement of Bearing B Axial Displacement
(3) By utilizing bolts or nuts to allow adjustment of
the axial preload. In this case, the starting torque
should be measured to verify the proper preload.
Fig. 10.4 Axial Displacement with Position Preload Fig. 10.6 Comparison of Rigidities and Preloading Methods
A 96 A 97
