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 Purpose for Preload
For example, when a ball bearing is used in a motor, it has "Zero"
radial clearance when an axial load is applied. If there is any radial
clearance, vibration and noise of the balls will occur, and the stiffness
of the ball bearing will be very low. This force that is applied in the
axial direction is known as preload. An optimum preload should be individvally
specified for each ball bearing size. If the Preload is applied excessively,
Bearing Fatigue Life will be short and will increase raceway noise as well.
Bearing starting and running torque will also be high. If the applied Preload
is insufficient, fretting corrosion can occur. This happens as a result
of vibration causing the balls to resonate and abrade on the raceways.
Therefore, obtaining the correct Preload is very important.
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Optimum Preload
Optimum Preload is normally recommended after calculating the optimum operating
surface stress at the contact ellipse. The contact ellipse is the area
of contact between the ball and raceway that occurs as a result of elastic
deformation of both parts under load.
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Regarding the figure, the contact ellipse area (S) between the ball and raceway is formulated as
S = πab
(a: the major axis of the contact ellipse area, b: the minor axis of the contact ellipse area).
Operating surface stress (P) is given by Q/S, where Q = Ball load or load on the raceway (Perpendicular to the area of contact),
and S = Surface area of the contact ellipse. Generally, the unit is shown "MPa" (Kgf / mm2).
The aim for the surface stress is below. The following is one of the guidelines
for noise life.
If the noise life requirement is over 10,000 hours, the Preload can be
calculated based on an optimum surface contact stress that does not exceed
800 MPa {80 Kgf/mm2}.
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For general applications with a noise life requirement between 5,000 and
10,000 hours, the optimum Preload can be calculated using a contact ellipse
stress that does not exceed 1000 MPa {100 Kgf/mm2}.
For stiffness critical applications requiring an operating noise life of
less than 5,000 hours, a surface stress of less than 1500 MPa {150 Kgf/mm2} should be used.
A way of looking at the Preload from the Basic Dynamic Load Rating (Cr)
| Over 10,000 hours noise life requirement: |
0.5/100 - 1/100·Cr |
| 5,000 - 10,000 hours noise life requirement: |
1/100 - 1.5/100·Cr |
| Less than 5,000 hours noise life requirement: |
1.5/100 - 2/100·Cr |
If a surface stress of 2700 MPa {270 Kgf/mm2} is applied to a high carbon chromium bearing, permanent raceway and ball
deformation will occur. It is possible that stresses below 2700 MPa {270 Kgf/mm2} will result in no permanent raceway or ball deformation, but we would
recommend to use a maximum safe operating stress of 1600 MPa {160 Kgf/mm2}.
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Preload and Stiffness
There are two basic methods of Preloading: Solid Preload and Spring Preload. Solid Preload can be obtained by mechanically locking all of the rings in postion while
under an axial load. The advantages of this type of design are that the
components remain simple and the stiffness is high. The disadvantage is
high variation in Preload under temperature variation, and that the Preload
can reduce with wear. Spring Preload (or Constant Pressure Preload) can be applied using a coil spring or a spring wave washer, etc. An advantage
of Spring Preload is that it maintains consistent Preload with temperature
variation.The disadvantages are that the designs are more complex and normally
have lower stiffnesses.
The Preload can be applied in two directions, Duplex face to face (DF) and Duplex back to back (DB).When considering stiffiness, DB is stiffer under moment loads than DF. |
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