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3.1
Bearing life
After
certain period of the bearing running, the accuracy has been lowered,
the noise and vibration have been increased with lubricating grease
ageing. The running face has been striped because of fatigue. Therefore
the bearing can not be used any more. The service life of this kind
of bearing is called the bearing life in a broad sense. They are
respectively named as accuracy life ,noise life, lubricating grease
life , rolling fatigue life etc.
3.1.1
rated fatigue life
Rated
fatigue life refers to total frequency of bearing running with 90%
reliability to same model bearings under the same operating conditions.
Under the circumstances of definite rotating speed, it usually indicates
the rated fatigue life by total running period. It is common to
only take the fatigue life into consideration for the research of
bearing life.
3.2
basic dynamic load rating
3.2.1
basic dynamic load rating functions as the constant load to the
external ring static bearing. Under this kind of load, the rated
fatigue life is 1 million revolutions, r.p.m.
3.2.2
basic rated life of ball bearing
(total
rotary number) L10=(c/p )3……(3.1)
It
indicates the relations amid basic rated transient load, equivalent
moving load and basic rated life.
(time)
L10= 106/60n (c/p)3……(3.2)
It
is more convenient to indicate the life by time when the bearing
is rotated by Constant
speed, as it is shown in formula(3.2)
In
the formula,
L10
indicates the basic rated life.
L10h
is indicated by hour to show the basic rated life
P
indicates the equivalent moving load, N(kgf)
C
indicates the basic dynamic load rating, N(kgf)
N
indicates the rotating speed, rpm
3.2.3
revise the basic dynamic load rating according to the specific temperature
when the rolling bearings have been used under heat conditions,
the bearing hardness will be reduced, the fatigue life will be lowered
than that for use in normal temperature. Therefore the basic dynamic
load rating should be assessed a little smaller correspondingly.
Ct=ft×C……(3.3)
In the formula:
Ct indicates the
basic dynamic load according to the temperature correction.
Ft indicates the
temperature coefficient (Table 3.1)
C
indicates the basic dynamic load rating.
When
the bearing will be used under the high temperatures above 120℃,
it should comm.only be implemented with size stability treatment.
| Bearing
temperature℃ |
125 |
150 |
175 |
200 |
250 |
| Temperature
coefficient ft |
1.00 |
1.00 |
0.95 |
0.90 |
0.75 |
3.2.3 revised rated fatigue life
The basic formula of the rated fatigue life
Ball bearing L10=(c/p )3……(3.3)
L10 is rated fatigue life with 90% reliability. Following the
improvement of the rolled steel adopted by the bearings, the fatigue
life will be extended accordingly. It can use the following compensation
coefficient to revise the rated fatigue life. Ln=a1 a2 a3 L0…….(3.4)
Ln indicates the fatigue life with the considerations of the steering
response, material modification, lubricating condition.
L10 indicates the rated fatigue life with 90% reliability
a1 indicates the reliability coefficient
a2 indicates the bearing characterization factor
a3 indicates the service condition coefficient
please refer to table 3.2 for a1 value with high than 90% of the
reliability.
Reliability coefficient
a1 value |
| Bearing
temperature℃ |
125 |
150 |
175 |
200 |
250 |
| Temperature
coefficient ft |
1.00 |
1.00 |
0.95 |
0.90 |
0.75 |
If the bearing is not tilted, and it uses the lubricating oil
with high viscosity, 2 can be set to (a2× a3) value.
3.3 calculation if bearing load
3.3.1 load coefficient
Though it can calculate the radial load and axial load, the load
which actually effects the bearing is usually larger than the calculated
value because of the mechanical vibration impact. The load value
can be worded out according to the following formula:
Fr=fw.Frc……3.5
Fa=fw.Fac……3.6
Among them, Fr, Fa indicate the load (N,kgf) which effects the
bearing.
Frc, Fac show the theoretically calculated load (N,kgf)
Please refer to Table 3.3 for the load coefficient of fw.
| Running condition |
Use occasion examples |
fw |
| Non-impact smooth
running |
Motor, machine tool
air-conditioner |
1-1.2 |
Ordinary
running |
Air blower, compressor,
elevator, crane,
paper-making machinery |
1.2-1.5 |
Running with vibration,
impact |
Construction machinery,
stone crusher
(abrasive wheel),
vibrating screen, calender |
1.5-3 |
3.4 equivalent dynamic load
Most of the bearing undertake the synthetic load of radial load
and axial load. It also has a varied kind of load conditions. Therefore
it can not directly compare the actual bearing load with the basic
dynamic load rating. Therefore convert the actual load to the imaginary
load with defined size and orientation which passes through the
bearing center and make analysis & comparison. The bearing has
the same life as the actual load under conditions of the imaginary
load.
Such kind of the converted imaginary load is called the equivalent
moving load.
3.4.1 the calculation of radial bearing can be calculated according
to the following formula:
P=XFr+YFa…..3.7
Among them:
P indicates the equivalent dynamic load (N,kgf)
Fr indicates the radial load (N,kgf)
Fa indicates the axial load (N,kgf)
X indicates the radial load coefficient
Y indicates the axial load coefficient
3.5 basic rated static load and
equivalent static load
3.5.1 basic rated static load
Basic rated static load (Co) is a static load which makes the
bearing create the following calculated contact stress between the
raceway groove contact part and ball that endures the max. stress.
Ball bearing 4200 Mpa (428kgf/mm2)
In the contact part which bears such kind of contact stress, the
sum total of the permanent deformation volume from the ball and
raceway groove is about 0.0001 times of the ball package diameter.
3.5.2 Equivalent static load
Equivalent static load is an imaginary load. When the bearing
is in a static mode or the rotating speed is extremely low, under
conditions of this imaginary load, the contact stress of ball and
raceway groove which bear the max, load is the same as that under
actual load conditions.
The equivalent static load of radial bearing will adopt the larger
value which results from the following two formulas.
Po= xofr+yofa…….3.8
Po= fr……3.9
Among them:
Po indicates the equivalent static load (N,kgf)
Fr indicates the radial load (N,kgf)
Fa indicates the axial load (N,kgf)
Xo indicates the static radial load coefficient
Yo indicates the static axial load coefficient
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