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Bearing life

May 22, 2019

Bearing life
I. Rated life and rated dynamic load
1, bearing life
The number of revolutions or hours experienced by a bearing before pitting occurs under a certain load is called bearing life.
Due to the difference in manufacturing precision and material uniformity, even the same material, the same batch of bearings of the same size, used under the same working conditions, the lifespan is not the same. If the statistical life is 1 unit, the longest relative life is 4 units, the shortest is 0.1-0.2 units, and the ratio of the longest to the shortest life is 20-40 times.
To determine the bearing life standard, the bearing life is linked to reliability.
2, rated life
A group of bearings of the same specifications (model, material, process) are used under the same working conditions, 90% of the bearings do not produce pitting, and the number of revolutions or hours experienced is called the bearing rated life.
3, the basic dynamic load rating
In order to compare the bearing capacity of the bearing against pitting, when the rated life of the bearing is specified to be one million revolutions (106), the maximum load that can be withstood is the basic dynamic load rating, expressed in C.
That is to say, under the action of the rated dynamic load C, the bearing has a reliability of 90% (100) without pitting failure, and the larger the C, the higher the bearing capacity.
Basic dynamic load rating
(1) Centripetal bearing refers to pure radial load
(2) Thrust Ball Bearing refers to pure axial load
(3) Centripetal Thrust Bearing refers to the radial component that produces pure radial displacement.
Second, the calculation formula of bearing life:
Luoyang Bearing Factory has carried out a large number of experimental studies on 208 bearings and established a digital relationship line between load and life. In the formula:
L10--the basic rating life (106 rpm) when the bearing load is P
C--Basic dynamic load rating N
Ε--index
For ball bearings: ε=3
For roller bearings: ε=10/3
P--Equivalent dynamic load (N)
The load subjected to the bearing under actual conditions: A, R, is converted into the equivalent dynamic load. The load is variable for the bearing component. In the experimental study, the bearing life is 106 rpm. The unit is convenient (counter), but in actual production, the general life is expressed in hours, which must be converted
L10×106=Lh×60n
and so 
The rolling bearing life calculation is divided into:
1. Know the bearing model, load and shaft speed, calculate Lh;
2, known load, speed and life expectancy, calculate C, select the bearing model.
Usually the middle repair or overhaul limit of the machine is the design life of the bearing, generally taking Lh'=5000. For the bearing working at high temperature, the temperature coefficient ft should be introduced.
Ct=ftC
t ≤120 125 150 200 300
Ft 1 0.95 0.90 0.80 0.60
The last two forms become:
For radial bearings
For thrust bearings
Third, the calculation of the equivalent dynamic load P
In actual production, the working conditions of the bearing are various. For this reason, the load under the actual working conditions is converted into the experimental load with the same imaginary life--the equivalent load.
For N0OOO, NU0OOO, NJ0OOO, NA0000 only bear radial load: Pr=Rfp
For 51000, 52000 only bear axial load: Pa=Afp
For other types of bearings 2OOOO, 10000, 20OOO, 60000, 70,000, 30000, 29000
Pr=fp(XR+YA)
In the formula:
R--the radial load that the bearing actually bears
A--the axial load that the bearing actually bears
X--radial conversion load factor
Y--axial conversion load factor
Fp--load factor, considering load and stress changes, machine inertia, etc.
Fourth, the calculation of the axial load of the radial thrust bearing
1. Pressure center
The outer ring is the intersection of the reaction force line and the axis line
For force thrust bearings
Where: Dm=0.5 (D ten d)
For shafts with large spans, the simplified pressure calculation assumes that the pressure center is at the center of the bearing width.
2. Axial load calculation
First introduction: bearing dressing figure 13-13 b), uranium bearing assembly diagram 13-13a)
When the radial thrust bearing is subjected to radial load, a derivative axial force S is generated.
Calculated according to Table 13-7:
70000C: S=0.4R 70OOOAC: S=0.7R 70OOOB: S=R
30OOO: S=R/(2Y)
Figure 13-13 shows the shaft supported by a pair of radial thrust bearings with an applied load of Fr and Fa.
In order to calculate the equivalent dynamic load P on each bearing, it is necessary to first find R1, A1, and R2, A2. According to Fr, it is easy to find R1 and R2. When calculating A1 and A2, not only Fa, but also derived axial force S1, S2 should be considered.
Figure b) shows the formal dressing, taking the shaft, the inner ring and the rolling body as separate bodies. Under the action of Fr, the bearing reaction force N of the outer ring of the bearing is decomposed into R, S.
Figure S2 and Fa in the same direction
1) If Fa+S2=S1
To maintain balance A1=Fa+S2 A2=S1
2) If Fa+S2>S1, the axis has a tendency to sway to the left; to maintain balance, the bearing must be subjected to a balance force Fb1 of the outer ring of the bearing.
Bearing 1 is pressed: A1=Fa+S2=S1+Fb1
Bearing 2 is relaxed: A2=S1+Fb1-Fa=S2
3) If Fa+S2
Pressed bearing 2: A2=S1-Fa=S2+Fb2
Relaxed bearing 1: A1=Fa+S2+Fb2=S1
The following summarizes the method for calculating the axial load A of 30,000 and 70,000 bearings:
(l) According to the bearing installation structure, firstly determine the direction of the total axial force on the shaft, distinguish the pressed and loosened bearings, and the resultant force is pressed by the bearing with the surface pointing back.
(2) When the bearing is pressed, the axial force A is equal to the algebraic sum of the other axial forces in addition to the inherent axial force.
(3) The bearing is loosened, and the axial force A is equal to its own axial force.
Five, the static load of the rolling bearing
For bearings with low or no rotation, the rolling contact surface is too large due to excessive contact stress, resulting in permanent over-sized pits, called plastic deformation, resulting in shock vibration. To this end, the bearing size should be selected according to the static strength, and the rated static load is also used to characterize the bearing's ability to resist plastic deformation.
Rated static load: The specification specifies the sum of the permanent deformation of the loaded maximum rolling element and the weaker ferrule raceway, equal to the load of one ten thousandth of the diameter of the rolling element, as the rated static load is indicated by C0.
The C0 values for each type of bearing are listed in the manual.
Static strength calculation
C0≥S0P0
1. Equivalent static load P0
(l) 6OOOO, 30OOO, 70OOO, l0OOO, 200OOO
P0=X0R+Y0A
Where: X0, Y0 are shown in Table 13-8
P0 obtained if P0 < R, take P0 = R
(2) Thrust bearing
P0A=A+2.3tgα

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