HANDLING OF FK BALL BEARING UNITS

8.1 Bearing life

Even in bearings operating under normal conditions, the surfaces of the raceway and rolling elements are constantly being subjected to repeated compressive stresses which cause flaking of these surfaces to occur. This flaking is due to material fatigue and will cause the bearings to fail. The bearing life of a insert bearing is usually defined in terms of a insert bearing can undergo before flaking.

Some insert bearing failure is caused by seizing, abrasions, cracking, chipping, gnawing, rust etc, since these are caused by improper installation, insufficient or improper lubrication, faulty sealing or inaccurate bearing selection, they must be considered separately from bearing life.

8.1.1 Basic load rating and rated life

Basic load rating includes basic dynamic load rating and basic static load rating. The load applied to the insert bearing operating under a speedy rotating (n> 10r/min) condition is defined as dynamic load C, while the load applied to the bearing operating under a static or slow oscillating and rotating (n≤10r/min) condition is defined as static load Co. Insert bearing is a kind of radial ball bearing, mainly take radial force. So, the basic load rating is radial basic dynamic load Cr and radial basic static load Cor.

Basic dynamic load rating Cr: the basic dynamic load rating is an expression of the load capacity of a bearing based on a constant load which the bearing can sustain for one million revolutions.

Basic static load rating Cor: the maximum applied radial load for contact stress occurring at the rolling element and raceway contact points.

——4600MPa for self aligning ball bearing

——4200MPa for radial ball bearing

——4000MPa for radial roller bearing

The load capacity of the bearing is expressed by the basic dynamic load rating and basic static load rating which is shown in the bearing dimension page.

Life: The life of a rolling bearing is defined as the total number of revolutions which the bearing is capable of enduring before the first evidence of fatigue flaking develops on any one of the rings or rolling elements.

Reliability: The reliability is the percentage of the bearing of a group of apparently identical bearings operating under identical conditions which can expect to attain or exceed a certain defined life. The reliability of an individual bearing is the probability of the bearing to attain or exceed a defined life.

Basic rating life: For a group of apparently identical rolling bearings operating under identical conditions, the basic rating life is defined as the total number of revolutions that 90% of the bearings can be expected to complete or exceed.

According to national standard GB/T6391-2003 (equaling to ISO281: 1990) , the basic rating life of radial ball bearing is calculated by following formula:

Where: L10 : basic rating life (106r)

Cr: basic dynamic load rating

Pr: equivalent dynamic load

Equivalent dynamic load Pr: the equivalent dynamic load is a constant load with a fixed direction under which the bearing life is identical to that of the bearing operating under which the bearing life is identical to that of the bearing operating under actual load.

For a insert bearing operating with a constant rotation speed, the basic rating life can be expressed terms of hours of operation, and is calculated in following formula:

Where: L10h = basic rating life (hours)

n = bearing rotation speed (r/min)

If the bearing operates under indeterminate loads and rotation speed, the following formula should be applied when calculating bearing rating life:

Where: Pm = mean equivalent dynamic load

P = equivalent dynamic load

N = total revolution numbers within one load changing cycle

8.1.2 Calculation method of equivalent dynamic

load

The basic equivalent dynamic load is

determined under a hypothetical condition.

When calculating the bearing life, the actual load

has to be converted into equivalent dynamic load

which is in confirm with the load condition

determining the equivalent dynamic load rating.

General equation for calculating the equivalent dynamic load :

P = XFr+YFa

Where P = equivalent dynamic load (N) ;

Fr = actual radial load (N)

Fa = actual axial load (N)

X = radial factor

Y = thrust factor

The axial load which insert bearing can carry is determined by the mounting method of the bearings on the shafts.

For the setscrews locking type or eccentric locking collar type bearings, if flexible shafts are applied and the setscrews are tightened enough, the axial load Fa which the bearing can carry must not surpass 20% of the radial load Fr.

For the adapter sleeve locking type bearing, if the nuts are properly tightened, the axial load Fa can be maximum 15% ~ 20% of the radial load Fr.

The values of radial and thrust factors X and Y for insert bearings can be obtained from the following table:

When twist load is applied to the bearings, the equivalent dynamic bearing load is calculated by：

Pm = fm•P

Where: Pm = equivalent dynamic load when considering twist load

fm = when twist load is big :fm=2

When shocking load is applied, equivalent dynamic load can be calculated by：

Pd = fd • P

Where: Pd = equivalent dynamic load when considering shocking load (N)

fm = shocking load factor; which is defined as follows:

When no shocking load or minor shocking load is applied:

fd = 1~1.2

When adequate shocking load is applied:

fd = 1.2 ~ 1.8

8.1.3 Adjusted rating life equation

Normally the basic rating life L10 can be applied to calculate the bearing rating life, the bearing life is with 90% reliability.

However, in some applications a bearing life over 90% reliability may be required, moreover, the effect of bearing quality and operation conditions are expected to take into consideration when calculating bearing life, the adjusted bearing life Lnm (n means failure rate, (100-n) means reliability) meet these requirements.

Bearing life Lnm, is adjusted bearing life under (100-n) % reliability、specified bearing quality and operation conditions, it can be calculated by:

Lnm = a1axyzL10

Life adjustment factor for reliability a1 please refers to following table.

Life adjustment factor axyz include followings:

——material,

——lubrication,

——environment,

——Impurity particle,

——Internal stess,

——mounting,

——bearing load.

The bearing life is affected by any of above factors, so all factors must be taken into consideration when

selecting bearing to avoid failure. Please refer to national standard GB/T6391-2003 for Bearing life calculating method.

8.1.4 Example of insert bearing selection

One ball bearing is to operate at at rotation speed of 800r/min, under only a radial load of Fr = 3000N, with a

basic rating life of at least 30000 hours, select the bearing.

Solution 1:

According to formula

From L10h = 30000 hours, rotation speed = 800г/m,

Under only a radial load, i.e. P = Fr = 3000N,

Therefore，C = 33877N.

Solution 2:

By connecting n(800г/m) and the required basic rating life L10h (30000hours) with a straight line on the fig, it can be found that C/P value is 11.3, C/Р = 11.3, P = Fr = 3000N, thus the required basic dynamic load rating is C = 33900N

8.2 Selection of ball bearing units

As the excellent characteristics of ball bearing unit is recognized, its application fields are always expanded and at present it is used in all aspects of industrial activities in general.

Its expected service life can be extended twice by using the ball bearing unit correctly. On the contrary, inappropriate selection and handling will shorten the expected service life.

Therefore, it is necessary to examine the following items thoroughly, when the ball bearing unit is selected.

1. Size and nature of the working load.

2. Desirable minimum expected service life.

3. Operating speed of the shaft.

4. Bearing number and parallel application arrangement on the shaft in question.

5. Available space for assembling and disassembling work.

6. Appearance at the place to be used.

7. Gas generation and dust condition at the installation place.

8. Ambient temperature at the installation place.

9. Machining precision of the facility, to which the bearing is applied.

10. Maintenance and control, including the lubrication system.

The above items are regarded as the selection conditions, and the items 1,2 and 3 can be examined by the service life calculation of the ball bearing unit.

As to the item 4, such a type as allows the alignment adjustment through the installation modification, must be selected, since the mutual alignment work becomes necessary even in the case of automatic alignment adjusting type, where many sets of bearing can be applied to one shaft.

Regarding the item 5, it must be examined if enough installation space is available or not, in order to know in what manner the installation work can be done.

Item 6 may suggest the necessity of the clean and aesthetic design, depending on the application purpose of the machine involved. For example, such consideration will be needed for the application to the electric appliance or sewing machine.

Items 7 and 8 mean that it must be studied if the gas and chemicals, or high temperature, which are harmful to ball bearing, are existing or not.

As suggested in Item 9, the ball bearing unit must suit to the processing precision of the installation section.

Item 10 covers the maintenance and inspection problem, namely, how easily the maintenance can be done, or if the unit is installed inside the machine where the lubrication can hardly be done or if the lubrication must be and how etc. The optimum selection of bearing unit, right unit for right place, will ensure the full development of performance of ball bearing unit.

8.3 Selection of Shafts

The ball bearing unit is provided with hexagonal hollow set screws at two spots located at 120° one side of inner ring. Mounting on the shaft normally adopts loose fit. In this case, the following relationship between the shaft and the inner bore is recommended.

However, if the ball bearing unit is used at high rotation speed or under heavy load, the shaft fit must adapt a tight fit.

The bearing can be also installed to the shaft by use of the adapter assembly. This is convenient method that can be used as the intermediate bearing of relatively long shaft or a slight difference is found at the shaft dimension. In this method, the bearing inner diameter makes 1:12 taper and the corresponding tapered adapter sleeve is applied, followed by nut tightening:

Therefore, a slight difference in shaft diameter does not cause much trouble.

8.4 Limiting speed

The limiting speed of ball bearing units are mainly determined by the fit between the bearings and the shafts. Normally, clearance fit is used between setscrews type and eccentric collar type bearing units and shafts, then h7 shaft tolerance is selected. h8 or h9 tolerance is applied for light load and slow speed application. And tighter j7 tolerance is applied for heavy load and high speed. The shaft applied to the adapter sleeve bearing is h9 with IT5 class tolerances.

The speed ratings for insert bearing of CS200-2RS series are the same as deep groove ball bearings which are shown in following table.

The limiting speeds for the ball bearing units with different fits are shown in following table

Note: 1. The JS7(h9/IT5) column fit for adapter sleeve type ball bearing units, and the rest j7 ~ h9 column fit for the setscrews type and eccentric locking collar type ball bearing units.

2. Above table data is reference for J type seal, H type seal, SL type dual seal, F type seal products only.

2025 October 3rd Week FK Product Recommendation:

722500 DA/DB:

The 722500 DA/DB is a type of Plummer Block (bearing housing) designed to support rotating shafts with rolling bearings. It features:

Robust cast iron or steel housing, suitable for heavy-duty applications.

DA/DB variations may refer to different sealing or mounting designs (e.g., DA with set screws, DB with double seals).

Easy installation and maintenance, ideal for industries like conveyors, motors, fans, and mining equipment.