Main Dimensions
In dimensional plan, every diameter of a hole d has several outer diameters D and various widths B associated to it. The bearings with the same diameter of a hole as well as equal outer diameter fall into the same diameter group. Every diameter group includes bearings of various width groups ranked according to their width. The diameter and the width groups make up the dimensional group, which is indicated by double figure number, and where the first figure indicates width group and the other the diameter group.
Designation
Designation of bearings consists of numeric and alphabetic characters indicating type, dimension and design of bearings.
The bearings of the basic version are marked by basic type designation, indicating bearing design and dimensions for the nominal hole diameter d.
The extension characters before that basic indicate other material than plain steel used for roller bearings and incompleteness of bearing. The extension characters following the basic designation indicate differences in internal construction, dimensions, designation of shields, design varieties of bearing rings, cage, degree of accuracy, clearance, vibration levels, heightened security, aggregating of bearings, stabilization for higher temperatures, friction, lubricant and so on.
Accuracy of Bearings
It is understood as accuracy of bearings‘ dimensions and operation. Bearings are manufactured in the following sizes: P0, P5, P5A, P4, P4A, P2, SP and UP. The accuracy P0 is basic, lower number in designation indicates higher accuracy of bearing.
Internal Clearance
Clearance in a bearing is the value of the shifting distance of one bearing ring in regard to another ring from one extreme position to the other. Usually we can detect smaller clearance in built-in bearings. The smaller clearance is caused by bearing ring interference rates in journal and hole of the body. Further reducing, mainly of the clearance, occurs during operation due to the effect of temperature.
For the bearings of common design, the clearance is set so that one of the bearing rings can be fitted firmly, which is sufficient for the most of operating conditions of bearing. In special cases of fits with other requirements for radial play, the bearings designated as C1 to C5 are manufactured.
Cage
Cage in roller bearing fulfils the following functions :
- distributes the rolling bodies evenly around the circumference
- prevents the rolling bodies from mutual contact and sliding
- prevents the rolling bodies from falling out
According to the construction and material, cages are divided into those pressed or those solid.
Covers
The bearings with covers on one side or the other are produced with the shields Z, 2Z, ZR, 2ZR or with the sealings RS, 2RS, RSR, 2RSR.
Shields make clearance seal. The seal consisting of packing rings from the rubber precured on metal braces is very effective.
Design of Support
General Principles of the Rolling Bearing Support Design
Rotating shaft or other part supported on rolling bearings is led in radial or axial direction so that basic precondition of exactness of its motion is fulfilled. The part should be supported radially in two points and axially in one. The bearing must allow some movement of the shaft in axial direction so that undesirable axial prestress is avoided.
The supports in which higher radial and axial loads act at higher rotational frequencies should be arranged so that the bearings absorb radial or axial forces only.
Two bearing support design of which enables absorbing both the radial and axial loads, is another often used arrangement.
Fitting of Bearing
Radial and axial fitting of bearing to journal and hole of a body is directly connected to the whole design arrangement of support. It is important to take characteristics of the acting forces, operating temperature etc. into account, when deciding about type of support to be used.
Bearing is fitted in radial direction in the fitted cylindrical surface of the journal and the hole in the body. The correct radial fitting of the bearing to the journal and the body has substantial effect on making use of its load capacity and its correct work in the fitting.
Basically, both bearing rings should be firmly fitted in order that the radial support around all the circumference is ensured. To facilitate assembling and disassembling, it is possible to have one of the rings push fitted.
When choosing proper radial bearing fitting we judge and take note of the following effects: load, size and type of bearing, its material, parts, heat, accuracy, assembling, disassembling, axial shift and fitting.
Sealing
Sealing of bearing space is very important, because harmful substances surrounding a bearing may damage it. Simultaneously sealing prevents the lubricant to leak out of bearing.
Types of sealings:
- non-contact
- frictional
- combined
Lubrication of Bearings
Appropriate lubrication of bearings has direct influence on bearing life. Lubricant creates a lubricating film, preventing metals to contact one another, between rolling bodies and bearing rings. Moreover, the lubricant lubricates the areas affected by friction; it has cooling effect, preserves the bearing from corrosion and in many cases also seals the space of the bearing. In most cases, approximately 90% of all of them, bearings are greased by plastic grease or oil. In special conditions, other lubricants may be used.
Lubricating with Plastic Lubricant
In designers‘ practice, lubricating with plastic lubricants is preferred to lubricating with oil for the simplicity of support arrangement, the sealing properties of plastic lubricants and easy bearing maintenance.
Lubricating with Oil
Lubricating with oil is useful if rotational frequency at operation is that high that period of final lubricating with plastic lubricant is too short. Other reason for using it can be either the necessity to remove heat out of the bearing or high temperature of environment preventing the use of plastic grease or the condition when neighbouring parts of a bearing had already been lubricated with oil.
Lubricating with Greases
The first lubricants mentioned above are used for lubricating bearings in the situations when plastic lubricants or oils cannot meet the requirements for reliable lubricating in the conditions of limiting friction or considering their resistance to high operational temperatures, chemical effects and so on.
Bearing Lateral
Its Meaning and Importance
To realize the full performance potential of the tapered roller bearing, it is necessary to obtain and maintain in service the proper range of mounted bearing lateral (i.e., bearing lateral as installed on an axle). Due to the geometrical design of the tapered bearing, lateral is directly related to the radial clearance between rollers and races. This radial clearance determines the number of rollers in the bearing which will carry the applied radial and thrust loads. It is the resulting individual roller / race loads that determine the bearing's fatigue life and to at least some degree the operating temperature of the bearing assembly and its effect upon grease and seal life. For these reasons, mounted lateral is critical to bearing performance.
Benefits of Proper Bearing Lateral
Improved rolling contact fatigue life is one of the benefits of proper mounted lateral.
Figure 1 and Figure 2 show radial roller load distributions for two mounted lateral; zero (free turning) and 0.030" lateral. The theoretical contact fatigue life of the bearing can be determined from such roller load distributions.
As explained in BRENCO Technical Forum 90-1, "Bearing Life", fatigue life is highly dependent on load. Excessive mounted lateral can cause high peak roller loads, as a result of fewer rollers sharing the load, which can significantly reduce expected fatigue life. This is illustrated graphically in Figure 3 wherein the bearing fatigue life for a range of mounted laterals is expressed as a ratio of the rated L10 life at zero mounted lateral. Note that the theoretical life for an in service mounted lateral of 0.020" is approximately 43% of the life predicted for zero lateral.
In fact, it may be seen that some small amount of preload gives maximum fatigue life. However, because of the rapid drop in predicted life as preload increases, spacers are selected during bearing assembly to give a mounted lateral within the allowable range of zero (free turning) to 0.020".
Fundamentals
Several factors, some within the control of the shop assembling the bearing and some within the control of the shop installing the bearing, determine mounted lateral. These factors include:
* spacer size selection * journal diameter
* cone bore diameter * mounting practice
When new or reconditioned railroad bearings are assembled, the cone spacer width is selected to produce a predetermined amount of clearance (C1) between the roller sets and the cup rolltracks (Figure 4). This clearance permits a small lateral movement (LB) of the cup over the roller sets. This movement is called bench end play or bench lateral.
When the bearing is pressed on the axle journal producing an interference fit, there is an increase in the cone race diameter. For the typical railroad bearing cone sizes, approximately 80% of the fit is transferred into cone diameter expansion. The increase in cone race diameter reduces the clearance (C2) between the roller sets and the cup rolltracks, in effect reducing the mounted lateral movement (LM) of the cup over the roller sets (Figure 5).
Taking into consideration the nominal 20° included angle of the railroad bearing cup rolltracks, the 80% press fit to cone diameter change, and two cone assemblies per bearing, each 0.001" of interference fit will reduce bench lateral by approximately 0.0045".
Mounted Lateral Limits vs. Bench Lateral Limits
The manufacturers of freight car roller bearings have established the acceptable mounted lateral range as zero (free turning) thru 0.020". This lateral range and the acceptable cone bore to axle journal interference fit range (0.0045" maximum with new cones and 0.0015" minimum with reconditioned cones) dictates the bench lateral range.
The 0.0045" interference fit will reduce bench lateral by 0.020"; therefore, minimum bench lateral was established at 0.020" to avoid a harmful preload on the bearing. The 0.0015" interference fit will reduce bench lateral by 0.0067"; therefore, maximum bench lateral was established as 0.026". The relationship of bench lateral and interference fit to mounted lateral is shown graphically in Figure 6. Upper limits on bench lateral are established with a view towards minimizing the loss of life potential reflected in Figure 3, but with practical regard given to the fact that reconditioned cone bores and used axle journal diameters produce a trend toward minimum interference fit.
It should be noted at this point that lateral ranges for journal roller bearings are given for both hand operated lateral devices (0.020" - 0.026") and power driven lateral devices (0.023" - 0.029"). The higher range for power lateral devices recognizes the ability of these devices to thoroughly seat the roller of the cone assembly against the cone thrust rib and measure the full movement of the cup with respect to the rollers. The bench lateral range used in this Forum refers to hand lateral.
How to Achieve Proper Lateral
With this understanding of the benefits of proper mounted lateral and the theoretical bench lateral limits, how can these limits be achieved in practice?
Both hand operated and power driven lateral equipment require lightly lubricated bearing components and a moderate thrust load to "seat" all of the rollers in the roller set against the cone thrust rib. Fully seating the rollers permits approaching the true geometrical (or minimum) outside diameter of the cone assembly. It is more difficult to achieve the fully seated roller condition with the hand lateral than with the power lateral. In addition, the application of a generally higher thrust load with power lateral will usually cause some small elastic enlargement of the bearing cup. In recognition of these differences, the limits for the power driven equipment are set 0.003" higher than those for hand operated devices.
As a practical guide to achieving consistent and accurate bench lateral measurements, care should be taken to insure that cup and cone assemblies are clean and lightly oiled rather than dry when measurements are made. Furthermore, regular quality control checks should be made to verify that lateral equipment and dial gages are in proper working order and calibrated. It is also good practice to maintain a "master" bearing of known lateral to serve as a reference check for both the operation and the equipment.
Cone Bore Fit
It should be clear from the preceding discussion that mounted lateral is directly related to cone bore fit, which is determined by both the cone bore and the journal diameter. These dimensions must be within the specified tolerance ranges if proper mounted lateral is to be achieved. As previously pointed out in Technical Forum 88-3, "Proper Mounting of Bearing Enhances Retention", undersize journals have been identified as a substantial problem area in service. Likewise, oversize journals have the potential for causing an undesirable preload in some bearings. Therefore, careful checking of journal size using a snap gage is a prerequisite to achieving both proper mounted lateral as well as good bearing retention.
Bearing Installation Practice
In order to achieve full bearing performance and service life, checking the bearing's mounted lateral upon installation is an absolute must. The correct procedure for checking mounted lateral is shown in BRENCO's video tape titled "Proper Bearing Mounting Procedures" which is available upon request.
Maintaining Proper Lateral in Service
Any evidence of loss of bearing retention in service, such as loose backing rings or loose or missing cap screws, is an indication that excessive lateral has probably developed as well. Furthermore, as required by the AAR Field Manual, whenever wheelsets are removed from cars, the bearing laterals must be checked and bearings removed from service if the laterals are greater than 1/16" (0.061").
In summary, correct lateral is essential to bearing fatigue life in service. In order to maximize this fatigue life, the mounted lateral should be as close as possible to zero without any preload. Such mounted laterals are dependent upon close control of bench lateral as well as careful attention to correct size cone bores, axle journals, and correct bearing installation practice.
CASE-CARBURIZED VS. THROUGH-HARDENED
Which Material is Right for Your Application?
When you specify bearings, you want to be sure you choose those that are appropriate for the operating environment and technical requirements of your application. One decision you may be faced with is whether to use case-carburized or through-hardened product.
In making the decision between case-carburized and through-hardened product, technical and commercial issues must be considered. Technical issues include required service life, loading, amount of debris, alignment and maintenance practices. Price and warranty are two prime commercial considerations.
Case-carburized Bearings endure Heavy Shock Loads
Case-carburized bearings have a tough, ductile core combined with a hard, wear-resistant outer surface. The core enhances the ability to endure heavy shock loads without damage. It also improves the ability to operate under misalignment and where dirt and debris are present.
Through-hardened Bearings Offer appropriate Levels of Performance
Through-hardened bearings feature a more uniform hardness throughout. They also remain more rigid under misalignment which can generate higher stress levels within the bearing. If the application is free of shock loads, is relatively clean and is not highly misaligned, then through-hardened bearings may be a cost efficient alternative.
In order to provide you with a variety of alternatives for their application performance needs, we will always offer you the consultancy on deciding whether to use through hardened or case-carburized bearings. For more product and performance information on what type of bearing is right for your application, contact our sales representative.