Tapered Roller Thrust Bearings
Tapered roller thrust bearings enable axially very compact bearing arrangements to be produced which can carry very heavy axial loads, are insensitive to shock loads and are stiff. SKF produces single direction tapered roller thrust bearings (fig 1) and double direction tapered roller thrust bearings (fig 2). The screw-down bearings (fig 3) constitute a special design of single direction tapered roller thrust bearing and have a full complement of rollers. They are used for rolling mill screw-down arrangements.
SKF tapered roller thrust bearings are produced with the logarithmic contact profile between raceways and rollers to guarantee optimum stress distribution in the bearing, thus enhancing bearing life.
SKF tapered roller thrust bearings with cage are of separable design so that the two washers and the roller and cage thrust assembly can be mounted separately and easily. The full complement screw-down bearings are held together by special retaining components and, to simplify handling, the washers have threaded holes for eye bolts.
Single direction tapered roller thrust bearings
The small single direction tapered roller thrust bearings are used almost exclusively for the kingpin bearing arrangement of commercial vehicles (fig 4). They have a full complement of rollers and are held together by a sheet steel cover (fig 5). Most of these bearings are fitted with rubbing seals of wear and oil-resistant nitrile (NBR) rubber (fig 6). They are lubricated for life with a multi-purpose grease and require no maintenance. There are several designs which differ only in the design of the housing washer and the cover (fig 7).
The larger single direction bearings differ in the execution of the washers; some have dimensionally identical shaft and housing washers (fig 8) and some have washers of different design (fig 9).
The washers of the bearings with shaft and housing washers having the same dimensions (fig 8) both have a tapered raceway and one retaining flange. Bearings of this design are to be preferred where very heavy axial loads are to be supported as, for example, in the swivels of drilling rigs or in crane hooks. The bearings have a cage which is centred on both washers.
Bearings of the other design (fig 9) have a shaft washer with tapered raceway and retaining flange whilst the housing washer is plane (flat). Therefore, bearings of this design are able to accept minor eccentricities of the shaft with respect to the housing bore without any detrimental influence on bearing performance. With the exception of a few sizes which have a full complement of rollers, the bearings are fitted with a roller centred cage.
Double direction tapered roller thrust bearings
Tapered roller thrust bearings of the double direction type are mainly used together with multi-row cylindrical roller bearings in rolling mill applications (fig 10). Those bearings with a plane shaft washer (fig 11) are the more popular as they are able to accommodate slight eccentricities of the shaft with respect to the housing bore, of the order of the radial internal clearance of the radial bearing with which they are used. A spacer sleeve is arranged between the two housing washers. This is dimensioned so that the cover screws of the chock can be fully tightened and no special spring adjustment of the bearing is required.
The double direction tapered roller thrust bearings with tapered raceways on the shaft and housing washers (fig 12) generally have a higher axial load carrying capacity than the bearings with a plane shaft washer and are able to radially locate the shaft to a certain extent. If the load carrying capacity of these bearings is inadequate with respect to the requisite life, double row tapered roller bearings with large contact angle of the TDIS design can be used instead, see under "Double row tapered roller bearings.
As double direction tapered roller thrust bearings are usually mounted with a loose fit on the roll neck as well as in the chock, the shaft washer of most bearings has one or two locating slots. It is then possible to prevent the shaft washer from rotating on its seat by using keys which engage the slots, or by similar means.
Tapered roller thrust bearings of this type must be able to accommodate extremely heavy axial loads and also permit angular movement of the screw spindle with respect to the support. The bearings are therefore generally of the full complement type and have extremely long rollers of large diameter. To be able to accommodate the alignment movements of the chock without detrimental effect on bearing performance, the bearings have one sphered washer. This may be either the shaft washer (fig 13) or the housing washer (fig 14).
The necessary alignment movements take place in bearings having a sphered shaft washer above the roller complement between the sphered shaft washer and a correspondingly concave sphered screw spindle or pressure plate (fig 15). The pressure plate can also be supplied by SKF.
Where the housing washer is sphered, the alignment movements are accommodated below the roller complement between the housing washer and the pressure plate. The pressure plate for these bearings is always supplied with the bearing.
SKF produces many designs of screw-down bearing; they are tailored to meet the particular requirements of the specific application.
The bore and outside diameter tolerances of SKF single and double direction tapered roller thrust bearings are produced, with very few exceptions, with Normal tolerances to ISO 199-1979. The tolerances for the height and the running accuracy differ. Details of the values for a particular bearing as well as the tolerances of screw-down bearings will be sent on request.
Those SKF tapered roller thrust bearings which do not have a full complement of rollers are fitted with
- a machined brass cage (fig 16),
- a pin-type steel cage (fig 17), or
- a steel window-type cage (fig 18).
To achieve satisfactory operation, tapered roller thrust bearings, like all ball and roller bearings, must always be subjected to a given minimum load, particularly if they are to operate at high speeds or are subjected to high accelerations or rapid changes in the direction of load. Under such conditions the inertia forces of the rollers and cages, and the friction in the lubricant, can have a detrimental influence on the rolling conditions in the bearing arrangement and can cause damaging sliding movements to occur between the rollers and raceways.
The minimum axial load to be applied to single direction tapered roller thrust bearings can be estimated using
Fam = minimum axial load [kN]
A = minimum load factor (see product data)
n = rotational speed [r/min]
When starting up at low temperatures or when the lubricant is highly viscous, even greater loads may be required. The weights of the components supported by the bearing, together with the external forces, generally exceed the requisite minimum load. If this is not the case, the single direction tapered roller thrust bearing must be subjected to an additional axial load.
It is not necessary to check the minimum axial load for screw-down bearings as these are preloaded via O-rings or, in reversing mills, by springs. When using O-rings to produce the preload, the preload force should compress the diameter of the O-ring by some 20%.
When the bearing is preloaded by springs, the minimum load is dependent on the weight of the associated components. Appropriate values will be supplied on request by the SKF application engineering service.
Equivalent dynamic bearing load
For single direction tapered roller thrust bearings which are subjected to purely axial dynamic loads
P = Fa
Equivalent static bearing load
For single direction tapered roller thrust bearings
P0 = Fa
Comparative load ratings
For rolling mill applications, load ratings are often used which are not calculated according to ISO 281:1990 but by a different method based on a rating life of 90 million revolutions (500 r/min for 3 000 operating hours). As a direct comparison of these load ratings with ISO load ratings is not possible, even if they are converted for 1 million revolutions (ISO rating life definition), "comparative" load ratings calculated by the same non-ISO method are given in the product tables for double direction tapered roller thrust bearings as these bearings are frequently used in rolling mills.
These comparative load ratings may only be used together whit the life and equivalent load equations specified below; they may not be used to calculate an ISO rating life.
Comparative life determination
The comparative life is calculated using the comparative load rating CF, as follows:
LF10 = 90 (CF/PF) 10/3
LF10h = (CF/PF)10/3 (1 500 000/n)
LF10 = comparative rating life, million revolutions
LF10h = comparative rating life, operating hours
CF = comparative dynamic load rating which gives a rating life of 90 million revolutions, kN
PF = equivalent dynamic bearing load, kN
(PF = Fa)
n = constant operating speed, r/min
Deep groove ball bearings
Needle roller bearings
Combined cylindrical roller/taper roller bearings
Cylindrical roller bearings
Angular contact ball bearings
Combined needle roller bearings
Spherical roller bearings
Tapered roller bearings
Thrust ball bearings
CARBŪ toroidal roller bearings
Cylindrical roller thrust bearings
Angular contact thrust ball bearings
Tapered roller thrust bearings
Needle roller thrust bearings
Track runner bearings
Spherical roller thrust bearings
Indexing roller units
Backing bearings for cluster mills
Other SKF rolling bearings