Four-Row Tapered Roller Bearings

Four-row tapered roller bearings (fig) are used successfully worldwide for rolling mill bearing arrangements where rolling speeds are slow to moderate. Because of their design they are able to take up those axial loads which occur in such applications in addition to radial loads and generally no separate thrust bearings are required in such arrangements. This means that the roll necks can be relatively short and the chocks at both sides of the roll can have the same design (fig).

Four-row tapered roller bearings, because of their design, must be installed as complete bearings in the chocks. To enable quick roll changes to be made they must therefore have a loose fit on the roll necks. Loose fits have the disadvantage that the inner rings wander on their seats under the prevailing load conditions causing wear to the seats which normally have a lower hardness than the rings. To alleviate the effects of a loose fit to some extent, i.e. to reduce wear, SKF bearings have a helical groove in the bore and/or lubrication grooves in the side faces of the inner rings (fig). These grooves enable lubricant to be supplied to the contact surfaces of the inner ring and seat. Additionally, the grease-filled grooves can absorb the wear particles.

SKF produces an extensive range of four-row tapered roller bearings which covers not only the conventional designs with intermediate rings between the outer and/or inner rings (fig), but also a new design with modified rings which integrate the intermediate rings (fig). The particular advantages of these four-row tapered roller bearings without intermediate rings which have been developed by SKF include

• simpler mounting because of fewer components
• improved load distribution over the four rows of rollers, leading to less wear and longer service life
• simpler axial location on the roll necks because of reduced inner ring width tolerances
• dimensional equivalence to conventional four-row tapered roller bearings with intermediate rings.

To enable these advantages to be exploited in all applications, those bearings of conventional design with intermediate rings produced by SKF are gradually being converted to the new design.

SKF four-row tapered roller bearings are produced in the TQO (fig) and TQI (fig) designs. The main difference is the configuration, or arrangement, of the roller and cage assemblies and the number of inner and outer rings dictated by this. TQO bearings are, however, used almost exclusively.

Sealed four-row tapered roller bearings

SKF supplies many sizes of four-row tapered roller bearings with integral radial shaft seals at both sides (fig). They are available in several designs depending on bearing size as well as on the open (non-sealed) basic design.

Whenever possible, sealed bearings should be used for rolling mills. Compared with open bearings they offer the following advantages

• longer service life
• reduced grease consumption (by up to 90%)
• extended maintenance intervals
• grease is retained within the bearing
• less surplus grease to contaminate the milling emulsion, to improve both the rolling process and product quality
• reduced environmental impact.

Sealed bearings can simply replace open bearings as part of a rebuild or refurbishment because the boundary dimensions are the same.

Sealed bearings are fitted with specially designed, C-shaped, radial shaft seals on both sides (fig). This enables the bearing to incorporate rollers that are equal to, or just slightly shorter than the rollers used in an open bearing. Consequently, the load carrying capacity of a sealed bearing is the same or very similar to an open bearing. The seals permit high sliding velocities and are intended for operating temperatures between �20 and +140 �C.

SKF Explorer four-row tapered roller bearings have sheet steel reinforced radial shaft seals made of hydrogenated acrylonitrilebutadiene rubber (HNBR) that are snapped into the groove in the outer ring.

Standard bearings contain sheet steel reinforced seals made of fluoro rubber (FPM) that must be staked into a groove in the outer ring. Fluoro rubber seals require special handling (see “WARNING! Safety precautions for fluoro rubber”).

O-rings inserted in grooves in the outer ring outside surface prevent dirt or water from entering between the outer rings and the chock bore that otherwise would contaminate the lubricant.

Standard TQOSN design sealed bearings are supplied with window-type, stamped steel cages, without spacer rings (fig, fig, fig). However, spacer rings are available in the TQOS design (fig + seals).

Larger size sealed bearings in the TQOSN.1 design are for applications that require extremely high load carrying capacity. These bearings have a special pin-type steel cage that uses pierced rollers (fig + seals). Sealed four-row tapered roller bearings are also produced with extended inner rings which can serve as counterfaces for radial shaft seals. These bearings are supplied as standard without spacer rings in the TQOESN design (fig + seals) or with spacer rings in the TQOES design.

Depending on the relubrication needs, sealed four row-tapered roller bearings can be supplied to three standard design variants

• Designation suffix VA901 (standard bearings) or E1 (SKF Explorer bearings) These sealed bearings (fig) can be relubricated via vertical slots in the outer rings. Bearings without spacer rings have a steel reinforced acrylonitrile-butadiene rubber seal between the two inner rings. Bearings with spacer rings have two O-rings that act as seals between the inner rings, similar to those shown in the fig. The O-rings have a permissible operating temperature range from �40 to +100 �C
• Designation suffix VA902 (standard bearings) or E2 (SKF Explorer bearings). These sealed bearings (fig) have no lubrication slots and cannot be relubricated. Otherwise they have the same features as bearings with designation suffix VA901 or E1.
• Designation suffix VA903 (standard bearings) or E3 (SKF Explorer bearings) These sealed bearings (fig) have vertical lubrication slots in the outer rings. They do not have a seal between the inner rings.

Bearings of the TQO configuration

Bearings of this design have two pairs of tapered roller and cage assemblies, the two rows of rollers of the pair being arranged face-to-face. They are the most widely used of four-row tapered roller bearings and are produced by SKF in a correspondingly wide range of sizes and designs. The main SKF focus is on the bearings without intermediate rings which have four outer rings and, of course, on the sealed bearings.

The SKF manufacturing programme comprises the bearings which are briefly described in the following and shown in the product tables. These are also available in several design variants.

TQON design Bearing with two double row inner rings with roller and cage assemblies. Steel window-type cages and four single row outer rings (fig).

TQO design Bearing with two double row inner rings with roller and cage assemblies. Steel window-type cages. One double row outer ring and two single row outer rings. Two intermediate rings between the outer rings and one between the inner rings

(fig).

TQON.1 design Bearing with two double row inner rings with roller and cage assemblies. Pierced rollers and pin-type cages of steel. Four single row outer rings (fig).

TQO.1 design Bearing with two double row inner rings with roller and cage assemblies. Pierced rollers and pin-type cages of steel. One double row outer ring and two single row outer rings. Two intermediate rings between the outer rings and one between the inner rings (fig).

TQOEN design Bearing with two double row inner rings with roller and cage assemblies; the inner rings are extended at one side. Steel window-type cages and four single row outer rings (fig). The extensions on the inner ring are designed as concentric lands for radial shaft seals and are ground.

TQOE design Bearing with two double row inner rings with roller and cage assemblies; the inner rings are extended at one side. Steel window-type cages. One double row outer ring and two single row outer rings. Two intermediate rings between the outer rings and one between the inner rings (fig). The extensions on the inner ring are designed as concentric lands for radial shaft seals and are ground.

TQOT design Bearing with two double row inner rings with roller and cage assemblies. The inner rings have a tapered bore, taper 1:12. Steel window-type cages. One double row outer ring and two single row outer rings. Two intermediate rings between the outer rings (fig). Bearings with a tapered bore are used when an interference fit is required for the inner rings on the roll neck, e.g. when rolling speeds are high.

TQOSN design Bearing sealed at both sides with radial shaft seals. Two double row inner rings with roller and cage assemblies. Steel window-type cages and four single row outer rings (fig).

TQOSNP design Bearing sealed at both sides with radial shaft seals. Two double row inner rings with roller and cage assemblies. Steel window-type cages and four single row outer rings. With spacer sleeves between the two inner rings and the two middle outer rings (fig).

TQOS design Bearing sealed at both sides with radial shaft seals. Two double row inner rings with roller and cage assemblies. Steel window-type cages. One double row outer ring and two single row outer rings. Two intermediate rings between the outer rings and one between the inner rings (fig).

TQOSN.1 design Bearing sealed at both sides with radial shaft seals. Two double row inner rings with roller and cage assemblies. Pierced rollers and pin-type cages of steel. Four single row outer rings (fig).

TQOESN design Bearing sealed at both sides with radial shaft seals. Two double row inner rings with roller and cage assemblies; the inner rings are extended at one side. Steel window-type cages. Four single row outer rings (fig). The extensions on the inner ring are designed as concentric lands for additional seals and are ground.

TQOES design Bearing sealed at both sides. Two double row inner rings with roller and cage assemblies; the inner rings are extended at one side. Steel window-type cages. One double row outer ring and two single row outer rings. Two intermediate rings between the outer rings and one between the inner rings (fig). The extensions on the inner ring are designed as concentric lands for additional seals and are ground.

Bearings of the TQI configuration

Bearings of the TQI design have two paired rows of rollers arranged back-to-back (fig). Their use is limited, being primarily where high stiffness is required and considerable tilting moments have to be accommodated. The bearings are generally mounted with an interference fit on the roll necks and therefore often have a tapered bore. The bearings are also used for the vertical rolls of universal mills; in this case the outer rings are mounted with an interference fit in the chocks and the inner rings have a loose fit on their seat.

Because of their limited application, only a few sizes and designs are produced by SKF. The designs described in the following and shown in the product tables belong to the SKF standard manufacturing programme.

TQIN design Bearing with two single row inner rings and one double row inner ring with roller and cage assemblies. Steel window-type cages and four single row outer rings (fig).

TQI design Bearing with two single row inner rings and one double row inner ring with roller and cage assemblies. Steel window-type cages and two double row outer rings with an intermediate ring between them (fig).

TQI.1 design Bearing with two single row inner rings and one double row inner ring with roller and cage assemblies. Pierced rollers and pin-type cages of steel. Two double row outer rings with an intermediate ring between them (fig).

TQIT design Bearing with two single row inner rings and one double row inner ring with tapered bore (taper 1:12 or 1:30) with roller and cage assemblies. Steel window-type cages and two double row outer rings with an intermediate ring between them (fig). Bearings with a tapered bore are used when an interference fit is required for the inner rings on the roll neck, e.g. when rolling speeds are high.

Design variants To assist identification of the different design variants, letters or letter combinations are used which appear in the product tables under the heading “Design” (fig). They follow the design identification, e.g. TQON/GW. The significance is explained in the following.

SKF Explorer class bearings

SKF Explorer four-row tapered roller bearings are available as open bearings or as sealed bearings and are shown with an asterisk in the product tables. The helical groove in the bore and case hardened bearing rings are standard for SKF Explorer bearings. Each bearing is marked with the designation suffix E, the box is marked with the name “EXPLORER”

Comprehensive inspection facilities and efficient refurbishment Besides the SKF Explorer class features like higher load carrying capacity and longer service life, the SKF Explorer four-row tapered roller bearings offer comprehensive inspection and maintenance facilities:

• The newly developed cage allows the cage and roller assemblies to be removed from the inner rings and to be reinstalled. It is now possible to inspect the inner rings completely and if necessary, to refurbish them.
• Cage and roller assemblies and inner and outer rings of various bearings can now be combined as desired to form “new” bearings. All that is required is to regrind the ring side faces, which is now also possible for inner rings.
• The seals also have been redesigned as to be more maintenance-friendly. They are simply snapped into their retaining grooves. Dismounting and reinstallation could not be easier.

Dimensions The boundary dimensions of the four-row tapered roller bearings (TQO / TQI) have not been standardized by ISO. The dimensions of many of the cones and cups of the inch bearings do, however, conform to the AFBMA Standard 19-1974 (ANSI B3.19-1975). This standard has subsequently been withdrawn.

Tolerances SKF four-row tapered roller bearings are produced with dimensional accuracy corresponding to

• Normal tolerances for metric bearings
• Normal tolerances for inch bearings.

The running accuracy of all bearings is to tolerance class P5 specifications

The Normal and class P5 tolerances for the metric bearings conform to ISO 492-2002. The values of the Normal tolerances for inch bearings follow tolerance class 4 according to ANSI/ABMA Standard 19.2-1994. The ISO Standard 578:1987, which also covered this tolerance class for inch bearings was withdrawn in 1997.

Internal clearance SKF four-row tapered roller bearings are delivered as ready-to-mount bearing units with an axial internal clearance adapted to the actual application. The bearing components must be arranged in the prescribed order and may not be interchanged with components of another bearing.

Four-row tapered roller bearings having a clearance other than that of the basic design are identified by the designation suffix C followed by a three or four-figure number which is the mean value of the axial internal clearance expressed in �m.

Influence of operating temperatures on bearing material SKF four-row tapered roller bearings are subjected to a unique heat treatment process that enables the bearings to be operated up to +150 �C without any inadmissible dimensional changes occurring.

Cages SKF four-row tapered roller bearings are fitted with either

• four pressed steel window-type cages (fig) or
• four steel pin-type cages (fig).

Minimum load To achieve satisfactory operation, four-row tapered roller bearings, like all ball and roller bearings, must always be subjected to a given minimum load. Otherwise 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 may cause damaging sliding movements to occur between the rollers and raceways.

The requisite minimum load to be applied can be obtained from

Frm = 0,02 C

where

Frm = minimum radial load [kN]

C = basic dynamic load rating [kN] (see product data)

The weight of the components supported by the bearing, together with external forces, usually exceeds the requisite minimum load. If this is not the case, the four-row tapered roller bearing must be subjected to an additional radial load.

Equivalent dynamic bearing load

P = Fr + Y1Fa when Fa/Fr = e

P = 0,67 Fr + Y2Fa when Fa/Fr > e

The values for the calculation factors e, Y1 and Y2 can be found in the product tables.

Equivalent static bearing load

P0 = Fr + Y0Fa

Values for the calculation factor Y0 can be found in the product tables.

Comparative load ratings For rolling mill applications, load ratings are typically not calculated according to ISO 281:2007 but are calculated 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 life definition) “comparative” load ratings calculated by the same non-ISO method are provided in the product tables.

These comparative load ratings may only be used together with the life and equivalent load equations specified below; they may not be used to calculate an ISO rating life.

Comparative life calculation The comparative life is calculated using the comparative load rating CF as follows:

LF10 = 90 (CF/PF)10/3

or

LF10h = (CF/PF)10/3 (1 500 000/n)

where

LF10 = comparative rating life, million revolutions

LF10h = comparative rating life, operating hours

CF = comparative dynamic load rating to give a rating life of 90 million revolutions, kN

PF = equivalent dynamic bearing load (for conditions please refer to table 1), kN

n = constant operating speed, r/min

For load cases 1a) and 1b), see table 1, it is necessary to use the load rating for one roller row when using PFL. This load rating (for 1 row) can be obtained from

CF(row) = 0,29 CF(bearing)

Supplementary designations The designation prefixes and suffixes used to identify SKF four-row tapered roller bearings are explained in the following.

Bearings to the SKF Explorer class specifications are identified by the suffix E and have inner and outer rings of case hardened steel as standard. The helical groove in the bore is also standard. Therefore, the relevant suffixes (HA1 and G) are not used for these bearings.

Design of bearing arrangements

Roll neck requirements In most rolling mill applications four-row tapered roller bearings are mounted with a loose fit on the roll neck. The roll neck journal and the axial abutment for the inner rings must have a certain minimum hardness. The recommended hardness is

• 45 Shore (� 34 HRC) for the roll neck surface and
• 60 Shore (� 45 HRC) for the axial abutments for the inner rings.

Axial location of inner rings on the roll neck The inner rings must not be axially clamped. There must remain a total clearance between the bearing rings and their abutments of 0,2 to 1,3 mm (fig).

Mounting instructions When mounting four-row tapered roller bearings, the components must be assembled in the correct order. To facilitate this, they are marked with letters (fig). All the components of one bearing are also marked with the same serial number so that mixing is avoided if several bearings are being mounted at the same time.

As in the majority of applications the direction of load is constant with respect to the outer ring only approximately one quarter of the outer ring raceway will be under load. For this reason the side faces of the outer rings are divided into four zones marked I to IV. The markings for zone I are also joined by a line across the outside diameter of the rings (fig). When the bearings are first installed it is customary for zone I to lie in the direction of the load. Depending on the operating conditions, the outer rings should be turned through 90� after a given period of service when the rolls are changed so that a new zone comes under load.

Detailed mounting and maintenance instructions will be supplied on request. If skilled personnel is not available to mount the bearings it is advisable, particularly where large bearings are concerned, to request the assistance of SKF service personnel. Further details of the SKF mounting service will be supplied on request.

Tapered roller bearings | Single row tapered roller bearings | Paired single row tapered roller bearings | Double row tapered roller bearings | Four-row tapered roller bearings