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CARB® Toroidal Roller Bearings

The CARB toroidal roller bearing is a completely new type of radial roller bearing (fig 1). This compact self-aligning roller bearing was developed by SKF and introduced on the market in 1995. In a unique design, it combines the self-aligning capability of the spherical roller bearing with the unconstrained axial displacement ability of the cylindrical roller bearing. It can also have the compact cross section normally associated with the needle roller bearing.

Demo, self-aligning

Demo, axial displacement

The applicability of CARB bearings covers a wide range with regard to radial loads. They are intended exclusively as non-locating bearings and as such they are excellent with their combination of self-aligning and axial displacement properties, opening up completely new opportunities to save space, weight and production costs. By deliberately displacing the rings axially with respect to each other it is possible to accurately set the radial internal clearance in the bearing.

CARB bearings permit smaller and lighter bearing arrangement designs offering the same or improved performance in a particularly impressive manner, e.g. in planetary gearboxes. They simplify the bearing arrangement design for long shafts that are subjected to temperature variations. When using CARB bearings, it has also been proven that vibration levels are reduced, e.g. in paper machines or fans.

The CARB bearing is a single row bearing with long, slightly crowned symmetrical rollers. The raceways of both the inner and outer rings are concave and situated symmetrically about the bearing centre. The attained optimal combination of both raceway profiles provides a favourable load distribution in the bearing, as well as low frictional running.

The rollers of the CARB bearing are self-guiding, i.e. they will always adopt the position where the load is evenly distributed over the roller length - irrespective of whether the inner ring is axially displaced and/or misaligned with respect to the outer ring.

The load carrying capacity of the CARB bearing is very high even when it has to compensate for angular misalignment or axial displacement. This results in an operationally reliable bearing arrangement with long service life.

Open bearings

CARB toroidal roller bearings are produced to two basic designs depending on bearing size and series as

  • bearings with cage (fig 2a)
  • full complement bearings (fig 2b).

The load carrying capacity of the full complement CARB bearing is appreciably higher than that of the caged bearing. Both designs are available with a cylindrical bore as well as with a tapered bore. Depending on bearing series the tapered bore has a taper of either 1:12 (designation suffix K) or 1:30 (designation suffix K30).

Sealed bearings

Today, the range of sealed bearings (fig 3) consists of small and medium size full complement bearings for low speeds. These bearings with seals on both sides are filled with a grease for high temperature and long life, and they are maintenance-free.

The double lip seal, suitable for high temperature operations, is sheet steel reinforced and made of hydrogenated-acrylonitrile butadiene rubber (HNBR). It seals against the inner ring raceway. The outside diameter of the seal is retained in an outer ring recess and provides proper sealing also in applications with outer ring rotation. The seals can withstand operating temperatures in the range of –40 and +150 °C.

The sealed bearings are filled with premium quality grease, with a polyurea thickener and synthetic ester base oil. This grease has good corrosion inhibiting properties and can be used at temperatures between –25 and +180 °C. The base oil viscosity is 440 mm2/s at 40 °C and 38 mm2/s at 100 °C. The grease fill is 70 to 100% of the free space in the bearing.

Sealed bearings with other lubricating greases or degrees of grease fill can be supplied on request.

Bearings for vibratory applications

For non-locating bearing positions in vibratory applications SKF manufactures CARB bearings with a surface hardened pressed steel cage in the C 23/C4VG114 series with a cylindrical bore. These bearings have the same dimensions and product data as the bearings in the C 23 series. They enable a press fit on the shaft to avoid possible fretting corrosion caused by a loose fit on the shaft. Using CARB bearings in vibratory applications on the non-locating side of the bearing arrangement will result in a self-aligning bearing system with better performance and reliability.

For additional information on CARB bearings in the C 23/C4VG114 series, please consult the SKF application engineering service.

SKF Explorer class bearings

All CARB bearings are manufactured to the SKF Explorer performance class.

Bearings on sleeves

CARB bearings with a tapered bore can be mounted on smooth or stepped shafts using

  • an adapter sleeve (fig 4), see product table
  • a withdrawal sleeve (fig 5), see product table.

Where appropriate, modified adapter sleeves of the E, L and TL designs are available for CARB bearings, to prevent the locking device from chafing the adjacent cage:

  • With the E-design sleeve, the standard KM lock nut and MB locking washer are replaced by a KMFE nut (fig 6a), and the standard lock nut HM 30 is replaced by an HME 30 nut with a recess at the outside diameter (fig 6b).
  • The L-design sleeve differs from the standard design in that the standard KM lock nut and MB locking washer have been replaced by a KML nut and MBL locking washer; implying lower sectional height (fig 6c).
  • With the TL-design sleeve, the standard HM .. T lock nut and MB locking washer have been replaced by the corresponding HM 30 nut and MS 30 locking clip; implying lower sectional height (fig 6d).

Where larger axial displacements can occur, it is recommended to observe the information in the section "Space at the sides of the bearing".

More detailed information on the SKF sleeves will be found in the section "Bearing accessories".

Appropriate bearing housings

The combination of a CARB bearing and an appropriate bearing housing constitutes an economic, interchangeable and reliable non-locating bearing arrangement, which fulfils the demands for easy maintenance. SKF standard housings are available for almost all CARB bearings of diameter series 0, 1, 2 and 3. Two bearing arrangement types are possible without requiring special measures:

  • CARB bearing on an adapter sleeve and smooth shafts.
  • CARB bearing on cylindrical seat and stepped shafts.

Following housings may be used for CARB bearings:

  • SNL plummer block housings in the 2,3, 5 and 6 series (fig).
  • Large SNL plummer block housings in the 30 and 31 series (fig).
  • SDG plummer block housings (fig).
  • SONL plummer block housings for oil lubrication (fig).
  • SAF pillow (plummer) block housings for inch shafts (fig).
  • SDAF pillow (plummer) block housings for inch shafts (fig).
  • SBD plummer block housings (fig).
  • FNL flanged housings (fig).
  • 7225(00) flanged housings (fig).
  • THD take-up housings (fig).

Detailed information on housings can be found in the catalogue "Bearing housings".

The fig shows a CARB bearing C 2220 in an SNL plummer block housing. (Note that the outer ring has to be secured in the housing using the two locating rings.)

DimensionsThe boundary dimensions of CARB bearings (open bearings / sealed bearings) are in accordance with ISO 15:1998. The dimensions of the adapter and withdrawal sleeves correspond to ISO 2982-1:1995.

TolerancesSKF CARB bearings are manufactured as standard to Normal tolerances. Bearings up to and including 300 mm bore diameter are produced to higher precision than the ISO Normal tolerances. For example

  • the width tolerance is considerably tighter than the ISO Normal tolerance; the tolerance is the same as for Explorer spherical roller bearings
  • the running accuracy is to tolerance class P5 as standard.

For larger bearing arrangements where running accuracy is a key operational parameter, SKF CARB bearings with P5 running accuracy are also available. These bearings are identified by the suffix C08. Their availability should be checked.

The values of the tolerances are in accordance with ISO 492:2002.

Internal clearanceCARB bearings are produced as standard with Normal radial internal clearance and most are also available with a larger C3 clearance. Many bearings can also be supplied with a smaller C2 clearance or with a much greater C4 or C5 clearance.

The radial internal clearance limits are listed for bearings with

  • cylindrical bore in table 1
  • tapered bore in table 2.

The limits are valid for bearings before mounting under zero measuring load, and with no axial displacement of one ring relative to the other.

Axial displacement of one ring relative to the other will gradually reduce the radial internal clearance in a CARB bearing. The amount of axial displacement encountered in cases without external heating of the shaft or foundation will have little effect on the radial internal clearance, see section "Axial displacement".

CARB bearings are often used together with spherical roller bearings. The clearance of the CARB bearing is slightly larger than that of the corresponding spherical roller bearing having the same clearance class. An axial displacement of the inner ring relative to the outer ring of 6 to 8% of the bearing width will reduce the operational clearance to approximately the same value as a spherical roller bearing of the same size.

MisalignmentDuring operation, angular misalignment of up to 0,5° between the inner and outer rings (fig 7) can usually be accommodated by a CARB bearing without any negative consequences for the bearing.

Demo

However, misalignment values greater than 0,5° will increase friction and influence bearing service life. For misalignment greater than 0,5° please consult the SKF application engineering service. The ability to compensate for misalignment when the bearing is stationary is also limited. For CARB bearings with a machined brass cage centred on the inner ring, designation suffix MB, the misalignment should never exceed 0,5°.

Misalignment displaces the rollers axially, causing them to approach the side faces the bearing rings. Therefore, possible axial displacement should be reduced, see section "Axial displacement".

Axial displacement

CARB toroidal roller bearings can accommodate axial displacements of the shaft with respect to the housing within the bearing. The axial displacement can result from thermal expansion or deviations from determined bearing positions.

Misalignment as well as axial displacement influences the axial positions of the rollers in a CARB bearing. Axial displacement also reduces the radial clearance. SKF recommends checking that the axial displacement is within acceptable limits, i.e. the residual clearance is great enough, and that the rollers do not protrude outside the side face of a ring (fig 8a) or contact any locking ring (fig 8b) or seal. To accommodate the displacement of the roller and cage assembly, provide free space at both sides of the bearing as described in the section "Free space at the sides of the bearing" and indicated in fig 10.

The axial displacement from the normal position of one bearing ring in relation to the other is limited by

  • the displacement of the roller set, or by
  • the reduction of clearance.

The maximum possible axial displacement is obtained from the smaller of these two limitations.

Limitation caused by the displacement of the roller setThe guideline values s1 and s2 for axial displacement (fig 8) shown in the product tables are valid provided

  • there is a sufficiently large operational clearance in the bearing before shaft elongation, and
  • the rings are not misaligned.

The reduction in the axial displacement caused by the misalignment can be estimated using

smis = k1 B a

where

smis = reduction in axial displacement caused by misalignment [mm]

k1 = misalignment factor (see product data)

B = bearing width [mm] (see product data)

a = misalignment [degrees]

Assuming a sufficiently large operational clearance, the maximum possible axial displacement is obtained from

slim = s1 – smis

or

slim = s2 – smis

where

slim = possible axial displacement relative to the movement of the roller set caused by misalignment [mm]

s1 = guideline value for the axial displacement capability in bearings with a cage or in full complement bearings when displacing away from the snap ring [mm] (see product data)

s2 = guideline value for the axial displacement capability in sealed or full complement bearings when displacing towards the seal or snap ring respectively [mm] (see product data)

smis = reduction in axial displacement caused by misalignment [mm]

Calculation example 1For bearing C 3052 with

  • a width B = 104 mm
  • a misalignment factor k1 = 0,122
  • a value for the axial displacement s1 = 19,3

with an angular misalignment a = 0,3° between the inner and outer ring, the possible axial displacement can be obtained from

slim = s1 – smis

slim = s1 – k1 B a

slim = 19,3 – 0,122 × 104 × 0,3 = 19,3 – 3,8

slim = 15,5 mm

Limitation caused by the reduction of clearanceThe radial clearance reduction corresponding to axial displacement from a centred position can be calculated using

In cases where the reduction of the clearance is greater than the radial clearance before shaft elongation, the bearing will be preloaded. If instead a certain radial clearance reduction is known, the corresponding axial displacement from a centred position can be calculated using

where

scle = axial displacement from a centred position corresponding to a certain radial clearance reduction [mm]

Cred = reduction of radial clearance as a result of an axial displacement from a centred position

k2 = operating clearance factor (see product data)

B = bearing width [mm] (see product data)

See also calculation examples 2 and 3 below.

The axial displacement capability can also be obtained using diagram 1, which is valid for all CARB bearings. The axial displacement and radial clearance are shown as functions of the bearing width.

From diagram 1 it can be seen (dotted line) that for a bearing C 3052 K/HA3C4, for an operational clearance of 0,15 mm which corresponds to approximately 0,15% of the bearing width, an axial displacement of approximately 12% of the bearing width is possible. Thus, when an axial displacement of approximately 0,12 × 104 = 12,5 mm has taken place, the operational clearance will be zero.

It should be remembered that the distance between the dotted line and the curve represents the residual radial operating clearance in the bearing arrangement.

Diagram 1 also illustrates how it is possible, simply by axially displacing the bearing rings relative to each other, to achieve a given radial internal clearance in a CARB bearing.

Calculation example 2For bearing C 3052/HA3C4 having

  • a width B = 104 mm
  • an operating clearance factor k2 = 0,096
  • an operational clearance of 0,15 mm,

the possible axial displacement from the central position of one ring to the other until the operational clearance equals zero can be obtained from

scle = 12,7 mm

The axial displacement of 12,7 mm is below the guideline value, s1 = 19,3 mm, shown in the product table. An operating misalignment of 0,3° is also permissible, see also example 1 in section "Limitation caused by the displacement of the roller set".

Calculation example 3For bearing C 3052, which has

  • a width B = 104 mm
  • an operating clearance factor k2 = 0,096

the reduction in operational clearance Cred caused by an axial displacement scle = 6,5 mm from the central position is calculated using

Cred = 0,039 mm

MountingWhen mounting a CARB bearing onto a shaft or in a housing, both bearing rings and the roller complement must be centred with respect to each other. For this reason SKF recommends mounting CARB bearings when the shaft or housing is in the horizontal position.

When mounting a CARB bearing onto a vertical shaft or into a vertical housing, the roller complement together with the inner or outer ring will move downwards until all clearance has been removed. Unless proper clearance is maintained during and after installation, the expansion or compression forces resulting from an interference fit on either the inner or outer ring will create a preload. This preload can cause indentations in the raceways and/or prevent the bearing from turning altogether. To prevent this preload condition from occurring during vertical mounting, a bearing-handling tool, which keeps the bearing components centred, should be used.

Detailed instructions for a selected bearing can be found at skf.com/mount.

Mounting bearings with a tapered bore

Bearings with a tapered bore are always mounted with an interference fit. The reduction in radial internal clearance, or the axial displacement of the inner ring on its tapered seat is used as a measure of the degree of interference.

Suitable methods for mounting CARB bearings with a tapered bore are:

  • Measuring the clearance reduction.
  • Measuring the lock nut tightening angle.
  • Measuring the axial drive-up.
  • Measuring the inner ring expansion.

Small bearings with bore diameter up to 100 mm can be properly mounted by measuring the lock nut tightening angle.For larger bearings the SKF Drive-up Method is recommended. This method is more accurate and takes less time than the procedure based on clearance reduction or the lock nut tightening angle. Measuring the inner ring expansion i.e. applying the SKF SensorMount® Method, enables large size bearings to be mounted simply, quickly and accurately, since a sensor is integrated into the bearing inner ring.

Detailed instructions for a selected bearing can be found at skf.com/mount.

Measuring the clearance reductionThis method, which uses feeler gauges for measuring the radial internal clearance before and after mounting bearings, is applicable for medium and large-sized bearings.

Before measuring, rotate the outer ring a few times. Make sure that both bearing rings and the roller complement are centrically arranged with respect to each other.

For the first measurement, a blade should be selected which is slightly thinner than the minimum value for the clearance. During the measurement, the blade should be moved back and forth until it can be inserted to the middle of the roller. The procedure should be repeated using slightly thicker blades each time until a certain resistance is felt when moving between

  • outer ring and uppermost roller (fig 12a) - before mounting
  • inner or outer ring and lowest roller depending on the cage (fig 12b) - after mounting.

For larger bearings, especially those having a rather thin-walled outer ring, the measurements may be affected by the elastic deformation of the rings, caused by the weight of the bearing or the force to draw the feeler gauge blade through the gap between the raceway and an unloaded roller. To establish in such cases the "true" clearance before and after mounting, the following procedure should be followed (fig 12c):

  • Measure the clearance "c" at the 12 o'clock position for a standing bearing or at the 6 o'clock position for a bearing hanging on a journal.
  • Measure clearances "a" at the 9 o'clock position and "b" at the 3 o'clock position without the bearing being moved.
  • Obtain the "true" radial internal clearance with relatively good accuracy from 0,5(a + b + c).

Recommended values for reduction of radial internal clearance are provided in table 3.

Detailed instructions for a selected bearing can be found at skf.com/mount.

Measuring the lock nut tightening angle

Mounting small to medium-size bearings on tapered seats is easy when the tightening angle a of the lock nut (fig 13) and the method that is described in the following is used. Recommended values for the tightening angle a are provided in table 3.

Before starting the final tightening procedure, the bearing should be pushed up on the tapered seat until the bore of the bearing or the sleeve is in contact with the seat on the shaft around its whole circumference, i.e. the bearing inner ring cannot be rotated relatively to the shaft. By then turning the nut through the given angle a, the bearing will be pressed up the tapered seat. The residual clearance of the bearing should be checked, if possible.

If using a KM nut, unscrew the nut and place the locking washer in position. Tighten the nut firmly again and lock it by bending one of the locking washer tabs into the nut slot. If using a KMFE nut, lock the nut by tightening the grub screw with the recommended tightening torque.

Detailed instructions for a selected bearing can be found at skf.com/mount.

Measuring the axial drive-up

Mounting bearings with a tapered bore can be done by measuring the axial drive-up of the inner ring on its seat. Recommended values for the required axial drive-up "s" for general applications are provided in table 3.

The most suitable method in this case is the SKF Drive-up Method. This mounting method provides a very reliable and easy way to determine the starting position for a bearing from which the axial displacement is to be measured. For that, the following mounting tools (fig 14) must be used

  • an SKF hydraulic nut of the HMV .. E design
  • a hydraulic pump
  • a pressure gauge, appropriate to the mounting conditions
  • a dial gauge.

Applying the SKF Drive-up Method the bearing is pushed up its seat to a defined starting position (fig 15) using a given oil pressure (corresponding to a given drive-up force) in the hydraulic nut. In this way, part of the desired reduction in radial internal clearance is achieved. The oil pressure is monitored by the pressure gauge. The bearing is then driven up from the defined starting position through a given distance to its final position. The axial displacement "ss" is accurately determined using the dial gauge mounted on the hydraulic nut and can be calculated for individual bearings at skf.com/mount where also detailed mounting instructions are available. Values for some common conditions are given in table 4.

NoteThe axial displacement "ss" is not the same distance as the distance "s" in table 3.

Demo Video

Measuring the inner ring expansion

Measuring inner ring expansion enables large size CARB bearings with a tapered bore to be mounted simply, quickly and accurately without measuring the radial internal clearance before and after mounting. The SensorMount Method uses a sensor, integrated into the CARB bearing inner ring, and a dedicated hand-held indicator (fig 17).

The bearing is driven up the tapered seat using common SKF mounting tools. The information from the sensor is processed by the indicator. Inner ring expansion is displayed as the relationship between the clearance reduction (mm) and the bearing bore diameter (m).

Aspects like bearing size, smoothness, shaft material or design - solid or hollow - do not need to be considered.

For detailed information about the SensorMount Method, please contact the SKF application engineering service.

Detailed instructions for a selected bearing can be found at skf.com/mount.


Bearing terminology | Y-bearings | 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 | CARB® 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 | Bearing accessories | Engineering products | Mechatronics

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