Four-point contact ball bearings

Four-point contact ball bearings are radial single row angular contact ball bearings with raceways that are designed to support axial loads acting in both directions. Radial loads can be supported up to a certain fraction of the axial load. These bearings take up considerably less axial space than double row bearings.

SKF four-point contact ball bearings of the QJ design (fig) have a contact angle of either 35� or 45�. The inner ring is split. This enables a large number of balls to be incorporated in the bearing thus giving the bearing high load carrying capacity. The bearings are of separable design, i.e. the outer ring with ball and cage assembly can be mounted separately from the two inner ring halves.

Both inner ring halves of SKF Explorer four-point contact ball bearings have a recessed shoulder (fig). This improves the oil flow when the bearing is used together with an SKF cylindrical roller bearing (fig). In addition, these recesses can be used to facilitate dismounting..

Bearings with locating slots

In many applications a radial bearing is used in combination with a four-point contact ball bearings which acts as a pure thrust bearing and is mounted with radial clearance in the housing (fig). To restrain the outer ring from turning in the circumferential direction, bearings with two locating slots (designation suffix N2) in the outer ring positioned at 180� to each other are available (fig).

SKF Explorer class bearings

High performance SKF Explorer four-point contact ball bearings are shown with an asterisk in the product table. SKF Explorer bearings retain the designation of the earlier standard bearings, e.g. QJ 309 N2MA. However, each bearing and its box are marked with the name “EXPLORER”.

Dimensions The boundary dimensions of SKF four-point contact ball bearings are in accordance with ISO 15:1998.

Tolerances SKF four-point contact ball bearings are produced as standard to Normal tolerances. Some sizes are also available with enhanced precision to tolerance class P6 specifications.

SKF Explorer four-point contact ball bearings are made to P6 tolerance class, except that the width tolerance is reduced to 0/�40 �m.

Standard large bearings, with an outside diameter of approximately 400 mm and above, are manufactured to Normal tolerances for dimensional accuracy and to P6 tolerances for running accuracy.

The values for tolerances correspond to ISO 492:2002.

Internal clearance SKF four-point contact ball bearings are supplied with Normal axial internal clearance as standard, but some sizes are available with greater or smaller clearance, or with reduced clearance limits.

The actual clearance limits are provided in table 1 and are valid for bearings before mounting under zero measuring load.

Misalignment The ability of four-point contact ball bearings to tolerate misalignment of the inner ring with respect to the outer ring, and consequently the ability to compensate for misalignment in the application or to tolerate shaft deflections is limited. It depends on the internal clearance in operation, bearing size and the magnitude of the forces and moments acting on the bearing. The interrelationship of these factors is complex and no general rules can be provided.

Any misalignment will lead to increased running noise, cage stresses and reduced bearing service life.

Influence of operating temperature on bearing material

SKF four-point contact ball bearings undergo a special heat treatment. When fitted with a steel, brass or PEEK cage, they can operate at temperatures up to +150 �C.

Cages SKF four-point contact ball bearings are fitted with one of the following cages

• a machined window-type brass cage, outer ring centred, designation suffix MA (fig)
• an injection moulded window-type cage of glass fibre reinforced polyetheretherketone (PEEK) with lubrication grooves in the guiding surfaces, outer ring centred, designation suffix PHAS.

If bearings with a PEEK cage are required, other than listed, please contact the SKF application engineering service.

Minimum load To achieve satisfactory operation, four-point contact ball 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 inertia forces of the balls and cage, and the friction in the lubricant, can have a detrimental influence on the rolling conditions in the bearing and may cause sliding damaging movements to occur between the balls and raceways.

The requisite minimum load to be applied to four-point contact ball bearings can be estimated using

where

Fam = minimum axial load [kN]

ka = factor depending on bearing series (see product data)

C0 = basic static load rating [kN] (see product data)

n = rotational speed [r/min]

dm = mean bearing diameter

= 0,5 (d + D) [mm]

When starting up at low temperatures or when the lubricant is highly viscous, even greater minimum loads may be required. The weight of the components supported by the bearing, together with external forces, generally exceeds the requisite minimum load. If this is not the case, the four-point contact ball bearing must be subjected to an additional axial load, for example, by means of springs.

Equivalent dynamic bearing load

If four-point contact ball bearings are arranged as locating bearings and have to accommodate both radial and axial loads, the equivalent dynamic bearing load is obtained from

P = Fr + Y1Fa when Fa/Fr = e

P = XFr + Y2Fa when Fa/Fr > e

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

It should be noted that four-point contact ball bearings will only function properly when the balls contact the raceways at one point per raceway. This is the case when the axial load

Fa = 1,27 Fr for bearings which have a contact angle of 35� (series 2, 3 and 10), or

Fa = 1,78 Fr for bearings which have a contact angle of 45� (series 12).

If the four-point contact ball bearing is arranged with radial freedom in the housing to act as a thrust bearing in combination with a radial bearing, the usual arrangement for these bearings (fig), then the equivalent dynamic bearing load for bearings with a contact angle of 35� (series 2, 3 and 10) becomes

P = 1,07 Fa

and for bearings with a contact angle of 45� (series 12)

P = 0,81 Fa .

Equivalent static bearing load

P0 = Fr + Y0Fa

The value of Y0 depends on the bearing series and can be found in the product table.

Supplementary designations The designation suffixes used to identify certain features of SKF four-point contact ball bearings are explained in the following.

Design of bearing arrangements

The outer ring of bearings, which are arranged as thrust bearings with radial clearance in the housing, should not be clamped (fig). Otherwise the outer ring cannot compensate for thermal movements, which will cause additional force in the bearing. If clamping the outer ring cannot be avoided, the outer ring must be at least carefully centred during mounting.

Angular contact ball bearings | Single row angular contact ball bearings | Double row angular contact ball bearings | Four-point contact ball bearings | Other SKF angular contact ball bearings