Spherical ball bearings are bearings with two rows, two divided raceways on the inner ring and one shared concave raceway on the outer ring. They are particularly suitable for compensation of misalignments between shaft and housing. Spherical ball bearings can absorb radial and axial forces and are suitable for smooth and cool operation at increased speeds due to low friction.
KRW supplies spherical ball bearings with normal tolerance (PN) in accordance to DIN 620-2 (Tolerances for roller bearings) and ISO 492 (Radial bearings - Dimensional and geometrical tolerances). All other deviating or special tolerances must be specified with the order.
Versions of spherical ball bearings
Spherical ball bearings are self-latching bearings that cannot be disassembled. They can absorb high radial forces as well as low axial forces from both directions. The minimum load of the bearing is very low. Spherical ball bearings can be delivered with cylindrical or tapered bore.
By default, spherical ball bearings by KRW come with a two-piece roller-riding solid brass cage (suffix: M). Other cage designs are available on request or chosen for specific applications and labelled accordingly on the bearing.
Compensation of angular misalignment in a spherical ball bearing
Compensation of Angular Misalignments
Spherical ball bearings are suitable for compensation of misalignments between inner and outer bearing rings due to shaft deflections, angular errors and housing deformation. The admissible angular misalignment with static and circulating inner ring differs between versions. The maximal angular adjustment α is available on request.
KRW distinguishes between kinematic limiting speed nG and thermal reference speed nth. The kinematic limiting speed is a practical mechanical limit value and is based on the mechanical fatigue strength of the rolling bearing as a function of its installation situation and lubrication. The limit speed must not be exceeded even under optimum operating conditions without prior consultation with KRW.
The thermal reference speed represents the equilibrium between the heat generated in the bearing by friction and the heat flow dissipated. It is standardised in DIN ISO 15312 (Rolling bearings - Thermal reference speed).
Admissible Operating Temperatures
The admissible operating temperature of a bearing is limited by cage material, dimensional stability of the bearing components (ball race and rolling elements), as well as lubrication. By default, KRW bearings are stabilised up to 200°C (S1). KRW provides roller bearings for higher operating temperatures on request.
For dynamically loaded bearings
The service life formula according to ISO 281 L10 = (C/P)p for dynamically loaded bearings requires an equivalent load (P) from constant direction and size. To calculate P, calculation factors and the ratio of axial and radial load are required.
Equivalent dynamic bearing load P
The equivalent bearing service life for spherical ball bearings depends on the ratio Fa/Fr (axial force / radial force). The equivalent dynamic bearing load can then be determined using the following formula:
|P||equivalent dynamic load||[kN]|
|Fr||dynamic radial force||[kN]|
|Fa||dynamic axial force||[kN]|
|e||calculation factor, see chart||[-]|
|Y1||calculation factor, see chart||[-]|
|Y2||calculation factor, see chart||[-]|
For statically loaded bearings
Dynamic dimensioning loses its validity for bearings rotating at very low speeds (n x dm ≤ 4000 mm/min). The static load safety factor S0 is calculated as follows:
|S0||static load safety factor||[-]|
|C0||basic static load rating (from bearing chart)||[kN]|
|P0||equivalent static bearing load||[kN]|
|dm||mean bearing diameter [dm = (D+d)/2]||[mm]|
Static load capacity
Axial load capacity
The axial load capacity of spherical ball bearings is low due to the broad osculation on the outer ring.
When using spherical ball bearings with clamping sleeves on smooth shafts it must be made sure that, despite enough security, the max. occurring axial loads are lower than the pressed connection between shaft and bearing bore. For a professional installation, the following formula can be used to estimate the admissible axial load capacity:
|Fa,zul||max. admissible axial load||[kN]|
|B||bearing width, see product chart||[mm]|
|d||bore diameter, see product chart||[mm]|
Minimum Radial Load
A minimum load is required for the reliable operation of a rolling bearing. If the minimum load is not reached, slippage may occur. The minimum radial load for spherical ball bearings is roughly assumed to be 1% of the static load rating C0 of the bearing. If the value falls below this value, consult KRW Application Engineering.