Dynamic axial load carrying capacity

Double row cylindrical roller bearings with flanges on both the inner and outer rings can support axial loads in addition to radial loads. Their axial load carrying capacity is primarily determined by the ability of the sliding surfaces of the roller end / flange contact to support loads. Factors having the greatest effect on this ability are the lubricant, operating temperature and heat dissipation from the bearing.

Calculating the dynamic axial load carrying capacity

Under normal operating conditions the axial load carrying capacity can be estimated using the equations below. Conditions that are considered typical for normal bearing operation are:
  • a certain temperature
    There is a difference of 60 °C between the bearing operating temperature and the ambient temperature.
  • a specific heat loss from the bearing
    There is a flow of 0,5 mW/mm2 °C; with reference to the bearing outside diameter surface (A = π D B).
  • adequate lubrication
    A viscosity ratio κ ≥ 2 is required (see the section Lubrication conditions - the viscosity ratio κ). For grease lubricated applications, the viscosity of the base oil in the grease may be used. If κ is less than 2, friction and wear will increase. These effects can be reduced at low speeds, for example, by using lubricants with AW (anti-wear) or EP (extreme pressure) additives. In grease lubricated applications where axial loads act for long periods, the bearings should be relubricated frequently with a grease that has good oil bleeding properties at normal operating temperature (> 3% according to DIN 51 817).
  • sufficient radial load
    The value of the radial load should be at least twice the value of the axial load. A lower ratio (axial versus radial load) is possible, but should be checked by the SKF application engineering service.
  • limited misalignment
    Where misalignment between the inner and outer rings exceeds 1 minute of arc, the action of the load on the flange changes considerably. Therefore the safety factors included in the guideline values may be inadequate. In these cases, contact the SKF application engineering service.
For bearings with a heat emitting reference surface area Ar ≤ 50 000 mm2, the permissible axial load can be calculated with sufficient accuracy from



Start calculation
For bearings with a heat emitting reference surface area Ar > 50 000 mm2, the permissible axial load can be calculated with sufficient accuracy from



Start calculation
When circulating oil lubrication provides efficient cooling, the permissible axial load can be raised by

ΔFap = k1 ΔTs Vs 15 × 104 / (n (d + D))

where

Ar=heat emitting reference surface area in accordance with ISO 15312:2003
=π B (D + d) [mm2]
Fap=permissible axial load [kN]
ΔFap=raise for permissible axial load due to cooling [kN]
C0=basic static load rating [kN]
Fr=actual radial bearing load [kN]
n=rotational speed [r/min]
d=bearing bore diameter [mm]
D=bearing outside diameter [mm]
B=bearing width [mm]
ΔTs=temperature difference between oil inlet and outlet [°C]
Vs
oil flow through the bearing [l/min]
k1=a factor
= 0,5 for oil lubrication
= 0,3 for grease lubrication
k2=a factor
= 0,05 for oil lubrication
= 0,03 for grease lubrication
The values for the permissible load Fap obtained from the equations are valid for a constant and continuous axial load provided there is an adequate supply of lubricant to the roller end / flange contacts. Where axial loads act only for short periods, the values can be multiplied by 2. For shock loads the values can be multiplied by 3, provided the following limits relative to flange strength are not exceeded. A short period can last anywhere from several seconds to a few minutes. It is characterized by a temperature spike of not more than 5 °C after which time the bearing returns to normal operating temperature. As a rule of thumb, a short period is considered as the time it takes for the bearing to make 1 000 revolutions. Values calculated according to the above equations are not hard limits. If higher axial load carrying capacity than calculated is required, contact the SKF application engineering service for detailed analysis.

Axial load limit relative to flange strength

To avoid the risk of flange fracture, the constantly acting axial load applied to the bearings should never exceed

Famax = 0,0023 D1,7

where

Famax=maximum constantly acting axial load [kN]
D=bearing outside diameter [mm]
Where axial loads act only for short periods, the values for Famax can be multiplied by a factor of 2 while shock loads can be multiplied by a factor of 3.

Requirements for abutments

In applications where double row cylindrical roller bearings are subjected to heavy axial loads, axial runout and the size of the abutment surfaces of adjacent components can affect flange load and running accuracy. To obtain an even flange load and provide proper running accuracy, use the values provided in the table. For the diameter of the abutment surfaces, SKF recommends supporting the inner ring at a height corresponding to half the flange height (fig 1).
das = 0,5 (d1 + F)

where

das=shaft abutment diameter [mm]
d1=inner ring flange diameter [mm]
F=inner ring raceway diameter [mm]
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