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Single 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 gradient
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).
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| – |
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.
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| – |
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.
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For bearings with a heat emitting reference surface area Ar ≤ 50 000 mm2, the permissible axial load
can be calculated with sufficient accuracy from
Fap = k1 C0 104 / (n (d + D)) - k2 Fr
For bearings with a heat emitting reference surface area Ar > 50 000 mm2, the permissible
axial load can be calculated with sufficient accuracy from
Fap = k1 C0
2/3 7,5 × 104 / (n (d + D)) - k2 Fr
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
= 1,5 for oil lubrication
= 1 for grease lubrication
|
| k2
|
= |
a factor
= 0,15 for oil lubrication
= 0,1 for grease lubrication
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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 = 0,0045 D1,5 (bearings in the 2 Diameter Series)
or
Famax = 0,0023 D1,7 (bearings in other Dimension series)
| 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 single 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).
das = 0,5 (d1 + F)
| where |
| das
|
= |
shaft abutment diameter [mm] |
| d1
|
= |
inner ring flange diameter [mm] |
| F |
= |
inner ring raceway diameter [mm] |
|
)
fig
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