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Design considerations

Verification of axial displacement

The actual internal clearance can limit the possible axial displacement. Misalignment reduces the possible axial displacement. Therefore, the actual axial displacement should be verified.

1. Determine the required axial displacement

  • Thermal expansion of the shaft can be estimated using
    sreq = α L ΔT
  • Where additional effects need to be considered, advanced simulation or tests may be required.

2. Determine the maximum misalignment

  • Estimate the misalignment β of the housing seats based on specified tolerances.
  • Where additional effects need to be considered, advanced simulation or tests may be required.

3. Check the permissible axial displacement

Check the permissible axial displacement in both directions, depending on the bearing used:

  • open bearing with a cage (fig. 1)
  • full complement bearing with a retaining ring (fig. 2)
  • sealed bearing (fig. 3)

sreq < s1 - β k1 B

or

sreq < s2 - β k1 B

Where sreq is too large, consider Offset mounting.

4. Check the internal clearance

  • Determine the clearance reduction caused by axial displacement.
    Clearance reduction caused by axial displacement
  • Determine the amount of clearance reduction from other effects and evaluate the residual clearance (→ Selecting initial internal clearance).

Symbols

Bbearing width [mm]
Cred
reduction of radial clearance as a result of an axial displacement from a centred position [mm]
k1
misalignment factor (→ product table)
Lshaft length between the bearings [mm]
s1axial displacement limit in bearings with a cage (fig. 1) or in full complement bearings (fig. 2) when displacing away from the retaining ring [mm] (→ product table)
s2axial displacement limit in sealed (fig. 3) and full complement bearings (fig. 2) when displacing toward the seal or retaining ring respectively [mm] (→ product table)
sreqrequired axial displacement from a centred position [mm]
αthermal coefficient of expansion [°C–1]
= 12 x 10–6 for steel
βmisalignment [°]
ΔTtemperature difference [°C]

Calculation example

Application (fig. 4):
  • Bearing C 3040
    - d = 200 mm
    - D = 310 mm
    - B = 82 mm
    - Normal clearance: min. 170 μm
    - s1 = 15,2 mm
    - k1 = 0,123
    - k2 = 0,095
  • Shaft length L = 3 000 mm
  • Temperature range for the shaft: 20 to 90 °C (70 to 195 °F)
  • Max. misalignment: 0,46°

Verification of axial displacement:

  1. Required axial displacement
    sreq = α L ΔT
    sreq = 12 x 10-6 x 3 000 x (90 - 20) = 2,5 mm
  2. Max. misalignment
    Input provided: 0,46°
  3. Checking the permissible axial displacement
    sreq < s1 - β k1 B
    2,5 < 15,2 - 0,46 x 0,123 x 82 ≈ 10,5
    → okay
  4. Checking the internal clearance
    Clearance reduction caused by axial displacement
    Calculation example
    Min. internal clearance when the bearing is displaced:
    170 - 7 = 163 μm

    Determine the clearance reduction caused by other effects (e.g. interference fit, temperature difference between inner and outer rings) and evaluate the residual clearance (→ Selecting initial internal clearance)
fig. 1
fig. 2
fig. 3
fig. 4
Free space on both sides of the bearing
fig. 5

To enable axial displacement of the shaft relative to the housing, free space must be provided on both sides of the bearing as indicated in fig. 5. The value for the width of this free space is based on:

  • the value Ca (→ product table)
  • the expected axial displacement of the bearing rings from the central position during operation
  • the displacement of the rings caused by misalignment

Calculating the free space required on both sides of the bearing

Careq = Ca + 0,5 (s + β k1 B)

where

Bbearing width [mm]
Caminimum width of space required on both sides of the bearing [mm] (→ product table)
Careqwidth of space required on both sides of the bearing [mm]
k1misalignment factor (→ product table)
srelative axial displacement of rings, e.g. thermal shaft expansion [mm]
βmisaligment [°]
Offset mounting

Where considerable thermal changes in shaft length are a possibility, the inner ring can be mounted offset, relative to the outer ring, up to the axial displacement limit s1 or s2 (fig. 6) in the direction opposite to the expected axial displacement (fig. 7). The extended permissible axial displacement is used, for example, in the self-aligning bearing arrangements of drying cylinders in paper machines.

fig. 6
fig. 7
Bearings on sleeves

CARB bearings with a tapered bore can be mounted with:

  • an adapter sleeve on plain or stepped shafts (fig. 8 or fig. 9):
    • Adapter sleeves are supplied complete with a locking device.
    • Use appropriate SKF adapter sleeve assemblies to prevent the locking device from interfering with the cage (→ product table).
  • a withdrawal sleeve on stepped shafts (fig. 10)

Check axial displacement carefully, as it could be that s1 (→ product table) cannot be fully realized.

For additional information about sleeves, refer to Bearing accessories.

fig. 8
fig. 9
fig. 10
Appropriate bearing housings

SKF standard bearing housings are available for most CARB bearings in the C 30, C 31, C 22 and C 23 series.

The two common arrangements when using standard housings are:

  • CARB bearings with a tapered bore on an adapter sleeve and a plain shaft
  • CARB bearings with a cylindrical bore on a stepped shaft

The comprehensive assortment of SKF bearing housings is provided in the following tables:

  • split plummer (pillow) block housings (table 1)
  • non-split housings (table 2)
  • application-specific housings (table 3)
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