Requisite bearing size

To determine the requisite size of a spherical plain bearing or rod end, it is necessary to know the requisite rating life for the application. This depends on the type of machine, the operating conditions and the demands regarding operational reliability. The following steps can be used to determine requisite bearing size:
  1. Use the guideline values of the load ratio C/P provided in table 1 to obtain a requisite basic dynamic load rating C. Compare this value with the basic dynamic load rating of the bearings listed in the product tables.
  2. To check whether the sliding contact surface combination of the selected bearing or rod end can be used under the actual load p and sliding velocity v conditions use the pv diagrams:
    • for steel/steel sliding contact surface combination (diagram 1)
    • for SKF Explorer steel/steel bearings (diagram 2)
    • for steel/bronze sliding contact surface combination (diagram 3)
    • for steel/PTFE sintered bronze sliding contact surface combination (diagram 4)
    • for steel/PTFE fabric sliding contact surface combination, metric bearings (diagram 5)
    • for steel/PTFE fabric sliding contact surface combination, inch bearings (diagram 6)
    • for steel/PTFE FRP sliding contact surface combination, FS and FBAS designs (diagram 7)
    • for steel/PTFE FRP sliding contact surface combination, F design (diagram 8)
  3. The specific bearing load p and the sliding velocity v needed to perform this check can be calculated as explained in the following sections.
    • If the pv diagram indicates that the basic rating life equation can be used, proceed to step 4.
    • If the pv diagram shows that the pv range is exceeded, select a bearing with a higher load carrying capacity.
  4. Calculate the Basic rating life and proceed as follows:
    • If the calculated rating life is shorter than the requisite rating life, a larger bearing or rod end should be selected and the calculation repeated.
    • If the calculated rating life is larger than the requisite rating life, the bearing or rod end can be selected for the application.
The bearing or rod end size is often determined by the dimensions of the associated components. In these cases, check the pv diagram to determine if the product is suitable.

Specific bearing load

The magnitude of the specific bearing load can be calculated using

p = K P/C

where
p=specific bearing load [N/mm2]
K=specific load factor depending on the bearing design and sliding contact surface combination (table 2) [N/mm2]
P=equivalent dynamic bearing load [kN]
C=basic dynamic load rating [kN]

Mean sliding velocity

The mean sliding velocity for constant movement can be calculated using

v = 5,82 × 10-7 dm β f

where
v=mean sliding velocity [m/s]
When the operation is intermittent (not continuous), the mean sliding velocity should be calculated for a cycle of operation.
dm=
inner ring mean diameter [mm]
  • for radial spherical plain bearings and rod ends dm = dk
  • for angular contact spherical plain bearings dm = 0,9 dk
  • for spherical plain thrust bearings dm = 0,7 dk
  • dk = inner ring sphere diameter [mm], refer to the relevant product table
β=half the angle of oscillation (fig. 1) [degrees, °], for rotation β = 90°
f=frequency of oscillation [min-1], or rotational speed [r/min]
For intermittent movement, the angle of oscillation is usually given per unit time. In this case the mean sliding velocity can be calculated using

v = 8,73 × 10-6 dm 4β / t

where
β=half angle of oscillation (fig. 1) [degrees, °]
t=time taken to pass through complete oscillation [s]
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