Requisite bearing size

Composite bushings and washers

To determine the requisite size of composite bushing or thrust washer, start with the basic rating life for the application. The basic rating life will take the type of machine, the operating conditions, and operational reliability for that machine into consideration. The following steps can be used to determine requisite bearing size: 

  1. As a first approximation a guideline value of the load ratio C/P = 2 can be used to obtain the requisite basic dynamic load rating C. A suitable bearing can then be selected from the product tables. 
  2. Check whether the bushing or thrust washer can be used under the actual load p and sliding velocity v conditions. use the pv diagrams:
  3. The specific bearing load p and sliding velocity v needed to perform this check can be calculated as explained in the equations below.
    • 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 or contact the SKF application engineering service.
  4. Calculate the basic rating life (equation see below, alternatively the XXXX ) 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.
For flanged bushings check the suitability of the bushing and flange separately. To calculate the specific load of the flange, replace C with Ca in the equation above. Values for Ca are provided in the product table.

Specific bearing load

The magnitude of the specific bearing load can be calculated using

p = K F/C

where

p=specific bearing load [N/mm2]
F=dynamic bearing load [kN]
C=basic dynamic load rating [kN]
K=specific load factor [N/mm2]
= 80 for PTFE composite material (suffix E)
= 120 for POM composite material (suffix M)

Mean sliding velocity

The mean sliding velocity for constant movement can be calculated using

v = 5,82×10-7 d β f

where

v=sliding velocity [m/s]
d=sliding diameter [mm]
= bore diameter d for bushings
= 0,5 (d + D1) for the flange of flanged bushings
= 0,5 (d + D) or dimension Ja for thrust washers
f=frequency of oscillation [min-1], or rotational speed [r/min]
β=half the angle of oscillation (fig. 1) [degrees, °], for rotation β = 90°

Basic rating life calculation

Many factors influence the life of a plain bearing, e.g. load, sliding velocity, operating temperature, surface roughness of the counter surface against which the composite sliding layer runs. Any calculation is therefore only approximate.
The values obtained using the basic rating life equations are attained by the majority of bearings and are often exceeded. This has been confirmed by rig tests and field experience.
The basic rating life for SKF PTFE and POM composite plain bearings can be obtained from:

Gh = c1 c2 c3 c4 c5 KM/(p v)n

Perform calculation

Consider limiting values for product (p v) in this life calculation as follows:

  • PTFE materials: if p v < 0,025 then 0,025 should be used
  • POM materials: if p v < 0,1 then 0,1 should be used

where
Gh=service life [operating hours]
c1=load factor (diagram 3)
c2=speed factor (diagram 4)
c3=temperature factor (diagram 5)
c4=surface roughness factor (diagram 6)
c5=factor for the type of load
= 1 for stationary load (i.e. the loaded zone is always at the same position on the bearing circumference)
= 1,5 for rotating load (i.e. the loaded zone moves round the circumference of the bearing)
KM=factor depending on material and bearing type
= 480 for PTFE composite bushings
= 300 for PTFE composite thrust washers
= 1 900 for POM composite bushings and thrust washers
p=specific bearing load [N/mm2]
v=sliding velocity [m/s]
n=exponent
= 1 for PTFE materials
= 1 for POM materials if p v ≤ 1
= 3 for POM materials if p v > 1


Bronze, polyamide, filament wound bushings and strips

The occurring loads, stresses as well as sliding velocity should not exceed the permissible values provided under the relevant bearing data sections:

Furthermore, for filament wound bushings (PWM series), diagram 7 indicates acceptable pv combinations of specific bearing loads and sliding speeds for this bushing material. If the values for the specific bearing load and the sliding speed are positioned below the curve in this pv diagram, the bearing is suitable for the application. Under favourable conditions, operating ranges above the curve may also be possible. In this case, contact the SKF application engineering service.

The specific bearing load p and sliding velocity v needed to perform these checks can be calculated as explained in the equations below.

Specific bearing load

The magnitude of the specific bearing load can be calculated using

p = F/A

where

p=specific bearing load [N/mm2]
F=bearing load [N]
A=load carrying cross section [mm2]
= d (B - 2) for filament wound bushings (fig. 2)
= d B for bronze and polyamide bushings
= (D/2)2 - (d/2)2 Π for thrust washers


Mean sliding velocity

The mean sliding velocity for constant movement can be calculated using

v = 5,82×10-7 d β f

where

v=sliding velocity [m/s]
d=sliding diameter [mm]
= bore diameter d for bushings
= 0,5 (d + D1) for the flange of flanged bushings
= 0,5 ((d + D) or dimension Ja for thrust washers
f=frequency of oscillation [min-1], or rotational speed [r/min]
β=half the angle of oscillation (fig. 1) [degrees, °], for rotation β = 90°


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