Selection of Bearing Size

The selection is usually made step by step, repeated if necessary, by comparing -

1. the load ratio involved with the normal minimum values for that ratio
2. the forces affecting the bearing and the maximum permitted load of the bearing proposed
3. the maximum surface pressure and the surface pressure on the proposed bearings
4. the maximum slip speed and the slip speed involved of the bearing proposed
5. the specific performance of the bearing involved with the published catalogue limits.

re 1:
The load ratio (C/F) is a value for a specific use of a bearing according to formula (5):

(5)

(C/F)vorh (C/F)min

[kN]

The common minimum values for (C/F) for different anti-friction surfaces as listed in
table 3, can be used to establish the required dynamic load rating C in accordance with
formula (5a) by changing formula (5). By this means a suitable bearing size can be
selected.

(C/F)min   

Steel/Brass Steel/Bronze Steel/Steel Steel/PTFE-
Bronze Fabric
Steel/PTFE-
Nylon Fabric
20 2 2 1,75 1,5

Tabelle 3: Typical Load Ratios

(5a)

Creg (C/F)min  . Fexist

[kN]

re 2:
When the existing force affecting the bearing is a static load, it can be used as is for a comparison.
When it is a dynamic load, it can be calculated by using formula (2), (3) or (4).

When a Rod End is mounted with a locking not or retransfered with two nuts, the additional tensile
stress at the male thread or the connecting rod has to be taken into consideration.

However the static or dynamic load must always be smaller than the maximum permitted load,
which is calculated from the static load rating Co using formula (6). This might have to be
further reduced by the load factor fB (Illustration 1) and the temperature factor fT (Table 4).


Illustration 1: Load Factors - check fB

Temperature in Celsius 80o 100o 150o 200o 250o
lubricated 1 1 1 0,8 0,5
maintenance free 1 1 0,8 0,5 0,3
Temperature in Fahrenheit 176o 212o 302o 392o 482o
lubricated 1 1 1 0,8 0,5
maintenance free 1 1 0,8 0,5 0,3

Table 4: Temperature Factor FT

(6)

Fr, max = Co  . fB  . fT

[kN]

(6a)

Fa, max = a  . Fr, max

[kN]

If no bearing size is given on the application the required static load rating can be
established by changing formula (6) and a Rod End can be selected from the tables
accordingly.

(7)

[kN]

re 3:
The load on a mating surface can be worked out by using formula (8). It must be less
than the standard value for surface load according to the anti-friction combination of
of materials, selected as listed in table (1).

Pzul
[N/mm2]

Steel/Brass Steel/Bronze   Steel/Steel      Steel/PTFE-
Bronze fabric 
Steel/PTFE-
Nylon fabric   
50 50 50 150 50

Table 1: Maximum surface pressure

(8)

Pexist = Pmax /(C/F)exist

[N/mm2]

Pmax  according to table (1), F according to formula (2), (3) or (4)

re
4:
The existing average slip speed Vm is calculated according to formula (9) using the
frequency of rotation of the crank K and the slip part of the Spherical Plain Bearing G.
(At one rotation of K it corresponds to the double arc b between the center 1 and 2 in
Illustration 5 and thus to the double maximum oscillating angle
b).


Illustration 5: Oscillating angle
b relative to crank rotation

(9)

[m/s]

Diameter of Ball dK [mm] (Table 12) and f [1/min]

In case where the bearing rotates fully
b needs to be substituted by 180o slip speed
has to be less than the speed permissible listed in table 5.

Vmax [m/s] Oscillation Revolution
Steel/Steel 0,15 0,10
Steel/Bronze or Brass 0,25 1,00
Maintenance Free 0,25 0,35

Table 5: Maximum slip speed

re 5:
The product p . v can be defined as a specific bearing performance PL (Formula10).
Thus, an estimated value for the heat build-up per pro mm2 of the Spherical Plain
Bearing's surface is available, mainly dependent on the anti-friction material
combination, the lubrication/cooling applied and the surface pressure and slip
speed.

(10)

PL, exist = pexist  . Vexist

Slip speed v to (9)
Surface pressure p to (8)

After the selection of the bearing the following table 6 is valid: PL, exist PL, max

PL, max
[W/mm2]

Steel/Bronze, (Brass), (Steel)

 maintenance free

0,5

1,3

Table 6: Maximum specific bearing performance


Table 12: Ball diameter for Rod Ends and Spherical Plain Bearings

Series K Series E
Size dk Size dk
2 6,00 6 10,0
3 7,94 8 13,0
4 9,52 10 16,0
5 11,11 12 18,0
6 12,70 15 22,0
8 15,87 17 25,0
10 19,05 20 29,0
12 22,22 25 35,5
14 25,40 30 40,7
16 28,57 35 47
18 31,75 40 53,0
20 34,92 45 60,0
22 38,10 50 66,0
25 42,86 60 80,0
30 50,80 70 92,0
35 57,15 80 105,0
40 65,96 0 0