REXNORD - MARBETT - MCC
B74
L
10
L
10h
C
P
n
C
P
L
10
= (—)
3
C
P
L
10h
= ——— • (—)
3
1666
n
4000 ÷ 8000
10000 ÷ 20000
20000 ÷ 30000
40000 ÷ 80000
Calculation data
Bearing dimensioning
The bearing dimensions, for a given application, should be chosen on the basis of the
loads to be supported, expected service life and reliability. ±n most cases the reason
for choosing a bearing is the fact that the shaft has already been sized during the
design of the plant.
Calculation procedures for
self-aligning bearings
The calculation procedure for self-aligning bearings consists in ensuring a satisfactory
operating life of the unit:
1) The nominal service life is calculated using the formulas indicated below, keeping in
mind the type of stress on the bearing (dynamic or static).
2) The nominal life of the bearing should reLect the expected service life of the plant as
indicated in Table 1 hereunder.
Service life
The dimensioning of the bearing requires the knowledge of the appropriate life
expectancy of the project in relation to the speciIc application. This depends on the
type of plant, daily/yearly working hours and on the required reliability.
In the absence of practical experience Table 1 gives our recommendations of the
average expected plant life under various conditions.
Table 1
- Recommended plant expected life ²
10h
for MB support bearings
Dynamically stressed bearings
Bearings which rotate under load are considered dynamically stressed (one ring of the
bearing makes one full rotation against the other). ±n most cases, self-aligning supports
are dynamically stressed.
Expected life calculation formula
The service life of a bearing is expressed in the number of revolutions or hours of
operations which can be expected without breakdown (erosion or laminations of the
bearing revolving parts).
Correct expected life
±n the majority of cases, for self alinging MB bearings, it is sufIcient to calculate ²
10
,
²
10h
using the formula indicated above. The new theory of life calculation permits
to establish the correct life expectancy keeping into consideration the effects of
lubrication, pollution of solid particles and of the fatigue limit load Pu (values are
reported in the bearing tables). The calculation of the correct expected life requires
therefore the assistance of the technical commercial staff.
= life in millions of revolutions.
= life in hours.
= dynamic load coefIcient (N). Values are indicated in the support dimensional tables.
= equivalent dynamic load (N). See calculation formula on Table 2.
= revolutions (RPM).
²ife in Millions revolutions.
²ife in hours.
The calculating procedure for nominal duration is valid whatever the type of bearing:
When bearings rotate at constant speed it is more practical to calculate the life in
number of hours:
Plant type
Plant expected life ²
10h
hours
Seasonal Operation plants
Daily operated plant, 8 hours/day, not completely utilised
Daily operated plant, 8 hours/day, completely utilised
Daily operated plant, 24 hours/day