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TRB Bearing Selection

Bearing selection

Bearing ratings for dynamic radial capacity (Cr) and static radial capacity (Cor) shown here are in accordance with ISO 281-1990 and ISO 76-1987 respectively.

Radial and axial loads must be considered together as combined ‘equivalent loads’, as explained below. Generally, maximum instantaneous loading is governed by housing strength rather than bearing static capacity.

Dynamic rating

Expected bearing life is calculated by the following equation:

L10 = [Cr/(P x fd)] (10/3)
where L10 = expected life of 90% of similar bearings under similar operating conditions (in millions of revolutions)
C r = radial dynamic rating
P = equivalent dynamic load
fd = dynamic or service factor, generally from 1 for steady loading to 3.5 for heavy shock, reciprocation or vibration 

The dynamic equivalent load is calculated as follows: 

when Fa/Fr ≤e : P = Fr + Y1Fa
when Fa/Fr >e : P = 0.67Fr + Y2Fa 

where:
Fr = applied radial load
Fa = applied axial load

and calculation factors Y1, Y2 and e are given in the product data tables.

Life calculation with multiple load conditions

Where varying loads are experienced in operation, using the maximum load condition may lead to an unrealistically low calculated life. For n load conditions constituting the full load cycle (at constant speed), an overall dynamic equivalent load may be calculated as follows:

where:
Pi = dynamic equivalent load under load condition i
pi = proportion of time load condition i is applicable

Where the load is continuously variable it may be broken down into a discrete approximation to the actual load cycle. 

Minimum loading

In order to avoid excessive skidding of the rollers, sufficient loading must be applied to the bearing, as follows:

P ≥ 0.01Cr

Static rating

The static load rating is defined as the load at which a contact stress of 4GPa occurs at the centre of the most heavily loaded contact, and at which a permanent deformation of 0.0001 times the roller diameter is sustained at the location of the contact. This has been found to not cause significant deterioration in bearing performance under typical operating conditions.

The suitability of a bearing for sustaining a specified static (or instantaneous maximum) load is determined as follows:

Cor ≥ So x Po
where:
Cor = static radial capacity
So = static safety factor
Po = Fr + YoFa where calculation factor Yo is given in the product data tables. 

Temperature characteristics

Standard bearings can operate at between -20°C and 100°C. Operation outside this range is possible but may require special treatment of bearings parts – please contact our technical department.

The difference in temperature between the shaft and housing should not exceed 40°C.

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