# Loads

 Minimum load For additional information → Requisite minimum load Single and double direction tapered roller thrust bearings: Perform calculations Screw-down bearings: Because these bearings are preloaded via O-rings or, as in reversing rolling mills, by springs, it is not necessary to check the minimum axial load. Where the bearing is preloaded by O-rings, the preload force should compress the O-ring diameter by approximately 20%. Where the bearing is preloaded by springs, the minimum load depends on the weight of the associated components (→ contact SKF). Equivalent dynamic bearing load For additional information → Equivalent dynamic bearing load, P P = Fa Equivalent static bearing load For additional information → Equivalent static load P0 = Fa

## Symbols

 A minimum load factor (→ contact SKF) Fa axial load [kN] Fam minimum axial load [kN] n rotational speed [r/min] P equivalent dynamic bearing load [kN] P0 equivalent static bearing load [kN]

## Comparative load ratings

In the oil and gas industry, bearing size is often selected based on load ratings that are not in accordance with ISO 281. To assess SKF tapered roller thrust bearings in accordance with methods used in the oil and gas industry, comparative load ratings are listed in the product table for Tapered roller thrust bearings, single direction. These load ratings are:

• Dynamic load rating C90
This represents the load that will result in an L10 rating life (→ Bearing rating life) of 90 million revolutions (3 000 hours at 500 r/min). It is typically expressed in kN.
• Dynamic load rating C1
This represents the load that will result in an L10 rating life of 1 million revolutions. It is converted from the C90 load rating and is typically expressed in kN.
• Main thrust rating Ws
This represents the calculated maximum load for the bearing. The main thrust rating is based on the load that will result in an L10 rating life of 18 million revolutions (3 000 hours at 100 r/min). It is defined in API 8C, section 9.9 Rotary Swivels, and is typically expressed in short tons.

Comparative load ratings must neither be used for bearing rating life calculations in accordance with ISO 281, nor for calculating the SKF rating life as described under Selecting bearing size. Comparative load ratings cannot be converted into nor compared with basic dynamic load ratings in accordance with ISO 281, not even when both of these load ratings are calculated for 1 million revolutions.