In certain applications, the speed limits are superseded in importance by other considerations, such as low speeds and oscillating movements.
Low speedsAt very low speeds, it is very difficult for an elasto-hydrodynamic lubricant film to be built up in the contact areas between the rolling elements and raceways. In these applications, lubricants containing EP additives should be considered (→ Grease lubrication). Alternatively, consider the use of Solid Oil.
Oscillating movementsWith this type of movement, the direction of rotation changes before the bearing has completed a single revolution. As the rotational speed is zero at the point where the direction of rotation is reversed, a full hydrodynamic lubricant film is difficult to maintain. As a result, SKF recommends using a lubricant containing an effective EP additive to maintain a boundary lubricant film capable of supporting the applied loads. Hybrid bearings perform well under insufficient lubrication conditions and can therefore provide favourable results in applications where rapid accelerations, decelerations and load reversals (directional changes) occur.
In general, it is not possible to give a limit or a rating for the speed of oscillating movements, as the upper limit is not dictated by a heat balance but by the inertial forces that come into play. With each reversal, there is a danger that inertia causes the rolling elements to slide for a short distance and smear the raceways. The effects of accelerations and decelerations depend on the mass of the rolling elements and cage, the type and quantity of lubricant, the operating clearance and the loads on the bearing.