The rotational speed of rolling bearings is limited. Usually, the rotational speed is limited by the temperature limit of the lubricant or the material of the bearing parts. The speed at which the bearing reaches the operating temperature limit depends on the heat generated by the bearing, any external heat, and the heat that can be removed from the bearing. The heat generated by the bearing depends on the type and size of the bearing, internal design, load, lubrication, and alignment. But also influenced by other factors, including cage design, accuracy and internal clearance. In the product table, the two speeds are generally listed: reference speed (related to heat) and limit speed (related to mechanical structure). In general, the limit speed of the bearing is higher than the reference speed. However, for some bearing series, the reference speed may be higher than the limit speed because the friction characteristic is better than the mechanical ability of the bearing to keep the high speed. However, in any case, the bearing's ultimate rotational speed must always be considered, even if the bearing is operating under the most favourable working conditions.