This equation is not readily applied to rolling element bearings because of the difference in stress states and difficulty in determining the number of stress cycles that any point in the bearing components experiences per revolution of either race or bearing. One equation for bearings gives the effect of load on bearing life, L, where P is the equivalent load, n=3 for ball bearings, and C is the dynamic load capacity (or the basic load rating), which is the load the bearing can carry for a million inner ring revolutions with 90% chance of survival. A bearing of large D has a large number of rollers which, for each turn of the shaft, subjects the bearing race to more cycles of strain than the small-bore bearing would. Jet engines operated in the range of DN between 1.5 and 2 million up to the 1980s. An increase in speed from 1.8 to 4.2 million DN reduces the life of a 120 mm bearing by 90% at a load of 2000 pounds and 98% at a load of 4000 pounds. This usually results in bearings being made of steel of higher than 60 Rc hardness. The tempering temperature of hardened steel limits their operating temperature range to between 400 and 600°F depending on the type of steel. Compressive residual stress increases bearing life. Prenitriding and/or pre-over-stressing doubles bearing life. Alternate materials: Ceramic bearings are suggested for high temperature service. Alumina, titanium carbide, silicon carbide, and silicon nitride have been used. Homogeneity and porosity are the biggest problems.