Spherical roller bearings are self-aligning, double-row, combination radial and thrust bearings. They use a spherical or crowned roller as the rolling element. The outer ring contains a spherical race which allows for some misalignment of the shaft and housing. Spherical roller bearings are unequalled in their capacity for high loads and tolerance to shock loads, but have limited speed capabilities. They perform consistently, even under conditions of extreme speed, application-specific stress, and marginal lubrication. The cage of a spherical roller bearing provides high dimensional accuracy and functionality. According to some estimates, spherical roller bearings have twice the running life of conventional bearings. Most spherical roller bearings are made of alloy steels or low-carbon steels. Chrome-plated products are also available. Some applications require the use of case-hardened or thorough-hardened, high-carbon, bearing-quality steel. High-carbon grades of steel do not require carburizing and can be case-hardened by induction heating or thorough-hardened by conventional heating methods. When low-carbon, carburized grades of steel are used, carbon is introduced after the spherical roller bearings are machined to a depth sufficient to produce a hardened case that can sustain bearing loads. The addition of carbon and alloys ensures the proper combination of a hard, fatigue-resistant case and a tough, ductile core.