Metal balls are rolling, spherical elements that are used in check and ball valves, bearings, and other mechanical devices that provide rotary or linear motion. They are usually made from alloy steel, carbon steel, stainless steel, tool steel, cobalt or cobalt alloys, copper or copper alloys, nickel or nickel alloys, titanium or titanium alloys, specialized ferrous alloys, or miscellaneous nonferrous metals. Specifications for metal balls include outer diameter, allowable deviation, surface roughness and basic diameter tolerance. Outer diameter (OD) is the overall width or average diameter. Allowable deviation is the greatest radial distance in a radial plane between a sphere that is circumscribed around the metal ball surface and any point on the metal ball surface. Surface roughness measures the irregularities which form the surface relief, but do are not significant deviations of form. Basic diameter tolerance is the maximum allowable deviation mean diameter from the diameter specified. In terms of features, some metal balls are heat resistant or corrosion resistant. Others are hollow or wear resistant.