car to be driven for extended periods of time. The failed bearing is not likely to damage the crankshaft. In more modern practice, the crankshaft and connecting rod big end bearings in a modern automobile engine are made of a replaceable steel shell, keyed to the bearing caps. The inner surface of the steel shell is plated with a coating of bronze, which is in turn coated with a thin layer of Babbitt metal as the bearing surface. The process of laying down this layer of metal is known as Babbitting. Alternative bearings In many applications, rolling-element bearings, such as ball or roller bearings, have replaced Babbitt bearings. Though such bearings can offer a lower coefficient of friction than plain bearings, their key advantage is that they can operate reliably without a continuous pressurized supply of lubricant. Ball and roller bearings can also be used in configurations that are required to carry both radial and axial thrusts. However, rolling-element bearings lack the beneficial damping and shock-load capability provided by fluid-film bearings, such as the Babbitt. Babbitt alloys The science of bearing Babbitt selection The engineering of a bearing's Babbitt lining is usually completed during the design of the machine. In selecting the proper type of Babbitt for a particular job there are a number of factors to take into consideration, the most important of which are as follows: Surface speed of the shaft Load that the bearing is required to carry There is no doubt that if a bearing is to be highly loaded in relation to its size, a high tin alloy is desirable; whereas for much slower speed work and less heavily loaded bearings, a lead-Based Babbitt may be employed, and is far more economical. 1. Surface speed of the shaft: (The number of feet traveled per minute by the shaft circumferentially.) Formula: (Pi x D x RPM) / 12 = S Example: Determine the surface of a 2 inch diameter shaft going 1,400 revolutions per minute (RPM) (Pi x D x RPM) / 12 = (3.1416 x 2 x 1,400) / 12 = 733.04 Ft/min Where: Pi = 3.1416, D = Diameter of shaft, S = Surface speed of the shaft 2. Load bearing is required to carry: (the weight which is being exerted through the combined weights of the shaft and any other direct weights on the shaft and measured in pounds per square inch.) Formula: