BEARING, TRUE. The angle between the geo graphical meridian and the direction of the object observed. All bearings are measured in points degrees each) or degrees. The designation of true and magnetic bearings, either in points or degrees, is the same as for compass bearings, but the record of the bearing is fol lowed by p. c. (i.e. per compass) for compass bearings; by may. (i.e. magnetic) for magnetic bearings; and by true for true bearings; for examples, N. by E. (p. c.), N. 16° W. (p. c.), N. by E. (wag.), S. S. W. (true). See COM In practical mechanics, bearing is the name given to the surfaces of contact between a moving-piece, such as a shaft or a connecting rod and its support. For reciprocating motion the surfaces of the bearings must be perfectly rectilinear in the line of the motion. Such bear ings are usually called slides, and examples are found in the crosshead guides of a locomo tive, and also in most other forms of steam engines. For rotary motion the bearing-surfaces must be surfaces of revolution; examples of such bearings are found in the hangers for mill shafting. in the pivots of turntables, etc. The bearing-surface of the support is often made of special metal to reduce friction. For similar
purposes the rotating-surface is often construct ed to run on steel balls, or cylindrical or conical rollers held by the support. The bear ings of a bicycle are familiar examples of ball bearings, and both roller and ball bearings are used for motor-vehicle wheels. Some bearings are used in screw-jacks, many machine tools, elevators, etc., in which the moving part is threaded like a screw and the support is threaded like a nut. Such bearings allow rotary motion and reciprocating motion at the same time. The thrust-bearings of large steamship propeller shafts consist of a nut with incised threads engaging with collars on the shaft. These bearings differ from screw-bearings in that the threads are not spiral. but run straight around the nut and shaft, and so only rotary motion is possible. In machinery ball thrust bearings are often employed. A roller thrust bearing was made by the Ball-Bearing Company of Boston. Mass., in 1900, capable of carrying a thrust of SO tons.