In many machines there is a shaft or axle that is constantly turning, or rotating. A part that carries the weight of a rotating shaft is called a bearing. The purpose of a bearing is to carry the weight of the shaft without slowing it down any more than is necessary. If you put a book on a table that is covered with a tablecloth, and try to push the book across the table, it probably will not slide at all. Remove the tablecloth and the book will slide, because the wood of the table is smooth. Put two round pencils under the book and it will roll easily across the table. Put marbles under the book instead of pencils and again it will roll easily\u0097this time in all directions. A little oil or grease put on the book, the table, the pencils, or the balls (marbles) will make the book slide even more easily. These are all ways of reducing friction (about which there is a separate article). Any surface will resist the movement of another surface rubbing against it, and this resistance is called friction. Friction creates heat, as you can find out even while you are reading this, by rubbing your hands together. Bearings use all these ways of reducing friction in a machine. The simplest kind of bearing is a journal bearing. In machinery, the word journal means the part of a shaft that touches another surface while it rotates. A journal bearing is a hole into which the shaft is fitted. This hole is usually lined with a metal\u0097bronze, or a special metal called Babbitt\u0097that is very smooth, and reduces friction just as the smooth surface of the table did in your experiment. In a roller bearing, metal rods are fitted into the hole; in a ball bearing, metal balls are used; and these further reduce friction, just as the pencils and marbles did on the table. Roller bearings are used on the wheels of railroad cars and automobiles; ball bearings are used in roller skates and bicycles. In a watch, jewels are used as bearings. When you read of a "17-jewel watch," it means the watch has seventeen bearings made of jewels. These are the same kind of jewels as are used in rings and other jewelry, except that they do not have to be beautiful jewels. A jewel can be made so smooth that no oil or grease is needed, and it is so hard that friction will not cause it to wear out for Home Use Washing Machine Drain Motor Tractor many years.