tailieunhanh - Ebook Basic machines and how they work: Part 2

(BQ) Every concept is clearly defined, and discussions always build easily from elementary theory to specific applications familiar to anyone with the slightest interest in mechanics. Important concepts, machine components, and techniques are clearly illustrated in more than 200 diagrams, drawings, and cross-sections that reveal inner workings — all of these help to clarify even further an already clear and well-organized presentation | CHAPTER MACHINE ELEMENTS 11 AND BASIC MECHANISMS MACIIINE ELEMENTS Any machine, however simple, utilizes one or more basic machine elements or mechanisms in its makeup. In this chapter we will take a look at some of the more familiar elements and mechanisms used in naval machinery and equipment, BEARINGS ln chapter 7 we saw that wherever two objects rub against each other, friction is produced. If the surfaces are very smooth, there will be little friction; if either or both are rough, there will be more friction. FRICTION is the resistance to any force that tends to produce motion of one surface over another. When you are trying to start a loaded hand truck rolling, you have to- give it a hard tug (to overcome the resistance of static friction) to get it started. Starting to slide the same load across the deck would require a harder push than starting it on rollers. That isbecauserollingfrictionis always less than slidii friction. To take advantage of this fact, rollers or bearings are used in machines to reduce friction. Lubricants onbearing surfaces reduce the friction even further. A bearing is a support snd guide which carries a motig part (or parts) of a machine and maintains the proper relationship betweenthe moving or parts and the stationary part. It usually permits only one form of motion, as rotation, and prevents any other. There are two basic types of bearings: sliding type (plain bearings), also called friction or guide bearings, and sntifrictional type (roller and ball bearings). Sliding Qpe (Plain) Bearings In bearings of this type a film of lubricant separates the moving part from the stationary 69 part. There are three types of sliding motion bearings in common use: Reciprocal motion bearings, journal bearings, and thrustbearings. 1. RECIPROCAL MOTION BEARINGSprovide a bearing surface on which an object slides back and forth. They are found on steam reciprocating pumps, where connecting ?ods slide on bearing surfaces near their .