Читать книгу Farm Machinery and Equipment - Harris Pearson Smith - Страница 21

Cam. A cam (Fig. 4–27) is a device that produces intermittent motion. When an object is in motion part of the time and at rest between motions, the action is said to be intermittent. A cam may best be described as a wheel with a hump on one side (Fig. 4–27). The part that projects is called the nose. Anything resting against the cam will be moved only when the nose comes around to it; otherwise, it remains stationary.

Bearings. Bearings in farm equipment are required to hold the various power-transmission parts in position. The proper bearing to use is determined by the amount of wear, the speed at which the shaft is turning, the load it must carry, and the amount of end thrust. Bearings are divided into two general classes: friction, or plain; and antifriction.

Friction Bearings. Bearings of this type are shown in Figs. 4–28 and 4–29. In plain bearings, the revolving shaft is supported by, and is in direct contact with, a fixed bearing surface. For this reason, friction is high and the bearing should be lubricated with a fairly light oil. The bearing metal may be cast iron, babbitt, bronze, or other material.

Antifriction Bearings. Bearings of this type have balls or rollers placed between the shaft and the supporting bearing, thus reducing the friction. They are, therefore, called antifriction bearings. The lubrication of ball and roller bearings serves to preserve the polished surfaces from corrosion; to act as a cooling agent; and to protect the rubbing surfaces between the rollers, races, and separators. The selection of a lubricant for antifriction bearings is based on the type of bearing housing, the operating temperature, the speed of bearing rotation, and the requirements of the bearing. Some antifriction bearings are packed and sealed, thus requiring no further lubrication for the life service of the bearing. Do not use a detergent oil for lubricating antifriction bearings on electric motors. If used, the bearing is likely to fail in two or three months. Both ball and roller types of antifriction bearings are used extensively on almost all power-operated farm equipment.

FIG. 4–27. A cam shape at left, and the application of a cam at right.

FIG. 4–28. Solid bearing.

FIG. 4–29. Plain or split bearing.

Ball bearings are bearings having one or more rows of small balls placed in a cage or holder (Fig. 4–30). The balls are separated slightly and held in position by a retainer. Because of the small amount of surface in contact between the balls and the shaft, friction is reduced to a very low point. Ball bearings are designed to carry (1) radial loads at right angles to the shaft, (2) thrust forces that are parallel to the shaft or tend to shift the shaft out of position, and (3) a combination of radial and thrust loads. There are several types of ball bearings. They are designed to carry the various types of loads listed above (Fig. 4–30). Ball bearings have many applications in all types of farm equipment. Pillow blocks for ball bearings are shown in Fig. 4–31.

FIG. 4–30. Types of ball bearings: A, double-row; B, single-row; C, single-row with ring seal; D, end-thrust.

FIG. 4–31. Types of pillow blocks for ball bearings.

Roller bearings differ from ball bearings in that small cylindrical rollers are substituted for the balls. This gives a much larger bearing surface, such as is necessary for a heavy load. There are also cages to hold the rollers apart as in the ball bearings. Figures 4–32 to 35 illustrate several types of roller bearings. Roller bearings may be straight or tapered, depending on the shape and placement of the rollers.

FIG. 4–32. Parts of a plain roller bearing.

FIG. 4–33. The various parts of two applications of a spiral roller bearing. (Hyatt Roller Bearing Co.)

Straight roller bearings can be further divided into (1) the plain roller (Fig. 4–32), (2) the spiral roller (Fig. 4–33), and (3) the needle bearing (Fig. 4–34). The plain, straight roller bearing consists of a number of solid-cylinder steel rollers assembled in a cage with an inner and outer race (Fig. 4–35). One further classification of plain roller bearings is determined by the manner in which the parts are assembled, as a bearing with a separable inner race, a bearing with a separable outer race, and a bearing with nonseparable parts.

FIG. 4–34. The various parts of a needle roller bearing. (Torrington Roller Bearing Company.)

The spiral, or wound, straight, hollow roller bearing is shown in Fig. 4–33. Types are designed to operate with inner and outer races or with no inner race and a split outer race.

The needle-type roller bearing consists of a hardened outer shell containing a number of hardened rollers with pointed ends (Fig. 4–34). The needle bearing has the greatest radial load capacity possible for a given housing bore. It can also be used where the bearing space is limited.

FIG. 4–35. The various parts of a tapered roller bearing. (Timken Roller Bearing Company.)

FIG. 4–36. Application of tapered roller bearings on a disk harrow. (Timken Roller Bearing Company.)

Tapered roller bearings are designed to carry radial, or thrust loads, or a combination of both. The conical rollers are set in the cage at an angle between the inner and outer races as shown in Fig. 4–35. It is usually necessary to mount tapered roller bearings in pairs, to balance the radial and thrust loads (Fig. 4–36). Their greatest application is for wheel bearings, but there are many other applications.

Maintenance of Antifriction Bearings. Good maintenance of antifriction bearings is essential to obtain a long, trouble-free life service. All persons who operate and repair farm equipment should obtain a bearing-maintenance handbook from a bearing manufacturer. These handbooks describe and illustrate replacement of bearings, their care, cleaning, and selection, and the use of lubricants.

FIG. 4–37. Two types of bearing bushings: top, straight bushings showing types of grooves for oil; bottom, types of grooves for graphited oilless bearing.

Bushings. A bushing is a replaceable lining for a bearing. It may consist of wood, babbitt, bronze, chilled iron, or other material. Figure 4–37 shows two types of bearing bushings with different types of grooves for distributing the oil along the shaft.

Keys. Keys are of two kinds: first, those that fit into a slot in both the shaft and pulley, holding the two firmly together and causing them to turn as a unit; second, the cotter or split keys that are put through a hole in the end of a bolt or pin to hold the nut and washer on.

Bolts. The great variety of bolts used in the construction of farm machinery may be classified as follows: machine, carriage, stove, and plow bolts.

FIG. 4–38. Types of bolts: A, machine; B, carriage; C, plow; D–G, stove.

FIG. 4–39. Types of special bolts.

Machine bolts are used for holding two pieces of metal together. They have a square or hexagonal head with the stem of the bolt fitting into the head without any change of diameter, as Fig. 4–38.

Carriage bolts (Fig. 4–38), unlike machine bolts, have a rounded or oval-surfaced head with a square shoulder underneath extending out some half an inch, varying according to the size of the bolt.

Plow bolts may have many different kinds of heads, but practically all of them have from one to four shoulder-like points that fit into a groove prepared for them in whatever material they are placed. The undersides of the heads of plow bolts are always countersunk (Fig. 4–38), so that the head may go deep enough into the material to fit flush with the surface. Such bolts are used for holding plowshares.

Stove bolts, as shown in Fig. 4–38, are rather short bolts having threads running down close to the head, which may be either flat or round. Most stove bolts also have a slot cut across the heads, so that screw drivers may be used to prevent them from turning. This type of bolt is used for bolting thin metal together. Some special bolts are shown in Fig. 4–39.

Nuts. The most common types of nuts used on farm machinery are shown in Fig. 4–40. The square nut is used on the cheaper machines, but the hexagon nut is used on the higher-class machines. Castellated nuts are used where vibration is likely to cause the nut to work loose. Wing nuts are used where it is necessary to remove a part frequently. Lock nuts are constructed so that they automatically lock themselves in place.

FIG. 4–40. Types of nuts: A, square; B, hexagon; C, castellated; D, wing; E, square lock.

Screws. Many types of screws are also used in the construction of farm machinery. They may be classified as follows: set, cap, lag, and wood.

Setscrews (Figs. 4–41 to 4-43) may have several different shapes for the point. They are so called because they extend through the collar, allowing the point to come in contact with the shaft so that the collar and shaft will be fastened rigidly together and turn as a unit. They are also used in the same way to prevent various parts from moving out of place.

FIG. 4–41. Types of screw points, lag and cap screws.

Cap screws (Fig. 4–42) may have square, hexagonal, flat, or button types of heads. Such screws resemble closely a machine bolt with the exception that they do not have a nut on the threaded end; instead, the end passes through whatever it is to hold into a threaded hole which serves as a nut, for example, the cylinder head of an automobile.

Lag screws (Fig. 4–41) have heads like a machine bolt, while the other end is sharp. The threads are coarse and similar to an ordinary wood screw. They are used to attach machinery to floors or beams. The coarse threads, when started, will draw themselves into the wood as the screw is turned with a wrench.

FIG. 4–42. Types of screws: A, wood; B, machine; C, self-threading or tapping; D, cap.

Wood screws, unlike lag screws, are rather small and have slots across the head so that a screw driver can be used to force them into the wood. (Fig. 4–42).

Washers. Different kinds of washers are used extensively in connection with bolts in farm machinery. They may be used on either the end beneath the head of the bolt or beneath the nut. Washers are of various kinds as follows: flat malleable-iron, cast-iron, wrought-iron, and spring-lock washers. There is very little difference between malleable- and cast-iron washers, both being rather thick, oftentimes 1/2 inch, and placed where there is a considerable amount of wear. Wrought-iron washers are round discs with holes in the center to allow their being placed under the nut. Lock washers are made of spring steel with one side split from the edge to center of the hole. The ends of the split parts are turned in such a manner that they will allow a nut to be turned down easily but resist any effort to turn it off. A quick-repair washer is shown in Fig. 4–44.

FIG. 4–43. Hollow-head setscrew and wrench.

FIG. 4–44. Quick-repair washers: A, side-latch; B, over-latch.

FIG. 4–45. Types of springs.

Springs. Springs (Fig. 4–45) play an important part in the operation of farm machinery. Extension springs aid in lifting and adjusting heavy implements. Compression and torsion springs facilitate the operation of certain parts of a machine.