Farm Machinery and Equipment. Harris Pearson Smith
friction clutch. The two notched plates are pressed together by springs of sufficient force to transmit power for normal working loads. Should an overload occur, the clutch will slip with a snapping action and prevent damage to the working unit (Fig. 4–23). This type of safety clutch is used on many power-operated machines, such as corn pickers, forage harvesters, and haying machinery.
Positive clutches consist of two parts which have jaws so shaped and placed that when they are brought together they engage positively as a unit with no slippage (Fig. 4–24). These clutches should be engaged before applying power to the driving side. They are used when light loads are transmitted at slow speeds, as in drives for planters and grain drills.
FIG. 4–24. Positive-type clutch: A, clutch parts disengaged; B, clutch parts engaged to transmit power.
The ratchet-and-pawl arrangement, as found in the hubs of horse-drawn mowing machines and in the drives of manure-spreader aprons (Fig. 4–25), is a type of clutch. The ratchet is composed of gearlike teeth. These teeth generally have an equal slope from the vertex of the teeth on each side, or they may take a form similar to circular-saw teeth. The pawls, or dogs, consist of small pieces of cast iron that fit into recesses in a casting rigidly fastened to the axle. The outer ends of the pawls are held against the ratchet teeth by small coil expansion springs. As the machine moves forward, the three or four pawls engage the ratchet teeth and the axle turns with the wheel. A movement backward allows the pawls to slip over the teeth with a distinct clicking noise. Therefore, this type clutch operates in only one direction.
FIG. 4–25. Ratchet-and-pawl drive for manure-spreader apron. Note the eccentric drive for moving the pawl forward and backward.
The overriding clutch is a type of ratchet-and-pawl arrangement (Fig. 4–26). In some types, balls placed in tapered recesses take the place of the rectangular metal pawl. The overriding-type clutch is often used to operate fans on cotton-picking machines and forage harvesters. When the power is disengaged, the fan keeps turning to clear out any material that may be in the pipes, until it finally comes to a stop from the lack of power.
Another type is the belt-tension clutch. When an idler on a belt is mounted so it can be moved to release the tension around the pulleys, there is not enough friction on either the drive pulley or the driven pulley to transmit power. Thus, the tension applied by the movement of the idler pulley can be made to serve as a clutch to engage and disengage the power.
FIG. 4–26. Overriding- or overrunning-type clutch. (John Deere.)
COMPONENT PARTS OF MACHINES
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