Farm Machinery and Equipment. Harris Pearson Smith
Useful Rules for Belts. To find the length of a flat belt for two pulleys of unequal size; add the diameters of the two pulleys together, divide this sum by 2, multiply by 3, and to this product add twice the distance between the centers of the two shafts.
To calculate the speed or size of a pulley: the revolutions per minute of the driving pulley times its diameter equals the revolutions per minute of the driven pulley times its diameter. If three of the quantities are known, the fourth can be easily determined.
S × D = S × D
where S = r.p.m.
D = diameter
Another expression is
FIG. 4–10. Variable speeds are obtained by changing the sheave pitch of alternate sheaves.
The speed or revolutions per minute can be determined by substituting the known and unknown quantities as
To find the speed of a belt, multiply the circumference of the pulley by the number of revolutions at any given time. This disregards slippage and creep. The speed of a flat belt should not exceed 5,000 feet per minute. A good speed is 3,500 to 4,000 feet per minute.
General Precautions Pertaining to the Use of Belts:
1. Belts that are too tight cause injurious strains on the belts and machinery, resulting in hotboxes and broken pulleys.
2. Belts that are too loose have a flappy, unsteady motion.
3. All belts should be kept free from dirt and moisture.
4. Mineral oils should not be used on leather and rubber belts.
5. Boiled linseed oil, or resin mixed with tallow and oil, makes a good belt dressing.
6. Belts should be run horizontally or as nearly so as possible.
7. The lower side of a belt should be the driving side, as this gives a greater arc of contact.
8. Idler pulleys should be placed on the slack side of the belt and nearer to the driven pulley.
9. The arc of contact should be 180 degrees and over if possible.
10. A pulley that is too narrow should never be used.
Sprocket Wheels and Chains. Hook-link and roller are the two types of chain used in transmitting power on farm equipment. Sprocket wheels are designed to fit each type of chain.
FIG. 4–11. The proper method of running a hook chain on the sprockets.
The hook-link chain may be made of either malleable iron or crimped steel (Figs. 4–11 and 4-12). Hook-link chains are used where the power requirements are low and the speed relatively slow. The steel hook-link chain is most extensively used. In the operation of hook-link chains, the hook of the chain link should be run with the open lip away from the sprocket wheel and leading in the direction of travel, as shown in Figs. 4–11 and 4–12. There may be exceptions to this rule when the drive pulley is small. Figure 4–13 shows a special clamp for disconnecting pressed-steel hook-chain links.
FIG. 4–12. Pressed-steel hook chain.
FIG. 4–13. Clamp for disconnecting pressed-steel hook-chain links. (Huron Tools, Inc.)
FIG. 4–14. Types of roller-chain links: A, offset link; B, cottered connecting link; C, slip-spring connecting link; D, roller-chain pin extractor. (Link-Belt Company.)
Roller chains are made of a special high-grade steel and can be used at high speeds. The various parts of the chain, as shown in Fig. 4–14, are finished, polished, and hardened. Standard sizes of roller chain are designated by the pitch and number. The pitch of a chain is the length of each link from center to center of the pins. Pitches of 3/8, 1/2, 5/8, 3/4, and 1 carry corresponding numbers of 35, 40, 50, 60, and 80. Larger sizes are available. Single-width roller chains are more commonly used on farm equipment, but double-, triple-, and quadruple-width types are made. For the average application on farm equipment, the single-width roller chains give satisfactory service and are usually more economical. Generally, it is best to use the smallest pitch chain that will accommodate the horsepower and load requirement. Where light loads are transmitted at relatively low speeds, a double-pitch chain may be used (Fig. 4–15). If possible, operate the chain with the tight span on top. For special applications of roller chain, consult the manufacturer.
FIG. 4–15. Three types of roller chain and sprocket with taper-lock hub: top, cotter pin; middle, riveted; bottom, double-pitch. (Link-Belt Company.)
FIG. 4–16. Where transmission shafts are close together, gears are used to transmit power from one shaft to another.
Gears. When the machine is rather compact and the shafts are close together, gears may be employed to transmit the power, as shown in Fig. 4–16. The various types of gears are shown in Fig. 4–17.
Often there is a combination of either spur or bevel, and other type. If the power is transmitted parallel to the shaft, helical, or spur, gears are employed; but if the shafts are at right angles, the beveled, or worm gears, must be employed. The use of gears makes a more substantial construction and eliminates a great amount of lost motion; however, the cost is greater, especially in the case of repairs. It is much cheaper to replace one or two links in a chain than to replace a complete gear. When one tooth is broken and all the others remain, the gear cannot be used.
Spur gears have their shafts parallel. The teeth that make up the gear have their surfaces parallel to the shaft. In an internal spur gear (Fig. 4–17C), the teeth are on the inside of the rim. An external spur gear (Fig. 4–17A) has teeth on the outside of the rim. For every internal spur gear, it is necessary to have an external spur gear to operate it; but two external gears may be used together without an internal spur gear.