Читать книгу Farm Machinery and Equipment - Harris Pearson Smith - Страница 19

The methods of transmitting power in connection with farm equipment are (1) direct drive, (2) pulleys and belts, (3) sprocket wheels and chain, (4) gears, (5) shafts and universal joints, and (6) flexible shafting.

Direct Drives. When a machine is driven directly from the shaft of an electric motor or internal-combustion engine, this is termed a direct drive or direct connection. Feed mills and centrifugal water pumps are often driven in this manner. There is usually a clutch between the power source and the machine.

Pulleys and Belts. A belt of flexible material forming a band about two or more pulleys is a simple method of transmitting power in farm equipment. Belts can be used in many intricate patterns over several pulleys on parallel shafts as shown in Fig. 4–1. The pulleys and belts may be either flat or V-shaped.

Flat Belts. The most common belting materials are leather, rubber, and canvas (Fig. 4–2). The principal use of flat belts on field equipment is in elevator chutes to convey harvested crop material from the harvester to a trailer. These belts are made mostly from rubber and canvas belting. Leather belting is expensive, must be kept dry, and is not commonly used on farm equipment. The standard belt speed for farm tractors should be 3,100 feet per minute ± 100 feet per minute.¹ Metal fasteners are generally used to join the two ends of flat belts (Fig. 4–3).

FIG. 4–1. Belt pattern on a combine.

V Belts. The trapezoidal-shaped or V belts are so named because the sides of the belts are beveled to fit into the V slot of a pulley or sheave. The frictional contact between the sides of the belt and the sheave flanges results in less belt slippage and in better power transmission than is obtained with flat belts. Figure 4–4 shows the actual cross-sectional sizes and dimensions of the five V-belt sizes. The size of a V belt is referred to by the letter designation of A, B, C, D, and E, with A being the smallest and E the largest. Double-sided V belts are almost hexagonal in shape. This type is designed to drive from either or both sides.

FIG. 4–2. Different kinds of belting: a, leather; b, stitched canvas; c, balata; d, rubber; e, solid woven.

The designer who wishes to use V belts on a piece of farm equipment calculates the power requirements for the various units and selects the V-belt size needed to transmit the necessary power. If large amounts of power are required, two or more belts may be used with multiple-groove sheaves (Fig. 4–5). V belts can be used to transmit power between sheaves for distances ranging from a few inches to several feet, in many different arrangements (Figs. 4–1 and 4-6).

FIG. 4–3. Methods of closing metal belt laces: Alligator above, Clipper below.

FIG. 4–4. The actual cross-sectional sizes and dimensions of five sizes of V belts.

FIG. 4–5. Adjustable and nonadjustable multiple-groove sheaves for V belts.

FIG. 4–6. V belts can be used to transmit power around corners.

A V belt should ride with the top surface almost flush with the top of the sheave groove. There should be at least 1/8 inch clearance under the belt in the bottom of the sheave groove.

Generally, V belts are made in endless lengths and used where the belts can be permanently installed or put on the sheaves without dismantling parts of the machine. However, special V-belt fasteners (Fig. 4–7) make it possible to use open-end V-belt applications over drives that would be costly to dismantle.

FIG. 4–7. Connection for open-end V belts. (Flexible Steel Lacing Company.)

FIG. 4–8. The relation between center distance and V-belt length.

How to Measure the Length of a V Belt. When the drive consists of two sheaves (Fig. 4–8), the relation between the center distance between shafts and the belt length can be determined by the following formula:

where L = effective length of belt, inches

C = distance between centers of sheaves, inches

D = effective outside diameter of large sheave, inches

d = effective outside diameter of small sheave, inches

Pulleys and Sheaves. Pulleys for flat belts are manufactured from wood, cast iron, steel, and composition fiber. The diameter of a flat pulley is slightly larger at the center than at the edges. This is called the crown of the pulley.

Sheaves for V belts are made from cast iron, cast semisteel, and diepressed steel. Many single- and multiple-groove sheaves are adjustable, to permit the belt to ride higher or lower in the groove and give a variable speed ratio between sheaves (Figs. 4–9 and 4-10). The belt should not be run lower in the adjustable sheave than its approximate thickness or lower than the angled sides of the groove.

FIG. 4–9. Variable speeds are obtained by changing the sheave pitch. The pitch of alternate sheaves can be changed.

Some 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.

FIG. 4–17. Types of gears: A, spur; B, cluster; C, internal spur; D, herringbone; E, helical; F, worm and worm-wheel; G, straight-bevel; H, straight-bevel gear set; I, spiral-bevel set used in tractors; J, hypoid gear set; K, spline-shaft gear.

Beveled gears (Fig. 4–17H) have their shafts at right angles or nearly so. Where the power has to turn a corner, beveled gears are used. The teeth are on an incline which varies according to the difference in diameter of the gears meshing together. Beveled gears tend to wear so that their teeth do not fit one another closely. For this reason there should always be some method of adjustment. Miter gears have an equal number of teeth cut at the same angle (Fig. 4–17G).

Worm gears (Fig. 4–17F) consist of a shaft, called the worm, with screw-like threads which run spirally around it. This meshes with a helical spur gear called the sector. As the worm turns, the teeth of the sector, which fit in the grooves, are turned slowly. This type of gear is used to a limited extent in farm machinery.

Helical gears (Fig. 4–17E) may take the form of either spur gears or beveled gears, but they do not have straight teeth. The teeth are more or less curved so that they will remain in mesh or in contact longer than straight teeth. In the spur gear, they are called helical spur gear; in the beveled type they are called helical beveled gear. When helical gears are used, much noise is eliminated, because of the fact that the teeth remain in contact longer, giving an even, constant pressure at all times.

FIG. 4–18. Proper application of hitch and universal joints. Power-take-off-driven machines have been standardized so that all makes of tractors and machines may be connected with greater safety.

A pinion is the smaller gear of any two gears that are meshing together; it may be a spur, bevel, or helical gear.

Shafts and Universal Joints. In the operation of many farm machines, the tractor is used to move the machine forward and at the same time furnish the power for its operation. The power is transmitted from tractor to machine by means of a shaft which is usually termed a power-take-off shaft. If the travel of the tractor and the machine were always in a straight line, a solid shaft could, in many cases, be used. Field operation requires turning of corners in harvesting broadcast crops and back-and-forth trips for row crops. Thus, the power-take-off shaft must be equipped with at least two universal joints to permit these turns (Figs. 4–18 and 4-19). The complete shaft including universal joints is called a power-take-off drive. Shafts and universal joints are frequently employed, also, to transmit power at various angles on a particular machine.

In 1946, the American Society of Agricultural Engineers and the Society of Automotive Engineers approved standards establishing the dimensional relationships necessary to permit any tractor to be hitched successfully to any implement.¹ These standards require that the location of the power-take-off shaft shall be within the limits of 3 inches to the right or left of the center line of the tractor. The standard speed of a power-take-off shaft is 536 r.p.m. ± 10 r.p.m. A shield is required for the power-take-off shaft of the tractor and must be strong enough to support the weight of the operator. The manufacturer of the driven machine must furnish shielding for the shafting furnished.

FIG. 4–19. Complete power-take-off shaft with three universal joints, safety snap clutch, and shaft support. The safety shield has been removed to show all parts of the assembly. (Be-Ge Mfg. Company.)

FIG. 4–20. Torque-meter chart showing the effect of a frictional slip coupling designed to operate at relatively high torsional loads. (M. Hansen, Agr. Engin., 33(2): 69, 1952.)

TABLE 4–1. POWER-TAKE-OFF TORSIONAL LOADS

* Safety clutch in P.T.O. line slipped, limiting torsional load to this value.

SOURCE: Agr. Engin., 33:68, 1953.

The maximum instantaneous-torque starting values are more important considerations in designing a power-take-off drive than the average horsepower requirements of the operation. This is illustrated in Fig. 4–20. The data in Table 4–1 show the power-take-off torsional loads for several implements performing different farm operations.

Where flexibility is desired in transmitting light loads a distance of a few inches, a piece of thick-walled rubber hose may serve as a universal joint.

FIG. 4–21. Flexible shafting operating a side-delivery hay rake. The inset shows the construction of one type of flexible shafting. (Stow Mfg. Company.)

Flexible Shafting. A strong and durable flexible shaft can be used in many cases for the transmission of power in farm power equipment, replacing exposed universal joints and shafts. Figure 4–21 shows an application of flexible shafting for transmitting power from a tractor to the reel of a side-delivery hay rake.