Farm Machinery and Equipment. Harris Pearson Smith
times in the case of a self-propelled combine, where the source of power is an engine mounted on the machine. Power must be transmitted to a slow-revolving reel and to a high-speed fan. Rotating movement must be changed into back-and-forth movement for the knife on the cutter bar and to an oscillating or shaking movement for the straw rack and grain pan. All this is done by means of pulleys and belts, sprocket wheels and chain, gears, and shafts. The operation is made possible by having good bearings to support the shafts and parts. The various parts of the machine are held together by different kinds of bolts and screws. Therefore, it is well to learn something about all these units to have an appreciation of their use in the design and construction of farm equipment.
METHODS OF TRANSMITTING POWER
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