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.
TYPES OF CLUTCHES
A clutch is a gripping device between a power source and a machine, or between working the parts in a machine, whereby the units can be connected or disconnected. In the operation of farm power equipment, clutches permit the starting of the engine with the machine disconnected. The clutch is then engaged and the power transmitted to the machine by means of shafts, gears, belts, and other devices.
Clutches are either friction or positive types. Friction clutches are used in tractors, trucks, and other equipment such as self-propelled combines and cotton harvesters in which power must be applied gradually to the load (Fig. 4–22). This type of clutch usually has flat plates covered with friction material which are held against each other by springs so that they turn as a unit, thus transmitting power.
FIG. 4–22. Friction clutch.
FIG. 4–23. Safety snap clutch.
Slip or