1. Field of the Invention
The present invention relates generally to automated sprinkler irrigation systems, and more particularly to a fluid driven traveling sprinkler unit.
2. Prior Art
Traveling sprinkler units, called "travelers," are widely used in the irrigation of grain crops, tree crops, vegetables, pasture, sod and the like. A typical traveler includes a frame supported on forward and rearward wheels. A power driven winch is carried on the frame and pulls the unit across a field by reeling in a cable. The cable has one end anchored at one end of a predetermined path of travel or "cable path" to be followed by the unit. A steering gear guides the forward wheels to follow the cable path. The unit carries a riser pipe for supplying pressurized water to a rotating sprinkler head mounted atop the riser pipe. As the unit travels along the cable path, the sprinkler head rotates to distribute irrigation water in a predetermined radius around the unit.
Some travelers carry internal combustion engines to drive the winch. Others use water powered reciprocating pistons. Still others drive the winch with a water powered turbine. A transmission is interposed between the winch drive and the drum which reels in cable to provide a range of selectable travel speeds. Travel speed determines the amount of water applied to a field as the unit moves along a cable path.
Pressurized water is supplied to a traveler through a long length of flexible hose. Hoses used with travelers typically have a length within the range of about 500 to 660 feet, and a diameter within the range of about 2 1/2 to 5 inches. The hose trails along behind the traveler as the traveler traverses a cable path. Drag forces imposed on the traveler by the hose help keep the drive cable taut.
The length of a cable path which can be traversed by a traveler is determined by the length of the supply hose. Cable path length is maximized in usual practice by providing the hose supply header connection near the midpoint of the cable path. During the first half of the travelers' movement along a cable path, the traveler moves toward the supply header. It then passes by the supply header and moves away from the header during the second half of its movement along the cable path. Maximum cable path length attainable with this system is about twice the length of the supply hose.
Two opposing factors operate to influence the speed of movement of a traveler as it traverses a cable path. The first is an increase in the effective diameter of the winch drum as more and more cable is wound onto the drum. This effect tends to increase the speed of travel of the unit as more and more cable is reeled in by the winch.
The second factor is increasing drag force load imposed on the traveler as the length of hose being dragged increases. This factor tends to decrease travel speed as the unit progresses along a cable path. When the traveler is initially positioned at one end of a cable path, the supply hose is laid out behind the traveler for a distance of 20 or 30 feet, then makes a wide radius U-turn, and extends forwardly along one side of the cable path for connection to the supply header. When the traveler starts moving along the cable path, it must drag only about 20 or 30 feet of water filled hose. When the traveler reaches a midpoint along the cable path, it is then dragging about half the length of the supply hose. As the traveler approaches the end of the cable path, it is dragging nearly the full length of the supply hose. The range of hose drag force loadings imposed on a traveler extends from only a few pounds at the outset of traveler movement to several thousands of pounds as the traveler approaches the end of a cable path. A 5 inch diameter water filled hose 660 feet long typically imposes about a 5,500 pound load on the traveler as the traveler nears the end of a cable path.
Designing a suitable drive system to power the winch of a traveling sprinkler unit poses several unique challenges. The drive must have a sufficient capacity to be able to accommodate drag force loadings of several tons. The drive must be able to maintain a substantially constant travel speed despite the tendency of increasing effective cable drum diameter to increase travel speed, and despite the tendency of increasing hose drag force loads to significantly reduce travel speed. Moreover, the drive should be light in weight, simple in construction, and should require minimal maintenance.
While internal combustion engines have been used on some travelers to power the winch, they add significantly to the weight of the traveler, require a separate fuel supply, are expensive to purchase and maintain, and often become quite temperamental with age.
Water powered reciprocating piston drives have the disadvantage of requiring a heavy duty ratchet linkage that must carry high loads and is subject to severe wear. Piston drives are relatively heavy, complex and costly, and require some auxiliary means of disposing of the water used to power them.
Turbine drives have the advantages of being substantially lighter in weight, simpler in construction and less expensive to operate and maintain. Moreover, the water used to power a turbine drive requires no separate means of disposal.
Axial flow turbines proposed for use in traveler winch drive systems have the disadvantage of providing an increasing resistance to fluid flow as their speed of operation slows. Stated in another way, axial flow turbines decrease in efficiency as the load imposed on them increases. This characteristic is opposite to that which is needed in a traveling sprinkler unit if travel speed is to be maintained despite significantly increasing hose drag loads. Still another problem with axial flow turbines is that they provide a relatively low torque, high speed output that is not well suited for traveler application.
One traveler drive proposal uses a radial inflow turbine to power the traveler's winch. In this proposal, the turbine has no "scroll," i.e., no housing portion which defines a spiral waterway of decreasing cross section which converges on the turbine wheel or "runner." Without a scroll, water admitted to the runner differs in velocity depending on the sector of the turbine volute from which it is admitted to the runner. The efficiency obtainable with such a turbine is much lower than desired and the torque output attainable is undesirably low.
A problem common to all previously proposed traveler winch drive systems is that they have not responded properly to increasing drag forces and to increasing effective winch drum diameter to maintain substantially constant travel speeds. Other problems include the inability of previously proposed drives to cope with wide variations in flow rates and torque output requirements that are encountered in different sprinkler installations. Still another drawback of proposed drive systems is their failure to provide an easy means of changing the drive ratio to provide a wide range of travel speeds in any given sprinkler installation.