This invention relates generally to an apparatus for improving traction efficiency and reducing soil compaction of agricultural wheeled tractors. Reference is made to U.S. Pat. Nos. 4,402,357 and 4,508,150 the disclosure of which is incorporated herein by reference. In both of these documents, the shortcomings of a pneumatic tire for a tractor's drive wheels are discussed.
As is noted in the above mentioned patent application, in the past few decades there has been a trend in the farm tractor industry toward larger, heavier and more powerful two-wheel drive tractor models as well as four-wheel drive type tractors. The reason for this evolution may be said to be two-fold: (a) to offset the inherently modest performance characteristics of agricultural drive tires in most field conditions in response to the market's demand for higher productivity; and (b) to compensate for increased resistance by the pulled implement which is caused by gradually more compacted soils.
In a publication entitled "Fighting World Hunger - Farm Productivity" (TRUCK & OFF-HIGHWAY INDUSTRIES, Sept. 1981), the farm tractor is singled out and, notably, attention is focused on the subject of traction and tire slippage. Industry experts are discussing monitoring wheel slip to automatically reduce wheel tractor slippage, and usage of dual wheels and front wheel assist as means for improvement.
In a recent paper entitled "Soil Compaction: An Increasingly Costly Problem" (published by ELANCO PRODUCTS COMPANY, Indianapolis with technical assistance from Purdue University), it is stated that " . . . (S)oil compaction is becoming an increasing problem . . . about 7.7 million of Indiana's 12 million acres of row cropland are susceptible to severe compaction . . . (I)n southern Michigan . . . excessive compaction was seen in 95 percent of the soybean fields and 90 percent of the corn fields surveyed!". And, on costliness of compaction: " . . . In 1982 corn experiments, Purdue University scientists showed that compaction reduced . . . yields by about 19 percent. Yields were 160 bushel per acre on non-compacted soil; only 130 bushels on compacted plots . . . Ohio scientists reported a 30-percent reduction in corn yields . . . Also in Illinois, Elanco Products Company researchers demonstrated a 60 percent yield reduction--from 159 bushels to only 96 bushels--due to compaction."
In addressing the issue as to why compaction is increasing, the tractor, again notably, is termed the main culprit. A photo of an articulated type tractor, having no less than twelve drive tires, is shown and it is stated: "Even with wider tires, heavier axle loads cause greater compaction." And, as to how compaction increase costs, it is stated: "Compacted soils are harder to till. Research in Illinois showed that 92 percent more power was required to plow a severely compacted soil. Larger more powerful tractors required to till compacted soils, in turn, cause more compaction. It's a costly cycle."
The findings reported in the above reference papers--said to be "published in the interest of better crop production"--serve to explain at least one reason for the need of bigger drawbar tractors on farms. If, for example, a 100 horsepower tractor was needed to plow a non-compacted field, then to plow the same field at equal tractor wheel speed when it has become severely compacted, it would require 192 horsepower, according to the research in the Illinois test. Further, at equal tractor wheel speed, the 92% more power required means 92% more vertical load on the drive wheels. This, in turn, adds to soil compaction--thus the "costly cycle."
To achieve this added vertical load on the drive wheels, the industry practice is to add ballast weight, either in the form of auxiliary cast-iron wheel weights or by means of a liquid solution carried within the drive tires. Typically, an agricultural drive tire will hold almost five times its empty weight in calcium chloride/water solution. The SAE (Society of Automotive Engineers) Recommended Practice J884c states that a 18.4-38 size tire, properly filled with 31/2 lb. CaCl.sub.2 per gallon of water, means 1,113 lbs. of liquid ballast. This size tire, per se, generally weighs less than 250 pounds. From the photo referred to in the paper, total ballast weight may well approximate some 13,000 pounds for the twelve drive tires--thus the captioned " . . . heavier axle loads cause greater compaction."
The immediate foregoing relates to reason (b) in the above. In regard to reason (a), reference is made to a publication entitled "The Comparative Performance of Some Traction Aids" (P. H. Bailey, National Institute of Agricultural Engineering, Silsoe, England; JOURNAL OF AGRICULTURAL ENGINEERING RESEARCH, Vol. 1, No. 1, 1956). This study reports traction performance, i.e., wheel slippage, tractive force, rolling resistance and overall efficiency for a number of traction enhancing devices operating on various types of field and compared to bias-type pneumatic tire. Included in these tests were the spade-lug steel wheel of early tractors.
From the Bailey publication above, the following can be deduced, pertaining to operation on "wheat stubble on clay, surface moist, fairly wet and firm below". At point of maximum terra-dynamic efficiency for both tire and for steel wheel--both occurring at ten percent slippage--the Coefficient of Drawbar Pull, i.e., ratio of horizontal pull developed to vertical load imposed, was 0.25 for tire versus 0.53 for steel wheel. Thus, at equal weight on a tractor's drive wheels and at equal rate of wheel slippage, the tractor with steel wheels will provide 112 percent more drawbar pull (0.53/0.25=2.12) on this type of field condition, not atypical for Midwest and Western States in the U.S.
The reason for the steel wheel's higher Coefficient of Drawbar Pull stems from its sharp spade-lugs, as opposed to the much wider tire lugs of pneumatic agricultural drive tire. Thus, the steel wheel will penetrate into the sub-surface layer of soil, having higher soil values and thus more shear strength for developing thrust for propulsion.
The grousers of a track-type vehicle provide the same penetrating characteristics as do the spades of steel wheels of early tractors. The track of such track-type vehicles provide an additional favorable aspect of traction, namely, low ground pressure due to its substantial width and length, thus resulting in less soil compaction. Agricultural tractors, however, must have great mobility and be capable of travel on paved roads for maximum utility. Neither the track-type vehicles nor the steel-wheel type tractors fulfill this requirement.
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