Mechanics of Tillage and Traction 3(2+1)

LESSON 9. DESIGN OF TILLAGE TOOLS

The pressing need for design information has demanded that methods for design be developed. In fact, the need is so great that qualitative procedures have been and still are widely used. The qualitative procedures have often been based on art rather than science (121, 269). That these procedures must be changed if progress is to be made in tillage tool design, is clearly demonstrated by the history of tillage tools.

Basic tools such as the forked stick date back into antiquity; yet, they are still found in their original form in many parts of the world. Even in more advanced societies, today, the moldboard plow is designed by empirical methods. Generally, these empirical methods are trial-and-error attempts; the tool is varied in some manner and acceptable designs are identified when the resulting soil condition is adjudged to be satisfactory. Quantitative descriptions or representations of the final soil condition are seldom used and, in addition, the forces required to move the tool are frequently not quantitatively assessed. Generally, no effort is made to describe the reaction of the soil. Consequently, design today merely accepts what occurs; it does not control what occurs. Thus, even though the need for design is great, design in the true sense of the word is not accomplished and probably will not be accomplished until quantitative information is available.

To illustrate the pressing need for design information, consider the economic possibilities of the results of better design. In the United States, more than 250 billion tons of soil are estimated to be stirred or turned each year ( 268 ). To plow this soil once requires 500 million gallons of gasoline costing $105 million. If proper design could decrease the draft of the plow only 1 percent, a savings in direct operating cost of $1 million per plowing would result. If soil manipulation can be controlled by proper design so that subsequent operations may be minimized or even eliminated, additional savings would result. Control cannot be assigned realistic dollar values today because its economic effects are not known. The benefits of control in road building, land leveling, and plant growth, however, must be considered.

9.2. DESIGN FACTORS

In tillage tool design, a limited number of abstract factors become of primary importance. In order to utilize the capability of soil for some specific purpose, the soil must be manipulated (changed, moved, or formed) to a desired condition. The manipulation is accomplished with a tillage tool by moving the tool through the soil. To obtain different final soil conditions, only the shape of the tool and the manner of moving the tool can be varied.

The major design factors that define the soil manipulation are,

1.  initial soil conditions,

2.  shape of the tillage tool, and

3.  manner of moving the tool

4.  forces required to move the tillage tool through the soil

5.  results of the manipulation ie., the final soil conditions

The first three factors are not clearly defined in a quantitative sense although qualitatively they represent distinct and complete elements in tillage tool design. Hence, they are called abstract design factors.

Unlike the above three design factors, the forces can be quantitatively defined. These forces are not those that are applied to the soil; they are those that must be applied to the tool to move it. The tool, in turn, applies equal but opposite forces to the soil. The five design factors represent the five elements that are of direct concern and, hence, of importance to a tillage tool designer.

Relations between the various design factors provide a means for designing a tillage tool. These relations can be qualitatively determined from available knowledge of the physical action of a tool that is manipulating soil. The concept of a mathematical function is useful in representing the relations. Two real variables are mathematically defined to be functionally related within some range if a definite single value of one of the real variables corresponds with a definite single value of the other variable according to some rule. The rule that prescribes the corresponding value is the functional relation.

The concept can be extended to several variables. A functional relation exists if a definite single value for each independent variable corresponds to a definite single value for a dependent variable according to some rule. For example, if a dependent variable is a function of four independent variables, specifying the value of each of the four independent variables determines the value of the dependent variable. Consider the number of single value correspondence rules possible for the five design factors. If tool shape is physically varied but the manner of movement, and the initial soil conditions are kept constant, the forces required to move the tool and the resulting soil conditions vary as tool shape is varied. Furthermore, for each “value” of tool shape, a definite “value” of the forces and final soil condition exists. If a definite value does not exist, no unique law of nature exists. Available knowledge indicates that some kind of law does exist and tool shape does affect tool forces and the resultant soil condition. Thus, in mathematical terms, shape, forces, and final soil condition are functionally related.