Mechanics of Tillage and Traction 3(2+1)

LESSON 3. MECHANICS OF TILLAGE TOOLS

Mechanics of tillage tools have been developed where simple tools or simple actions are involved and where forces and reactions can be described. This chapter presents several approaches that have been used to develop simple forms of soil-tillage tool mechanics. Only homogeneous soil conditions are considered. Although this approach is completely unrealistic, it does not negate the results of the studies. Complete knowledge of reactions for a homogeneous soil will provide a basis for solving problems dealing with layered soils. Interactions of importance will probably occur, but they should not present insurmountable obstacles. The approaches discussed in this chapter do not represent any final solution of the problems that are posed. The approaches, however, do represent those that have been utilized and those that may contribute to the development of a successful mechanics of tillage tools.

3.1.1. The Reaction of Soil to Tillage Tools

The reaction of soil to a tillage tool can be quantitatively described only by a mechanics. The soil can be visualized as a continuous semi-infinitive mass composed of air, water, and solids arranged in some homogeneous manner. As a tool advances in the soil, the soil reacts to the tool and some action occurs. For example, the soil may move as a mass, the solids may displace the air or water, or the solids may break apart. The action of the soil’s response can be described by such qualitative terms as plowing, cultivating, and harrowing. When a quantitative description is desired, however, numbers must replace the qualitative terms.

The behavior equations were developed to quantitatively describe simple reactions of the soil to forces and also to define dynamic parameters that assess the soil. Behavior is defined here as any phenomenon that can be identified, isolated, and studied so that a behavior equation can be written to quantitatively describe the phenomenon. Thus, if an action such as plowing can be represented by the simultaneous occurrence of phenomena represented by behavior equations, a possible means is available for developing the desired quantitative description. Incorporating behavior equations into a system of equations that describes an action for a specific set of circumstances is one way to develop a mechanics. The equations of the mechanics will provide the desired quantitative description.

3.1.2. Principles for Developing Mechanics

The steps involved in the development of mechanics based on behavior equations are:

  i. The action to be quantitatively described must be defined.

 ii. The behavior involved in the action to be described must be recognized.

iii. In most circumstances the behavior must be incorporated into a mechanics that describes the action.

The action to be described is defined by interest from outside the action. A problem to be solved, curiosity, or merely a quest for knowledge are sources of interest. In the example of the projectile, interest determines whether the path of motion of each mathematical point of the projectile must be described or whether only the path of motion of the center of mass must be described. No set procedure can be established for defining an action because the procedure usually embodies simply defining the problem. Personal interest and the nature of the action itself will influence the definition. Until the action (defined here as the doing of something) has been at least qualitatively defined, however, the problem of quantitatively describing the action cannot be undertaken.

Because no unique structure exists, because of the mathematical complexity of the structure, and because more than one behavior is always involved, two guidelines for choosing behavior involved in an action are indicated. First, the choice of behavior must be arbitrary. In other words, for any specific action most of the behavior can be ignored. Second, the mathematical complexity suggests choosing behavior where the inputs and outputs of the behavior equation are as close as possible to the factors that will describe the action. For example, stress and strain do not lend themselves to describing the path of motion of a projectile.

When more than one behavior equation is required, a mechanics is required to combine the behavior equations. Just as no specific procedure can be given for defining an action, so no specific procedures can be given for combining behavior equations. Each situation has its own peculiarities. As suggested in the example of the projectile, including a second behavior equation may so change the result of the mechanics that little similarity remains. While the details of procedure will vary, combining behavior equations usually involves considering the equations simultaneously with boundary conditions. Simultaneous solution of the system of equations results in the desired mechanics.

3.1.3. The Complete Soil-Tillage Tool Mechanics

The reactions of soils to forces applied by tillage tools are affected by the resistance to compression, the resistance to shear, adhesion (attraction forces between the soil and other material) and frictional resistance. These are all dynamic properties in that they are made manifest only through movement of soil. Acceleration forces are not a property of soil but are also present. Nichols has shown that reactive forces of all classes of soils are dominated by the film moisture on the colloidal particles and are thus directly related to the soil moisture and colloidal content.

By following the principles, a soil tillage tool mechanics can be developed in progressive stages (fig. 1.1).

The purpose of the mechanics is to quantitatively describe the action of tillage on the soil. In the initial recognition phase, the action is observed and noted to be repetitive. The recognition phase is gradually supplanted by a qualitative phase, in which the general forces are identified and specific reactions are observed. Nearly all of the world’s literature on tillage research falls into the qualitative phase as defined here. The tool size and shape, width and depth of operation, speed of operation, and soil conditions are varied and the soil reaction is noted. The procedure involves trial-and-error methods of solving problems. The qualitative phase has been habitually utilized for problem-solving purposes; unfortunately, relations based on trial-and-error results rarely explain the underlying basic principles. Hence, the relations generally may not be used to satisfactorily explain new and untried situations, and more trial and error studies must be made.

3.2. TILLAGE TOOL DESIGN FACTORS

The purpose of the tillage tool is to manipulate a soil as required to achieve a desired soil condition. There are three abstract design factors namely, i. initial soil condition, ii. tool shape and, iii. manner of tool movement. These three design factors control or define the soil manipulation. The results of these three input factors are evidenced by two output factors, namely, i. the final soil condition and, ii. the forces required to manipulate the soil. All five factors are of direct concern to a tillage implement designer.

Of the three input factors, the designer has complete control only on the tool shape. The user may vary the depth or speed of operation and may use the tool through a wide range of soil initial conditions. However, tool shape cannot be considered independently of the manner of movement or initial soil condition. The orientation of a tool shape with respect to the direction of travel must be defined. Different initial soil condition sometime requires different shapes. For example many different shapes of the mould board plows has been developed for different soil types and conditions.

The shape that is of concern in design is the surface over which the soil moves as a tillage tool is operated. Gill and Vanden Berg classify three shape characteristics as i. macroshape, ii. edgeshape and iii. microshape. The term macroshape designates the shape of the gross surface. The edgeshape refers to the peripheral and cross-sectional shape of the boundaries of the soil working surface. Notched and smooth disk blades have different edgeshape but the macroshape may be the same. The microshapes refer to the surface roughness.

Most tillage tool have been developed by cut-and-try methods on the basis of qualitative analysis. The manipulation-shape relation has received has greatest emphasis in the development of the mould board plow bottoms, whereas force shape relations have been of concern in subsoilers and chisel type tools. Mathematical description of the shapes are the most versatile means of representation, but tools such as mouldboard plough have complex shapes that cannot easily be representative in mathematical form. Graphic representation is often employed for plow bottoms, although mathematical analysis has been attempted and computer analysis of plow-bottom shapes is increasing.

The shape of the cutting edge can materially affect draft as well as vertical and lateral components of soil forces.  For example, disk blades sharpened from the concave side penetrate more readily than blades sharpened from the convex side.  Worn plowshares reduce the vertical downward force V, tend to cause soil compaction, and sometimes substantially increase draft. 

The roughness of a surface over which soil slides (microshapes) influences friction forces.  Surface roughness is related to the initial polish and the effect of abrasive wear, and may result locally from the rust, scratches, or small depressions.  Frictional resistance can account for as much as 30% of the total draft of a mould board plow.  Microshapes can also have an important effect on other aspects of soil movement, such as scouring.

3.2.1. Scouring

One of the most important aspects of sliding action of soil is scouring of a tool while it is being operated. Since the coefficient of soil-metal friction of nonadhesive soil is normally less than that of soil-soil friction, less force is required to move a tool through soil if sliding occurs along the metal surface. Scouring is defined as the shedding or self-cleaning of the soil through a sliding action; but scouring also requires that the soil moves fast enough so that “too much congestion" does not occur. Thus, scouring is a relative term, rather than an exact term that designates the exact point where sliding begins. In normal operation where scouring is adequate, soil flows over a tool alone: a path that is determined by the shape of the tool. In adhesive soils, when sticking occurs, a layer of soil may build up along the surface of the tool so that soil flows over a layer of soil attached to the surface of the tool. In incipient cases of sliding, the soil moves across the tool so slowly that the soil on the tool acts as a rigid body which is driven through the soil mass. Soil does not flow smoothly across the plow when this occurs.

Definition

Doner and Nichols (1934) defined the scouring  at any point on a sliding surface as being approximately equal to the tangential force of the sliding added to the shear resistance of the soil minus the frictional force at the same point. They concluded from their studies that plow curvature at the wing rather than at the share would reduce soil sticking.

Factors affecting scouring

Payne and Fountaine (1954) studied the mechanics of scouring along simple surfaces and concluded that the following factors affect the scouring of a tool in soil:

1.  The coefficient of soil-metal friction

2.  The coefficient of soil-soil friction

3.  The angle of approach of the tool

4.  The soil cohesion

5.  The soil adhesion

Manner of movement involves orientation of the tool, its path through the soil, and its speed along the path.  For tools that travel in a straight line (i.e. not rotary or oscillating tools.), the path is usually identified by merely specifying the depth and width of cut.  Orientation of a tool having a particular shape may significantly influence both the soil manipulation and the forces.  Often the linkage system used to position of a tool affects both depth and orientation. When sufficient power is available, speed is the easiest design factor to vary.  Increasing the speed generally increases draft but also affects soil movement and breakup.