A tillage implement is pulled by a power unit like tractor, power tiller or animal source.
While pulling the implement, three types of forces are found to be acting on the implement:
1. Force of gravity
2. Soil reaction on the implement and
3. Pull of the power unit
These forces must be in equilibrium for satisfactory operation of the implement. These forces have been represented in Fig. 13.12. F is the force exerted by the power unit which can have components in all the major planes and associated with it is a couple. The force F can be resolved into three components L, V and S in three planes. The component L represents the draft of the implement which is used for pulling loads.
Fig. 13.12 Forces on a tillage tool
If Ɵ is the angle of inclination of force F in the vertical plane with the horizontal surface and ɸ is the angle of inclination of F in the transverse plane with the horizontal then-
L = F cos Ɵ. cos ɸ
The vertical component V has the effect of adding load to the tractor rear wheels. This removes loads from the front wheels of the tractor. It has a profound effect upon the tractive ability, stability and steer ability of the tractor. It affects the implement power to penetrate and maintain the working depth.
V = F sin Ɵ. cos ɸ
The side force S maintains directional stability on the tractor and implement. It has effect on the draft of the implement because of frictional forces.
S = F cos Ɵ. cos ɸ
In case of mounted implements, supported and pulled by tractor, the force P between the implement and tractor in the vertical plane is the force containing L and V components which is given by the relation.
The sum of the components of all the forces in vertical plane can be separately equated to zero so that:
Ʃ H = 0, Ʃ V = 0 and Ʃ M = 0
Where, Ʃ H represents the horizontal component, Ʃ V represents vertical component and Ʃ M represents the moments due to force P and the reactions at lower hitch point and forces in the top link of the tractor.
Mechanics of Tillage:
When a tillage tool moves in the soil, pressure is exerted to the soil with the help of inclined planes or wedges. As the tool advances, the soil is subjected to compressive stresses which result in shearing action in a friable soil. While shearing, the soil reaction may extend for some distance on either side of the shear plane due to internal friction of soil particles and the cohesive forces.
It is defined as the force that holds two particles of the same kind together. It is the frictional force occurring due to interlocking of particles within the soil mass.
Internal friction and cohesion are parameters of shear stresses, which are represented by the following equation-
S = C + N tan ɸ
S = shear stress at soil failure surface
C = cohesion of soil
N = normal stress on the plane of shear failure, and
ɸ = angle of internal friction.
When normal load is zero, the cohesion becomes equal to shear stress of the soil S, and ɸ may be determined by measuring the shear stress for several values of normal stress N. Shear strength of soil has an important influence on the draft of a tillage tool.
All tillage operations involve a sliding action of soil over some surface of the tool. Friction of soil against a tool affects draft requirement.
Soil friction is usually assumed to follow the law for simple friction:
µ = F /N
= tan Ɵ
µ = coefficient of friction
F = frictional force, tangent to the surface
N = normal force (perpendicular to the surface), and
Ɵ = angle of friction
It is the force between soil and metal. This is mostly due to moisture film.
Effect of Speed on Draft:
Draft increases with increase in speed in most of the tillage implements. This may be due to rapid acceleration of any soil that is moved appreciably.
Soil acceleration increases draft because:
1. Acceleration force increases the normal loads on soil engaging surfaces thereby increasing the frictional resistance.
2. Kinetic energy is imparted to the soil.
Draft can be expressed as Ds = D0 + KS2
Ds = draft at speed S
DO= static component of draft independent of speed
S = forward speed, and
K = constant, depending upon type of implement and soil condition.
There are three design factors for tillage implements:
1. Initial soil condition
2. Tool shape and
3. Manner of tool movement
These factors control or define the soil manipulation which is the aim of tillage implements.
The results of these three input factors are evidenced by:
i. Final soil condition and
ii. Force required manipulating the soil. Implement designer is directly concerned with these factors.