Dynamics of rigid soil-engaging implements and parts of implements : lateral motion of soil-engaging implements and vertical motion of seed drill coulters
In this thesis the lateral motion of rigid, soil-engaging implements hitched to a tractor with a one-point or a three-point hitch, as well as the vertical motion of seed drill coulters for use in tilled seedbeds are investigated. The motion of most tractor-hitched soil-engaging implements is heavily overdamped, it was therefore described by a first order model, based on a quantitative description of the lateral properties of the implement and a functional description of the hitch. For each hitch-implement combination an effective hitch-point, towards which the implement tends to move, was defined and a method to calculate the effective hitch length was developed. The return of a laterally disturbed implement was shown to be exponential. The disturbance is reduced to 10 % of its initial value when the implement moves forward a distance of 2.3 times the effective hitch length. The tractional hitch point and the directional hitch point coincide for one-point hitches but not for three-point hitches. This gives the three-point hitches unique properties: The angular as well as the lateral deflection of a three-point hitched implement will normally be smaller compared to those of a one-point hitched version of the same implement. This is true both when it is used on a hillside and when it is used with an asymmetric load. The three-point hitch is also shown to be versatile enough to enable the construction of a laterally movable and, for both directional and non-directional implements, self-centring front mounted three-point hitch. The horizontal and vertical soil forces on coulters with different rake angles (70-135°) were measured under both pseudostatic (no vertical velocity and no accelerations) and fully dynamic conditions. Most of the measurements were made in a soil bin with dried sand, but some of the pseudostatic measurements were made in a clay field. Empirical models were developed for the soil forces acting on the coulter under both pseudostatic and dynamic conditions. These models were used to simulate coulter equilibrium depth and coulter penetration. The simulations led to the following conclusions: Coulters with a large rake angle (≥1l0°) have several advantages when compared to coulters with smaller rake angles; their equilibrium depth is less sensitive both to speed and to soil parameters and their penetration is faster, better damped and less sensitive to suspension parameters (pivot arm length and pivot axis height) and to suspension type (single armed or parallel armed). The coulter penetration was found to improve with decreasing speed, decreasing moment of inertia and increasing depth of operation. No reasons were found for complicating the coulter suspension by using a parallel arm suspension or by using separate pivot axes for front and rear row coulters. The above conclusions apply to well tilled seedbeds without stones or clods. If there are frequent stones or clods in the seedbed the present recommendation of using coulters with rake angles around 90° seems valid.
Tractor; hitch; one-point hitch; three—point hitch; three-point linkage; front mounted three-point hitch; dynamics; seed drill coulter; furrow opener; soil forces; dynamic soil force model
Rapport - Sveriges lantbruksuniversitet, Institutionen för lantbruksteknik
Utgivare: Institutionen för lantbruksteknik, Sveriges lantbruksuniversitet
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