Frontmonterade redskaps rörelse i sidled : teorier och experiment
Stability criteria of front-mounted lateral movable three-point hitches have been known for several years. The steering ability of the tractor will be improved by this kind of implement hitch, compared with an implement hitch unable to move sideways. This is applicable particularly when a soil-working implement is used. There are several other advantages with front-mounted lateral movable three-point hitches. In spite of all advantages, this kind of implement hitch is not used in practice. This work shows that the lateral motion of front-mounted directional implements mostly follows the simplified theory proposed by Jönsson (1983). Also the lateral motion of front- mounted non-directional implements follows the simplified theory, provided that the area within which the non-directional hitch-point is moving can be approximated by a point. Another necessary requirement for the simplified theory is that the friction in the links can be neglected. According to the simplified theory, directional implements are moving towards the directional hitch-point and non-directional implements are moving towards the non-directional hitch-point. The lateral disturbances, are exponentially reduced according to the equation (Jönsson, 1983): Ls = Ls0 ∙e^-(x/Lh) where Ls side disturbance X travelled distance Lh hitch length The simplified theory assumes that the areas within which the hitch points are moving, when implements are moving laterally, can be approximated by points. This assumption implies that the hitch length can be approximated as constant. This approximation was shown to be fulfilled when the implement was mounted on a rear-mounted three-point hitch. In this case, the force in the upper link was relatively small. This was shown by a study of two different pulled non-directional implements. One implement with a draught force in the upper link and one with a compressive force in the upper link. The return of the implement from the lateral disturbance was almost similar for both implements. Thus, small variations in the upper link force were shown to have only minor influence on the lateral motion of the implement. To get front-mounted three-point linkages laterally stable and self-centring a large draught force in the upper link is required. This complicates the application of the simplified theory to front-mounted laterally movable three-point hitches, because the simplified theory has so far neglected the influence of the force in the upper link. The influence of different hitch geometry variables on the areas, within which the hitch points move when the implement moves laterally, has been studied in this work. For laterally stable implement-hitch combinations it is normally possible to approximate the area, within which the directional hitch point moves, by a point. The area, within which the non-directional point moves, is often larger. However, some combinations of hitch variables made it possible to reduce this area, within which the non-directional hitch-point moves, so much that also this area could be approximated by a point. Variable changes that contribute to reduce this area are: 1 Reducing the convergence of the lower links or moving forward the convergence point of the lower links 2 Reducing the length of the upper link 3 Increasing the relation between the upper link draught force and the soil force 4 Increasing the length of the lower links The influence on the lateral motion of the implements from these variables was studied partly by simulating the lateral motion of different implements and partly by measuring the lateral motion of different test hitch-implement combinations. In the simulations, the motion of the implement was always directed towards the momentary hitch point. The motion of the implements was then compared with the lateral motion of the implements according to the simplified theory. As long as the hitch points were moving within a small area, the lateral motion of the implement followed the simplified theory.
Rapport - Sveriges lantbruksuniversitet, Institutionen för lantbruksteknik
Publisher: Institutionen för lantbruksteknik, Sveriges lantbruksuniversitet
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