Självcentrerande frontmonterad trepunktskoppling : principer och funktion
Interest in front-mounted implements has increased during the last couple of years. One drawback of the commercially available front linkages is, however, that they have been unable to move sideways. Jönsson (1983) formulated the conditions that must be fulfilled to build a front-mounted three-point hitch that allows lateral movements of the implement since it centres the implement. This is done in two different ways, depending on the implement. Directional implements are centred if the lower links of the hitch converges in the direction of travel. This was first shown by Makanjuola & Cowell (1970). The return velocity of the implement depends on the convergence of the lower links. Non-directional implements are more difficult to centre. While directional implements centre due to the linkage geometry, non-directional implements centre due to the internal forces in the linkage. Jönsson (1983) formulated a criterion of stability: (FT/FR) > (LC/LA) where FT= Force in upper link. FR= The implement's resulting force on the linkage. LC= Lateral distance between the linkage’s instantaneous centre and the centre of resistance. LA= Distance between the instantaneous centre and the upper link forces. If the stability criterion is true in every possible position that the three-point linkage can reach, a non-directional implement will centre after a. disturbance. In this work, a front-mounted self-centring three-point hitch of that kind was built and tested. The hitch was tested in combination with five different implements: 1. A one-furrow fully-mounted plough. During the work the plough proved to be self-centering and steerable. The plough was, however, very sensitive to the linkage adjustments. This was primarily due to the short upper link and mast height. On the other hand, the short top link functioned as a depth control. As the working depth of the plough changed, the change of the share pitch brought the tool back to its original depth. 2. Hoe. With this implement this front-coupling was superior to the conventional rear-mounted type. According to the driver's wish, the linkage was easily adjusted to slow or fast reaction. A slowly reacting implement compensates better for bad driving in rowcrops than a fast reacting one. On the other hand, the turning radius increases. It is also better from the ergonomic point of view to have this kind of implement in front of the driver. 3. Harrow. During the tests the harrow usually, but not always, centred itself. In extreme conditions the harrow's clod-crusher nullified the centring ability. A harrow with a jointed cross-shaft ought to be self-centering. The joint will permit vertical movement of the implement. Thereby the clod-crusher cannot interfere with the upper link-forces. 4. Single tine. The front coupling was also tested with a single tine. This was done to test a pure non-directional implement. With the tine the centring was good and the front-mounted three-point hitch behaved as expected. 5. Single disc. In order to test a pure directional implement, a single disc was mounted to the hitch. During the tests the return velocity of the implement was measured at different cross-shaft lengths. A dependence between the length of the cross-shaft (the convergence) and the return velocity was indicated, in spite of severe disturbances during the tests. The tested type of front-linkage probably has a greater developing potential than the ones available on the market today. The self-centring front-linkage reduces the stress on the tractor, the linkage and the implement more than the conventional front-linkages. The driver ergonomy is improved by using front-mounted implements instead of rear mounted. A lot of work is however needed, before this type of linkage can be commercially available. The adjustment of the coupling needs to be simplified. Some kind of regulating device for working depth is also needed to fully use the advantages of front-mounting.
Rapport - Sveriges lantbruksuniversitet, Institutionen för lantbruksteknik
Publisher: Institutionen för lantbruksteknik, Sveriges lantbruksuniversitet
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