WO2002011518A2

WO2002011518A2 - Harrow for tractor

Info

Publication number: WO2002011518A2
Application number: PCT/KR2001/001370
Authority: WO
Inventors: Jung Ho Kim
Original assignee: Kim, Dae, Ho; Daeho Farming Machinery Co., Ltd.
Priority date: 2000-08-10
Filing date: 2001-08-10
Publication date: 2002-02-14
Also published as: AU2001277808A1; WO2002011518A3; JP2004504857A; JP4931322B2; CN1388740A; KR200212529Y1

Abstract

The invention relates to a harrow for tractor which has a simple and safe structure with efficient levelling work through free adjustment of the inclination angle of a harrow plate relative to the ground with safety. The harrow for tractor comprise a plurality of connecting rods (10) hinge-connected to the rotavator (2), a harrow plate(20) hinge-connected to the hinge axises (23a) of the rear ends of the connecting rods (10), a connecting arm(22) formed projectingly upward on the harrow plate(20) and a lifting cylinder (50) mounted on the bracket(4) extending upward from the rotavator (2) and hinge-connected to the hinge axis (22a) of the top end of the connecting arm (22), wherein the hinge axis (22a) for connecting the harrow plate (20) and the lifting cylinder (50) is arranged so as to be positioned above the hinge axises (23a) for connecting the harrow plate (20) and the connecting rods (10).

Description

HARROW FOR TRACTOR

FIELD OF THE INVENTION

The present invention relates to a harrow for tractor and more particularly to a novel harrow for tractor which has a simple structure to reduce the product price and transportation cost, permits efficient levelling work through free adjustment of the inclination angle of a harrow plate relative to the ground and is able to protect the harrow plate against a damage even when the harrow plate is overloaded or inadvertently collide against a field levee or the like during a levelling work.

BACKGROUND OF THE INVENTION

The conventional harrow for tractor was usually composed of a single harrow plate with a finite width in consideration of the mobility for the tractor. Thus, when levelling work was conducted in a paddy field using such a harrow, working efficiency was low due to the restricted work amount from the narrow width, and the relatively increased traces of track also resulted in the depressed ground compared to the surrounding, so that there was a problem of inefficient levelling work.

Thus, recently a folding type harrow plate was developed, wherein the the outer harrow plates are foldably pivoted on the opposite site of central harrow plate so that leveling work of wider area at one time may be possible.

Figure 1 shows the perspective view of a conventional foldable harrow for tractor. Referring to that figure showing the harrow for tractor, in the top area of the rotavator 2 in the rear part of the tractor, there is provided a transverse shaft 3, at the center of which brackets 4 for mounting a cylinder 50 extending upward are provided, and a lifting cylinder 50 is connected, in a hinged manner, to the top parts of the brackets 4 and further a rectangular frame 90 connected to the lifting cylinder 50 is provided, likewise in a hinged manner, so as to be swingable and liftable on the transverse shaft 3. In addition, to the rear lower section of the frame 90, a central harrow plate 81 is coupled by means of pins 98, wherein a pair of outer harrow plates 85 are hinge-coupled to opposite ends of the central harrow plate 81 to be capable of swinging and folding forward.

However, because in such a harrow for tractor, the harrow plate 80 is connected to the frame 90 by the pin 98 extending forward and backward, the left and right side ends of the harrow plate 80 can swing vertically about the pin 98 along the curvature of a paddy field, and nevertheless the proper adjustment of the inclination angle of the harrow plate 80 in accordance with the hardness or curvature of the paddy field is difficult, because the inclination angle for the blade part of the harrow plate 80 with regard to the ground surface is always fixed.

Further, such a harrow plate 80 tends to experience overload during a soil pushing work for removing a great deal of soil at a time, with the harrow plate 80 inclined at a more upright angle relative to the ground, because of the wider breath compared to a single harrow plate, leading to a bending damage. Moreover, even during a levelling work for a waterly field, with the harrow plate 80 lying close to the ground surface, the harrow plate 80 would be forcefully dragged by the tractor with a large chance of undergoing a damage, when a worker carelessly causes the harrow plate 80 to collide with an obstacle like a field levee.

In addition, there is also a problem of increased steps for manufacturing and assembling with the above-described harrows for tractor, wherein the frame 90 and central harrow plate 81 should be formed by respectively welding longitudinal and lateral pipes substantially in a rectangular form and connections for the pin 98 should be provided in the central position to combine the frame 90 and central harrow plate 81 in a mutually swingable manner. Furthermore, frames and central harrow plates are not appropriate for home delivery or packed delivery service enterprises in the respect of collective transport and so their delivery should depend on agents or individual car transportation by consumers, resulting in increased incidental costs. The situation gets even worse, because such harrows are usually distributed to farming houses located in remote places.

On the other hand, because the distance between the front wheels 1 and the harrow plate 80 is long in such a harrow for tractor, it is required, to conduct a leveling work for a deep corner of a field, that the direction of the tractor be changed, with the harrow plate 80 lifted upward from the ground, the harrow plate 80 be lowered on the ground for continued work.

However, if the harrow plate 80 is not lifted high enough in consideration of time saving at the time of changing the direction and so happens to pass an irregular ground surface, the side ends of the harrow plate 80 would be swung vertically and ultimately one end could be stuck in the paddy field and damaged.

SUMMARY OF THE INVENTION

The present invention is intended to resolve the problem as described above and so the object of the invention is to provide a novel harrow for tractor which has a simple structure to reduce the cost of product itself and transportation, permits an efficient levelling work through free adjustment of the inclination angle of a harrow plate relative to the ground and is able to protect the harrow plate against a damage even when the harrow plate is overloaded or inadvertently collide with a field levee or the like during a levelling work.

The above object is achieved according to an aspect of the invention by a harrow for tractor comprising a plurality of connecting rods(lθ) hinge-connected to the rotavator(2) in the rear of a tractor to be lif table and swingable, a harrow plate (20) hinge-connected to the hinge axises(23a) of the rear ends of the connecting rods(lθ) to be swingable vertically, a connecting arm(22) formed projectingly upward on the harrow plate (20) and a lifting cylinder (50) mounted on the bracket(4) extending upward from the rotavator (2) and hinge-connected to the hinge axis(22a) of the top end of the connecting arm(22), wherein the hinge axis(22a) for connecting the harrow plate(20) and the lifting cylinder(50) is arranged so as to be positioned above the hinge axises(23a) for connecting the harrow plate(20) and the connecting rods(lθ), and so that an extension of the lifting cylinder(50) causes the harrow plate(20) to move down toward the ground surface, under the state of the harrow plate(20) lifted to an elevated position, and a further extension of lifting cylinder(50) even after the contact of the harrow plate(20) with the ground surface causes the increase of the inclination angle of the blade part of the harrow plate(20) relative to the ground surface.

According to another aspect of the invention, the harrow plate (20) is provided with an upper and lower stoppers (24,25) for limiting the swinging angle(20) of the harrow plate(20) by interfering with the connecting rods(lθ) as the harrow plate(20) swings.

The above object is also achieved by a harrow for a tractor wherein the harrow plate(20) consists of a central harrow plate(21) connected to a lifting cylinder (50) and a pair of outer harrow plates (27) arranged at opposite ends of the central harrow plate(21), the central harrow plate(21), the outer harrow plates(27) are connected in swingable manner by connector rods(70) respectively provided with hinged axises(71a, 77a) at the opposite ends, the hinged axises(71a,77a) being oriented to be offset to each other, and safety elements (73) are provided at the connection areas of the central and outer harrow plates(21, 27) so that the elements may be broken when the outer harrow plates(27) are overloaded.

Preferably, the safety elements (73) consist of a pair of safety plates(74) mounted by safety bolts(75) each passing through an outer harrow plate(27) and connector rod(70), one side of the safety plates(74) being contacted with the front surface of central harrow plate(21), the safety ρlates(74) or the safety bolts(75) being formed weaker compared to the harrow plate (20).

Further, preferably, the harrow plates(20, 30) consist of a main harrow plate(20) hinge-connected to connecting rods(lθ) and a pair of auxiliary harrow plates(30) hinge-connected to opposite ends of the main harrow plates(20), and the auxiliary harrow plates(30) are connected to the main harrow plate (20) by means of springs (40) so as to outstretch or to be folded on the main harrow plate(20) at the opposite ends of the main harrow plate(20) under the elastic restoring force of the springs(40).

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the perspective view of a conventional harrow for tractor,

Figure 2 shows the front view of a conventional harrow for tractor according to an embodiment of the present invention,

Figures 3 and 5 show the side views for the embodiment as described above,

Figure 6 shows the front view illustrating the case wherein the harrow plates are overloaded in the state of the harrow plates laid for the embodiment as described above, and

Figure 7 shows the rear view illustrating the case wherein the harrow plates are overloaded in the state of the harrow plates erected for the embodiment as described above. DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the invention will be described in detail below by referring to the accompanying drawings.

Referring to Figures 2 to 7, which show the front, side and rear views of a harrow for a tractor according to an embodiment of the invention, harrow plates 20 and 30 are connected, through a pair of connecting rods 10 and a lifting cylinder 50, to a rotavator, so that as seen in Figures 3 to 5, the harrow plates 20 and 30 are lifted off the ground surface as the lifting cylinder 50 is contracted, and after the harrow plates 20 and 30 have touched the ground, the inclination angle of the harrow plates 20 and 30 with reference to the ground surface is adjustable with the extension or contraction of the lifting cylinder 50.

Further, the harrow plates 20 and 30 are, as seen in Figure 2, composed of a central harrow plate 21 hinge-connected to connecting rods 10, a pair of outer harrow plates 27 hinge-connected to opposite ends of the central harrow plate 21 so as be foldable upward and forward, and a pair of auxiliary harrow plates 30 hinge-connected to outer ends of the outer harrow plates 27 so as to be foldable rearward and upward. In the interface area between the central harrow plate 21 and the pair of outer harrow plates 27, connector rods 70 and safety elements 73 are provided, as seen in Figures 6 and 7, so that the safety elements 73 may be cut off under an overload to protect the harrow plates 20 and 30 at any inclination angle of the harrow plates 20 and 30 relative to the ground surface. Further, springs 40 are provided between the outer harrow plates 27 and auxiliary harrow plates 30, as seen in Figure 2, so that the auxiliary harrow plates 30 can be folded in the case of bumping against an obstacle like a levee to protect the harrow plates 20 and 30.

On the top of the rotavator 2, there is provided a transverse axis 3, at the central positions of which cylinder mounting brackets 4 are provided extending upward, wherein a lifting cylinder 50 is hinge-connected to the upper ends of the brackets 4 and a pair of connecting rods 10 are hinge-connected to the both sides of the lifting cylinder 50 in a pivotal and liftable manner. Further, the rear ends of the connecting rods 10 are connected to the harrow plates 20 and 30 also by means of hinges so that the harrow plates may be swingable vertically relative to the lifting cylinder 50.

As described earlier, the main harrow plate 20 includes the central harrow plate 21 and the outer harrow plates 27 hinge-connected to the central harrow plate 21 at its opposite ends so as to be foldable upward and forward as required, wherein the distal ends of the outer harrow plates 27 are connected to auxiliary harrow plates 30 under elastic support of springs 40, so that the auxiliary harrow plates 30 may be folded rearward and upward.

An upward projecting connecting arm 22 is formed on the upper middle position of the central harrow plate 21 to be connected to the lifting cylinder 50 and on the both side of the arms, a pair of brackets 23 are formed in hinged manner at the rear ends of the connecting rods 10 to allow vertical swinging movement. As seen in Figure 3, a pair of upper and lower stoppers 24 and 25 are provided on the bracket 23 to locate the extending connecting rod 10 between the stoppers, so that the harrow plates 20 and 30 can be laid or raised by the action of the lifting cylinder 50 until the movement of the connecting rod 10 is limited by the upper or lower stopper 24 or 25.

Further, the first hinges 71 are provided at the opposite ends of the central harrow plate 21, as seen in Figures 6 and 7, to allow the rotation of the connector rods 70 about the hinge axises 71a substantially in the extending direction of harrow blades 21a and 27a, wherein the connector rods 70 are connected to the outer harrow plates 27.

The connector rods 70 substantially in the form of rectangular pipe, which connect the central harrow plate 21 with the outer harrow plates 27 in swingable manner, are connected, at the inner ends of the rods 70, to the first hinges 71 of the central harrow plate 21 to allow for the outer harrow plates 27 to swing in the direction of extension of the harrow blades 21a and 27a, while the second hinges 77 positioned at the outer ends of the connector rods 70 are so oriented as to allow the rotation of the outer harrow plates 27 in the direction substantially perpendicular to the direction of extension of the harrow blades 21a and 27a. The safety bolts 75 are secured in the holes 70a formed in the middle of the connector rods 70 to fix the safety plates 74.

The outer harrow plates 27 provided at the opposite ends of the central plate 21 also to be able to be folded upward and forward at about right angle are provided, on their surfaces, with a pair of brackets 78a and 78b, with which the connector rods 70 are coupled by means of pins to form the second hinges 77. Further, the safety elements 73 are provided at inward ends of the outer harrow plates 27, so that the elements may contact to the front surface of the central harrow plate 21 and thus the outer harrow plates 27 can be maintained at unfolded state. As described before, the rotation axises of the first hinges 71 and the second hinges 77 are oriented to be offset to each other, so that the outer harrow plates 27 can be widely changed in the direction, as long as outer harrow plates 27 can rotate in the rear side relative to the advancing direction, independent of the inclination angle of the outer harrow plates 27. Further, the first and second hinges 71 and 77 can be embodied by a variety of ways including hinge axises, pins, bolts and the like.

The safety elements 73 are each composed of a safety plate 74 fixed to the connector rod 70 and outer harrow plate 27 to contact an the front surface of the central harrow plate 21, and a safety bolt 75 which passes through the holes 70a and 27a of both the connector rod 70 and outer harrow plate 27 to fix the safety plate 74. The safety plates 74 or the safety bolt 75 are made of material weaker than the harrow plates 20 and 30. The central harrow plate 21 are provided, at its opposite ends, with stoppers 26 with a predetermined length extending outward and positioned in the front regarding the safety plates 74. The bracket 78a positioned in the rear, out of the pair of brackets 78a and 78b taking part in forming a second hinge 77 is formed with a hole 27a. The safety bolt 75 is inserted into this hole 27a. The safety plates 74 are fastened to the ends of the central harrow plate 21 through chains 79, so that they are prevented from falling to the field even when safety bolts 75 are broken.

The safety element 73 may be realized, for example, by a safety pin or safety plate formed with weak parts beside the combination of safety plate 74 and safety bolt 75 as described above, if only they can be broken before the harrow plates 20 and 30 are damaged in the case of an overload.

The auxiliary harrow plates are hinge-connected to the ends of the outer harrow plates 27 so that they can be swung rearward and upward, wherein brackets 29 and 32 are provided across and on the top or front surface of the outer harrow plates 27 and auxiliary plates 30 and additionally hinge connectors 33 are formed on the brackets 29 and 32 in such a manner that the auxiliary harrow plates 30 can be unfolded to extend outward at the ends of outer harrow plates 27 or folded onto the outer harrow plates 72 following backward swing motion.

Springs 40, which are provided on the brackets 29 and 32 at front and lower positions relative to the hinge connectors 33, act to keep the auxiliary harrow plates 30 stretched straight by pulling the harrow plates forward and downward. Further, the distance between the pair of brackets 29 and 32 over which distance the spring runs gets longer as the harrow plate 30 is folded until it swings 90° to the backward and upward position and then gets shorter as the harrow plate 30 is further folded until it completely overlaps on the outer harrow plate 27. Therefore, the springs 40 serve to keep the auxiliary harrow plates 30 either unfolded or folded.

Further, the harrow plates 20 and 30 comprise substantially rectangular pipes 21b, 27b and 30b provided, at finite interval, with a number of harrow pieces 21a, 27a and 30a. The harrow pieces 27a and 30a mounted on the outer and auxiliary harrow plates 27 and 30 are each formed to protrude downward at the bottom center-side edge, so that the soil and water carried along with the harrow plates 20 and 30 may be collected toward the central side without diverting to the outer ends of harrow plates 20 and 30 and thus lower ground may be filled to help efficient levelling when the harrow plates 20 and 30 later pass that area. The gaps formed between respective adjacent harrow pieces 27a and 30a act to prevent excessive soil or water from being carried by the harrow plates 20 and 30. The harrow pieces 21a on the central harrow plate 21 are each formed to be inclined to the front direction at the free end and the harrow pieces 21a with higher and lower inclination angle are alternately arranged.

On the other hand, on the inner ends of the pair of outer harrow plates 27, there are provided in opposite relation two rotation brackets 28 with a finite length, between the inner ends of which brackets and the central part of the lifting cylinder 50, supporting rods 60 extensible under the elastic tension of springs 63 are disposed, so that the outer harrow plates 27 can be folded upward and forwardly when the harrow plates 20 and 30 are lifted. These supporting rods 60 are each composed of a rod 65 slidingly fitted inside a pipe 61 with a finite length, wherein a spring 63 is inserted between the rod and pipe to elastically support the rod in the direction of expanding the rod 65. Accordingly, when the harrow plates 20 and 30 are lifted upward by contracting the lifting cylinder 50, the harrow plates 20 and 30 are pulled toward the lifting cylinder 50, while the rotation brackets 28 of outer harrow plates 27 elastically supported by the supported rods 60 are pushed relatively backward from the central harrow plate 21 and the outer harrow plates 27 are folded upward and forward with regard to the central harrow plate 21.

Besides, the supporting rods 60 function also to prevent the harrow plates 20 and 30 from a hard vertical vibration along the curvature of the ground, by elastically pressing the rear edge of the harrow plates 20 and 30 against the ground surface by the help of the springs 63 positioned between the rods 65 and pipes 60.

As the result of the above construction, when the cylinder 50 is contracted to raise the harrow plates 20 and 30 of a tractor, first the harrow plates 20 and 30 are laid at an angle relative to the ground surface, about the hinge connectors 23 with the connecting rods 10, and subsequently begin to be lifted from the ground when the lower stoppers 24 interfere with the connecting rods 10 in motion. However, in the state of the harrow plates 20 and 30 lifted-off, the harrow plates 20 and 30 can not be swung downward to either side even during the direction change of the tractor and can stay stable in the lifted-off state, because the upper stoppers 25 of the harrow plates 20 and 30 are stably restrained by the connecting rods 10.

In contrast, when the lifting cylinder 50 is extended, with the harrow plates 20 and 30 lifted-off, the harrow plates 20 and 30 are lowered to touch the ground. When the cylinder 50 is further extended, the harrow plates 20 and 30 are caused to be erected at a finite angle to the ground with regard to the hinge connectors 23 with connector rods 10, so that the inclination angle of the harrow plates 20 and 30 can be freely adjusted to suit the circumstance of a paddy field to achieve efficient levelling work.

Subsequently, when the lifting cylinder 50 is further extended, the lower stoppers 24 of the harrow plates 20 and 30 are interfered with the lower surface of connecting rods 10, and with a further extension of the cylinder 50, the harrow plates 20 and 30 are driven in the paddy field, so that a relatively more amount of soil at one time can be collected to fill lower grounds afterward.

In such a process, when the lifting cylinder 50 is contracted, the harrow plates 20 and 30 are pulled toward the lifting cylinder 50, while the rotation brackets 28 of outer harrow plates 27 elastically supported by the supported rods 60 are pushed relatively backward from the central harrow plate 21 and simultaneously the outer harrow plates 27 are folded forwardly and upwardly with regard to the central harrow plate 21. On the contrary, when the harrow plates 20 and 30 are lowered by expanding the lifting cylinder 50, the outer harrow plates 27 are unfolded by their own weight.

On the other hand, when a harrow for tractor is acted on its outer harrow plate 27 by an overload by encountering an obstacle in the course of levelling work, with the harrow plates 20 and 30 laid transversely on the ground surface, as shown in Figure 6, the outer harrow plate 27 and the connector rod 70 concurrently are caused to be swung in the direction rearward to the advance, with the first hinge 71 at one end of the central harrow plate 21 as the center. In contrast, the safety plate 74 in turning motion interfere, on its end part, with the stopper 26 provided at the end of the central harrow plate 21 and prevented from further rotation, and therefore the safety plate 74 is separated from the outer harrow plate 27, pulling the safety bolt 75 combining the safety plate with the harrow plate, and ultimately the safety bolt 75 is broken due to excessive tensile force, protecting the harrow plates 20 and 30 against a damage.

Further, when a harrow for tractor is acted on its outer harrow plate 27 by an overload or by encountering an obstacle in the course of soil dragging work, with the harrow plates 20 and 30 erected, as shown in Figure 7, only the outer harrow plate 27 is turned rearward about the second hinge 77, touches the rear brackets 78a on the connector rod 70 in the course of turning and causes the breakage of the safety bolt 75 combined with the connector rod 70, protecting the harrow plates 20 and 30 against damage. In addition, there could be the cases in which an outer harrow plate 27 is turned rearward under the simultaneous action of the first and second hinges 71 and 77 at a certain rotation angle of the harrow plates 20 and 30, so that both tensile and shear stress are acted on the safety bolt 75 to break that bolt, protecting the harrow plates 20 and 30.

Besides, the harrow for a tractor according to the embodiment of the invention as described can be mounted by means of a pair of connecting rods 10, and therefore the structure is very simple and accordingly product cost and overhead costs are also reduced. Specially, the harrows can be shipped in packs because they do not include voluminous and heavy components like the conventional frames and the costs related with the material movement can be remarkably reduced, because direct individual transportation to remote farming houses is not necessary.

According to the present invention, although the harrow plates are often lifted above with the extension or contraction of the lifting cylinder, the harrow plates can be controlled in their inclination angle relative to the ground surface once they have contacted the ground, whereby efficient levelling work for paddy fields in accordance with the circumstance of the fields can be performed.

Further, even when the harrows for tractors are acted on the harrow plates by overload or carelessly collide, at either side end, with obstacles such as levees or the like, safety bolts are broken or auxiliary harrow plates are swung and folded rearward upwardly by overcoming the elastic force of springs , so that the harrow plates can be prevented from damage due to bending or so and thus interruption in work can be also prevented, while prolonging the service life of the products.

Claims

l.A harrow for tractor comprising a plurality of connecting rods(lθ) hinge-connected to the rotavator(2) in the rear of a tractor to be liftable and swingable, a harrow plate(20) hinge-connected to the hinge axises(23a) of the rear ends of the connecting rods(lθ) to be swingable vertically, a connecting arm(22) formed projectingly upward on the harrow plate(20) and a lifting cylinder(50) mounted on the bracket(4) extending upward from the rotavator (2) and hinge-connected to the hinge axis(22a) of the top end of the connecting arm(22), wherein the hinge axis(22a) for connecting the harrow plate(20) and the lifting cylinder(50) is arranged so as to be positioned above the hinge axises(23a) for connecting the harrow plate(20) and the connecting rods(lθ), and so that an extension of the lifting cylinder(50) causes the harrow plate(20) to move down toward the ground surface, under the state of the harrow plate(20) lifted to an elevated position, and a further extension of lifting cylinder(50) even after the contact of the harrow plate(20) with the ground surface causes the increase of the inclination angle of the blade part of the harrow plate(20) relative to the ground surface.

2. A harrow for tractor of claim 1, wherein the harrow plate(20) is provided with an upper and lower stoppers(24,25) for limiting the swinging angle(20) of the harrow plate(20) by interfering with the connecting rods(lθ) as the harrow plate(20) swings.

3. A harrow for tractor of claim 1, wherein the harrow plate(20) consists of a central harrow plate(21) connected to a lifting cylinder(50) and a pair of outer harrow plates(27) arranged at opposite ends of the central harrow plate(21), the central harrow plate(21), the outer harrow plates(27) are connected in swingable manner by connector rods(70) respectively provided with hinged axises(71a, 77a) at the opposite ends, the hinged axises(71a,77a) being oriented to be offset to each other, and safety elements (73) are provided at the connection areas of the central and outer harrow plates(21, 27) so that the elements may be broken when the outer harrow plates(27) are overloaded.

4. A harrow for tractor of claim 3, wherein the safety elements(73) consist of a pair of safety plates(74) mounted by safety bolts(75) each passing through an outer harrow plate(27) and connector rod(70), one side of the safety plates(74) being contacted with the front surface of central harrow plate(21), the safety plates(74) or the safety bolts(75) being formed weaker compared to the harrow plate(20).

5. A harrow for tractor of claim 1, wherein the harrow plates (20, 30) consist of a main harrow plate(20) hinge-connected to connecting rods(lθ) and a pair of auxiliary harrow plates(30) hinge-connected to opposite ends of the main harrow plates (20), and the auxiliary harrow plates (30) are connected to the main harrow plate (20) by means of springs(40) so as to outstretch or to be folded on the main harrow plate(20) at the opposite ends of the main harrow plate(20) under the elastic restoring force of the springs(40).