Abstract:
The invention concerns a method for clamping a knife of a disc chipper, wherein the knife is pressed from the knife disc via knife a clamp against a wear plate in a diagonal position (α) with respect to the plane of the knife disc with a compressive force (N) substantially parallel with the axis of the knife disc. According to the invention, the compensating force of the force directed to the knife clamp, caused by the diagonal position (α) of the knife and transversal to the compressive force (N) is exerted to the wear plate.

Description:
FIELD OF THE INVENTION 
   The present invention concerns a method for clamping a knife of a disc chipper against a wear plate by means of a knife press between the wear plate and the knife disc. The invention also concerns a knife clamp, that is pressed by a device supported from the knife disc and that is supported to the knife and the wear plate. 
   BACKGROUND OF THE INVENTION 
   Disc chippers are generally used in the wood processing industry for chipping wood prior to the further processing. The rotating disc of a chipper is equipped with knives attached evenly distributed thereto cutting chips from a log against a counter knife. The knives are generally attached to the disc through separate knife presses that are locked in position generally by means of a compressive force provided by bolts. The knife press by means of which the knife is pressed in its position is in this publication referred to as knife clamp. 
   When attaching a knife in its position in the knife disc according to a method of prior art, the knife clamp and the knife are pressed with bolts against the wear plate. The method is described e.g. in the publication U.S. Pat. No. 6,056,030. The bolts are parallel with the axis of the knife disc. The bolts have their support from the knife disc and move the knife clamp and the knife in the axial direction of the knife disc, away from the knife disc towards the inclined surface of the wear plate, against which the knife is positioned. This positioning surface of the knife in the wear plate deviates due to its inclination from the direction perpendicular to the pressing motion of the clamp, whereby the knife clamp and the knife are subject to forces from the wear plate, that tend to move the knife clamp aside from the direction of the desired pressing motion. Against this transversal motion the knife clamp is supported from the knife disc near the compressing bolt. A friction force is generated to the point of support resisting the movement of the knife clamp during the compressing process. 
   SUMMARY OF THE INVENTION 
   The method in accordance with the present invention is characterized in, that the point of support preventing the transversal movement of the knife clamp is positioned in the wear plate on the area of the plane between the knife and the knife clamp, said area facing the knife, whereby the transversal force required for attaching the knife increases the compressive force between the knife clamp and the knife. The knife clamp in accordance with the invention substantially takes the support of the knife clamp against the transversal movement from the wear plate by means of a bracket of the knife clamp. Due to the method, the compressive force of the knife clamp can be provided also with other methods than bolt attachment, because a supporting surface of the knife disc under the knife clamp is not essential. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention and its details will now be described in more detail with reference to the enclosed drawings, where 
       FIG. 1  shows a traditional knife assembly and knife clamp of a disc chipper, 
       FIG. 2  shows the forces directed to the knife clamp, 
       FIG. 3  shows a knife clamp according to the present invention, 
       FIG. 4  shows forces directed to the knife clamp according to the present invention, and 
       FIG. 5  shows a preferred construction according to the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a generally used knife assembly. Knife  1  is attached in its place by pressing the knife by means of a knife clamp  2  against a wear plate  3 . Bolts for attaching the wear plate to the knife disc  4  is not shown in the figure. The knife clamp  2  is pressed in the direction of arrow N against the knife  1  by means of a bolt  5 . Bolt  5  is supported from the knife disc  4  by means of a hardened threaded bushing  6 . The surface  7  of the knife clamp placed against the knife  1  is formed slightly concave, so that the knife clamp is supported against the knife at points  8  and  9 . Thus, the knife  1  has the best possible support from the knife clamp  2  despite of small inaccuracies in manufacturing. The most important thing is to the have the knife supported in the best possible way near the point of the knife by means of surface  8 . 
   It has been noticed in the practice that the point portion  11  of the wear plate  3  slightly yields due to the force N. Thus, also the knife  1  and the knife clamp  2  must turn from 0.2 to 0.3 mm at the point portion  12  thereof. Turning of the knife clamp and tensioning of the bolts  5  are contributed by the rounded ends  13  of the bolts. 
   Against the direction perpendicular to the pressing motion, the knife clamp is supported by means of the surface  14 . The surface  14  is supported against the counter surface  15  in the knife disc and a friction force caused by the supporting force is generated between these surfaces. 
   During the chipping process the point  11  of the wear plate is in a small continuous bending motion and the size of said friction force varies continuously. In practice this variation causes a pulsating effect to the compressive force of the point  8  of the knife clamp. Function of the knife clamp is to keep the knives firmly attached to the chipper and another main function is to maintain an adequate compressive force on the area of the point  8 . Decreasing of said compressive force is substantially influenced not only by the bending of the wear plate but also the force turning the knife clamp caused by the compressive force between the surfaces  14  and  15 , said force being dependent on the height h and the angle α of the clamp, as shown in FIG  1 . The height of the clamp is dependent on the adequate stiffness of the clamp and the distance between the screws  5 . Angle α of the knife is in general about 36-40 degrees and it is influenced by the sharpening angle of the point of the knife. 
   Referring to  FIG. 2 , the arrows describe forces exerted by the knife clamp  2  to the other parts of the knife disc. For simplicity of the force figure, the concavity of the surface  7  of the knife clamp against the knife is not taken into account. The compressive force caused by the bolt is P 1  (=N). The correspondingly big supporting force P 1 ′ exerted to the knife  1  is divided into component P 2  effecting against the knife and component P 3 +P μ  effecting in the direction of the knife. Force P 3  parallel with the surface of the knife, which is a residual force taking into account the friction force caused by P 2 , tends to move the knife clamp from its place. The size of the force is dependent on the friction between the knife and the knife clamp. The surface of the knife clamp facing against the knife of the knife clamp is effected by the force P 3  coming from the knife, and force P 3 ′ affecting the knife disc is formed to the supporting surface  14 . Forces P 3μ ′ and P +  parallel with the surface  14  are formed thereby. 
   Force P 2  effects in perpendicular direction to the surface of the knife and it is divided into counter forces P 4  and P 5 , which are located on their respective areas of influence  8  and  9 . Most important is that the force P 4  maintains adequate. Distance of the above mentioned pair of forces is c and the size of this distance varies to some extent according to inaccuracies of the surfaces of the knife  1  and the knife clamp  2 . Force P 3  moving aside the knife clamp from the direction of effect of force P 1  gives a decreasing effect (P 4 ) and an increasing effect (P 5 ) according to formula (b/c) P 3  to the pare of forces P 4 -P 5 . This results in additional need of force P 1 , when a certain level of force P 4  is needed. 
   A bigger height h ( FIG. 1 ) also increases the dimension b and requires increasing of the clamping force P 1 . Harmful effect of this problem increases during the operation of the chipper, whereby the vibration decreases the influence of the friction and the compressive force of the area  8  can decrease as much as 50%. A minor effect increasing the force P 1  comes from force P +  according to formula (a/c) P + . This increasing portion of the main force P 1  ranges from 3 to 10%. When the knife  1  gets narrower due wearing, the dimension a is 0 and there is no favourable effect P + . 
   A knife clamp  2 ′ according to the invention is shown in  FIG. 3 . The knife clamp  2 ′ is supported against the perpendicular direction parallel with the compressive motion N by means of the surface  17  of the bracket  16 . Otherwise the knife clamp  2 ′ corresponds to that shown in  FIG. 1 . Counter surface for the bracket  16  is the surface  18  of the groove  20  in the wear plate  3 . The system according to the present invention has that considerable benefit that no supporting surface  15  is needed and the knife clamp according to  FIG. 3  can also be used with other attaching methods than bolts. 
   In the knife clamping system of  FIG. 3  according to the present invention, the support of the transversal force effected by the clamping force N of the knife clamp has been moved over to the other side of the surface  30  between the knife  1  and the knife clamp  2 ′ compared with the system shown in  FIG. 1 . The force pattern of the construction of  FIG. 3  shown in  FIG. 4  shows that correspondingly force P 3  parallel with the surface of the knife as well as a corresponding supporting force P 3 ′ are generated in this construction to the surface  17 . The force increasing the force P 2  against the knife can be calculated from the  FIG. 4  by formula (d/e) P 3 ′. As it is evident from said formula, that a long surface  7 ′ against the knife is unfavourable with respect to the increasing of force P 2 . A wide knife  1  according to  FIG. 3 , however, requires a wide knife clamp  2 ′. 
   The advantages of the present invention become evident from the construction in accordance with  FIG. 5 , where the knife  24  is led in position through the groove  25  by means of a projection  22  of the knife clamp  23 . Turn knife  24  equipped with two points is much narrower than the knife of  FIGS. 1 and 3  and requires thereby a better support. Thereby the supporting of the knife  24  is a much more demanding task and the knife clamp according to the invention and  FIG. 5 , where the relation of dimensions f/g is bigger than the relation d/e according to  FIG. 3  is a suitable solution for this purpose. An adequate strength is also reached without any problems, because height h′ does not matter. 
   A wide knife as shown in  FIG. 3  requires two supporting regions  8 ′ and  9 ′ between the knife clamp and the knife. There the bracket  16  cannot be used for supporting the force P 2 , because a three-point or three-area supporting is not successful due to fabrication inaccuracies. The knife clamp according to  FIG. 5 , instead, forms only one supporting area  28  against the knife from the bottom of the groove  25 . Another balancing support is provided by the bottom surface  26  of the groove  20 ′, against which the corresponding surface  21  of the bracket  16 ′ is supported, whereby bracket  16 ′ receives the force P 3 ′ and a part of the force P 2 . Thereby an adequate clamping of the knife can be achieved with a reasonable force N. 
   The arrangement in accordance with  FIG. 5  has an additional advantage therein that the knife disc  4  has a surface  29  with no groove. This means considerable savings in costs of machining. 
   Oversized compressive forces require a large number of clamping screws and big forces result in durability problems of threads and bending of the wear plate  3  as well as in permanent deformation, which causes functional disturbances and need of changing components. By means of a method and a knife clamp in accordance with the present invention, the disadvantages mentioned above can be avoided.