Wood pulp grinder
BACKGROUND OF THE INVENTION 
     The present invention relates to a wood pulp grinder designed for maintaining pressure upon the wood in the grinder during loading of additional wood for being ground, and particularly to an arrangement including a plurality of independently operable press rams in the grinder housing for accomplishing that purpose. 
     Such a wood pulp grinder is known from German Pat. No. 297,810. In that wood pulp grinder, there are two parallel press rams, each guided in a respective pressing chamber. Those chambers also serve as loading chambers for the wood bolts, in order to feed them to the common grinder magazine. These press rams are acted on alternately by pressure. As a result of the propagation of the pressure in the pile of wood which is present in the press magazine and forms a heap, the pressure is propagated from bolt to bolt of wood to the surface of the grindstone. 
     In this connection, the press rams are withdrawn into the upper pressing position for the loading of the wood. After lowering of the other press ram upon its work stroke, the work stroke of the press ram which is associated with the individual magazine which has just been loaded is introduced. It is advisable to charge each individual magazine of the individual press rams in the direction parallel to the axial direction of the grindstone. Otherwise, dependable loading of the individual magazines would not be possible. The spacing near the grindstone of the lower edge of the central wall of the press magazines which is arranged between those magazines, i.e. which lower edge is also the bottom end of the stroke of the press rams, is, of course, to be so selected here that, corresponding to the angle of slope for the course of the pressure, the grinding pressure, exerted by a press ram can also spread out over the entire width of the press magazine in the region of the surface of the grindstone. 
     This arrangement has the disadvantage that the two individual magazines associated with the two respective individual press rams must be charged individually with the wooden bolts so that a separate respective charging apparatus is necessary for both of them. Furthermore, in order to assure the full pressing pressure and its uniformity upon each work stroke, closure plates must be provided in the region close to the grindstone on the individual magazine associated with each individual press ram. These are not mentioned in the patent. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a wood pulp grinder requiring only a single charging device for the press magazine. 
     Another object of the invention is to maintain the pressure upon the wood in the grinder housing at at least a predetermined level while a main press chamber is being charged with additional wood to be ground and while the main press ram is actively pressing the wood toward the grindstone. 
     A further object of the invention is to accomplish the foregoing objects using a plurality of rams in the grinder housing which all cooperate for maintaining pressure. 
     According to the invention, the grinder housing has an open end at the peripheral surface of the rotating grindstone at which the wood is pressed against the grindstone. Spaced up away from the grindstone is a main press chamber which is charged periodically with wood to be ground. A press ram is movable in the main press chamber toward and away from the grindstone. A cover plate is movable across generally the bottom end of the main press chamber for preventing wood from returning into the main press chamber upon retraction of the press ram therethrough. 
     Also in the grinder housing and preferably adjacent to and arranged parallel to the main press ram is at least one circumferentially offset auxiliary press ram which moves toward and away from the grindstone independently of the main press ram. The auxiliary press ram is operated to maintain pressure upon the wood in the grinding chamber when the main press ram is retracted, as in preparation for loading additional wood into the main press chamber. The auxiliary press ram has an active lower surface which is located, at least at the beginning of its active pressing stroke toward the grinding stone, near the bottom end of the stroke of the main press ram and near the bottom of the wall of the main press chamber. The auxiliary press ram is preferably located adjacent the main press chamber. The auxiliary press ram has a length, measured in the direction of its path of travel toward and away from the grindstone, that is equal to at least one-half the length of the active pressing stroke of that press ram and preferably equal to at least eighty percent of that pressing stroke. Further, the active pressing stroke of the auxiliary ram or rams is preferably at least one-half as great as the length of the active pressing stroke of the main ram. 
     The active pressing surface of the auxiliary rams is preferably oblique to the direction of motion of the auxiliary rams and is preferably oriented in a direction that would be tangent to the surface of the grindstone on the region of the grindstone at which a projection of the auxiliary ram would fall, that is the surface that would be contacted by the auxiliary ram if it moved all the way to the grindstone. This produces the largest possible component of pressing force by the auxiliary ram toward the axis of the grindstone. 
     In an alternate arrangement, the obliquely inclined active pressing surface of the auxiliary ram, at least in its region close to the main pressing chamber, has a large radius curvature toward the main press chamber. This radius may amount to at least four-fifths of the width of the auxiliary rams, measured transversely to the axis of the grindstone. 
     There may be two auxiliary press rams, one on each circumferential side of the main press chamber. The two auxiliary press rams are advanced and retracted simultaneously, independently of the main press ram. 
     In an alternate design, each auxiliary press ram may be developed as a laterally open box construction, open on the lateral sides axially of the grindstone. 
     Other objects and features of the invention will be explained below with reference to an embodiment which is shown in the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational cross section through the wood pulp grinder. 
     FIG. 2a is a partial longitudinal section through the closure plate of the main press chamber. 
     FIG. 2b is a cross section through the closure plate of the main press chamber. 
     FIG. 3 is a side cross section of the grinder. 
     FIG. 4 is a cross section through another embodiment of the wood pulp grinder of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, the main press chamber 2 in which the main press ram 3 is arranged lies in the center of the magazine arrangement, circumferentially around the grind stone 1. The ram 3 is adapted to be displaced vertically, i.e. radially of the grindstone, by the push rod 8. The drives for the main and auxiliary press rams 3, 5, 5&#39; have been omitted in each case. They are generally known hydraulic servomotors. 
     Laterally alongside in circumferential directions from the main press chamber 2, there are auxiliary press chambers 4 and 4&#39; each having a respective auxiliary press ram 5 and 5&#39; associated individually with it. Rams 5, 5&#39; are driven vertically in similar manner by push rods 8&#39;. The inner chamber walls 11, 12 are both parts of the main press chamber 2 and at the same time also form the inner walls of the auxiliary press chambers 4 and 4&#39;. The length, measured in the axial direction, i.e. the direction of their strokes, of the auxiliary press rams 5 is preferably approximately equal to their effective pressing strokes and, in general, to their strokes as a whole. The length of the active strokes of the auxiliary press rams 5 is at least one-half the length of the active stroke of the main press ram. 
     The positions of the auxiliary press rams 5 and 5&#39; shown in FIG. 1 are their initial positions prior to their pressing strokes, i.e. the beginning of the pressing of the wood by these press rams. To the extent that the auxiliary press rams 5 and 5&#39; are moved downward, i.e. toward the grindstone, the main press ram 3 may be retracted without any change in the pressure acting in the heap of wood bolts. For this purpose, a closure plate 7 is preferably pushed below the main press ram 3 before that ram is retracted so that the wood bolts are prevented from moving upward. After the main press ram 3 has been completely retracted, the main press chamber 2 is loaded again with bolts of woods, the closure plate 7 is again pulled out of the press magazine and the work stroke of the main press ram 3 commences. The auxiliary press rams 5 and 5&#39; are again moved back simultaneously, also in a manner such that the pressure on the bolts of wood remains substantially unchanged. 
     The lower active surfaces 6 and 17, respectively, of the auxiliary press rams, are inclined. Each is substantially parallel to a tangent to the surface of the grindstone in the region where the projection along the direction of the push rods of the auxiliary press rams comes on to the surface of the grindstone. The inclinations of the lower active surfaces may be selected to be somewhat different from the tangent since the inclination is selected to produce the largest possible component of force in the direction toward the axis of the grindstone or the top most point of the surface of the grindstone. This assures that the pressure acts as uniformly as possible in the pile of wooden bolts over the entire grinding zone. 
     The pushing of the closure plate 7 into position is to take place, insofar as possible, under the operating pressure of the main press ram 3. The edge of the plate 7 which lies toward the front of the closing movement is therefore sharpened. As shown in FIGS. 2a and 2b, the front edge of plate 7 preferably has a long inclined surface which is provided with file-like or rasp-like teeth 15 to rasp off the surface of the bolts of wood over which the closure plate slides and thus to reduce the resistance of the pushing-in of the closure plate. The chips and the emerging moisture of the wood on the rasped surfaces leads here to reduced frictional resistance. Of course, the pressing pressure of the press rams could also be removed or reduced for a short time to ease insertion of the plate 7. 
     FIG. 3 is a side cross sectional view of the press grinder in the region of the main press ram 3. A wood-advance tray 21 is provided. It forms part of a wood-advance chamber which in this case is open on top. The bolts of wood are stored on tray 21 with their longitudinal direction parallel to the axis of the grindstone 1. They are introduced into the main press chamber 2 by press ram 22, actuated by piston rod 23. After this introduction, the main press chamber 2 is closed by the lateral, vertically shiftable slide 25, which can also be sealed in pressure-tight manner in the event that an excess pressure is to be maintained in the grinder housing. 
     FIG. 4 shows a variant in which only one auxiliary press chamber 4&#34; and one auxiliary press ram 5&#34; are provided. In this case, the bottom surface 17&#39; of the auxiliary press ram 5&#34; is rounded convexly on its side facing the main press chamber 2 with a large radus which is about 4/5 the width of the auxiliary press chamber 4&#34; or slightly larger, measured in the circumferential direction around the grindstone. Upon the raising of the auxiliary press ram 5&#34;, the wood which is then pushed forward by the main press ram 3 or 3&#39; can distribute itself better in the wide grinding housing or magazine and there is no sudden lateral movement of individual bolts of wood. Thus the bolts of wood slide gradually toward the side of the withdrawing auxiliary press ram. 
     It may be possible to avoid the use of the closure plate 7 if the pressure of the auxiliary press ram can spread less laterally outward because the wall 11&#39; between the main and auxiliary press chambers is extended further downward. This presupposes that the magazine having the full width is relatively long so that the pressure can also spread out over the entire grinding zone. In any event, the embodiment according to FIG. 4 appears to offer advantages in this connection over that of FIG. 1. Furthermore, in this case, the drive mechanism for the auxiliary press ram is also lighter, since only a single auxiliary press ram need be driven. FIG. 4 indicates that the auxiliary press ram can be developed as a laterally open hollow-box construction, i.e. it is open at its ends across the axis of the grindstone. It can be stiffened within the box by ribs 19 and 20. 
     In general, it may be said that the length of the auxiliary press rams 5, 5&#39;, 5&#34; should preferably be at least one-half their total strokes and, at least equal to one-half their active working stroke. The length of the auxiliary press rams preferably even amounts to at least 80% of the active work stroke thereof and may be even the full length of the work stroke, as stated above. In general, the length of the auxiliary press rams will also amount to at least 80% of the total stroke thereof in order to avoid excessive frequency of switching. In order to save material, the hollow construction of the auxiliary press rams of FIG. 1 or the box construction thereof of FIG. 4 is then preferably selected. 
     The dot-dash lines in FIG. 1 indicate still another embodiment of the wood chute in which the lateral walls are inclined toward each other at an angle of between 5° and 10°. The grindstone is in this version also shown in dot-dash lines below the elongate lateral walls of the chute. The constricting of the chute assures more uniform and stronger pressing of the logs of wood against the grindstone by the press rams. In this case, of course, the wood chute would have to be made relatively long since the lateral walls of the chute must be parallel in the stroke region of the auxiliary rams 5 and 5&#39;. Furthermore, one could then also make the wood chute relatively wide in its wide portion so as to obtain, in its narrow part adjacent the surface of the grindstone, the same length of grinding zone as would be present without the use of the inclined chute walls. 
     Although the present invention has been described in connection with a plurality of preferred embodiments thereof, many other variations and modifications will now become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.