From the viewpoint of the quality of the pulp it is important to keep the amount of bark passing to chipping with the logs as small as possible. The peeling of the bark from the surface of the logs usually takes place in a debarking drum in which the mutual movement and abrasion of the logs against one another bring about the debarking of the logs. The loose, finely crushed bark falls for the most part through the bark apertures in the drum to a bark conveyor under the drum.

With some wood species, the bark peels off as long strips or large slabs (acacia, eucalyptus), in which case the peeled bark tends to come out together with the logs at the discharge end of the debarking drum. The bark may also be very tightly attached to the log, which means that debarking in a conventional debarking drum is slow. Many deciduous trees, such as eucalyptus, acacia, hickory and birch are this type of trees that are difficult to debark.

The type of apparatus mentioned above is known, among others, from the publication WO 99/00231 (Sunds Defibrator Woodhandling Oy). The apparatus described in it is based on various mutual movements of blades and the logs being treated accomplished by means of angle lever mechanisms, due to which the apparatus is structurally relatively complex and thus expensive to manufacture.

The object of the invention is to provide a structurally simple and reliable apparatus for preparing logs prior to the actual debarking, which apparatus will completely replace debarking in a conventional debarking drum in connection with logs which are easier to debark, and in which apparatus the replacement of the blades must be possible in as simple and rapid manner as possible.

To achieve this object, the apparatus relating to the invention is characterised in that the apparatus comprises a drum rotated about its central axis, through which drum the logs to be treated are fed, and inside the surface limited by the cylindrical inner sleeve of which drum is arranged to project a number of blades, the blades being arranged to cut or tear the surface of the logs to be treated by the effect of the mutual movement of the drum and the logs, and that on the inner sleeve of the drum is provided a through hole for each blade, which means that the blades are arranged to be replaced on the outside of the drum.

The operation of the apparatus is based on the fact that a log being treated, and any other logs possibly on top of it, create such a force towards the inner sleeve of the drum that when the blade rotates together with the drum, the blade makes a transverse cut on the bark with respect to the longitudinal direction of the log, which cut facilitates the peeling and cutting of the bark.

The operation of the apparatus can be made even more efficient by arranging for the drum to be rotated at such speed of rotation that for each individual log in the batch of logs being treated in the drum is in turn achieved, by the effect of the rotary movement of the drum, such a path that, following an essentially free fall, the log will collide with the inner sleeve of the drum or a part projecting from it.

The speed of rotation of the drum is thus selected to be so high that the centrifugal force causes an ejection movement of the log, which, therefore, means that the earth gravity directed at the log exceeds the centrifugal force in the region of the highest point of the drum. Due to the collision taking place as a result of the free fall of the log, the forces directed at the bark increase manifoldly, whereby also the peeling of the bark in the drum is further intensified.

According to the invention, the blade is attached to the drum, for example, by means of screws which are arranged to be opened and fightened on the outside of the drum.

Further preferable developments of the invention are disclosed in the dependent claims.

The invention is described in greater detail in the following, with reference to the appended drawings, in which Figure 1 shows the apparatus relating to the invention as a perspective view.

Figure 2 shows the apparatus relating to the invention as a side view.

Figure 3 shows a section along line 111-111 in Figure 2.

Figure 4 shows a blade projecting from the inner sleeve of the drum.

Figure 5 shows a section along line V-V in Figure 4.

Figure 6 shows a blade projecting from the inner sleeve of the drum according to a second embodiment of the invention.

Figure 7 shows a section along line VII-VII in Figure 6.

The apparatus for debarking logs or preparing logs for separately performed debarking-especially logs that are difficult to debark-is shown as a diagrammatic perspective view in Figure 1. The apparatus is shown from the discharge end side. Figure 2 shows a side view of the apparatus, where the direction of feed of the logs is marked with arrow A. Figure 3 further shows a section of Figure 2 along line 111-111.

The apparatus comprises a drum 3 rotating about its central axis 2, through the space limited by the cylindrical inner sleeve 4 of which drum the logs 1 to be treated are fed, whereby they are subjected to abrasion and impact blows to treat the bark. The drum 3 is supported into an inclined position in such a way that its central axis 2 forms a sharp angle a with respect to the horizontal plane. The magnitude of the angle a is within the range from 2 to 12°. A typical value is approximately 5°.

In the example shown in the drawing, the drum 3 is supported from the outside on wheels 12 and provided with means designated generally by reference numeral 13 for rotating the drum 3.

For the sake of clarity, both the conveyor by means of which the logs 1 are fed to the drum 3, and the conveyor by means of which the logs 1 discharged from the discharge end of the drum 3 are conveyed to further treatment stages, have been omitted from the drawings.

A number of blades 11 for treating the logs 1 fed through the drum 3 are arranged to project from the surface of the cylindrical inner sleeve 4 of the drum 3. The blades 11 tear the bark of the logs 1 being treated or make transverse cuts in the bark at suitable intervals, which means that the bark will peel more easily and at the same time as pieces of suitable size during preparation or subsequently in the actual debarking drum.

In the example shown in Figure 1, on the surface of the inner sleeve 4 of the drum 3 are arranged one, or most preferably several, ridge-like protrusions 8 winding in a spiral manner with respect to the central axis of the drum. In the example shown in Figure 1, where the drum 3 is arranged to rotate in the direction of arrow B, the ridge-like protrusion 8 forms a left-hand wind. In this case, the rotary movement of the drum 3 brings about a force directed at the discharge end of the drum 3 on the logs 1 resting against the protrusions 8, that is, a force in the direction of arrow A shown in Figure 2.

In principle, the angle of inclination a of the drum 3 is sufficient to bring about a progressive movement of the logs 1 in the direction of arrow A, if the angle of inclination a is sufficiently large. However, the protrusions 8 enhance the progression of the logs in the drum 3 and reduce strains on the inner sleeve 4 of the drum 3.

When using ridge-like protrusions 8, the blades 11 are arranged to project further from the surface of the inner sleeve 4 of the drum 3 towards the central axis 2 of the drum 3 than the ridge-like protrusions 8, so that when the drum 3 rotates and the logs 1 rest on the ridge-like protrusions 8, the blades 11 will cut or tear the surface of the logs 1 and thus cut the bark.

The operation of the drum 3 can be based solely on the mutual movement of the logs 1 and the blades 11 effected by the rotary movement of the drum, whereby cuts are made on the logs 1 due to the effect of the blades 11. The efficiency of the apparatus, especially as regards the peeling of the bark, can, however, be increased in such a way that the drum 3 is arranged to be rotated at such speed of rotation that for each individual log 1 in the batch of logs being treated in the drum is in turn achieved, by the effect of the rotary movement of the drum, such a path F that, following an essentially free fall, the log will collide with the inner sleeve 4 of the drum 3 or a part projecting from it. The speed of rotation of the drum 3 is selected to be so high that the centrifugal force generated by the drum 3 on the log 1 will cause an ejection movement of each log 1 in turn, whereby the log will fly over the other logs being treated, and due to the collision taking place as a result of this essentially free fall of the log, the forces directed at the bark will increase manifoldly, whereby also the peeling of the bark in the drum 3 is further intensified.

Figures 4 and 5 show an example of the attachment of the blade 11 to the drum 3. On the inner sleeve 4 of the drum 3 is formed an aperture 20 for the blade 11.

Around the aperture 20, outside the drum 3, is welded a support plate 21 having an aperture of corresponding size and matching with aperture 20, and in addition threaded holes 22 for the fixing screws 23 of the blade 11.

In the embodiment shown in Figures 4 and 5, the blade 11 is welded to a protrusion piece 24, which protrusion piece is in turn joined with an attachment plate 25. The attachment plate is attached to the support plate 21 by means of screws 23 in such a way that the blade 11 extends through the aperture 20 to the inside of the inner sleeve 4 of the drum 3. In order to provide the blade 11 with the desired projection a from the surface of the inner sleeve 4, the required number of spacer plates 26 has been fitted between the support plate 21 and the attachment plate 25.

Due to this arrangement, the blade 11 can easily be replaced on the outside of the drum 3 simply by unscrewing the screws 23.

The curve-edged blade 11 shown in Figures 4 and 5 can be turned by 180° by unscrewing the screws 23, whereby a new sharp portion of the cutting edge is made available for use.

Instead of one blade 11, there may also be several parallel blades, which may all be fixed to a joint protrusion piece 24.

Figures 6 and 7 show an embodiment in which the blade 11 is comprised of a blade disc provided with teeth 30, the said blade disc being arranged to be rotated about its central axis 31. In this solution is used a base plate 32 which may be fixed to the drum 3 by means of the support plate 21 welded outside the drum 3 shown in Figure 4 and screws 23 (not shown in Figure 6).

To the base plate 32 is attached, in the direction of the central axis 31, a square bar 33 provided with a hole. To the base plate 32 is in addition attached an end support 34, perpendicularly to the central axis 31 and at a distance from the square bar 33. At the opposite end of the square bar 33 is removably attached an end plate 35.

The blade 11 is supported rotatably at the end of the shaft 36. In the operating position of the blade 11 (Figure 7), the shaft 36 is supported at one end in a hole

37 provided in the end support 34, and the other end extends through a hole 38 provided in the end plate 35.

On the shaft 36 is arranged, in a fixed manner and non-rotatably with respect to the hole of the square bar 33, a support 39, whereby a spring 40 provided between the support 39 and the end plate 35 is arranged to move the shaft 36 and at the same time to press the support 39 against the blade 11 and further to press the blade 11 against the end support 34.

On the blade 11 is provided a number of holes 41 arranged on the same circular arc with respect to the central axis 31, and correspondingly, on the support 39 is arranged one or more flexible plungers 42. The plunger 42 acts as a positioning means for the blade 11. When the plunger 42 and the hole 41 are matched, the ball belonging to the plunger 42 pushes partly into the hole 41.

The actual locking of the blade 11 into its operating position takes place by means of the compressive force generated by the spring 40, the said force pressing the blade 11 through the support 39 against the end support 34.

When the blade 11 is to be rotated about the axis 31 for the purpose of bringing a new tooth 30 or new teeth 30 into the operating position, the end of the shaft 36 is pressed in the direction of arrow A with a suitable tool (not shown), whereby the compressive force directed by the spring 40 at the blade 11 is reduced.

Correspondingly, if it is desirable to replace the blade 11 completely, the shaft 36 is pressed further in the direction of arrow A, whereby the distance S between end plate 35 and the flange 43 at the end of the shaft 36 increases at the same time. Once the shaft 36 has been moved enough to make possible the removal of the blade, between the end plate 35 and the flange 43 is fitted a locking piece (not shown) of suitable thickness, which will keep the shaft 36 in place, whereby the blade 11 can be replaced, after which the locking piece can be removed.