Arrangements in barking machines

A barking machine having a hollow rotor (1) through which logs to be barked are fed is provided with arms (6) which are pivotally mounted on the rotor (1) and which carry barking tools (5). Spring means are used for bringing the barking tools into abutment with the log surfaces. According to the invention these spring means comprise hysteresis-free gas springs (16) arranged to work at a pressure above 25 bar.

The invention relates to an arrangement in a hollow-rotor type barker or 
rotation ring barker, in which logs to be barked are fed through the rotor 
or ring and the bark is removed by means of barking tools which are 
located around said logs and which are carried by pivotable arms which are 
acted upon by spring means arranged to forcefully urge the barking tools 
into abutment with the logs. 
One requirement of such spring means is that it shall have but very low 
inherent damping properties, i.e. it shall not exhibit any appreciable 
hysteresis. In addition, the spring means shall preferably have small 
dimensions and adjustable spring characteristics. The low inherent damping 
characteristics are necessary in order to enable the barking tools, which 
in hitherto known constructions are provided with a pointed tip which 
removes the bark, to follow the outer contours of a log, which contours 
are often highly irregular, without "jumping", and to enable the abutment 
pressure or tip 
pressure to be held substantially constant both when tensioning the spring 
means and when relaxing the same, at mutually similar diameters. The 
aforementioned adjustability of the spring characteristics is necessary in 
order to be able to urge the tip of a barking tool against the log surface 
at a given pressure throughout the whole range of log dimensions for which 
the barking machine is designed, and to enable adjustments to be made to 
suit differing types of bark. 
To these ends, a number of differing spring arrangements have been tested 
and brought into use. Known spring arrangements designed to exert pressure 
upon the aforesaid barking tools can be divided into the following groups: 
1. rubber belts or tubes; 
2. hydraulic piston-cylinder devices connected to liquid-gas accumulators; 
3. steel springs; and 
4. pneumatic piston-cylinder devices connected to pressurized-air sources 
located externally of the barking machine. 
Spring arrangements of the kind in which the spring element comprises 
rubber belts or the like have very high inherent damping, hysteresis, and 
with such arrangements it is possible, at normal temperatures, to lose 
from 30-50% of the tool tip-pressure, depending upon the quality of the 
rubber used, during transition of the tool from an outwardly directed 
movement to an inwardly directed movement. The tip pressure can even be 
lost completely, when the temperature is very low. This means that the 
tool tip may temporarily lose contact with the surface of the log, 
subsequent to passing a knot, twig or some other raised surface for 
example. This results in unbarked patches. When the pointed tool-tip is 
again brought into contact with the surface of the log, the force imparted 
to the tool is so great that the tip will often penetrate the bark layer 
and enter the wood therebeneath. Because of the angle of abutment of the 
tool with the log surface, there is produced upon movement of the log 
surface relative the tool an upwardly directed force which accelerates the 
tool outwards, thereby again to lift the tool from the log surface, 
leaving non-barked patches as a result thereof. This phenomenon is 
repeated continuously when the log surfaces are irregular. In addition to 
unsatisfactory barking results, log surfaces become torn and large 
quantities of wood are lost. Despite these drawbacks, however, such spring 
arrangements, for example of the kind described in U.S. Pat. No. 2,786,459 
have hitherto been used, because the other types of spring arrangements 
are encumbered with other and more difficult disadvantages. In order to 
reduce the consequences of hysteresis to the greatest extent possible, 
however, it is necessary to run the barking machine at low speeds, which 
means that a barking operation takes a long time to complete and therewith 
increases production costs. 
Although spring arrangements based on the use of steel springs have low 
inherent damping, hysteresis, and consequently are not encumbered with 
hysteresis related problems, one serious disadvantage with steel springs, 
prohibiting their practical use, is that in order to obtain the requisite 
spring force, the springs must be given large physical dimensions, which 
in turn means that the spring arrangements are greatly affected by the 
centrifugal forces created as the rotor rotates, so as to result in 
phenomena similar to those caused by hysteresis. 
Pneumatic piston-cylinder devices have very low inherent damping 
properties, and theoretically are well suited as spring elements in 
barking machines. The use of pneumatic piston-cylinder devices, however, 
requires the provision of a compressor or some other pressurized-air 
source. Because such pressurized-air sources are relatively bulky and have 
a relatively large mass, they cannot be mounted on the rotor, but must be 
placed externally of the barking machine. Consequently, the pressurized 
air must be supplied to the cylinders via slide couplings, which due to 
the particular construction of hollow-rotor type machines must be given 
extremely large diameters. It is totally impossible to render such slide 
couplings completely free of leakage and consequently it is necessary to 
work at relatively low pressures, by which is meant pressures of the order 
of six (6) bars or less. This necessity of working at low pressures, means 
the cylinders must consequently be given extremely large diameters and 
that the machines must be made unnecessarily large, resulting in more 
expensive machines. The use of separate compressor plants means higher 
investment costs and high energy costs. 
Consequently, a prime object of the invention is to provide a spring 
arrangement of the abovementioned kind which exhibits low inherent 
damping, which has a spring characteristic which can be readily adapted to 
prevailing power requirements, and which has small physical dimensions. 
This object is achieved by means of the present invention, which is mainly 
characterized in that each spring arrangement comprises a gas spring 
incorporating a piston-cylinder device which is pivotally mounted on the 
rotor and which is arranged to operate at gas pressures above 25 bars. 
Such a gas spring has practically no hysteresis and will consequently 
follow completely all irregularities of the surface of a log, and exert on 
a barking tool a constant or substantially constant tool pressure at a 
predetermined diameter, irrespective of the magnitude of said 
irregularities and irrespective of whether the tools move inwardly or 
outwardly with respect to the centre of the rotor. The essential factor is 
that it is possible to use extremely high gas pressures, preferably within 
the range of 60-150 bars, with the aid of small piston-cylinder devices. 
The gas spring, the spring characteristic of which is not affected by 
centrifugal forces, is a cylinder-type pressure spring which is filled 
with gas and with which the force on the piston rod is determined by the 
gas pressure, which in the present case is high, and the diameter of said 
rod. Since the gas pressure is one of the force determining parameters, 
the tool force can be adjusted as required, i.e. in accordance with the 
type of bark to be removed, by altering the gas pressure in respective 
cylinders. This change in gas pressure can be accomplished by connecting 
all cylinders temporarily to an external pressure source which is common 
to all cylinders. Thus, the gas springs can be charged with gas to a given 
pressure in a single operation, such that all barking tools will bear 
against logs of similar diameter at mutually identical tool pressures, as 
distinct from the case with known spring arrangements, which only exert 
precisely the same pressure on all said tools under exceptional 
circumstances. As will be understood, it is also possible to regulate the 
pressure in respective individual cylinders to precise levels. 
As beforementioned, a gas spring operating at pressures according to the 
above obtains small physical dimensions and consequently can be readily 
incorporated in the rotor part of the barking machine at reasonable costs. 
The absence of hysteresis, or in all events the presence of only 
negligible hysteresis, means that the tools will steadily follow the 
surface of a log and effectively strip the bark therefrom.

In FIGS. 1 and 2 the reference 1 identifies an annular rotor which is 
journalled in a bearing 3 for rotation in a frame 2. In the illustrated 
embodiment, the rotor 1 is driven by a motor (not shown) via belts 4. Logs 
8 are fed sequentially in the direction of their long axes through the 
centre of the rotor 1, by means of conveyors or some other suitable 
transport means (not shown). Mounted on the rotor 1 are a number of 
barking tools 5 (three in the illustrated embodiment), which are carried 
by pivotable arms 6. In the illustrated embodiment, the tools 5 are 
assumed to be inserts detachably held by the arms 6. The tools 5 on the 
free ends of the arms 6 are urged continuously towards the centre of 
rotation 7 of the rotor 1. The barking tools 5 may be of any kind suitable 
for removing the bark from the logs 8 through a cutting action. The form 
of the pivotable arms 6 is such that when coming into contact with the end 
of an incoming log 8, they are automatically swung outwards, so as to 
glide over said end and up onto the surface of the bark. The arms 6 have 
shafts 9 which are journalled in bearings 10 arranged in the rotor 1. The 
shafts 9 are connected together by means of levers 11 having outwardly 
extending arms 12. Each such arm 12 is of bifurcate configuration and has 
a shaft 13, on which there is pivotally mounted an end piece 14 of the 
piston rod 15 of a gas spring 16. The gas spring 16 has an outer cylinder 
17, which is incorporated in a bearing house 28 mounted on a plate 29 
connected to the rotor, and housing a bearing 18. The bearing 18 has two 
mutually co-acting bearing surfaces 18a, 18b, which are arranged to permit 
the cylinder to swing within a cone-shaped area. In order to restrict the 
influence of centrifugal forces to the greatest possible extent, the 
angular radial bearing 18 is located as close as possible to the centre of 
gravity of the gas spring 16, with the aid of a ring nut 30 (FIG. 4). The 
gas spring 16, which is manufactured by Stromsholmens Mekaniska Verkstad 
AB, Tran.ang.s, Sweden, is illustrated in greatly simplified fashion in 
FIG. 3. As will be seen from this figure, the piston rod 15 is connected 
to a piston 19 slidably arranged in the gas-tight cylinder 17. As is 
known, the piston 19 is arranged to permit gas to flow between the two 
chambers 20, 21 defined by the upper and lower piston surface 
respectively, the chambers thereby being under the same prevailing 
pressure. Communication between the chambers 20 and 21 is effected through 
the passage 22 in piston 19. As mentioned in the introduction, the force 
is determined by the prevailing gas pressure in the cylinder and by the 
diameter of the piston rod. The force of the gas spring can be varied, by 
varying the pressure of the gas, which may be, for example, nitrogen qas 
or demoisturized air. The spring characteristic is determined by the 
volume of the chambers 20 and 21 and the diameter of the piston rod. 
As indicated in FIG. 3, each cylinder 17 is provided with a valve means 23, 
through which gas can be supplied from a gas source 26 and removed from 
the cylinder, thereby to change the load on the piston rod. 
FIG. 5 illustrates the respective hysteresis factor of a rubber spring and 
of a gas piston-cylinder device according to the invention. By hysteresis 
is meant here the force delay which occurs during the relaxation or 
de-tensioning of a spring arrangement and which is caused by the inherent 
damping properties of the spring material. This inherent damping is due to 
both the temperature of the spring element and to the quality of the 
rubber from which it is made. In FIG. 5, tensioning of the spring element 
is plotted along the X-axis, the origin O showing the smallest tension and 
the point M the maximum tension. Along the Y-axis there is plotted the 
static force developed by the spring arrangement, varying between a 
smallest force at origin O and a maximum force K. The shaded area A shows 
the temporary reduction in force of a rubber spring during a 
spring-relaxing sequence, and the hatched area B shows the possible 
variation which this reduction in force may exhibit as a result in 
variations in quality of the rubber material. It will be seen that 
hysteresis of a rubber spring can vary between 30 and 50 of the maximum 
force at normal temperatures. The area B also shows, together with the 
area C, the variation, 30-100%, in force reduction caused by a force delay 
at temperatures, which are lower, or considerably lower, than normal 
temperatures. In practice, the force delay at low temperatures results in 
a pure force reduction. Hysteresis of a gas spring working with pressure 
in accordance with the invention can only be measured with difficulty, and 
is indicated in the diagram by means of the reference .DELTA.. 
FIG. 6 illustrates the problems mentioned in the introduction and caused by 
hysteresis of the spring material, which material may comprise rubber 
belts or like spring elements. The reference R identifies the direction of 
movement of the rotor 1, and thus the tools 5, in relation to the logs 8. 
When the tool 5 passes a raised surface 30 on the log 8, the hysteresis 
factor will cause the tip pressure of the tool 5 to be relatively low in 
comparison with the tip pressure on the raised surface 30, which means 
that the area 31 will not be completely barked. As soon as the "delay" in 
the rubber element has ceased, the force on the tool tip will increase, 
causing the tool tip to penetrate the wood when it contacts the surface 32 
thereof. As beforementioned, because the tool 5 lies against the log 
surface at an angle .alpha., the tool will be lifted upon relative 
movement between log and tool, resulting in a further non-barked area 33. 
The only possibility of preventing the occurrence of non-barked surfaces 
and damaged wood is to lower the rotor speed to a minimum at which 
hysteresis losses can be counteracted. This will result in an unreasonably 
low barking capacity, however. 
A spring arrangement according to the invention enables the tool to follow 
the contours of a log substantially precisely, and completely eliminates 
the risk of the tool "jumping", provided that each gas spring is given a 
pressure of the aforementioned magnitude. Practical tests have shown that 
the invention enables the rotor to be driven at high speeds, i.e. provides 
a high barking capacity, without risk of the tools being caused to 
oscillate or to jump in the manner illustrated in FIG. 6. In addition to 
impairing the wood, these oscillations occurring with the spring elements 
used hitherto, i.e. rubber springs, also results in rapid fatigue of the 
tool arms and their force transmission means and suspension means. These 
disadvantages can also be eliminated by means of the present invention.