Abstract:
The invention concerns an arrangement to capture passively a broken saw chain of a chain saw with a guide bar that is arranged such that it can pivot, comprising a stationary screen that forms part of a sawing box and that covers a rear part of the saw. In order to achieve a cheap and efficient protection there is arranged between an end surface of the screen and the guide bar an energy-absorbing brake member that is sufficiently elastic or flexible to receive the kinetic energy from a broken saw chain, to direct kinetic energy away from the chain and to regain its normal form after it has suffered an impact from and been deformed by the chain.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Stage patent application of PCT/SE2013/051457, filed on Dec. 5, 2013, which claims priority to Swedish Patent Application No. 1251414-7, filed on Dec. 13, 2012, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention concerns an arrangement for the passive capture of a broken saw chain. The invention concerns also a tree harvester. 
       BACKGROUND OF THE INVENTION 
       [0003]    Saw chains of chain saws mounted at harvesters of the single-grip type for the felling and processing of tree trunks often run with large peripheral speeds around the guide bar. The saw arrangement that carries out the sawing is mounted in the protection of a shielded sawing box. A conventional saw arrangement comprises a frame, a saw motor, a driving wheel, a guide bar, a saw chain, a holder for the guide bar and an element to feed the guide bar out of and into the sawing box. The saw motor is arranged to drive the driving wheel and to rotate the saw chain around the guide bar that can pivot during the sawing operation. The guide bar is so located in the sawing box that its pathway of motion is not obstructed by the sawing box. The sawing box is provided with openings that allow sawdust to leave the box. 
         [0004]    A dangerous situation that can arise during sawing is breakage of the saw chain. Situations have arisen in which various parts have become loose from the broken chain and been thrown out at high speed as projectiles. A chain breakage in which parts are thrown out is generally known as a “chain shot”. The breakage of the chain is caused principally in that a part of the broken saw chain is thrown backwards and is cut off, cut or split due to the sudden whiplash effect that the said chain part exerts against some end surface in openings in the protective parts of the sawing box. The parts that are thrown out can include saw teeth, side links, driving links and other parts. Such parts that fly out into the surroundings can cause dangerous situations, in particular if people or other work machines are located in the vicinity of the chain shot. At a work location with several harvesters that work at the same time it is not always possible to ensure a minimum safety distance. 
         [0005]    In order to reduce the problem of chain shots, saw arrangements with various types of chain shot protection have been suggested. One known chain shot protection that is revealed by EP 1,528,852 C has a protective wall that carries out an active pivoting motion together with the guide bar at its rear part. A passive arrangement is known from SE 530,792 C2 that, instead of protecting against parts from a split chain, ensures as far as possible that a broken chain does not break into parts. In order to achieve this, the sawing box is equipped with a screen that has an end surface that has been given a convex curvature with a sufficiently large radius and such a hardness that a part of the broken saw chain is not cut off or split on contact with the end surface, but can slide around the end surface essentially unhindered. Due to the large radius, the arrangement demonstrates a lack of any sharp edges against which the chain can be split. Due to the hard surface, the links of the chain do not cut into the material and are not thus broken off. A timber working head for the cutting of tree trunks is known from SE 517,665. In order to prevent parts from a broken saw chain being thrown out, the sawing box is provided at its rear end edge with a shielding wall that carries out an active pivoting motion together with the guide bar. In one execution, the actively displaceable shielding wall may be manufactured from an elastic flexible material such as rubber. The shielding wall is pivoted by means of a turning motor that works synchronously with the guide bar during its pivoting motion during a cutting operation. The term “passive protection” is used to denote such protection as is mounted stationary in the sawing box. 
         [0006]    Each one of the arrangements described above has been shown to function well and has probably resulted in that it has been possible to avoid serious accidents. It should, however, be understood that tree harvesters are advanced units that are subject to heavy effects when working from impacts and collisions, and also from contaminants such as dirt, dust and sawdust that circulate in the air around the timber working head. Mechanisms that are components of the tree harvester can often be affected by disturbances, which lead to unnecessary interruptions in operation. It is, therefore, desirable to minimise as far as is possible the number of mechanisms, and in particular motor-driven mechanisms, that are components of the timber working head, which is in itself already very complex. In particular, with respect to the passive arrangement that is revealed by SE 530,792 C2, the extra saw protection mentioned in the introduction at the actively pivoting saw mechanism is unnecessary. 
         [0007]    There is, however, always an aspiration to improve and modify protective arrangements of this type with the aim of achieving both cheaper and more efficient protection. 
       SUMMARY OF THE INVENTION 
       [0008]    One purpose of the present invention is to achieve a protective arrangement of the passive type that is both cheaper and more efficient than currently known protective arrangements of corresponding type. A second purpose of the invention is to achieve a tree harvester that demonstrates an improved and more efficient protection against chain shot. 
         [0009]    These purposes of the invention are achieved through an energy-absorbing protective arrangement that demonstrates the distinctive features and characteristics that are explained below and a tree harvester of the type that is explained below. 
         [0010]    According to one examination of the invention, an energy-absorbing brake means that is mounted stationary in the rear part of the sawing box and that is sufficiently elastic or flexible to receive the kinetic energy from a part of a broken saw chain is used to direct away kinetic energy from the saw chain and to regain its normal form after having suffered an impact from and been deformed by a part of a broken saw chain. In one execution, a rear screen of the sawing box is provided with an indentation, in the opening of which is arranged an extended energy-absorbing element in the form of a flexible element such as a wire, chain, thread, rope, tape, or similar. In a second execution, the flexible element is constituted by an elastic element, an elastic line that is fixed at its ends and that cuts the opening in such a manner that it can capture and receive a broken saw chain in a flexible manner. When the elastic element suffers an impact from the saw chain it is bent backwards in a U-shaped curve. The elastic element is arranged to offer a braking force during the complete capture manoeuvre that can be progressive and that in this way undergoes a transition to become larger with increasing bending of the elastic element. Since the elastic element lacks mechanical moving parts and driving means, which means that it can be regarded as essentially passive, water, dirt and lubricants do not have any significant influence on the energy-absorbing properties of the elastic element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention will be described below in more detail with reference to embodiments that are shown in the attached drawings, of which: 
           [0012]      FIG. 1A  shows a view from the front of a tree harvester provided with a saw arrangement for the cutting of a tree trunk, which saw arrangement is equipped with a safety arrangement to capture a broken saw chain according to the present invention; 
           [0013]      FIG. 1B  shows schematically in an enlargement of detail a partly cut away rear part of a screen that is a component of the saw arrangement in which a safety arrangement according to the invention with the ability to receive the kinetic energy from a part of a broken saw chain is arranged, 
           [0014]      FIG. 2  shows a longitudinal section through a rear part of a saw arrangement viewed along the line II-II in  FIG. 3A , in a condition in which the kinetic energy from a broken chain has been captured by a safety arrangement according to the invention, 
           [0015]      FIG. 3A  shows a schematic perspective view of a partly cut away rear part of a saw arrangement with partly cut away parts in a condition in which the kinetic energy from a broken chain is captured by a safety arrangement according to the invention; 
           [0016]      FIG. 3B  shows schematically in an enlargement of detail a safety arrangement according to the invention illustrating a chain break of a saw chain and how it is captured by an elastic line. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    A tree harvester  1  of a common type is shown in  FIG. 1  provided with a safety arrangement according to the present invention. The tree harvester  1  has at what is its lower part in the drawing a chain saw  10  that is housed within a sawing box  11 . The chain saw  10  has a guide bar  12  around which runs a saw chain  14 . The saw chain  14  is driven around the guide bar  12  by a drive or a driving wheel  16  by a motor, not shown in the drawings, located in a rear part of the sawing box  11 . During a cutting operation, a pivot mechanism, not shown in the drawings, pivots the guide bar  12  out from the sawing box  11 , while the saw chain  14  that runs around the guide bar  12  at a high speed cuts off a tree trunk that is held fixed in the timber working head. The manner in which the guide bar  12  pivots into and out from the sawing box  11  during cutting is illustrated by the double arrow in  FIG. 3A . 
         [0018]    With reference also to  FIG. 3A , the guide bar  12  fixed into a holder  18  that can be pivoted around an axis of rotation A such that the guide bar  12  can pivot into and out from the sawing box  11  as has been described above, whereby the pivoting motion can be so great in certain cases that it reaches a value of 90°. The saw motor is of hydraulic type and has a shaft that drives, in turn, the driving wheel  16  of the saw chain  14 . The driving wheel  16 , which is located in a rear part of the sawing box  11 , transfers the power from the saw motor to the saw chain  14 , which runs forwards around the guide bar  12  to the vicinity of the driving wheel  16 . The driving wheel  16  is located at a rear end of the chain  14 . The saw chain  14  normally arrives in an essentially straight motion from the guide bar  12  to the vicinity of the driving wheel  16  (motion B) along a side of the guide bar that during a cutting operation interacts actively with the tree trunk, and rotates around the same. The saw chain  14  changes its direction of motion to one that is essentially the opposite, whereby the direction of motion is the opposite on the inactive side of the guide bar, which faces away from a tree trunk during a cutting operation (motion C). The driving wheel  16  is intended to move in a clockwise rotation. 
         [0019]    As is made most clear by  FIGS. 2 and 3A , it may occur that the saw chain  14  breaks during operation. A part of the chain, not shown in the drawings, can in this case be thrown out from the guide bar  12  when a chain part  14 A of the broken saw chain impacts an end edge  20  of a rear screen  22  of the sawing box  11  and is broken free through a whiplash effect that arises. The part that is freed from the chain part risks in this case being thrown out from the sawing box  11  at high speed as what is known as a “chain shot”. In order to avoid that a broken saw chain  14  is split and forms a chain shot, the present invention comprises a brake means, generally denoted by reference number  30 , that is located in so close proximity to the chain drive  16  in a rear part of the sawing box  11  that it can capture the said chain part  14 A of a broken saw chain  14 . The brake means  30  comprises a material that is sufficiently elastic to receive the kinetic energy from the broken chain part  14 A to direct away kinetic energy from the chain part and to regain its normal form after it has suffered an impact from the chain part and been deformed by the same. 
         [0020]    As is made most clear by the enlargement of detail in  FIG. 1B , the said screen  22  at the rear end of the sawing box  11 , in the close proximity of the driving wheel  16 , is provided with an indentation  31  whose width or whose gap is oriented in a plane that is perpendicular to the principal plane of the guide bar  12 . The “principal plane of the guide bar  12 ” is here used to denote the plane in which the guide bar  12  is pivoted into and out from the sawing box  11 . The indentation  31  is essentially U-shaped or V-shaped and it is limited by two side walls  31 A,  31 B that can be regarded as essentially opposite each other. The indentation  31  becomes narrower towards its bottom. In order to capture a saw chain  14  that breaks, an energy-absorbing brake means  30  is arranged in the indentation  31 . The brake means  30  is sufficiently elastic to receive the kinetic energy from the broken chain, to direct away kinetic energy from the chain and to regain its normal form after it has suffered an impact from the chain and been deformed by the same. 
         [0021]    In one execution, the brake means  30  comprises a line (or string) formed from an elastic material, in particular an elastic line  32  of the type that becomes progressively more rigid as the bending of it increases. The elastic line  32  may be arranged in any suitable manner in the sawing box in order to be able to capture a broken saw chain. The elastic line  32  is in the present embodiment located in the vicinity of the driving wheel  16 , preferably at a distance of 25-30 mm from the chain when the guide bar is positioned at its most outwardly pivoted position. The elastic line  32  extends perpendicular to the plane of motion of the guide bar  12  during cutting. The elastic line  32  can be located in a region in the said screen  20  of the sawing box  11  where the saw chain  14  approaches the driving wheel  16 . The indentation  31  demonstrates an opening that is located in the vicinity of the driving wheel  16  and that is turned towards the guide bar  12 . The indentation  31  forms an opening that is turned towards the part  14 A of the saw chain  14  that is returning to the drive. 
         [0022]    The elastic line described above that serves as brake means  30  is illustrated in greater detail in  FIG. 3B . In order to obtain the required properties, the elastic line comprises a core  33  that consists of an elastic material, a fibre reinforcement  34  that has been applied around this, and an outer covering layer  35  of elastic material, whereby the elastic material in the core and in the covering layer consists of synthetic rubber, and whereby the fibre reinforcement  34  consists of material that has poorer bending properties than the elastic material of the core. It is appropriate that the fibre reinforcement  34  be wound in a screw form around the core  33  at a specific angle of reinforcement to the longitudinal axis of the line and to the reinforcement projected perpendicular to it. It is preferable that the fibre reinforcement  34  consist of cords or tapes, preferably of polyester material. It is an advantage that the fibre reinforcement  34  include two or more layers. In this case, the layers can be wound in opposing directions. The elastic line  32  has a core of a suitable elastic material, for example EPDM rubber (ethylene propylene diene monomer, M-class, rubber). The elastic line can be provided with an outer shell of elastic material with particularly good resistance to weather, UV light and chemicals. A suitable elastic line can consist of a core of styrene butadiene rubber with a hardness of 75° IRH and it can demonstrate a diameter of 15 mm, comprising a screw-wound reinforcement consisting of threads of polyester cord with a suitable angle of reinforcement. The number of threads of polyester cord may be freely chosen, for example 24, and the threads are to be wound close to each other. Through selection of suitable curvatures for different angles of reinforcement, and curvatures that vary in accordance with this, the elastic line can acquire varying degrees of progression of the resistance of the line to bending, and the elastic line can in this way be adapted to capture a part  14 A of a broken saw chain  14  in as flexible as manner as is necessary to ensure that there is no risk that it splits and forms “chain shots”. 
         [0023]    As is shown in  FIGS. 2A and 2B , the elastic line  32  is fixed in a manner that is not shown in detail at its ends such that it extends across the indentation  31  in the rear screen  22  of the sawing box  11 . The locations of the indentation  31  and the elastic line  32  relative to the pivot mechanism of the guide bar have been selected such that the elastic line forms a fictive contact or an end section of the sawing box that can be said to limit the maximal pivoting motion of the guide bar  12  out from the sawing box, even if the said parts are not intended to come into contact with each other. 
         [0024]    In the event of a chain break, the brake means  30  will receive a broken part  14 A of a saw chain that may possibly also be falling somewhat downwards in a soft and flexible manner. The part  14 A of the saw chain that is under tension and that is subsequently broken continues its motion and impacts upon the brake means  30  and is collected against the same so softly that the chain part  14 A is not split. The elastic line  32  receives the kinetic energy from a broken chain  14 , directs the away kinetic energy from the chain and regain its normal form after it has suffered an impact from the chain and been deformed by the same. 
         [0025]    The present invention is not limited to that which has been described above and shown in the drawings: it can be changed and modified in several different ways within the scope of the innovative concept defined by the attached patent claims.