Patent Abstract:
A bar adjustment assembly for a chain saw includes a shaft, a bar adjustment pin that is journalled for rotation relative to the shaft, and a pin alignment spring that biases the pin toward an engagement position. A void is provided for accommodating the pin it is rotated away from the engagement position by a guide bar that has been incorrectly attached.

Full Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to guide bar adjustment mechanisms for chainsaws, and more particularly to an adjustment mechanism having a breakaway adjustment pin.  
         [0002]     Various mechanisms are known for adjusting the tension of the chain on a chainsaw. Nearly all of these mechanisms involve the movement of the guide bar upon which the chain rests. By lengthening the guide bar, the chain tension is increased. Conversely, shortening the guide bar decreases the chain tension.  
         [0003]     A commonly used mechanism for adjusting the position of the guide bar consists of an adjustment pin threaded onto an adjustment screw provided on the body of the chainsaw. The adjustment pin engages a hole in the guide bar. As the adjustment screw is rotated, the pin advances or retreats along the screw, moving the guide bar with it.  
         [0004]     One problem with these adjustment pin type chain tensioning mechanisms occurs during assembly of the guide bar onto the chainsaw body. The pin is not normally visible once the bar is placed onto the body. If the guide bar and the adjustment pin are not properly aligned during assembly, the adjustment pin and/or adjustment screw can be damaged.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     The present invention provides a bar adjustment assembly for a chainsaw including a self-aligning breakaway adjustment pin. The assembly comprises: a shaft, a bar adjustment pin journalled for rotation relative to the shaft, and a pin alignment spring biasing the rotation of the bar adjustment pin toward an engagement position.  
         [0006]     According to a further aspect of the present invention, the assembly further comprises a void for accommodating the bar adjustment pin when the bar adjustment pin is rotated away from the engagement position. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is an overall view of a chainsaw having a guide bar adjustment mechanism according to the present invention;  
         [0008]      FIG. 2  is a plan view of a portion of a chainsaw body that houses a guide bar adjustment mechanism according to the present invention;  
         [0009]      FIG. 3  is a plan view of the chainsaw body of  FIG. 2  having a misaligned guide bar attached thereto;  
         [0010]      FIG. 4  is a plan view of the chainsaw body of  FIG. 2  having a correctly aligned guide bar attached thereto;  
         [0011]      FIG. 5  is a cross section of the chainsaw body and guide bar adjustment mechanism taken along section line  5 - 5  of  FIG. 2  showing the assembly direction of the guide bar;  
         [0012]      FIG. 6  is a cross section of the chainsaw body and guide bar adjustment mechanism taken along section line  6 - 6  of  FIG. 3 ; and  
         [0013]      FIG. 7  is a cross section of the chainsaw body and guide bar adjustment mechanism taken along section line  7 - 7  of  FIG. 4 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     As shown in  FIGS. 1-7 , a guide bar adjustment mechanism  10  for a chainsaw includes a threaded shaft or adjustment screw  12 , an adjustment pin  14  and a pin alignment spring  16 . The adjustment screw  12  is mounted within a void or recess  18  provided in a portion of a chainsaw body  20 .  
         [0015]     The adjustment pin  14  is journalled on the adjustment screw  12 . Matching threads are provided on the adjustment pin  14  and the adjustment screw  12 . When the adjustment screw  12  is rotated by using an appropriate tool on the head  12 a of the adjustment screw  12 , the adjustment pin  14  moves back and forth along the length of the adjustment screw  12 . When the guide bar  24  is properly assembled onto the chainsaw body  20  ( FIGS. 4 and 7 ), the adjustment pin  14  normally engages an adjustment hole or slot  22  in a guide bar  24 . Due to the engagement of the adjustment hole  22  by the adjustment pin  14 , as the adjustment pin  14  moves along the length of the adjustment screw  12 , the guide bar  24  follows.  
         [0016]     During assembly of the chainsaw, as illustrated in  FIG. 4 , the guide bar  24  is placed onto the chainsaw body  20  so that the adjustment pin protrudes through the adjustment hole  22  of the guide bar  24 , as shown in  FIGS. 4 and 7 . Then, a cover portion  25  of the chainsaw is placed over the guide bar  24  and chainsaw body  20 . The cover portion  25  is secured in place by two bolts  26  or other fasteners (see  FIG. 1 ).  
         [0017]     As shown by the broken lines in  FIG. 5 , the recess  18  is shaped to allow the guide pin  14  to pivot fully into the recess  18 . Therefore, if the guide bar  24  is incorrectly positioned on the chainsaw body  20 , such that the adjustment hole  22  is not aligned with the adjustment pin  14 , the adjustment pin  14  can pivot out of the way, preventing damage to the adjustment mechanism  10  from occurring ( FIGS. 3 and 6 ).  
         [0018]     The pin alignment spring  16  is a helical torsion spring provided on the adjustment screw to keep the adjustment pin  14  in the correct orientation when the guide bar  24  is not attached. The pin alignment spring  16  is a helical torsion spring having first and second free ends ( 27 ,  28 ). The first end  27  of the pin alignment spring  16  engages the adjustment pin  14 , and the second end of the pin alignment spring  16  engages a surface of the chainsaw body  20  within the recess  18 . The pin alignment spring  16  is loosely wrapped on the adjustment screw  12 , such that it is journalled thereon, to allow the pin alignment spring  16  to travel along the adjustment screw  12  with the adjustment pin  14 . Alternatively, other types of springs, such as a helical tension spring, a helical compression spring, a spiral spring, a flat spring, etc., or other known types of biasing means such as elastic bands or straps, resilient foam or gel pads, etc., can be provided as the pin alignment means.  
         [0019]     The pin alignment spring  16  is positioned to bias the pivoting of the adjustment pin  14  out of the recess  18 . When the adjustment pin  14  is positioned fully upright in an engagement position, as shown in  FIGS. 2, 5  and  7 , a stop surface  30  meets a surface  31  of the chainsaw body  20  within the recess  18 . This meeting of the two surfaces  30 ,  31  prevents the adjustment pin  14  from pivoting any further. Alternatively, other stop mechanisms could be used.  
         [0020]     As described above, when the guide bar  24  is incorrectly positioned on the chainsaw body  20 , the adjustment pin  14  pivots into the recess  18 . When the guide bar  24  is subsequently removed from the chainsaw body  20 , the alignment spring  16  causes the adjustment pin  14  to pivot to its fully upright position so that the guide bar  24 , now being correctly aligned, can be reassembled without any manual repositioning of the adjustment pin  14 .  
         [0021]     It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.

Technology Classification (CPC): 8