Abstract:
A chain saw includes a housing, a sprocket wheel which is provided in the housing for driving a saw chain, and a chain guide for guiding the movement of the saw chain. The chain guide extends out longitudinally from the housing and is movable longitudinally relative to the housing. The chain saw includes a chain tensioning device. The chain tensioning device comprises a driven member arranged on the chain guide and a driving member which drives the driven member to move the chain guide longitudinally and a rotating device rotating the driving member. A stepless clutch device is arranged between the rotating device and the driving member. The chain tensioning device presses the chain guide while it tensioning the saw chain. Thus it prevents effectively the chain guide from loosing after the saw chain is tensioned. The operation of the chain tensioning device is labour-saving. The chain tensioning device has a stable structure and it is hard to be worn.

Description:
TECHNICAL FIELD 
       [0001]    The present invention generally relates to a chain saw, and more particularly to a chain tensioning device for a chain saw. 
       BACKGROUND OF THE INVENTION 
       [0002]    A PCT patent application WO9833631, which is published on Aug. 6, 1998 discloses a chain saw with a chain tensioning device, in which, the chain guide of the chain saw is fixed between two clamping plates and then fixed on the housing with them, and it can move longitudinally corresponding to the housing. Through a hole of the eccentric wheel and a rectangular hole on the housing, a bolt is tightened on the clamping plates which are guided and can move longitudinally on the housing, and its bolt head fixes the eccentric wheel on the housing. In order to tension the chain, first loosen the bolt and rotate the eccentric wheel, so that the bolt and clamping plates will move toward the direction of the chain guide top; when the chain has reached the expected tension, stop rotating the eccentric wheel, and then tighten the bolt again, so that the eccentric wheel and the clamping plates are fixed on the set position on the housing by force. This chain tensioning process is relatively complicated, and the fixation of the chain guide tends to loosen, which may cause the saw chain to be loose and increases the danger of the chain to jump from the chain guide. Therefore, this problem can only be solved by frequently conducting follow-up tensioning to the saw chain. 
         [0003]    In order to solve the chain saw tensioning problem mentioned above, a Chinese patent CN1174841C, which is published on Nov. 10, 2004 publishes a chain saw, which has a chain tensioning device. This chain saw has a housing as well as a sprocket wheel installed on the housing and a chain guide aiming at this sprocket wheel, this chain guide extends from the housing and guides the saw chain engaged with the sprocket wheel in a circular manner, this chain guide can be installed in a manner that it can be movably adjusted longitudinally against the housing and sprocket wheel by a tensioning device, and the tensioning device has a stop fixed on the chain guide and an eccentric wheel which can be driven by the rotation of a rotating element. This rotating element is composed of two housing-like internal and external hand wheels, the internal and external hand wheels are inserted to each other and rotate against each other around a common axis, during which, the internal hand wheel move along the axis corresponding to the housing and it can be fixed on the chain guide, the external hand wheel and the eccentric wheel are connected to each other in a carry-rotation way, and the internal and external hand wheels, the external hand wheel and the housing are connected by an offside clipping connector, and when the saw chain has been tensioned, this offside clipping connector can ensure relative static connection between the internal and external hand wheels. Through the structure mentioned above, after its chain has been tensioned, the chain guide of the chain saw won&#39;t get loosed easily, which can guarantee effective tension of the saw chain. However, during the tensioning process, it requires separate operations to the internal and external hand wheels, which are complicated and the human-machine function is weak. 
         [0004]    A Chinese patent CN1138620C has disclosed a chain saw, the chain saw includes a sprocket wheel movably installed on the housing which is used in the saw chain guided on the chain guide, an eccentric wheel that makes the chain guided move longitudinally and a chain tensioning device with a rotating device that makes the eccentric wheel rotate. of which, the rotation and connection device between the rotating device and eccentric wheel is constituted by a detent overload clutch, corresponding to the required chain tension to adjust its transposition clamping force. During the tensioning process of the chain, this structure only has to operate on one rotating device, and the chain can be tensioned and the chain guide can be clamped at the same time. It has convenient operation and strong human-machine function, but the detent overload clutch tends to be worn and the structure is not stable enough. In addition, during backward rotation of the rotating device to exchange the chain, efforts need to be made to overcome the clamping resistance between the detent elements of the detent overload clutch, and the operation is not easy. 
       SUMMARY OF THE INVENTION 
       [0005]    Regarding the above weakness the present invention is to provide a chain saw, which includes a chain tensioning device which is stable, anti-abrasive and labour-saving. 
         [0006]    In order to solver the above problems, the chain saw according to the invention comprises a housing; a sprocket wheel provided in the housing for driving a saw chain; a chain guide for guiding the movement of the saw chain, the chain guide extending out longitudinally from the housing and being movable longitudinally relative to the housing; and a chain tensioning device; the chain tensioning device comprises a driven member arranged on the chain guide, a driving member which drives the driven member to move the chain guide longitudinally, and a rotating device configured to rotate the driving member, and a stepless clutch device is arranged between the rotating device and the driving member. 
         [0007]    According to a preferred embodiment of the invention, the stepless clutch device is embodied as a friction clutch, whose friction force is set in accordance with the required saw chain tensioning force of the chain saw. 
         [0008]    According to an advantageous embodiment of the invention, a one-way driver is disposed between the rotating device and the friction clutch. The one-way driver comprises a clip member arranged on the rotating device, and a gear wheel arranged on the friction clutch for engaging with the clip member, the rotating device being configured to drive the friction clutch to rotate in a one-way direction via the clip member. 
         [0009]    According to an advantageous embodiment of the invention, the chain saw comprises a shield, the shield comprises a shield opening with tooth-shaped end circumference face. The rotating device comprises a clamping sleeve, the hand wheel being stationary relative to the clamping sleeve. The friction clutch device includes an anti-looseness device made of elastic material, the anti-looseness device comprises an inclined plane which is alternatively be engagable with the slope on the hand wheel, and a tooth shaped piece which is alternatively engagable with the tooth shaped end circumference face. 
         [0010]    According to an advantageous embodiment of the invention, the friction clutch comprises an elastic component providing with friction pressure and a friction plate subassembly biased by the elastic component. The friction plate subassembly comprises at least one friction plate and a rotating sleeve, the rotating sleeve being sleeved on periphery of the friction plate. The friction plate subassembly further comprises a friction plate base disposed between the driving member and the friction plate for driving the friction plate to rotate. The friction plate base connects the driving member with the friction plate, and the elastic component is disposed between the friction plate base and the friction plate. At least one clip member is arranged between the rotating device and the rotating sleeve for engaging with the gear wheel disposed on the periphery of the rotating sleeve, thus the clip member drives the rotating sleeve to rotate synchronously with the rotating device during the saw chain tensioning process. 
         [0011]    According to an advantageous embodiment of the invention, the driving member is configured as an eccentric wheel, and the driven member is configured as a mounting stud fixed on the chain guide. 
         [0012]    The chain saw according to the invention comprises a housing; a sprocket wheel provided in the housing for driving a saw chain; a chain guide for guiding the movement of the saw chain, the chain guide extending out longitudinally from the housing and being movable longitudinally relative to the housing; and a chain tensioning device. The chain tensioning device comprises a driven member arranged on the chain guide, a driving member which drives the driven member to move the chain guide longitudinally, a rotating device configured to rotate the driving member, a clutch device arranged between the rotating device and the driving member, and a one-way driver disposed between the rotating device and the clutch device. 
         [0013]    According to an advantageous embodiment of the invention, the one-way driver comprises a clip member arranged on the rotating device, and a teeth shaped member disposed on a gear wheel, the clip member engaging with the teeth shaped member in one-way direction. 
         [0014]    According to an advantageous embodiment of the invention, the clutch device is configured as a friction clutch, whose friction force is set in accordance with the required saw chain tensioning force of the chain saw. 
         [0015]    The clutch device can also be configured as a detent clutch, whose detent force is set in accordance with the required saw chain tensioning force of the chain saw. The driving member is configured as a gear wheel, and the driven member is configured as a rack engaged with the gear wheel. The detent clutch comprises a first clip member, a second clip member, and a spring piece which can generate acting force between the first clip member and the second clip member for supplying clip and clutch force. The rack component further comprises at least one anti-looseness tooth. Moreover, the tensioning device comprises an anti-looseness device disposed between the detent clutch and the chain guide, the anti-looseness device engages with the anti-looseness tooth for limiting the position the chain guide during the tensioning of the saw chain. 
         [0016]    The chain saw of the present invention includes a tensioning device, which can tension the saw chain and simultaneously tightly press against the chain guide, as well as preventing the saw chain from loosing and jumping off. The tensioning device is also stable, anti-abrasive and labour-saving. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The present invention will be illustrated further with reference to the accompanying drafting and the detail description. 
           [0018]      FIG. 1  is a partially principle view on a chain saw with the shield removed in the first embodiment. 
           [0019]      FIG. 2  is a sectional view of the tensioning device of the chain saw in the first embodiment. 
           [0020]      FIG. 3  is a explosive perspective view of the tensioning device of the chain saw in the first embodiment. 
           [0021]      FIG. 4  is a top view of the friction clutch in the first embodiment. 
           [0022]      FIG. 5  is a schematic perspective view of the hand wheel engaging with the rotating sleeve of the chain saw in the first embodiment. 
           [0023]      FIG. 6  is a sectional view of the tensioning device of the chain saw in the second embodiment. 
           [0024]      FIG. 7  is a explosive perspective view of the tensioning device of the chain saw in the second embodiment. 
           [0025]      FIG. 8  shows a partial enlarged view of the tensioning device of the chain saw in  FIG. 7 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0026]    As shown in  FIG. 1 , in one embodiment of the present invention, it involves a chain saw  5 , which comprises a housing  6 ; a sprocket wheel  7  provided in the housing for driving a saw chain  11 ; of which, this sprocket wheel  7  is driven by a power unit through a driver (not shown in the  FIG. 1 ), and this power unit can be a motor or an engine; this chain saw  5  also includes a chain guide  9 , one end of which is in the housing  6 , and the other end of which extends out longitudinally from the housing  6 ; the saw chain  11  is cased around the sprocket wheel  7  and chain guide  9 , and it can be guided along the edge of the chain guide  9  in cycles. In order to adjust the tension of the saw chain  11 , this chain guide  9  can move corresponding to the housing  6  along the longitudinal extension direction of this chain guide  9 , so that the distance between the chain guide  9  and the sprocket wheel  7  can be adjusted, and the saw chain  11  can be tensioned or loosed. 
         [0027]    As shown in  FIG. 2  and  FIG. 3 , in order to tension the saw chain  11  and fix the chain guide  9  on an appropriate position on the housing  6  in accordance with the adjusted tension of the saw chain  11 , this chain saw  5  is installed with a chain tensioning device, which is installed in a shield  13  used to enclose the housing  6 . Of which, this tensioning device includes a movable eccentric wheel  33  close to the side of the chain guide  9 , a rotating device  15  which controls the rotation of the eccentric wheel  33  and a friction clutch  10 ; an outer arc  33   a  of the eccentric wheel  33  leans against a mounting stud  91  of the chain guide  9 , and when the rotating device  15  operates the eccentric wheel  33  to rotate, this eccentric wheel  33  leans against the mounting stud  91  and propels this mounting stud  91  to move toward the housing  6  longitudinally along the longitudinal extension direction of the chain guide  9 , then the mounting stud  91  drives the chain guide  9  to move away from the housing  6  longitudinally along the longitudinal extension direction of the chain guide  9 , so the chain guide  9  is away from the sprocket wheel  7  and the saw chain  11  is tensioned; of which, the eccentric wheel  33  can also be structured as a worm gear or wheel gear, and the mounting stud  91  can be structured as a worm or a rack to match the the worm gear or wheel gear, as long as it can make the rotating device  15  to drive the chain guide  9  move against the housing  6  along the longitudinal extension direction of the chain guide  9  through transmission. 
         [0028]    The friction clutch  10  is a rotation and connection device installed between the rotating device  15  and the eccentric wheel  33 , which is used to connect the rotating device  15  and the eccentric wheel  33 . This friction clutch  10  generates friction force through its friction device, and the friction force refers to the limit value which the friction force on the friction device can reach when the friction clutch  10  is engaging and disengaging. The friction force corresponds to the ideal tension of the chain saw required by the chain saw  11  which has been adjusted in advance. During the tensioning process, when the saw chain  11  has completely fallen into a guide groove on the longitudinal edge of the chain guide  9 , the movement resistance for the chain guide  9  increases gradually, the rotating device  15  keeps moving until the constraint force of the saw chain  11  on the moving chain guide  9  exceeds the friction force by the friction clutch  10 , the friction clutch  10  makes the rotating device  15  unable to drive the eccentric wheel  33  to move, and the rotating device  15  keeps pressing against the chain guide  9  while the eccentric wheel  33  keeps a static state at this moment, i.e., the saw chain  11  has maintained a certain tension. Therefore, the friction clutch  10  can ensure that after tensioning the saw chain, the saw chain  11  can maintain the same tension; this tension is decided by the friction force of the friction clutch  10  which is preset in the factory, and it is not related to the personal feeling of the user, which makes it convenient for the user to reach the pre-designed tension state of the saw chain to realize optimum working process. 
         [0029]    Among the most preferred embodiments of the present invention, the rotating device  15  includes a clamping sleeve  152  and a hand wheel  151  relatively static to the clamping sleeve  152 ; the clamping sleeve  152  has a central axis  153 , and the eccentric wheel  33  and the clamping sleeve  152  can be connected through the friction clutch  10 ; the central axis  153  of the clamping sleeve  152  is coaxial with the rotation axis centre line of the eccentric wheel  33 . Inside of the clamping sleeve  152 , there is an internal thread  152   a , and through this internal thread  152   a , the clamping sleeve  152  matches with an external thread  37   a  of a bolt  37  with its one end fixed in the housing  6  which extends vertically from this housing  6  and passes through a rectangular hole  93  longitudinally extending on the chain guide  9 , which makes the clamping sleeve  152  set vertical to the chain guide  9 . Of which, the end of the clamping sleeve  152  close to the chain guide  9  is equipped with a press block  35 , which is used to limit the eccentric wheel  33  between the friction clutch  10  and the chain guide  9  as well as to press the chain guide  9  after the chain saw  5  has been tensioned to maintain the tensioning degree of the chain saw  5 ; in this way, during the work process, the saw chain  11  won&#39;t be loosened due to vibration of the chain saw  5 , which might cause danger to the user. In the meantime, the end of the clamping sleeve  152  close to the chain guide  9  is also equipped with a check ring  36 , which is used to limit the press block  35  to prevent the press block  35  from coming off the clamping sleeve  152  and ensure stability of the structure. The other end of the clamping sleeve  152  is enclosed by the hand wheel  151  relatively static to the clamping sleeve  152  for the convenience of the user to operate the clamping sleeve  152 ; to this end, the clamping sleeve  152  and the hand wheel  151  match each other to ensure that the hand wheel  151  drives the clamping sleeve  152  to move together. In addition, the inside of the hand wheel  151  is set with an enclosing wall  155  along its periphery, and the tail end of the enclosing wall  155  forms a slope  155   a.    
         [0030]    The rotating device  15  passes the shield  13  and the circular shield opening  131  coaxial with the clamping sleeve  152 ; this shield opening  131  matches with the shape of the hand wheel  151 , which makes it possible for the hand wheel  151  to be inserted into the shield opening  131  and drive the clamping sleeve  152  move corresponding to the shield  13 , and it is convenient for the operation to tension the saw chain. In addition, the lower end of the shield opening  131  also has a tooth end face  132  along the periphery of the opening. 
         [0031]    As shown in  FIG. 2  and  FIG. 4 , the friction clutch  10  comprises a friction plate subassembly, a friction plate base  31  and a prestressed clutch spring  29 ; of which, the friction plate subassembly includes a first friction plate  19 , a second friction plate  21 , a third friction plate  23 , a fourth friction plate  25  and a rotating sleeve  27 ; the friction plate base  31  is equipped with an eccentric wheel connection device  313  on the side close to the eccentric wheel  33 , the eccentric wheel connection device  313  and the friction plate base  31  are integral, which cooperates with the hole connection part  33   b  on the eccentric wheel  33  to connect the eccentric wheel  33 ; in this way, when the eccentric wheel connection device  313  has been connected with the eccentric wheel  33 , the eccentric wheel  33  and the eccentric wheel connection device  313  can maintain static to each other. In addition, on the back side of the friction plate base  31  against the eccentric wheel  33 , there is at least one friction plate connection  311 , and in the best embodiment of the present invention, the friction plate base  31  is equipped with three friction plate connections  311 , each of which is structured as a hollow threaded cylinder. Through a friction screw  18 , the first friction plate  19 , third friction plate  23  and fourth friction plate  25  pass through the connection hole on it which matches the friction plate connection  311 , and connect the friction plate connection  311 . Of which, the fourth friction plate  25  is close to the friction plate base  31 , and the second friction plate  21  and third friction plate  23  are between it and the first friction plate  19 ; the second friction plate  21  is clamped between the first friction plate  19  and the third friction plate  23 , and it is movably installed on the friction plate connection  311 ; the rotating sleeve  27  is installed on the back side of the friction plate base  31  against the eccentric wheel  33 , which can contain the first friction plate to the fourth friction plate  25 , and the rotating sleeve  27  comprises a gear wheel  273  on its periphery, its inner circumferential surface is installed with a circular friction lining  271  and a raised line  272  above the circular friction lining  271 ; of which, the circular friction lining  271  is installed between the third friction plate  23  and fourth friction plate  25 , which separates the third friction plate  23  and fourth friction plate  25 ; the raised line  272  can be connected to the notch  211  on the edge of the second friction plate  21  to make the second friction plate  21  to statically connect the rotating sleeve  27 . One end of the prestressed clutch spring  29  biases the friction plate base  31 , and the other end biases the fourth friction plate  25  to provide the friction plate pressure and generate corresponding friction force this way. What&#39;s worth mentioning is that: the first friction plate  19 , the second friction plate  21 , the third friction plate  25 , the fourth friction plate  27 , and the circular friction lining  271  installed on the friction plate base  31  all have bigger friction coefficients, and the size of the friction force can be adjusted through the biased pressures the prestressed clutch spring  29  has imposed on them. In the meantime, the first friction plate  19 , second friction plate  21 , third friction plate  23 , fourth friction plate  25  and the friction plate base  31  all have holes coaxial with the central axis  153  of the clamping sleeve  152 , and the clamping sleeve  152  passes through the holes in them to connect the bolt  37 ; in addition, a limit pressure spring  17  is installed between the hand wheel  151  of the clamping sleeve  152  and the first friction plate  19 , which biases the hand wheel  151  and the first friction plate  19  and gives the friction clutch  10  a limit force to prevent the friction clutch  10  from moving along the central axis  153  of the clamping sleeve  152  and causing instability to the tensioning device. See  FIG. 3  and  FIG. 4 , the main part of the friction plate base  31  is evenly installed with multiple anti-looseness devices  312  along its periphery; there are three anti-looseness devices  312  in this embodiment, each with an inclined plane  312   a  extruding toward the direction of the hand wheel  151 , and these inclined planes  312   a  can be selectively engaged with the slope  155   a  along the inner side  155  of the hand wheel  151 ; in addition, on the same side of each inclined planes  312   a , a tooth piece  312   b  is installed, and this tooth piece  312   b  can be selectively engaged with the tooth end face  132  configured on the shield opening  131 ; there are two tooth pieces  312   b  in this embodiment, and both the tooth pieces  312   b  and inclined planes  312   a  of this friction plate base  31  are made of elastic materials 
         [0032]    For the convenience of the hand wheel  151  to drive the friction clutch  10  to tension the saw chain and to avoid overcoming a strong friction force when loosening the saw chain  11 , as shown in  FIG. 3 , the inner edge of the hand wheel  151  is at least movably installed with one clip member  16  through the screw  17 ; in this embodiment, it is installed with three clip members  16  (see  FIG. 5 ), and this clip member  16  includes a connection part connecting the hand wheel  151  and a clipping part  16   a  which matches the gear wheel  273  in shape; in order to cooperate with the connection part of the clip member  16 , the inner periphery of the hand wheel  151  is installed with corresponding connection part  162  cooperated with the screw  160 ; a torsion spring  161  is installed between the clip member  16  and the connection part  162  of the hand wheel  151 , and this torsion spring  161  can give the clipping part  16   a  of the clip member  16  an elastic force toward the center of the hand wheel  151 , so that when the hand wheel  151  is connected to the friction clutch  10 , the clipping part  16   a  of the clip member  16  can be one-way engaged with the gear wheel  273  on the outer periphery of the rotating sleeve  27 , so a one-way driver will be formed between the rotating device  15  and the friction clutch  10 , i.e., the clip member  16  can only one-way drive the gear wheel  273  to rotate, while the clip member  16  cannot drive the gear wheel  273  to rotate during backward rotation. When this one-way driver rotates the rotating device  15 , the hand wheel  151  drives the clip member  16  to rotate, so the rotating sleeve  27  will be driven to rotate; when changing the saw chain, the chain guide has to be dismantled first, as long as to rotate the rotating device  15  backward at this moment, the clipping part  16   a  of the clip member  16  can crawl on the gear wheel  273  to make the rotating sleeve  27  maintain static while the hand wheel  151  keep rotating, which can also drives the clamping sleeve  152  to come off the bolt  37  installed on the housing  6 , so that the shield  13  can drive the tensioning device to come off the chain saw  5 . In addition to the single gripper ratchet mechanism disclosed in this embodiment, technicians in this field can also choose other one-way drivers, such as the OWC (one-way clutch), one-way bearing and other end-tooth ratchet mechanisms, internally connected ratchet mechanisms or fan-shaped ratchet mechanisms to realize one-way driving between the rotating device  15  and the friction clutch  10 . Of course, hydraulic or pneumatic one-way valve can also realize one-way hydraulic or pneumatic transmission. 
         [0033]    Through the structure mentioned above, the saw chain  11  tensioning and the chain guide  9  pressing of the chain saw in the present invention is realized in this way: when the user gets the chain saw  5  and needs to tension the saw chain  11 , the shield  13  needs to be covered at corresponding location on the housing  6  to place the rotating device  15  of the tensioning device installed on the shield  13  to aim at and pass through the bolt  37  of the chain guide  9 ; secondly, rotate the hand wheel  151  to make the hand wheel  151  drive the clamping sleeve  152  to move longitudinally toward the chain guide  9  on the bolt  37  through screw thread fit; in the meantime, the hand wheel  151  cooperates with the gear wheel  273  on the outer periphery of the rotating sleeve  27  through the clipping part  16   a  of the clip member  16  to drive the rotating sleeve  27  rotate synchronously, which makes the rotating sleeve  27  to drive the second friction plate  21  engaged with raised line  272  on it to rotate synchronously, and under the friction force of the prestressed clutch spring  29 , it drives the first friction plate  19  and third friction plate  23  to rotate synchronously; in the meantime, the rotating sleeve  27  also drives the third friction plate  23  and fourth friction plate  25  to rotate synchronously through friction force; in addition, because the first, third and fourth friction plates are fixed on the friction plate base  31  by the friction plate screw  18 , they can drive the friction plate base  31  to rotate synchronously, which makes the friction plate base  31  be able to drive the eccentric wheel  33  connected with it to rotate synchronously and drive the longitudinal movement of the chain guide  9 , and the saw chain  11  begins to be tensioned; at this moment, the tension of the saw chain  11  is smaller than the preset friction force generated by the spring  29  biasing the friction plate in the friction clutch  10 . Keep rotating the hand wheel  151 , and when the saw chain  11  reaches the optimum tension preset in the factory, the chain tensioning process of the chain saw  5  is completed; at this moment, the tension of the saw chain  11  forms a constraint to the reacting force of the chain guide  9 , and this constraint is transmitted to the eccentric wheel  33  which matches the mounting stud  91  through the mounting stud  91  installed in the chain guide  9 , which is further transmitted to the friction clutch  10  installed between the eccentric wheel  33  and the rotating device  15 ; at this moment, after the saw chain has finished the tensioning process, this reacting force exceeds the friction force mentioned above. Keep rotating the hand wheel  151 , under the reacting force generated by the eccentric wheel  33 , the friction clutch conducts engaging and disengaging; in this embodiment, i.e., the first friction plate  19 , the third friction plate  23  and the second friction plate  21  overcome the friction force between them and come off; in the meantime, the third friction plate  23 , the fourth friction plate  25  and the rotating sleeve  27  overcome the friction force between them and come off; in this way, the friction plate base  31  stops rotating, the eccentric wheel  33  maintains static, and the saw chain  11  remains its tension unchanged. What&#39;s worth mentioning is that: it is the optimum embodiment of the present invention to set four friction plates and a rotating sleeve  27  in order to stabilize the friction clutch  10 ; however, in fact, it only requires two frictional pieces to realize the role of the friction clutch  10  in the present invention through the friction force between them. The hand wheel  151  keeps rotating, and drives the clamping sleeve  152  and the press block  35  connected with the clamping sleeve  152  to keep moving toward the chain guide  9  and press the chain guide  9 . At this moment, under the pressing force, the inclined plane  312   a  on the anti-looseness device  312  and the slope  155   a  along the inner side  155  of the hand wheel  151  are mutual engaging and pressing, which causes an elastic deformation extruding along the radial direction of the anti-looseness device  312 , and the tooth piece  312   b  is engaged with the tooth end face  132  configured on the shield opening  131  along the radial direction, i.e., when the hand wheel  151  rotates to a state in which the chain guide is tensioned and tightly pressed, the friction clutch  10  and the shield  13  are automatically locked up together, which will ensure the chain guide  9  won&#39;t come off during the operation. When the saw chain needs to be changed, rotate the rotating device  15  backwardly to drive the clamping sleeve  152  move toward the position in which it can generate a pressing force against the bolt  37 , so that the slope  155   a  along the inner side  155  of the hand wheel  151  is disengaged from the inclined plane  312   a  on the anti-looseness device  312 , the anti-looseness device  312  recovers its elasticity, and the tooth piece  312   b  on the anti-looseness device  312  automatically comes off from its engagement with the tooth end face  132  configured on the shield opening  131 , i.e., the anti-looseness mechanism automatically unlocks; at this moment, keep rotating the hand wheel  151  backwardly until the clamping sleeve  152  is disengaged from the bolt  37  stalled in the housing  6 , so that the shield  13  can drive the tensioning device to come off the chain saw  5 . 
         [0034]    The friction clutch  10  adopted by this embodiment is a stepless clutch device, i.e., during engaging and disengaging, there is no requirement of locations of various clutch components and there is no abrupt change between them, which is different from the location requirement between the clip members of the clip clutch disclosed in the prior art. Of course, technicians in this field can also set other types of clutch devices in accordance with this embodiment. 
         [0035]    As shown in  FIG. 6 ,  FIG. 7  and  FIG. 8 , the second embodiment of the present invention relates to a chain saw which is generally the same as that in the first embodiment, and the difference is in the use of a different chain tensioning structure. For the convenience of description, further elaboration will not be conducted after using the same signs to present the same components, and different components will be expressed by new signs. 
         [0036]    This tensioning device includes a movable gear wheel  225  close to the side of the chain guide  9 , a rack component  215  fixed on the chain guide  9  to engage with the gear wheel  225 , a rotating device  218  which controls the rotation of this gear wheel  225  and a detent overload clutch  400 . The gear wheel  225  cooperates with the rack  251  of the rack component  215 , and when the rotating device  218  drives the gear wheel  225  to rotate, the gear wheel  225  engages with the rack  251  to promote said rack  251  to move longitudinally outside of the housing; then the rack component  215  drives the chain guide  9  to move in the same direction outside of the housing away from the sprocket wheel  7  and to tension the saw chain (not shown in the FIG.). The detent overload clutch  400  is a rotation and connection device installed between the rotating device  218  and the gear wheel  225 , which is used to connect the rotating device  218  and the gear wheel  225 . This detent overload clutch  400  includes a first clutch  222 , a second clutch  224  and a spring  220 ; in the detent overload clutch  400 , the gear wheel  225  is installed on the side of the second clutch  224  that is close to the rack component  215  with no relative rotation, while the other side of the second clutch  224  constitutes the first clip member  221 ; the first clip member  221  of the first clutch  222  and the second clip member  241  of the second clutch  224  are facing the clip and clutch device, and the spring  220  is installed between the first clutch  222  and the hand wheel  281 , the two ends of which leans apart against the first clutch  222  and the hand wheel, supply the first clutch  222  with a force toward the second clutch  224 . Through the cooperation between the first clutch  222 , the second clutch  224  and the spring  220 , a clip and clutch force is generated, in which, the clip and clutch force refers to the force generated from the engagement of the first clip member  221  of the first clutch  222  with the second clip member  241  of the second clutch  224 . The spring  220  biases the first clutch  222  in the axial direction of the first clutch  222  and the second clutch  224 , and leans on the second clutch  224 , so there will be a certain pressure between the first clutch  222  and the second clutch  224 . When the detent overload clutch  400  conduct engaging and disengaging, the pressure of the first clutch  222  imposed on the second clutch  224  will reach its limit value. The clip and clutch force is preset in accordance with the optimum tension of the saw chain, i.e., the spring force of the detent overload clutch  400  is preset in accordance with the tension required by the saw chain. This tension is determined by the clip and clutch force preset by the clip and clutch device  400  in the factory, and it is not related to the personal feeling of the user, which makes it convenient for the user to reach the pre-designed tension state of the saw chain to realize optimum working process. During the tensioning process, when the saw chain has completely fallen into the guide groove on the longitudinal edge of the chain guide  9 , the movement resistance for the chain guide  9  increases gradually, the rotating device  218  keeps moving until the constraint force of the saw chain on the moving chain guide  9  exceeds the clip and clutch force of the detent overload clutch  400 , the detent overload clutch  400  makes the rotating device  218  unable to drive the gear wheel  225  to move, and the rotating device  218  keeps pressing against the chain guide  9  while the gear wheel  225  keeps a static state at this moment, i.e., the saw chain  2  has maintained a certain tension. Therefore, the detent overload clutch  400  can ensure that after being tensioned, the saw chain can maintain the same tension, in which, the periphery outer wall of the first clutch  222  is installed with the teeth  274 . 
         [0037]    In this embodiment, the rotating device  218  includes a clamping sleeve  282  and a hand wheel  281  relatively static to the clamping sleeve  282 , and the clamping sleeve  282  and the hand wheel  281  are engaged in a way for the convenience of the operation and control of the operator; the clamping sleeve  282  has a central axis  283 , and along the central axis  283 , it passed through the spring  220  of the detent overload clutch  400 , the first clutch  222 , the second clutch  224  and the gear wheel  225  which is fixed on the second clutch  224 , and the central axis  283  of the clamping sleeve  282  is coaxial with the rotation axis centre line of said spring  220 , the first clutch  222 , the second clutch  224  and the gear wheel  225 . Of which, the inner edge of the hand wheel  281  is at least movably installed with one clip member  216  through the screw  217 , and this clip member  216  includes the clipping part  216   a  which matches the teeth  274  of the first clutch  222  in shape; in order to cooperate with the connection part of the clip member  216 , the hand wheel  281  is installed with corresponding connection part  262  to cooperate with the screw  217 ; a torsion spring  261  is installed between each clip member  216  and the hand wheel  281 , and this torsion spring  261  can supply the clipping part  216   a  of the clip member  216  with an elastic force toward the center of the hand wheel  281 , so that when the hand wheel  281  is connected to the detent overload clutch  400 , the clipping part  216   a  of the clip member  216  can be one-way engaged with the teeth  274  on the outer periphery of the first clutch  222 , so a one-way driver will be formed between the rotating device  218  and the detent overload clutch  400  This one-way driver makes the hand wheel  281  rotate and drive the clip member  216  to rotate when the rotating device  218  is rotating; while the clip member  216  can only one-way drive the teeth  274  to rotate, i.e., it drives the first clutch  222  to rotate, so clip connection between the first clutch  222  and the second clutch  224  will tension the saw chain  225 ; when changing the saw chain, the rotating device  281  has to be rotated backward, and the clipping part  216   a  of the clip member  216  only crawls on the teeth  274  while not driving the first clutch  222  to rotate; at this moment the first clutch  222  maintains static while the hand wheel  281  keeps rotating backwardly, which can also drives the clamping sleeve  252  to come off its engagement with the bolt  37  backwardly, so that the shield  13  can drive the tensioning device to come off the chain saw. Therefore, when changing the saw chain, just rotate the rotating device  218  in a reverse direction without the necessity of overcoming the clipping resistance between the first clutch  222  and the second clutch  224  of the detent overload clutch  400 . In addition to the single gripper ratchet mechanism disclosed in this embodiment, technicians in this field can also choose other one-way drivers, such as the OWC (one-way clutch), one-way bearing and other end-tooth ratchet mechanisms, internally connected ratchet mechanisms or fan-shaped ratchet mechanisms disclosed in the mechanic manual to realize one-way driving between the rotating device  218  and the detent overload clutch  400 . Of course, hydraulic or pneumatic one-way valve can also realize one-way hydraulic or pneumatic transmission. 
         [0038]    The tensioning device also includes an anti-looseness device  226  installed on the side of the clamping sleeve  282  that is close to the chain guide  2 , and this anti-looseness device  226  is installed between the detent overload clutch  400  and the chain guide  9 . Of which, the anti-looseness device  226  includes a hole  238  coaxial with the center axis  283  of the clamping sleeve  282 , and the clamping sleeve  282  passes through its opening  238  and is connected with the bolt  37 ; on the side of the anti-looseness device  226  that is close to the chain guide  9 , the periphery is at least installed with a second anti-looseness tooth  266 , the second anti-looseness tooth  266  can engage with the first anti-looseness tooth  252  which is disposed on the rack component  215 . The tensioning device also includes a check ring  228  installed on the end of the clamping sleeve  282  close to the chain guide  9 , which is used to limit the anti-looseness device  226  and the detent overload clutch  400  to prevent the anti-looseness device  226  and the detent overload clutch  400  from coming off the clamping sleeve  282  and ensure stability of the tensioning device. After the chain saw has been tensioned, during further rotation of the rotating device  218 , the clamping sleeve  282  and the anti-looseness device  226  connected to the clamping sleeve  282  keep moving toward the direction of the chain guide  9 , so as to press tightly against the chain guide  9 . At this moment, under the pressing force, the anti-looseness tooth  266  of the anti-looseness device  226  and the anti-looseness tooth  252  of the rack component  215  engage with each other and form a ratchet gear to keep a certain tension of the chain saw and a certain pressure against the chain guide  9 , as well as to effectively prevent the saw chain from loosening due to vibration during the operation of the chain saw  5 ; in the meantime, it can also prevent chain jamming due to loosening of the chain saw and ensure the safety of the user. 
         [0039]    The clamping sleeve  282  passes through a circular shield opening  239  of the shield coaxial with the clamping sleeve  282 ; this shield opening  239  matches with the shape of the hand wheel  281 , which makes it possible for the hand wheel  281  to be inserted into the shield opening  239  and drive the clamping sleeve  282  to move corresponding to the shield  13 . The check ring  228  is installed in the back side of the shield  13  of the hand wheel  281 , it limits the tensioning device to make this tensioning device installed in this shield  13 , and it can be removed from the housing  6  together with the shield  13 . 
         [0040]    Through the structure mentioned above, the saw chain tensioning and the chain guide pressing of the chain saw in this embodiment is realized this way: when the user gets the chain saw  5  and needs to tension the saw chain, firstly, the shield  13  and the tensioning device need to be covered at the corresponding location on the housing  6 , i.e., the shield  13  totally covers the sprocket wheel  7  on the housing  6  and the chain guide  9  partially contained in the housing  6 , and in the meantime, the clamping sleeve  282  aims at the bolt  37  which vertically extends from the housing  6 . Then, rotate the hand wheel  281 , and the hand wheel  281  drives the clamping sleeve  282  to move longitudinally toward the chain guide  9  on the bolt  37  through screw thread fit; in the meantime, the first clutch  222  relatively static to the clamping sleeve  282  drives the second clutch  224  engaged with it to rotate, which then drives the gear wheel  225  relatively static to the second clutch  224  to rotate; the gear wheel  225  drives the rack  251  of the rack component  215  fixed on the chain guide  9  to move longitudinally toward the direction away from the housing  6 , which makes the chain guide  9  to move correspondingly toward the direction away from the housing  6 , and the saw chain begins to be tensioned. During the tensioning process of the saw chain, the tension of the saw chain forms a constraint to the reacting force of the chain guide  9 , this constraint is transmitted to the gear wheel  225  which matches the rack  251  of the rack component  215  through the rack component  215  set on the chain guide, and it is further transmitted to the detent overload clutch  400  installed between the gear wheel  225  and the rotating device  218 . Rotate the hand wheel  281 , when the tension of the saw chain is smaller than the preset clip and clutch force of the detent overload clutch, the saw chain begins to be tensioned; when the saw chain reaches the optimum tension preset in the factory, keep rotating the hand wheel  281 , this reacting force becomes bigger than the clip and clutch force, the detent overload clutch is affected by this reacting force, and the first clip member  221  of the first clutch  222  and the second clip member  241  of the second clutch  224  in the detent overload clutch are disengaged; because the clip members in this embodiment are equipped with latch, so the first clip member  221  crawls on the teeth of the second clip member  241 , the first clutch  222  cannot drive the second clutch  224  to move, and there is disengagement in the detent overload clutch  400 . The gear wheel  225  keeps its current static state, and the saw chain keeps the tension degree unchanged. After the saw chain has been tensioned, keep rotating the hand wheel  281  to drive the clamping sleeve  282  and the anti-looseness device  226  connected to the clamping sleeve  282  to keep moving toward the direction of the chain guide  9  and press tightly against the chain guide  9 ; in the meantime, the second anti-looseness tooth  266  of the anti-looseness device  926  is engaged with the first anti-looseness tooth  252  of the rack component  215  to lock the position of the chain guide  9  and finish tensioning of the saw chain and locking of the chain guide  9 , and anti-looseness can be realized at the same time with convenient and simple operation.