Abstract:
Disclosed is an emergency shut-down device for a power tool. The shut-down device includes a push plate rotatably mounted to the power tool, a brake assembly for selectively stopping the operation of the power tool and coupled to the push plate so that rotation of the push plate in opposite directions causes actuation and de-actuation of the push plate, and a resistance generation device arranged between the push plate and the power tool. The resistance generation device is operatively coupled to the push plate to induce a predetermined resistance against the rotation of the push plate.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to an emergency shut-down device for power tools, especially power tools for gardening purposes, and in particular to a shut-down device that incorporates a mechanism to eliminate undesired shut-down operation by properly increasing mechanical resistance against manual operation of the shut-down device so as to ensure proper actuation of the shut-down device in emergency cases while eliminating inadvertent actuation/de-actuation of the shut-down device.  
         [0003]     2. The Prior Arts  
         [0004]     Gardening tools are usually powered by electrical motors for driving cutters to trim plants and mow lawns. To protect an operator of the power gardening tools, some power gardening tools are provided with emergency shut-down devices, which cut off power supply of the power tool and brake the rotating or moving cutter of the power tool.  
         [0005]     A conventional shut-down device for the power gardening tools comprises a push plate arranged at a position close to the hands of the tool operator. This allows the operator to immediately trigger the push plate for shutting down the power tool when an emergency happens. However, since the push plate is close to the operator&#39;s hand, it often happens that the push plate is inadvertently triggered by the operator&#39;s hand, who is not intending to stop the operation of the power tool. Although timely shutting down the power tool in an emergency is desired for protection of the operator or other persons from injury, undesired and inadvertent triggering the push plate may sometime cause accidents also.  
         [0006]     It is thus desired to provide an emergency shut-down device that can timely operate for safety protection, while eliminating inadvertent actuation/de-actuation of the shut-down device for further safety protection.  
       SUMMARY OF THE INVENTION  
       [0007]     An objective of the present invention is to provide an emergency shut-down device that allows for timely actuation to shut down a power tool while effectively preventing undesired and inadvertent actuation/de-actuation of the shut-down device.  
         [0008]     Another objective of the present invention is to provide an emergency shut-down device comprising a mechanism to induce a resistance against actuation/de-actuation of the shut-down device for eliminating inadvertent actuation/de-actuation, while allowing for timely actuation in emergency.  
         [0009]     In an aspect of the present invention, an emergency shut-down device for a power tool is provided, comprising a push plate rotatably mounted to the power tool, a brake assembly for selectively stopping the operation of the power tool and coupled to the push plate so that rotation of the push plate in opposite directions causes actuation and de-actuation of the push plate, and a resistance generation device arranged between the push plate and the power tool, wherein the resistance generation device is operatively coupled to the push plate to induce a predetermined resistance against the rotation of the push plate.  
         [0010]     In accordance with the present invention, the resistance generation device comprises first and second members physically engaging each other and movable with respect to each other. The second member is operatively coupled to the push plate by a link so that when the push plate is rotated, the second member is caused to move with respect to the first member. A biasing element applies a force to bias the first member against the second member for resisting the relative movement between the first and second members so as to induce the predetermined resistance against the rotation of the push plate.  
         [0011]     The present invention will become more obvious from the following description when taken in connection with the accompanying drawings, which show, for purposes of illustration only, preferred embodiments in accordance with the present invention. In the drawings: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is an exploded view of a power tool in which an emergency shut-down device constructed in accordance with the present invention is incorporated;  
         [0013]      FIG. 2  is a plan view, partially in section, of the power tool of  FIG. 1  in an assembled condition; and  
         [0014]      FIG. 3  is a plan view illustrating the condition where a resistance generation device of the emergency shut-down device of the present invention is actuated. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     With reference to the drawings, and in particular to  FIGS. 1 and 2 , a portion of a power tool, generally designated with reference numeral  1 , is shown. The other portion of the power tool is generally not involved in the present invention and is thus omitted in the drawings and will not be described herein. The power tool  1  comprises a power device  4  and a brake assembly  3  arranged inside a case (not labeled) and operatively coupled together. The brake assembly  3  constitutes an emergency shut-down device for the power tool  1 . The power device  4  comprises an electrical motor  41  having a spindle (not labeled) that supports and rotates a first gear  43 , which in turn drives a mated second gear  44 . An output shaft  42  is coupled to and driven by the second gear  44  to transmit torque to a cutter mounted or otherwise coupled to the output shaft  42 .  
         [0016]     A push plate  2 , serving as a manual trigger of the shut-down device, comprises a shaft  22  rotatably received in and supported by a bore  10  defined in the case of the power tool  1 . The push plate  2  forms an aperture  21  that fits over a projection  31  formed in the brake assembly  3 , which mechanically couples the brake assembly  3  to the push plate  2 . When the push plate  2  is rotated, for example in a forward, first direction, the brake assembly  3  is actuated by the engagement between the aperture  21  and the projection  31  to shut-down the operation of the power device  4 . When the push plate  2  is rotated in a reversed (backward), second direction, the brake assembly  3  is released, and the power tool  1  is allowed to resume operation.  
         [0017]     Still referring to  FIGS. 1 and 2 , the emergency shut-down device comprises a damper or a resistance generation device  5  that is mounted to the output shaft  42  and operatively coupled to the push plate  2  whereby when the push plate  2  is driven by an operator of the power tool  1 , the damper  5  induces a resistance against the manual operation of the push plate  2 . Thus, the operator must apply a great effort to operate the push plate  2  before he can actuate/de-actuate the brake assembly  3 . Thus, inadvertent actuation/de-actuation of the brake assembly  3  may be eliminated by the operator not applying the large effort to the push plate  2 .  
         [0018]     Also referring to  FIG. 3 , the damper  5  comprises first and second rings  52 ,  53  fit over the output shaft  42  and engaging each other in a relatively movable manner and a resilient element or biasing element  54  acting upon the first ring  52  to induce a resistance against the relative movement between the first and second rings  52 ,  53 . In the embodiment illustrated, the resilient element  54  comprises a helical spring fit over the output shaft  42 . A fastener  56 , such as a nut, fixes the first and second rings  52 ,  53  and the resilient element  54  on the output shaft  42  and also retains the helical spring  54  in position.  
         [0019]     The first ring  52  forms first teeth  521  and the second ring  53  forms second teeth  531 . The first and second teeth  521 ,  531  physically engage each other and form camming engagement therebetween. The helical spring  54  provides a biasing force acting upon the first ring  52 , which causes tight engagement between the teeth  521 ,  531 . The engagement between the teeth  521 ,  531  can be broken by applying a sufficient torque to the second ring  53  to such an extent causing the teeth  531  to slide over the teeth  521  with the aid of the camming engagement therebetween.  
         [0020]     A link  51  that is operatively coupled to the push plate  2  at a first end thereof has an opposite second end operatively coupled to the second ring  53 . When an external force is applied to the push plate  2 , the force is transmitted through the link  51  to the second ring  53 . Thus, the external force mush be large enough to overcome the resistance between the first and second rings  52 ,  53  induced by the biasing force of the resilient element  54  in order to break the engagement between the teeth  521 ,  531  of the rings  52 ,  53  before the push plate  2  can be further moved for actuation/de-actuation of the brake assembly  3 . This is applicable in both actuation and de-actuation of the brake assembly  3 .  
         [0021]     It is apparent to those having ordinary skills to replace the resilient element  54  with other resilient elements having different spring constants for different types of power tool.  
         [0022]     In accordance with an embodiment of the present invention, a washer  57  is fit over the output shaft  42  to support the rotation of the second ring  53  on the output shaft  42 .  
         [0023]     In accordance with a preferred embodiment of the present invention, a spring retainer  55  is provided between the fastener  56  and the resilient element  54 . The retainer  55  forms a cutoff  551  that receives and fixes an end of the helical spring  54 . An opposite end of the helical spring  54  is received and fixed in a hole (not shown) defined in the first ring  52  to retain the spring  54  between the retainer  55  (and thus the fastener  56 ) and the first ring  52 .  
         [0024]     An example of the retainer  55  is a washer-like member that fits over the output shaft  42  and defines the cutoff  551 . The fastener  56 , which is preferably a nut engaging a threaded section of the output shaft  42 , abuts against the washer  55  to fix the resilient element  54  and the rings  52 ,  53  in position.  
         [0025]     In another embodiment of the present invention, the link  51  forms a polygonal hole  511 , such as square hole, in the first end thereof. The shaft  22  of the push plate  2  forms, at an end thereof, a polygonal section  221  complementary in shape to and fit into the polygonal hole  511  of the link  51  to form operative coupling therebetween. The end of the shaft  22  defines an inner-threaded hole (not labeled) engaging with a bolt  59  that fixes the first end of the link  51  to the shaft  22  of the push plate  2 . Preferably, a washer  58  is arranged between the head of the bolt  59  and the link  51 .  
         [0026]     The second end of the link  51  forms a hook  521  fit into slot (not labeled) defined in the second ring  53  to form operative and driving coupling between the link  51  and the second ring  53 .  
         [0027]     Although the present invention has been described with reference to the preferred embodiments thereof, it is apparent to those skilled in the art that a variety of modifications and changes may be made without departing from the scope of the present invention which is intended to be defined by the appended claims.