Patent Publication Number: US-6338404-B1

Title: Locking device of power hand tool

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a rotary type power hand tool having a rotational output shaft, and in particular to a locking device for fixing the output shaft during a power failure so as to allow the hand tool to be driven manually. 
     BACKGROUND OF THE INVENTION 
     A conventional rotary type power hand tool is powered by an external power source, such as an electric main, or a built-in power source, such as a rechargeable battery set. A rotary type power hand tool comprises a rotational output shaft having an outer tip to which a chuck is mounted for gripping for example a drill bit or a screwdriver. In case of power failure for externally powered tools or insufficiency of power supply of an internally powered tool, the drill bit may get stuck in a work piece. To remove the drill bit from the work piece, a rotation of the drill bit in an opposite direction is usually performed manually. It is thus desired to drive the drill bit by manually rotating the power tool in case of power failure. 
     Conventionally, the output shaft is driven by a motor via a gear train. No means is provided for fixing the output shaft whereby when a user tries to drive the drill bit by manually rotating the hand tool, a relative rotation occurs between the output shaft and the hand tool, preventing the user to drive the drill bit by manually rotating the hand tool. 
     Thus, it is desired to provide a locking device for fixing the output shaft to overcome the above problem. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a locking device for a rotary type power hand tool which fixes an output shaft of the hand tool so as to allow a user to drive the output shaft by manually rotating the hand tool. 
     In accordance with the present invention, there is provided a locking device adapted to be arranged between a gear train and an output shaft of a power hand tool. The locking device comprises a fixed ring fixed in a housing of the hand tool. A disk drivingly engages the output shaft to be rotatable about a first axis. Separate locking pawls each having a second axis are concentrically arranged around the first axis and supported by a pair of retention rings to orbit about the first axis. Each pawl has a V-shaped projection received in a corresponding V-shaped notch defined in the disk whereby the pawl is capable of a very limited rotation about the second axis. The pawls are arranged in a central bore of the fixed ring with an outer face thereof opposing an inner circumference of the central bore with a tiny gap therebetween. A coupler drivingly engages the shaft and the retention rings whereby when the coupler is rotated by the gear train, the coupler drives the disk and the pawls simultaneously and thus allowing the shaft to be rotated without constraint. When a user manually rotates the output shaft, the disk is rotated while the pawls are prevented from rotation about the first axis which causes a rotation of each pawl about the second axis leading to an interference between the outer face thereof and the inner circumference of the fixed ring thereby preventing the shaft from being further rotated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be apparent to those skilled in the art by reading the following description of a preferred embodiment thereof, with reference to the accompanying drawings, in which: 
     FIG. 1 is an exploded view of a locking device constructed in accordance with the present invention; 
     FIG. 2 is an end view of the locking device of the present invention, showing the locking device in an unlocked condition; and a side elevational view of the locking device of the present invention; 
     FIG. 3 is a cross-sectional view of a power hand tool incorporating the locking device of the present invention; 
     FIG. 4 is similar to FIG. 2 but showing a locked condition of the locking device of the present invention; and 
     FIG. 5 is a plan view of a locking pawl of the locking device of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the drawings and in particular to FIG. 1, wherein a locking device constructed in accordance with the present invention is shown, the locking device is to be mounted in a rotary type power hand tool, such as a hand drill, between a torque source and an output shaft for locking the output shaft during a power failure. The locking device of the present invention comprises a fixed ring  1  adapted to be fixed inside a housing (not shown) of the hand tool. The fixed ring  1  defines a central bore  10  for accommodating a rotation prevention device. 
     The rotation prevention device comprises a central disk  6  defining a first axis and a plurality of locking pawls  3  arranged within the central bore  10  of the fixed ring  1 . Two retention rings  2  having an outside diameter greater than a diameter of the central bore  10  of the fixed ring  1  are respectively attached to opposite sides of the fixed ring  1  for retaining the central disk  6  and the locking pawls  3  in position. The central disk  6  defines an elongate central slot  62  into which a flattened inner end of an output shaft  5  is snugly and drivingly received. The flattened inner end of the output shaft  5  forms two opposite flat surfaces  51  for engaging opposite edges of the elongate slot  62 . An opposite outer end of the shaft  5  extends beyond the housing of the hand tool for engaging a working bit (not shown). 
     Also referring to FIGS. 2,  4  and  5 , a plurality of V-shaped notches  61  are defined in and substantially equally spaced along an outer circumference of the central disk  6  for receiving the locking pawls  3  therein. Each locking pawl  3  has a body (not labeled) forming an outer face  33  and a V-shaped projection  31  opposite to the outer face  33  for being received in the corresponding notch  61 . The V-shaped notches  61  of the central disk  6  and the V-shaped projections  31  of the locking pawls  3  are made so as to allow the pawls  3  to move in the notches  61 . 
     With the V-shaped projection  31  of each pawls  3  received in the corresponding notch  61  of the central disk  6 , the outer face  33  of the pawl  3  defines a radius R 3  which is very close to the inside diameter of the central bore  10  of the fixed ring  1  with a gap therebetween. 
     Each retention ring  2  defines a central bore (not labeled) and a plurality of retention holes  22  spaced along a circular path about a center of the retention ring  2 . A pair of triangular projections  21 , each forming two opposite inclining faces  211 , is formed on an inner circumference of the central bore of the retention ring  2  and diametrically opposite to each other. The retention holes  22  of the retention rings  2  are substantially aligned with each other for receiving pins  4  therein. Each pin  4  extends in a second axis through a hole  32  defined in the body of each pawl  3  for pivotally supporting the pawl  3  in the central bore  10  of the fixed ring  1  as shown in FIGS. 2 and 4 whereby when the retention rings  2  rotate, the pawls  3  orbit about the first axis. 
     Also referring to FIG. 3, a coupler  7  mechanically couples the locking device to the torque source of the power hand tool via a speed reduction gear train (not labeled). The coupler  7  comprises a cylindrical body forming an axial projection  73  through which a central bore (not labeled) is defined. A pair of triangular projections  71  having opposite inclining faces  72  is formed on an inner circumference of the central bore and diametrically opposite to each other. The flattened inner end of the shaft  5  is received between the triangular projections  71  with the surfaces  51  of the shaft  5  engaging the triangular projections  71  whereby when the coupler  7  is driven by the speed reduction gear train, one of the inclining faces  72  of each triangular projection  71  of the coupler  7  drivingly engages the corresponding surface  51  of the shaft  5  so as to transmit rotational motion to the output shaft  5 . In a normal operation, due to the engagement between the inner end of the shaft  5  and the disk  6 , the disk  6  is rotated in unison with the shaft  5 . 
     The axial projection  73  of the coupler  7  forms two parallel flat surfaces  731  on opposite sides thereof. The axial projection  73  is received in the central bore of one of the retention rings  2  whereby when the coupler  7  rotates, the flat surfaces  731  engage corresponding inclining faces  211  of the triangular projections  21  of the retention ring  2  for driving the retention rings  2  and the pawls  3  to rotate about the first axis with the coupler  7 . Thus, the shaft  5  is driven by the coupler  7  and the disk  6  is driven by the shaft  5  while the pawls  3  are driven by the retention rings  2  which are in turn driven by the coupler  7 . In other words, in a normal operation, both pawls  3  and disk  6  are rotated simultaneously thereby forming an unlocked condition as shown in FIG.  2  and causing no interference with the inner circumference of the central bore  10  of the fixed ring  1 . The shaft  5  is allowed to rotate without constraints. 
     In case that the coupler  7  is not rotated due to for example a power failure, when a user attempts to manually rotate the output shaft  5 , due to the engagement between the inner end of the shaft  5  and the disk  6 , the disk  6  is also rotated. The pawls  3 , however, are constrained by the retention rings  2  and are thus fixed, only being allowed to rotate about the pins  4  caused by the rotation of the disk  6 . This leads to an interference between the outer face  33  of each pawl  3  and the inner circumference of the central hole  10  of the fixed ring  1  forming a locked condition as shown in FIG. 4 thereby effectively preventing the shaft  5  from being further rotated. 
     Although the present invention has been described with respect to the preferred embodiment, it is contemplated that a variety of modifications, variations and substitutions may be done without departing from the scope of the present invention that is intended to be defined by the appended claims.