Patent Publication Number: US-6213224-B1

Title: Electric power tool with enhanced strength to axially-applied external force

Description:
This application claims priority on Japanese Patent Application No. 10-170304, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electric power tools. More particularly, the present invention relates to an electric power tool which has a split two-part housing for encasing a drive unit that includes a motor and a reduction mechanism for transmitting the rotation of the motor to a spindle protruding toward the top end of the tool. 
     2. Description of the Related Art 
     Various power screwdrivers and screwdriver/drills have been known in the art, as disclosed in Japan Published Examined Patent Application Nos. S59-52298 and H4-59112, both of which include a description of a split two-part housing made of synthetic resin that encloses a drive unit. Furthermore, the drive unit includes a motor and a reduction mechanism which is coupled to the motor by means of screws and transmits the rotation of the motor to a spindle protruding toward the top end of the housing. Additionally, a torque setting adjuster and a chuck are mounted on the part of the spindle protruding from the front of the housing. 
     When this arrangement is applied to an electric power screwdriver as in the above examples, the tool is subjected to a large force acting in the axial direction of the spindle when the tool bit is pressed against a screw or other workpiece. The split two-part housing made of synthetic resin sometimes fails to withstand the force, with the result being that the spindle and the housing become distorted or warped. This may in turn adversely affect gear engagement in the reduction mechanism and thus normal rotation of the spindle. 
     SUMMARY OF THE INVENTION 
     In view of the above-identified problems, an important object of the present invention is to provide an electric power tool with sufficient rigidity such that the tool can effectively withstand a large axial force applied thereto. 
     Another object of the present invention is to provide an electric power tool that can maintain normal rotation of the spindle when subjected to a large axial force. 
     The above objects and other related objects are realized by the invention, which provides an electric power tool, comprising: a spindle having an axis; a drive unit including a motor and a reduction mechanism for transmitting the rotation of the motor to the spindle of the tool; and a housing having a front end for containing the drive unit. In this tool, a portion of the drive unit is made of metal, with the metal portion of the drive unit exposed to the outside of the tool forward of the housing and connected to the front end of the housing. This construction imparts to the tool enhanced rigidity, particularly strength to external force applied in the axial direction, thus preventing the housing from twisting or deforming during operation. In addition, proper engagement, in the reduction mechanism is ensured so as to maintain normal torque transmission. 
     According to one aspect of the present invention, the metal portion of the drive unit is screwed to the front end of the housing in the axial direction of the spindle. This construction renders the drive unit strong enough to withstand axial external force applied to the unit. 
     According to another aspect of the present invention, the metal portion of the drive unit is integrally provided with a bearing portion for supporting the spindle. The integral metal bearing portion rigidly supports the spindle such that the spindle does not, easily twist or warp. 
     According to still another aspect of the present invention, the reduction mechanism is an epicycle reduction gear mechanism, and the drive unit includes a clutch means for interrupting the transmission of the rotation of the motor from the epicycle reduction gear mechanism to the spindle responsive to a load applied to the spindle. 
     According to yet another aspect of the present invention, the clutch means includes an adjusting member for adjusting the torque at which the transmission of the motor rotation to the spindle is interrupted. Furthermore, the metal portion of the drive unit is disposed between the adjusting member and the housing. 
     In accordance with another aspect of the present invention, the metal portion of the drive unit has a substantially cylindrical shape, and the tool optionally includes an auxiliary handle with a circular mount that can be fitted around the metal portion of the drive unit. 
     In a preferred embodiment, the metal portion of the drive unit may have an outer surface that is smoothly and continuously connected with an outer surface of the front end of the housing. 
     In another preferred embodiment, the metal portion of the drive unit is made of aluminum alloy. 
     To carry out the invention in one preferred mode, the housing further includes two substantially symmetrical casing halves fitted together in a plane in which the axis of the spindle is located. 
     Other general and more specific objects of the invention will in part be obvious and will in part be evident from the drawings and descriptions which follow. 
    
    
     BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS 
     For a fuller understanding of the nature and objects of the present invention, reference should be made to the following detailed description and the accompanying drawings, in which: 
     FIG. 1 is a partially cross-sectional side view of an essential part of an electric power screwdriver/drill according to the present invention; 
     FIG. 2 is a cross-sectional view of the gear assembly of the electric power screwdriver/drill of FIG. 1; and 
     FIG. 3 is an enlarged view of a portion of the electric power screwdriver/drill of FIG. 1, showing the connection of the second gear case to the main housing. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment according to the present invention will be described hereinafter with reference to the attached drawings. 
     FIG. 1 is a partially cross-sectional side view of an essential part of an electric power screwdriver/drill  1  which embodies the present invention. The screwdriver/drill  1  includes two split casing halves  3  and  4  screwed together to constitute a main housing  2 . Enclosed within the main housing  2  is a drive unit  5  comprising a DC motor  6  and a gear assembly  7  disposed in front of (to the right in the drawing), and coupled to the DC motor  6 . As shown in FIG. 2, the drive unit  5  further includes a cylindrical first gear case  9  which is secured to the front of a motor bracket  8  of the gear assembly  7 . The motor bracket  8  is screwed to the DC motor  6 . In addition, the DC motor  6  has a motor shaft  6   a  which protrudes into the first gear case  9 . The rotation of the motor shaft  6   a  is transmitted to a spindle  14  of the tool  1  via a epicycle reduction gear mechanism  10  that includes three stages of an internal gear  11 , a planetary gear  12 , and a carrier  13 , to which the gear  12  is coupled. 
     The drive unit  5  further includes a second gear case  15  secured to the front of the first gear case  9 . The second gear case  15  is a molded aluminum alloy component, including a small-diameter journal bearing  16  supporting the spindle  14  and a large-diameter connecting portion  17  which extends rearward from the journal bearing  16  and covers the front end of the first gear case  9 . A change ring  20  is fitted over the journal bearing  16  for rotatably operating a spring holder  19  that retains the front, end of a spring  18  mounted around the journal bearing  16 . The rear end of the spring  18  presses against and secures the front internal gear  11  by means of a flat washer  21  and two tiers of balls  22 . When the change ring  20  is manually rotated so as to screw-feed the spring holder  19  in the axial direction of the journal bearing  16 , the force fixing the internal gear  11  can be adjusted so as to control the torque corresponding to the load applied to the spindle  14  at which the internal gear  11  starts to rotate idly. Reference numeral  23  indicates a drill chuck mounted at the front end of the spindle  14 . 
     The external shape of the connecting portion  17  of the second gear case  15  is formed such that the connecting portion  17  connects smoothly and continuously with the exterior of the front end of the main housing  2 . As shown in FIG. 3, the connecting portion  17  is secured to the main housing  2  by means of four screws  24  in the axial direction toward the rear portion of the tool during assembly. This also secures the drive unit  5  to the main housing  2 , exposing for manual access the connecting portion  17  as well as the change ring  20  to the outside of the electric power tool  1  in front of the main housing  2 . Referring again to FIG. 1, a circular mount  25  of an auxiliary handle can be fitted around the connecting portion  17 . When the auxiliary handle is attached, protrusions  26  formed on the outer surface of the connecting portion  17  engage detent protrusions (not shown) formed on the inner surface of the circular mount  25  so as to prevent the rotation of the circular mount  25 . Reference numeral  27  is an alignment piece fitted in the inner surface of the main housing  2  in order to position the second gear case  15  relative to the main housing  2  during assembly. 
     As described above, since the metal second gear case  15  is exposed and directly connected to the front end of the split main housing  2 , the electric power screwdriver/drill  1  is provided with increased rigidity, making it strong enough to withstand external force applied to the tool, especially in the axial direction. Accordingly, the main housing  2  does not twist or deform when the tool bit is pressed against a workpiece with a large force to tighten screws or to drill holes, thus maintaining precise gear engagement in the epicycle reduction gear mechanism  10 . This also ensures proper transmission of torque to the spindle  14 . In addition, the drive unit  5  remains properly secured to the main housing  2  via the second gear case  15 . 
     Furthermore, as the second gear case  15  is secured to the main housing  2  by the screws  24  in the axial direction of the spindle  14 , the second gear case  15  is connected to the main housing  2  with a sufficiently high rigidity to withstand axial external force. Neither does the spindle  14 , rigidly supported by the journal bearing  16  of the second gear case  15 , easily become deformed or twisted. Since an auxiliary handle can be attached to the exposed second gear case  15 , the operability and the work efficiency of the electric power screwdriver/drill  1  are not only greatly enhanced, but the auxiliary handle itself can also be more securely attached to the tool due to the strength of the metal gear case  15 . 
     In the above-described embodiment, the drive unit  5  integrates the DC motor  6  and the gear assembly  7 . However, the present invention is applicable to a tool in which a motor and a gear assembly may be separately assembled. 
     A person of ordinary skill in the art will also appreciate that the second gear case  15  need not be connected to the main housing  2  in the manner described above. Any change or alteration in the design can be made to suit any particular application. Depending on the design of the reduction mechanism  10 , the connecting portion  17  of the gear case  15 , for example, may have a larger diameter than that of the front end of the main housing  2  so that the connecting portion  17  partially covers the main housing  2 . Alternatively, the connecting portion  17  of the gear case  15  may have a smaller diameter than that of the front end of the main housing  2  so that the main housing  2  partially covers the connecting portion  17 . In the above embodiment, the second gear case  15  is connected to the main housing  2  by the axially extending screws  24 ; however, other means may be adopted and/or the direction of the screws can be changed to match the external force likely to be applied to the tool  1 . 
     Those skilled in the art will also appreciate that the present invention is applicable to tools, including but not limited to electric screwdrivers and drills, other than electric screwdriver/drills as in the foregoing embodiment, so long as they are of the type in which a reduction mechanism is held between and encased in right, and left casing halves. 
     It will thus be seen that the present invention efficiently attains the objects set forth above, among those made apparent from the preceding description. As other elements may be modified, altered, and changed without departing from the scope or spirit of the essential characteristics of the present invention, it is to be understood that the above embodiments are only an illustration and not restrictive in any sense. The scope or spirit of the present invention is limited only by the terms of the appended claims.