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

An electric power tool is provided including a main housing (2) assembled by fitting together right and left casing halves (3, 4), a drive unit (5), a spindle (14) coupled to the drive unit, and a change ring (20). The drive unit includes a DC motor (6), a gear assembly (7), and first and second gear cases (9, 15). The aluminum-alloy second gear case (15) is provided with a connecting portion (17) which has an outer shape that smoothly and continuously connects with the front end of the main housing and which is secured to the main housing by four screws (24) in the axial direction toward the rear portion of the tool. In addition, the second gear case includes a small-diameter journal bearing (16) which extends forwardly from the front end of the connecting portion (17) within the change ring so as to support the spindle of the tool.

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.

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 6a which protrudes into the first gear
 case 9. The rotation of the motor shaft 6a 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.