Patent Publication Number: US-6708778-B2

Title: Hydraulic unit with increased torque

Description:
RELATED APPLICATION 
     This application claims the benefit and priority of Japanese Patent Application No. 2001-005478, filed Jan. 12, 2001, and Japanese Patent Application No. 2001-111685, filed Apr. 10, 2001, the contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to hydraulic units for use in electric power tools such as torque wrenches for generating pulsating instantaneous torque by means of hydraulic pressure. 
     BACKGROUND OF THE INVENTION 
     FIG. 6 shows a conventional hydraulic unit  50 . The hydraulic unit includes a cylindrical case  51  which integrally accommodates a liner  52  coupled to the output shaft of a tool motor for receiving torque therefrom. The hydraulic unit  50  further includes front and rear caps (not shown) as closing elements that plug the axial front and rear ends of the case  51 , thus forming a fluid chamber  53  therein. The front and rear caps also rotatably support a spindle  54  within the fluid chamber  53 . Furthermore, inserted radially in the spindle  54  is a pair of blades  55  that are biased generally outwardly in mutually opposing directions by a coil spring  62  so that the blades can be retracted into the spindle when inward pressure exceeding the biasing force of the coil spring is applied to the top surfaces of the blades  55 . The spindle  54  additionally includes a pair of ribs  56  which protrudes therefrom at diametrically opposite positions and which are 90 degrees phase-shifted from the blades  55 . Formed at the axial front and rear ends of the liner  52  are two generally oblong guide holes  57  along which the top surfaces of the blades  55  slide. Two axially extending first sealing bodies  58  are disposed between the guide holes  57 , with each sealing body  58  provided with a first sealing surface  59  which is flush with and conforms to the interior surface of the guide hole  57 . Additionally, two axially extending second sealing bodies  60  are disposed between the guide holes  57 , with each sealing body  60  provided with a second sealing surface  61  which also conforms to the interior surface of the guide hole  57 . The first sealing bodies  58  are 90 degrees phase-shifted from the second sealing bodies  60 . As shown in FIG. 6A, in the operation of the electric power tool, as the liner  52  rotates in the direction indicated in the arrow, the blades  55  rotate relative to the case  51  along the interior surfaces of the guide holes  57 . When the blades  55  reach the first sealing surfaces  59  and the ribs  56  reach the second sealing surfaces  61 , the fluid chamber  53  are divided into four partitions, creating alternate high and low pressure chambers. This differential pressure in the fluid chamber causes generation of impact torque (generation of a hydraulic impulse) to the spindle  54 . One example of such an hydraulic unit is disclosed in Japanese Published Examined Utility Model Application No. 6-27341. 
     In the foregoing hydraulic unit  50 , upon generation of a hydraulic impulse, the liner  52  continues its rotation, thus removing the blades  55  and the ribs  56  from the first and second sealing surfaces  59  and  61 , respectively. As the seal within the fluid chamber  53  is opened at this moment, no hydraulic impulse is generated, such that the liner  52  alone rotates (FIG.  6 B). As the liner  52  continues its rotation, the blades  55  slide along the interior surfaces of the guide holes  57 , approaching the second sealing surfaces  61 . As this gradually pushes the blades  55  into the spindle  54 , the basing force of the coil spring  62  against the blades  55  increases (FIG. 6C) until it peaks when the blades reaches the second sealing surfaces  61  (FIG.  6 D). Accordingly, the blades&#39; pressure on the interior surfaces of the guide holes  57  acts as rotational resistance to the spindle  54 , thus impeding its rotation. In addition, as illustrated, the cross section of the guide holes  57  is a combination of three circles such that the guide holes  57  have low axial ridges on both sides of each second sealing surface  61 , where the intermediate circle intersects the two side circles. Thus, as shown in FIG. 6D, when the blades  55  ride over the intersection points P, additional resistance to rotation of the blades  55  is created. 
     FIG. 8 is a graph showing a pattern of torque production in the conventional hydraulic unit  50 . Peaks “a” indicate intended torque produced by hydraulic impulses, whereas lower torque peaks “b” are produced between these hydraulic impulses by the above-described rotational resistance. Such useless low torque disadvantageously decreases the intended torque produced by hydraulic impulses. 
     FIG. 7 shows another conventional hydraulic unit  50 ′ similar to the foregoing conventional hydraulic unit  30 . FIGS. 7A-L are similar to FIGS. 6A-D, but they show the movement of the blades  55 ′ with respect to the case  51 ′ in a more detailed sequence, with each figure depicting unit&#39;s parts or elements in the position 10 degrees further rotated from the position in the immediately preceding figure. Additionally, identical or similar reference numerals or characters denote identical or similar parts or elements of those in FIG.  6  throughout the several views. Therefore, description of such elements is omitted. 
     As shown in FIGS. 7A-C, when the blades  55 ′ and the ribs  56 ′ reach the first and second sealing bodies  58 ′ and  60 ′, respectively, with the counterclockwise rotation of the case  51 ′ and the liner  52 ′, the fluid chamber  53 ′ is divided into four partitions or sub-chambers, thus producing impact torque (hydraulic impulse), as in the foregoing unit  30 . Referring to FIGS. 7D-L, following the production of impact torque, as the liner  52 ′ continues to rotate, the blades  55 ′ are gradually retracted into the spindle  54 ′ against the biasing force of the coil spring and eventually slide across the second sealing bodies  60 ′ over the ridges on the inner surfaces of the guide holes  57 ′. Compared to FIG. 6, FIGS. 7D-L illustrate in greater detail the increased resistance to the rotation of the spindle  54 ′ due to the cross section of the guide holes  57 ′ being a combination of three circles. 
     Moreover, as the cross section of the guide holes has a complex shape due to the combination of three intersecting circles, the interior surfaces of the guide holes  57 ′ requires high-precision polishing, thus increasing the number of manufacturing steps and resulting in higher cost. 
     In the foregoing hydraulic unit  60 ′, the cross section of the guide holes  57 ′ of the liner  52 ′ is a combination of three circles, and the first and second sealing bodies  58 ′ are required, thus making the entire structure of the liner complex. 
     SUMMARY OF THE INVENTION 
     In view of the above-identified problems, the present invention provides a hydraulic unit wherein the rotational resistance to the spindle can be effectively reduced except upon generation of hydraulic impulses, thus augmenting the torque produced by such hydraulic impulses. 
     The present invention also provides a hydraulic unit which has a simplified construction and thus a greater cost advantage over conventional hydraulic units. 
     In accordance with one embodiment of the present invention a hydraulic unit is provided including a generally cylindrical case containing working fluid, with the case including an interior surface, front and rear closing elements at two axial ends thereof, and at least one first blade-sealing surface and at least one second rib-sealing surface. The hydraulic unit further includes a spindle which is inserted into the case and includes front and rear ends rotatably supported by the front and rear closing elements, respectively, with the spindle further including at regular intervals at least one blade and at least one rib for circumferentially partitioning an interior of the case into a plurality of smaller fluid chambers whereby relative rotation between the case and the spindle causes top surfaces of the at least one blade and the at least one rib to slide along the interior surface of the case so as to create differential pressure among the small fluid chambers when the top surfaces of the blade and the rib reach the first and second sealing surfaces, respectively, thus generating instantaneous torque to the spindle. Additionally included in the hydraulic unit are a pair of pins provided on axial front and rear ends of each blade and cam recesses provided in opposing inner surfaces of the closing elements of the case. In this hydraulic unit, during rotation of the case, the cam recesses guide the pins and prevent the top surfaces of the blades from sliding on the second rib-sealing surfaces. This arrangement completely eliminates the rotational resistance created by the top surfaces of the blades riding over the sealing surfaces associated with the ribs, thereby maximizing the torque resulting from intended hydraulic impulses. It should be noted that as used herein, the term “oblong” is intended to include “elliptical” as well as “elongated circle.” 
     In accordance with one aspect of the present invention, the spindle includes first and second blades, the case includes two second blade-sealing surface, the first blade is provided with two first pins, the second blade is provided with two second pins shorter than the first pins, and each closing element includes in its inner surface a first oblong cam recess for guiding one of the first pins and a second oblong cam recess shallower than the first cam recess for guiding one of the second pins. In this aspect, each first cam recess shares a common longitudinal end portion with the second cam recess and has a shorter longitudinal axis than the second cam recess such that the first blade is prevented from coming into slidable abutment with one of the second blade-sealing surfaces by the first recess guiding the first pins. This ensures generation of one hydraulic impulse per rotation of the case, which further augments the unit&#39;s output torque each time torque is generated. 
     In accordance with another aspect of the present invention, while the first recesses prevent the first blade from coming into abutment with one of the blade-sealing surfaces, the second recesses cooperate with the second pins to permit the second blade to protrude into abutment with the other blade-sealing surface. 
     In accordance with yet another aspect of the present invention, the first and second blade are located diametrically opposite about the axis of the spindle, two ribs are positioned diametrically opposite about the axis of the spindle and 90 degrees phase-shifted from the blades, two rib-sealing surfaces are positioned diametrically opposite about the center axis of the interior surface of the case, the longitudinal axes of the first and second cam recesses are oriented orthogonal to a diameter of the case passing through the rib-sealing surfaces, and the widthwise axes of the second cam recesses pass through the axis of the spindle and are oriented orthogonal to the longitudinal axes of the first and second cam recesses, and the center of the second cam recess is located at the axis of the spindle. In this arrangement, when the case is at a first rotational position, the rib-sealing surfaces oppose the ribs and each second pin is located on the longitudinal axis of the associated second cam recess in the longitudinal end portion of the second recess not shared with the first recess while each first pin is located on the longitudinal axis of the first and second recess in the longitudinal end portion shared by the first and second recesses so as to allow the blades to be biased into abutment with the interior surface, thus producing instantaneous torque, and at a second rotational position of the case, rotated a further 180 degrees from the first rotational position, each second pin is located on the common longitudinal axes of the first and second cam recesses in the longitudinal end portion shared by the recesses and each first pin is located on the longitudinal axes of the first cam recess in the first cam&#39;s longitudinal end portion not shared with the second cam recess, thus preventing the first blade from coming into abutment with the interior surface. 
     In accordance with still another aspect of the present invention, the widthwise axes of the first and second cam recesses are selected so as to have a common and sufficiently short length to cause the blades to be retracted into the spindle when the case is at a third rotational position, rotated a further 90 degrees from the first position, where the first and second pins are located approximately on the widthwise axes of the second cam recesses, with the blades passing by the rib-sealing surfaces. 
     According to one feature of the present invention, each cam recess includes a pair of opposing semicircular walls and a pair of parallel liner walls connecting the semicircular walls, thus forming a continuous loop surface extending parallel with the axis of the spindle, and additionally, each of the aforementioned longitudinal end portions shared by the first cam recess and the associated second cam recess includes one semicircular wall and at least part of each liner wall. 
     According to another feature of the present invention, following the retraction of the blades into the spindle, when the case is at the third rotational position, the case returns to the first rotational position upon rotating a further 270 degrees, such that instantaneous torque is produced to the spindle once for each complete rotation of the case. 
     According to still another feature of the present invention, the hydraulic unit further includes a pair of coil springs disposed between the blades within the spindle for biasing the blades in outwardly radial directions, and the first and second pins are inserted in the respective first and second recesses. Additionally, the length of each second pin in the recesses is shorter than the portion shared by the first and second recesses and the length of each first pin in the cam recesses is shorter than the depth of the first cam recess and greater than the depth of the portion shared by the first and the second cam recesses. 
     According to yet another feature of the present invention, the case further includes a liner which is integrally rotatable with the case and defines the interior surface of the case, a transversal cross section of the interior surface of the case has an approximately oblong shape of a combination of three circles whose centers are located on a common straight line such that two pairs of axial ridges are symmetrically formed about the common line where the intermediate circle intersects the two side circles. The case further includes two rib-sealing surfaces, each of which is located at an intermediate position between the two ridges on either side of the common line and flush with the interior surface of the case, and the spindle further includes a large diameter section between the rear and front ends thereof, the large diameter section having a transversal cross section complementary to and snugly fitting in the intermediate circle, and the large diameter section includes two pairs of mutually parallel axial chamfers formed in an outer peripheral surface thereof to define one of the ribs between each pair such that when the rib-sealing surfaces of the case are displaced by rotation from the ribs, the chamfers undo the sealing provided by the rib-sealing surfaces opposing the ribs. In addition, the rib-sealing surfaces oppose the outer peripheral surface of the large diameter section except when the rib-sealing surfaces oppose the chamfers, whereas the case further including thereon two blade-sealing surfaces which are 90 degree phase-shifted from the rib-sealing surfaces. 
     In accordance with one embodiment, a hydraulic unit includes: a generally cylindrical case containing working fluid, with the case including an interior surface and front and rear closing elements at two axial ends thereof; a spindle which is inserted into the case and includes front and rear ends coaxially and rotatably supported by the front and rear closing elements, respectively, the spindle further including at least one axially extending sealing surface and at least one blade which is biased radially into abutment with the interior surface of the case for circumferentially partitioning a fluid chamber defined between the case and the spindle; at least one axially extending sealing body protruding from the interior surface of the case and opposing the at least one sealing surface of the spindle for sealing the fluid chamber when the case is at a predetermined rotational position; a pair of pins provided on axial front and rear ends of each blade; and cam recesses provided in opposing inner surfaces of the closing elements for guiding the pins during rotation of the case and retracting the blades into the spindle when the at least one sealing body passes by the at least one blade, in which while relative rotation between the case and the spindle causes a top surface of the at least one blade to slidably abut the interior surface of the case, the at least one sealing body opposes the at least one sealing surface so as to divide the fluid chamber into smaller chambers, thus creating differential pressure among the smaller chambers, thus producing instantaneous torque to the spindle. Furthermore, the interior surface of the case has a circular shape coaxial with an axis of the spindle. Since the interior surface of the case has a simple circular cross-section coaxial with the spindle, the case functions as a liner in conventional arrangements, thus reducing the number of components in the foregoing hydraulic unit. In addition, as the interior surface of the case need only be machined to a simple circular hole, eliminating the need for high-precision polishing, as is required for complexly shaped interior surfaces of conventional units, and significantly lowering the cost and number of steps required in manufacturing the hydraulic unit. 
     In accordance with one aspect of the present invention, the spindle includes first and second blades and the case includes two sealing bodies, the first blade is provided with two first pins, and the second blade is provided with two second pins longer than the first pins. Moreover, each closing element includes in its inner surface a first oblong cam recess for guiding one of the first pins and a second oblong cam recess deeper than the first cam recess for guiding one of the second pins. Each second cam recess shares a common longitudinal end portion with the first cam recess and has a shorter longitudinal axis than the first cam recess such that, following the retraction of the blades into the spindle, the second recesses prevent the second blade from coming into abutment with the interior surface of the case until the case further rotates a predetermined angle while the first recesses cooperate with the first pins to permit the first blade to protrude into abutment with the interior surface of the case. 
     In accordance with another aspect of the present invention, the first and second blade are located diametrically opposite about the axis of the spindle, two sealing surfaces are positioned diametrically opposite about the axis of the spindle and 90 degrees phase-shifted from the blades, and two sealing bodies are positioned diametrically opposite about the axis of the interior surface of the case. Additionally, the longitudinal axes of the first and second cam recesses are oriented orthogonal to a diameter of the case passing through the sealing bodies, the widthwise axes of the first cam recesses pass through the axis of the spindle and are oriented orthogonal to the longitudinal axes of the first and second cam recesses, and the center of the first cam recess is located at the axis of the spindle. In this arrangement, when the case is at a first rotational position, the sealing bodies oppose the sealing surfaces and each first pin is located on the longitudinal axis of the associated first cam recess in the longitudinal end portion of the first recess not shared with the second recess while each second pin is located on the longitudinal axis of the first and second recesses in the longitudinal end portion shared by the first and second recesses so as to allow the blades to be biased into abutment with the interior surface of the case, thus producing instantaneous torque. At a second rotational position of the case, rotated a further 180 degrees from the first rotational position, each first pin is located on the common longitudinal axes of the first and second cam recesses in the longitudinal end portion shared by the recesses and the second pin is located on the longitudinal axis of the second cam recess in the second cam&#39;s longitudinal end portion not shared with the first cam recess, thus preventing the second blade from coming into abutment with the interior surface. 
     In accordance with yet another aspect of the present invention, the widthwise axes of the first and second cam recesses are selected so as to have a common and sufficiently short length to cause the blades to be retracted into the spindle when the case is at a third rotational position, rotated a further 90 degrees from the first position, where the first and second pins are located approximately on the widthwise axes of the first cam recesses, with the blades passing by the sealing bodies. 
     In accordance with still another aspect of the present invention, the spindle includes an outer peripheral surface having a circular cross-section coaxial with the interior surface of the case. The spindle further includes two pairs of mutually parallel axial chamfers formed therein to define one of the sealing surfaces between each pair such that when the sealing bodies of the case are displaced by rotation from the sealing surfaces, the chamfers undo the sealing provided by the sealing bodies opposing the sealing surfaces. 
     In accordance with one aspect of the present invention, the sealing bodies oppose the outer peripheral surface of the spindle except when the sealing bodies oppose the chamfers. 
     In accordance with another aspect of the present invention, each cam recess includes a pair of opposing semicircular walls and a pair of parallel liner walls connecting the semicircular walls, thus forming a continuous loop surface extending parallel with the axis of the spindle. In addition, each of the aforementioned longitudinal end portions shared by each first cam recess and the associated second cam recess includes one semicircular wall and at least part of each liner wall. 
     In accordance with one aspect of the present invention, the hydraulic unit further includes a pair of coil springs disposed between the blades within the spindle for biasing the blades in outwardly radial directions. 
     In accordance with another aspect of the present invention, following the retraction of the blades into the spindle when the case is at the third rotational position, the case returns to the first rotational position upon rotating 270 degrees further, such that instantaneous torque is produced to the spindle once for each complete rotation of the case. 
     In accordance with still another aspect of the present invention, the first and second pins are inserted in the respective first and second recesses. Moreover, the length of each first pin in the recesses is shorter than the depth of the portion shared by the first and second recesses, whereas the length of each second pin in the cam recesses is shorter than the depth of the second cam recess and greater than the depth of the portion shared by the first and the second cam recesses. 
     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 DRAWINGS 
     For a fuller understanding of the nature and objects of the present invention, reference is made to the following detailed description and the accompanying drawings, in which: 
     FIG. 1A is a cross-sectional view of a hydraulic unit according to an embodiment of the present invention taken along the axial line; 
     FIG. 1B is a cross-sectional view of the hydraulic unit taken along line A—A in FIG. 1A; 
     FIG. 1C is a cross-sectional view of the hydraulic unit taken along line B—B in FIG. 1A; 
     FIG. 1D is a cross-sectional view of the hydraulic unit taken along line C—C in FIG. 1A; 
     FIG. 2 is a partially cross-sectional view of an impulse screwdriver incorporating the hydraulic unit shown in FIG. 1; 
     FIGS. 3A-E show in cross-section the movement of the blades with respect to the rotation of the case of the hydraulic unit of FIG. 1; 
     FIG. 4 is a cross-sectional view of a hydraulic unit according to an alternate embodiment of the present invention taken along the axial line; 
     FIGS. 5A-L show the movement of the blades with respect to the case of the hydraulic unit in FIG. 4; 
     FIGS. 6A-D show in cross-section the movement of the blades with respect to the rotation of the case of a conventional hydraulic unit; 
     FIGS. 7A-L shows the movement of the blades with respect to the case of a conventional hydraulic unit similar to the one shown in FIG. 6; and 
     FIG. 8 is a graph showing a pattern of torque production in the hydraulic unit of FIG.  6 . 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1A through 8, wherein like parts are designated by like reference numerals throughout, illustrate examples embodiment of the hydraulic unit according to the present invention. Although the present invention will be described with reference to the example embodiments illustrated in the figures, it should be understood that many alternative forms can embody the present invention. One of ordinary skill in the art will additionally appreciate different ways to alter the parameters of the embodiments disclosed, such as the size, shape, or type of elements or materials, in a manner still in keeping with the spirit and scope of the present invention. 
     First Embodiment 
     FIG. 1A is a cross-sectional view of a hydraulic unit  1  according to an embodiment of the present invention taken along the axial line, FIG. 1B is a cross-sectional view of the hydraulic unit taken along line A—A in FIG. 1A, FIG. 1C is a cross-sectional view of the hydraulic unit taken along line B—B in FIG. 1A, and FIG. 1D is a cross-sectional view of the hydraulic unit taken along line C—C in FIG.  1 A. The hydraulic unit  1  includes a cylindrical case  2 . Plugging the forward part of the cylindrical case  2  (with the front of the case shown as being on the left side of FIG. 1A) from the rear is a closing element such as a disk-shaped bottom cap  4  which is inserted into the cylindrical case  2  and abuts the rear surface of a restrainer  3 . A spring pin  5  passes through a gap in the restrainer  3 , penetrating the bottom cap  4  so as to rotatably integrate the bottom cap  4  with the case  2 . A bolt  6  screwed into the bottom cap  4  via a gap in the restrainer  3  provides a passage through which working fluid is supplied. 
     Additionally, a rotatable liner  7  disposed to the rear of the bottom cap  4  is integrally connected to the bottom cap  4  with a plurality of pins  8 . The liner  7  has a generally cylindrical shape, composed of a front plate  9  and a rear plate  10  connected to each other with an opposing pair of first sealing bodies  12  and an opposing pair of second sealing bodies  13 . Each of the front and rear plate  9  and  10  defines in its interior an approximately oblong or elongated circular guide hole  11  whose cross section is a combination of three circles. As illustrated, the first sealing bodies  12  oppose each other along the longitudinal axis of each guide hole  11 , whereas the second sealing bodies  13  oppose each other along the widthwise axis of each guide hole  11 . In addition, the first sealing bodies  12  are provided with mutually opposing first sealing surfaces  14  which generally are flush with and conform to the interior surfaces of the guide holes  11 . Likewise, the second sealing bodies  13  has axially extending center ridges  15  which are in turn provided with mutually opposing second sealing surfaces  16  which also conform to the interior surfaces of the guide holes  11 . In addition, a disk-shaped top cap  17  disposed at the rear of the liner  7  functions as a rear closing element that is both integrally rotatable with the case  2  and axially movable relative to the case and that is integrated in the rotary direction with the liner  7  by a plurality of pins  18 . Furthermore, a top nut  21  is screwed into the case  2  behind the top cap  17  with a disk spring  20  between the cap  17  and the nut  21 , such that by rotating the top nut  21  so as to cause the screw to travel in the forward direction, the biasing force of the disk spring  20  holds the top cap  17  against the rear of the liner  7 . Reference numeral  19  designates a cylindrical connector provided with a hexagonal opening protruding from the rear of the top cap  17 . 
     Reference numeral  22  designates the spindle of the hydraulic unit  1 . Disposed at the forward end of the spindle  22  is an output shaft  23  which penetrates the bottom cap  4  and protrudes forward of the case  2  so as to be rotatably supported by the bottom cap  4 . A column  24  is disposed at the rear of the spindle  22  and inserted into and rotatably supported by a closed-end hole formed in the front surface of the top cap  17 . Furthermore, formed in the center of the spindle  22  within the liner  7  is a large diameter section  25  the transversal or radial cross-section of which is complementary to or snugly fits in the intermediate circle of the guide holes  11  of the liner  7 . Provided through the large diameter section  25  is a pair of radially extending accommodating grooves  26  and a pair of axially disposed ribs  27  which are circumferentially 90 degrees phase-shifted from the accommodating grooves  26 . Furthermore, accommodated in each groove  26  is a blade  28  that has the same axial length as that of the large diameter section  25  and is slightly circumferentially tiltable. Two coil springs  29  are interposed between and bias the blades  28  outwardly in mutually opposing directions, thus bringing the front and rear portions of the top surfaces of the blades  28  into abutment with the interior surfaces of the guide holes  11  of the liner  7 . When the spindle  22  is in the rotated position shown in FIG. 1C, the contact between the blades  28  and the first sealing surfaces  14  of the liner  7  and the contact between the ribs  27  and the second sealing surfaces  16  result in the formation of four partitions in a fluid chamber  30  defined within the liner  7 . 
     Still referring to FIG. 1, a first oblong (elongated circle) cam recess  31  and a second oblong cam recess  32  which has a longer longitudinal axis than the recess  31  are formed in the opposing inner surfaces of the bottom cap  4  and the top cap  17  (four cam trecesses altogether in the hydraulic unit  1 ). The longitudinal axes of the first and second cam recesses  31  and  32  lie on the same plane as those of the guide holes  11  of the liner  7 . As shown in FIG. 1D, each first cam recess  31  has an oblong shape one semicircle of which is deviating or eccentric from the axis of the spindle  22 , generally surrounding the output shaft  23 , with its upper longitudinal end portion (as seen in FIG. 1D) located close to the outer peripheral surface of the large diameter section  25 . The second cam recess  32  has a longer oblong shape than the first cam recess  31  so that both of its longitudinal end portions are located close to the outer peripheral surface of the large diameter section  25 . In addition, the second cam recess  32  shares with the first recess  31  the upper (as seen in FIG. 1D) longitudinal end portion where the first recess  31  is deviated from the axis of the spindle  22 . As used herein, the term “longitudinal end portion” refers to the portion of a cam recess that includes a semicircular or curved wall portion and part of the two liner wall portions connected to the semicircle wall portion. In addition, the second cam recess  32  is formed shallower than the first cam recess  31 . 
     Provided on the front and rear end surfaces of one blade  28  are two first pins  33  which are inserted into the first cam recesses  31  and longer than the depth of the second cam recesses  32 . Likewise, provided on the front and rear end surfaces of the other blade  28  are two second pins  34  which are slightly shorter than the depth of the second cam recesses  32  and inserted into the second cam recesses. Accordingly, the upper (as seen in FIG. 1) blade  28  can only protrude from the large diameter section  25  up to a certain limit due to the interference of the first pins  33  with the inner peripheral surfaces of the respective first cam recesses  31 , whereas the lower blade  28  can only protrude from the large diameter section  25  up to a certain limit due to the interference of the second pins  34  with the inner peripheral surfaces of the respective second cam recesses  32 . When the blades  28  are at the rotational positions where they are oriented parallel to the longitudinal axes of the first and second cam recesses  31  and  32  while in contact with the interior surfaces of the guide holes  11  (FIGS.  1 C-D), the first and second pins  33  and  34  are detached from the inner peripheral surfaces of the first and second cam recesses  31  and  32 . However, when the blades  28  are at the rotational position where they are oriented parallel to the widthwise axes of the first and second cam recesses  31  and  32  (the position rotated 90 degrees from that of FIGS.  1 C-D), the first and second pins  33  and  34  abut the inner peripheral surfaces of the first and second cam recesses  31  and  32 , respectively, thus limiting the protrusion of the blades  28 . At this position, the top surfaces of the blades  28  are retracted further inward from the outer peripheral surface of the large diameter section  25  of the spindle  22  and detached from the interior surface of the guide holes  11 . 
     As shown in FIG. 2, for example, a hydraulic unit  1  thus constructed is installed within a housing  36  of an electric power tool such as an impulse screwdriver  35 . Specifically, the connector  19  of the top cap  17  of the unit  1  is integrally coupled to the top portion of a carrier  39  of an epicycle reduction gear mechanism  38  to which rotation of a motor  37  is transmitted, whereas the output shaft  23  of the spindle  22  protrudes from the top end of the housing  36  and is fitted with a chuck  40  for attaching a tool bit thereto. Thus, when the top cap  17  and the carrier  39  rotate with the rotation of the motor  37 , the liner  7  and the case  2  also rotate (rotation is counterclockwise in FIG.  3 A). As shown in FIG. 3A, due to the relative rotation between the blades  28  and the liner  7 , the top surfaces of the blades  23  slide on the interior surfaces of the guide holes  11  while tilted in the direction of rotation of the case  2 . Upon reaching the first sealing surfaces  14 , the blades  28  and the ribs  24  divide and seal the fluid chamber  30  into four partitions, thus creating alternate high and low pressure sub-chambers within the fluid chamber  30 . The differential pressure thus created in the fluid chamber  30  produces impact torque to the spindle  22  via the blades  28 , thus causing the spindle  22  to rotate (generation of hydraulic impulse). 
     Referring to FIG. 3B, as the case  2  continues its rotation, the first and second cam recesses  31  and  32  of the bottom cap  4  and the top cap  17  also rotate. Simultaneously, the first pins  33  of one of the blades  28  slide along the inner peripheral surfaces of the first cam recesses  31 , whereas the second pins  34  of the other blade  28  also slide along the inner peripheral surfaces of the second cam recesses  32 . As the points of contact between the pins  33  and  34  and the inner surfaces of the recesses  31  and  32  gradually approach the axis of the spindle  22 , the blades  28  are gradually retracted into the large diameter sections  25  by the recesses&#39; inner peripheral surfaces. When the blades  28  are at the position shown in FIG. 3C, where the liner  7  is about to complete approximately 90-degree rotation from the position of FIG. 3A, the blades  28  are detached from the interior surfaces of the guide holes  11 . At the position shown in FIG. 3D, where the liner  7  has rotated approximately 90 degrees, the distance between the first and second pins  33  and  34  becomes shortest due to the width (widthwise axis) of the first and second cam recesses  31  and  32 . This allows the blades  28  to be completely withdrawn into the large diameter portion  25  and pass by the second sealing surfaces  16  without touching these surfaces. 
     As the case  2  continues its rotation, one of the blades  28  gradually protrudes from the large diameter section  25  as the shorter second pins  34  are guided along the inner peripheral surfaces of the second cam recesses  32 . Referring to FIG. 3E, when the liner  7  has made 180-degree rotation from the position of FIG. 3A, that blade  28  comes into contact with the first sealing surface  14 . Concurrently, the longer first pins  33  of the other blade  28  are guided by the inner peripheral surfaces of the first cam recesses  31 , causing that blade  28  to continue to make relative rotation without protruding from the large diameter section  25  or functioning as a seal within the fluid chamber  30  as the blade remains detached from the first sealing surface  14 . Accordingly, no hydraulic impulse is generated at his position. The next hydraulic impulse is generated when the liner  7  has rotated another 180 degrees to return to the position of FIG. 3A, at which the first and second pins  33  and  34  abut the inner peripheral surfaces of the first and second cam recesses  31  and  32  again. This means that even with two blades  28  one hydraulic impulse is generated for each complete rotation of the case  2 . 
     As described above, according to the foregoing embodiment, the longer first pins  33  protrude from the end surfaces of one blade  28 , with the shorter second pins  34  protruding from the end surfaces of the other blade  28 , whereas the first and second cam recesses  31  and  32  are formed in the opposing inner surfaces of the bottom cap  4  and the top cap  17  so as to guide the first and second pins  33  and  34  during the rotation of the case and for preventing the top surfaces of the blades  28  from sliding on the second sealing surfaces  16  (which are associated with, or correspond to, the ribs  27  of the spindle  22  for sealing partitioned fluid chambers). This arrangement completely eliminates the rotational resistance created by the top surfaces of the blades (1) sliding on the interior surfaces of the guide hole  11  and being pushed into the large diameter section  25  and (2) riding over the second sealing surfaces  16 , thereby maximizing the torque resulting from intended hydraulic impulses. In other words, this arrangements eliminates torque “b” while augmenting torque “a” in FIG.  8 . 
     Furthermore, the hydraulic unit of the foregoing embodiment is formed such that the deeper first cam recesses  31  for guiding the longer first pins  33  are provided in combination with the shallower second cam recesses  32  for guiding the shorter second pins  34 . Additionally, each first cam recess  31  shares one curved wall portion and the liner wall portions, with its longitudinal axis shorter than that of the second cam recess  32 . This design allows the first recesses  31  to guide the first pins  33  during the operation of the tool so as to prevent that blade  28  from coming into contact with one of the first sealing surfaces  14 . This ensures generation of one hydraulic impulse per rotation of the case  2 , which further augments the unit&#39;s output torque. 
     As described above, in the foregoing embodiment, the depth of the first cam recesses  31  differ from that of the second cam recesses  32  such that these recesses  31  and  32  guide the first and second pins  33  and  34 , respectively, on the blades  28  in order to realize generation of a single hydraulic impulse for each rotation of the case  2 . However, only one cam recess may be formed in each of the bottom and top caps and pins of the same length may be provided on the blades in order to generate two hydraulic impulses per case rotation. Even in this case, the output torque of the electric power tool can also be increased by selectively preventing contact between the blades and the guide holes  11  of the liner  7 . 
     The number of blades need not be limited to two, as in the foregoing embodiment; the present invention can also be realized with one or three blades. Moreover, the shapes of the cam recesses are not limited to those described in the foregoing embodiment; instead, grooves having a sufficient width to accommodate the pins may be formed in an oblong loop. The recesses or the grooves may also be oval or elliptical rather than oblong as in the foregoing embodiment. 
     Second Embodiment 
     Another embodiment will be described hereinafter with reference to the attached drawings, in which identical or similar reference numerals or characters denote identical or similar parts or elements throughout the several views. Therefore, description of such elements may be omitted. 
     FIG. 4 is a cross-sectional view of a hydraulic unit  101  according to an embodiment of the present invention taken along the axial line, whereas FIG. 5 illustrates operation of hydraulic unit  101  in sequence. The hydraulic unit  101  includes a cylindrical case  102 . Plugging the forward part of the cylindrical case  102  (with the front of the case shown as being on the left side of FIG. 4) from the rear is a closing element such as a disk-shaped bottom cap  104  which is inserted into the cylindrical case  102  and abuts the rear surface of a restrainer  103 . The bottom cap  104  is additionally prohibited from rotation with respect to the case  102  by means of a rotation stopper (not shown). The case  102  also includes at its rear end a relatively large opening  105  into which a disk-shaped top cap  106  is inserted as a rear closing element. The top cap  106  is also prohibited from rotation with respect to the case  102  by means of a rotation stopper (not shown). Screwed into the opening  105  behind the top cap  106  is a top nut  107 . Accordingly, rotation of the top nut  107  causes the screw to travel in the forward direction, thus securing the top cap  106  in the case  102 . Reference numeral  108  designates a cylindrical connector provided with a hexagonal opening protruding from the rear of the top cap  106  through the top nut  107 . 
     Still referring to FIG. 4, reference numeral  109  designates the spindle of the hydraulic unit  101 . Disposed at the forward end of the spindle  109  is an output shaft  110  which penetrates the bottom cap  104  and protrudes forward of the case  102 . The output shaft  110  is rotatably supported by the bottom cap  104  and coaxial with circular interior surface of the case  101 . A column  111  is disposed at the rear of the spindle  109  and inserted into and rotatably supported by a closed-end hole  112  formed in the front surface of the top cap  106 . In addition, the column  111  is coaxial with the circular interior surface of the case  101 . Furthermore, formed in the center of the spindle  109  is a large diameter section  113  whose radial cross-section is circular and essentially fills the space between the bottom cap  104  and the top cap  106 . Provided through the large diameter section  113  is a pair of radially extending accommodating grooves  114  placed in communication with each other at the axial front and rear ends of the large diameter section  113 . Referring to FIG. 5, additionally provided on the large diameter section  113  is a pair of axially disposed ribs  115  which are circumferentially 90 degrees phase-shifted from the accommodating grooves  114 . The outer end surface of each rib  115  functions as a sealing surface (to be described in further detail below). Furthermore, accommodated in each groove  114  is a blade  116  that has the same axial length as that of the large diameter section  113  and is slightly circumferentially tiltable. Two coil springs  117  are interposed between the blades  116  in the large diameter section  113 , basing the blades  116  outwardly in mutually opposing directions, thus bringing the top surfaces of the blades  116  into abutment with the interior surfaces of the case  102 . A pair of sealing bodies  118  is disposed on the interior surface of the case  102  at diametrically opposite positions. Each sealing body  118  extends in parallel with the axis of the case  102  between the bottom cap  104  and the top cap  106 , with its inner end surface in contact with the outer peripheral surface of the large diameter section  113  of the spindle  109 . 
     When the spindle  109  is in the rotated position relative to the case  102  shown in FIG. 5C, where the blades  116  of the spindle  109  is 90 degrees phase-shifted from the sealing bodies  118  of the case  102 , the blades  116  are in abutment with the interior surface of the case  102  while the sealing bodies  118  oppose the ribs  115  on the large diameter section  113 , thus forming four partitions or sub-chambers in a fluid chamber  119  defined between the interior surface of the case  102  and the outer peripheral surface of the large diameter section  113 . Furthermore, two pairs of mutually parallel axial chamfers  120  are cut in the large diameter section  113  to define the ribs  115 , such that when the sealing bodies  118  of the case  102  are displaced by rotation from the ribs  115  of the large diameter section  113 , the chamfers  120  undo the sealing provided by the sealing bodies  18  and the ribs  15 . Referring to FIG. 4, a fluid feeding inlet  121  is provided in the output shaft  110  of the spindle  109  along the spindle&#39;s axis so as to be in communication with the front ends of the accommodating grooves  114 . Additionally, a closing screw  22  is tightened in the inlet  121  to permit supply of working fluid into the hydraulic unit by its removal. 
     A first oblong (elongated circle) cam recess  123  and a second oblong cam recess  124  which has a shorter longitudinal axis than the recess  123  are formed in the opposing inner surfaces of the bottom cap  104  and the top cap  106  (four cam recesses altogether in the hydraulic unit  101 ). Each first cam recess  123  has a longer oblong shape than the corresponding second cam recess  124 , and the center of the longitudinal axis of the first recess  123  coincides with the axis of the spindle  109 . Compared with the first recesses, each second cam recess  124  has a shorter oblong shape one semicircle of which is deviating or is eccentric from the axis of the spindle  109  so as to share with the first recess one semicircular (curved) wall portion and part of the two liner wall portions (the shared area defined by the semicircular wall portion and the part of liner wall portions is hereinafter referred to as the shared longitudinal end portion). The portion of each first recess  123  not shared with the second recess  124  is made shallower than the shared end portion. The first and second cam recesses  123  and  124  in the bottom cap  4  are configured symmetrically to those in the top cap  106 . 
     Provided on the front and rear end surfaces of one blade  116  are two first pins  125  which are inserted into the first cam recesses  123  and longer than the depth of the portion shared by the first and second recesses  123  and  124 . Likewise, provided on the front and rear end surfaces of the other blade  116  are two second pins  126  each of which has a length greater a greater length than the depth of each first cam recess  123  and is inserted into the portion shared by the first and second recesses  123  and  124 . 
     Accordingly, the lower (as seen in FIG. 4) blade  116  can only protrude from the large diameter section  113  up to a certain limit due to the interference of the first pins  125  with the inner peripheral surfaces of the respective first cam recesses  123 . Likewise, the upper blade  116  can only protrude from the large diameter section  113  up to a certain limit due to the interference of the second pins  126  with the inner peripheral surfaces of the respective second cam recesses  124 . As shown in FIG. 5C, when the second pins  126  are located in the portions shared by the first and second cam recesses  123  and  124  with the first and second pins  125  and  126  located on the longitudinal axes of the first and second recesses  123  and  124 , the blades  116  abut the interior surface of the case  102  and detach the first and second pins  125  and  126  from the inner peripheral surfaces (wall portions) of the first and second recesses  123  and  124 . Conversely, as shown in FIG. 5L, when the first and second pins  125  and  126  are located approximately on the widthwise axes of the first and second cam recesses  123  and  124 , the first and second pins  125  and  126  abut the inner peripheral surfaces (wall portions) of the first and second recesses  123  and  124 , thus limiting the amount of protrusion of the blades  116 . Simultaneously, the top surfaces of the blades  116  are retracted inside the peripheral surface of the large diameter section  113 . 
     For example, a hydraulic unit  101  thus constructed may be installed within a housing of an electric power tool such as an impulse screwdriver. Specifically, the connector  108  of the top cap  106  of the unit  101  is integrally coupled to the tool&#39;s output shaft to which rotation of the motor is transmitted, whereas the output shaft  110  of the spindle  109  of the hydraulic unit protrudes from the top end of the housing and is fitted with a chuck for attaching a tool bit thereto. Thus, when the top cap  106  rotates with the motor, the case  102  also rotates as indicated by the arrow (i.e., counterclockwise in FIG.  5 ), integrally rotating the spindle  109  via the fluid chamber  119 . As shown in FIGS. 5A-B, when the rotation of the spindle  109  starts to lag behind the case&#39;s  102  rotation due to an increased load on the output shaft  110 , the top surfaces of the blades  116  slide on and relative to the interior surfaces of the case  102  while tilted in the direction of rotation of the case  102 . As shown in FIG. 5C, upon reaching the ribs  115  on the large diameter section  113 , the sealing bodies  118  seal the fluid chamber  119 . Concurrently, the tilt of the blades  116  places the two partitioned sub-chambers which are located rotationally ahead of the sealing bodies  118  in communication with each other via the blade accommodating grooves  114 , increasing the pressure within these sub-chambers and thus creating alternate high and low pressure sub-chambers partitioned within the fluid chamber  119 . The differential pressure thus created in the fluid chamber  119  produces impact torque to the spindle  109  via the blades  116 , thereby causing the spindle  109  to rotate (generation of an hydraulic impulse). 
     Referring to FIGS. 5D-F, as the case  102  continues its rotation, the first and second cam recesses  123  and  124  of the bottom cap  104  and the top cap  106  also rotate. Simultaneously, the first pins  125  of one of the blades  116  slide on the inner peripheral surfaces of the first cam recesses  123 , whereas the second pins  126  of the other blade  116  also slide on the inner peripheral surfaces of the second cam recesses  124 . As the points of contact between the pins  125  and  126  and the inner surfaces of the respective recesses  123  and  124  gradually approach the axis of the spindle  109 , the blades  116  are gradually retracted into the large diameter sections  113  by the recesses&#39; inner peripheral surfaces (wall portions). At the position shown in FIG. 5G, the blades  116  are detached from the interior surfaces of the case  102 . As shown in FIGS. 5H-K, as the case  102  continues to rotate, the blades  116  are pulled into the large diameter section  113  by the first and second cam recesses  123  and  124 . At the position shown in FIG. 5L, where the case  102  has rotated approximately 90 degrees from the position of FIG. 5C, due to the length of the widthwise axis of the first and second cam recesses  123  and  124 , the blades  116  are completely retracted into the large diameter section  113  and pass by the sealing bodies  118  without interference with the bodies  118 . 
     As the case  102  continues its rotation, one of the blades  116  gradually protrudes from the large diameter section  113  and comes into contact with the case  102  again as the shorter second pins  123  are guided along the inner peripheral surfaces of the first cam recesses  123 . Concurrently, the longer second pins  126  of the other blade  116  are guided by the inner peripheral surfaces of the second cam recesses  124  (which has a shorter longitudinal axis), causing that blade to continue to rotate without protruding from the large diameter section  113  into abutment with the interior surface of the case  102 . Accordingly, when the case  102  rotates 90 degrees from the position of FIG. 5L, where the sealing bodies  118  reach the ribs  15 , the foregoing other blade  116  does not function as a seal within the fluid chamber  119 , thus generating no hydraulic impulse at this position. The next hydraulic impulse is generated when the case  102  rotates another 180 degrees to return to the position of FIG. 5C, where the second pins  126  are located in the portions shared by the first and second cam recesses  123  and  124  with the first and second pins  125  and  126  located on the longitudinal axes of the first and second recesses  123  and  124 . This means that even with two blades  116 , one hydraulic impulse is generated for each complete rotation of the case  102 . 
     As described above, according to the foregoing embodiment, the interior surface of the case  102  has a circular shape coaxial with the large diameter section  113  of the spindle  109  such that the case functions as a liner of conventional hydraulic units. Furthermore, the ribs  15  and the blades  116  of the spindles  109  cooperate with the sealing bodies  118  on the interior surface of the case  102  to provide sealing within the fluid chamber  119 , whereas the first and second cam recesses  123  and  124  are adapted to guide the first and second pins  25  and  26  to avoid interference between the blades  116  and the sealing bodies  118 . As the simpler circular cross-section of the interior surface of the case  102  eliminates the need for high-precision polishing, as is required for complexly shaped interior surfaces of conventional units, this reduces the number of components and steps of manufacturing the unit, thus greatly lowering the cost and time of manufacturing the hydraulic unit  101   
     As described above, in the foregoing embodiment, the depth of the first cam recesses  123  differ from that of the second cam recesses  124  such that these recesses  123  and  124  guide the first and second pins  125  and  126 , respectively, on the blades  116  in order to realize generation of a single hydraulic impulse for each rotation of the case  102 . However, the present invention is applicable to an arrangement in which only one cam recess is formed in each of the bottom and top caps and pins of the same length are provided on the blades in order to generate two hydraulic impulses per case rotation. 
     The number of blades need not be limited to two, as in the foregoing embodiment; the present invention can also be realized with one or three blades. Moreover, the shapes of the cam recesses are not limited to those described in the foregoing embodiment; instead, grooves having a sufficient width to accommodate the pins may be formed in an oblong loop. The recesses or the grooves may also be elliptical rather than oblong as in the foregoing embodiment. 
     It will thus be seen that the present invention efficiently attains the characteristics 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.