Abstract:
A compact and light-weight pneumatically operated screw driver providing high speed screw fastening with high operability. A rotary member rotationally driven by a pneumatic motor includes a main rotary member made from a plastic material and a sliding segment fixed to the main rotary member. A rotation slide member is axially slidably movable relative to the rotary member and is rotatable together with the rotation of the rotary member. A shut-off section is provided on which the rotation slide member is rotationally seated when the rotation slide member is moved toward the shut-off section. The rotation slide member is made from an elastic material. A sleeve like piston having a seal ring in sliding contact with a cylinder is provided. The seal ring shuts off a supply of compressed air to the pneumatic motor only by the seating of the rotation slide member onto the shut-off section.

Description:
CROSS-REFERENCE TO THE RELATED APPLICATION 
   The present application is closely related to the commonly assigned co-pending U.S. Patent applications titled “pneumatically operated screw driver” filed Sep. 3, 2004 (priority date: Sep. 19, 2003, Ser. No. 10/933,326 1297.44201X00), and to another commonly assigned co-pending U.S. patent application titled “pneumatically operated screw driver” (priority date Oct. 1, 2003, Base: JP2003-343293 and JP2003-343295) 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a pneumatically operated screw driver providing an axially driving force by a piston and rotational force by a pneumatic motor for screwing a threaded fastener into a woody member or the like. 
   U.S. Pat. No. 6,026,713 discloses a pneumatically operated screw driver including a driver bit engageable with a groove formed in a head of the fastener. The driver bit is connected to a piston which is driven in an axial direction of the driver bit upon application of a pneumatic pressure to one side of the piston. Further, a pneumatic motor is provided for rotating a rotary member. A rotation slide member is axially movable relative to the rotary member, and is rotatable together with the rotation of the rotary member. The piston is connected to the rotation slide member. Thus, the driver bit is axially movable while being rotated about its axis for screwing the fastener into a target. Further, a bumper is provided so as to absorb kinetic energy of the piston moving to its bottom dead center. An operation valve associated with a trigger is provided for opening a main valve in order to apply pneumatic pressure onto the piston. 
   The disclosed screw driver also includes a return chamber to which a compressed air is accumulatable for applying compressed air to the piston in order to move the piston and the driver bit to their initial positions. Accumulation of the compressed air into the return chamber is started when the piston is about to reach its bottom dead center. When the screw fastening operation is terminated upon abutment of the piston onto the bumper, the compressed air accumulated in the return chamber will be applied to an opposite side of the piston so as to return the piston and the driver bit to their original positions. Such conventional pneumatically operated screw driver is also disclosed in laid open Japanese Patent Application Publication No. H11-300639. 
   Recently, high speed screw fastening is needed, such as a screw fastening frequency the same as a nail driving frequency of a nail gun. In order to increase rotation speed of the driver bit, a pneumatic motor must provide high output. To this effect, new problems arise as to excessive frictional wear of components, particularly rotational components and heat generation of these components due to the excessive friction. To overcome the new problems, a material of the rotational components must be limited to a metal in view of heat resistivity. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide a compact and light-weight pneumatically operated screw driver providing high speed screw fastening with high operability. 
   Another object of the present invention is to provide such screw driver avoiding fuse-bonding and any generation of scratch at sliding surfaces of mutually sliding components due to frictionally wearing particles released from the components. 
   Still another object of the present invention is to provide such screw driver ensuring stop of a supply of compressed air to the pneumatic motor at the terminal phase of the screw driving operation. 
   Still another object of the present invention is to provide such screw driver capable of avoiding excessive rotation of the rotary member at a terminal phase of the screw driving operation in order to avoid excessive screwing operation. 
   These and other objects of the present invention will be attained by a pneumatically operated screw driver including an outer frame, a pneumatic motor, a cylindrical rotary member, a rotation slide member, a shaft member, a driver bit, and a cylinder. The pneumatic motor is disposed in the outer frame and is rotatable about its axis. The cylindrical rotary member extends in an axial direction of the pneumatic motor and is rotatable within the outer frame by the rotation of the pneumatic motor. The rotary member has an inner peripheral surface formed with a rotation transmission portion. The rotary member includes a main rotary member made from a plastic material and having an end at a side opposite to the pneumatic motor, and a sliding segment fixed to the end of the main rotary member and made from a metal. The rotation slide member is disposed within the rotary member and is slidable in the axial direction relative to the rotary member. The rotation slide member has an engagement portion engaged with the rotation transmission portion so as to be rotatable together with the rotation of the rotary member. The shaft member has one end portion connected to the rotation slide member and another end portion provided with a driver bit holding section and a piston section. The driver bit is connected to the driver bit holding section. The cylinder is fixedly disposed in the outer frame and extends in the axial direction. One of the outer frame and the cylinder provides a contact part with which an end face of the sliding segment is in rotational sliding contact. 
   In another aspect of the invention, there is provided a pneumatically operated screw driver including the outer frame, the pneumatic motor, a cylindrical rotary member, a rotation slide member, the shaft member, the driver bit, and a cylinder. The cylindrical rotary member extends in an axial direction of the pneumatic motor and is rotatable within the outer frame by the rotation of the pneumatic motor. The rotary member has an inner peripheral surface formed with a rotation transmission portion. The rotation slide member is disposed within the rotary member and is slidable in the axial direction relative to the rotary member. The rotation slide member includes a main section and an engagement portion protruding from the main section for engagement with the rotation transmission portion so as to be rotatable together with the rotation of the rotary member. At least the main section is entirely made from an elastic material. The cylinder is fixedly disposed in the outer frame and extends in the axial direction. The cylinder has an upper portion providing a shut-off section in sealing contact with at least the main section of the rotation slide member when the piston section reaches its bottom dead center for shutting off a compressed air passage directing to the pneumatic motor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  is a partial cross-sectional side view showing an initial state of a screw driver according to a first embodiment of the present invention; 
       FIG. 2  is a cross-sectional side view showing an essential portion of the screw driver in its screw driving phase; 
       FIG. 3  is a cross-sectional side view showing the essential portion of the screw driver and showing just a completion phase of the screw driving operation; 
       FIG. 4  is a perspective view showing a rotary member including a sliding member as a component of the pneumatically operated screw driver according to the first embodiment; 
       FIG. 5  is a perspective view showing a rotation slide member used in the pneumatically operated screw driver according to the first embodiment; 
       FIG. 6  is an enlarged cross-sectional view particularly showing a hole formed at a lowermost portion of a body; and 
       FIG. 7  is a partial cross-sectional side view showing a pneumatically operated screw driver according to a second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A pneumatically operated screw driver according to a first embodiment of the present invention will be described with reference to  FIGS. 1 through 6 . The directions used in the following description are defined based on a screw driver held in a vertical position with a driver bit extending downward and a grip extending rearward. Needless to say, the actual direction of the screw driver will be frequently changed due to its handiness when it is used. 
   A pneumatically operated screw driver  1  includes a body  5 . The body  5  constitutes an outer frame of a main body. The body  5  includes a handle  5 ′. The body  5  has an inside space defining a compressed air chamber  4  extending from the handle  5 ′ to an upper part of the body  5 . The body  5  is made from a metal such as a magnesium, an aluminum, and alloy thereof, and the body  5  has an inner peripheral surface  55 . The compressed air chamber  4  is in communication with an intake port  35  at the rear end of the handle  5 ′ for introducing the compressed air. A trigger lever  33 , an operation valve  30  opened or closed by the trigger lever  33 , and a main valve  28  opened or closed by the operation valve  30  are provided at the body  5 . 
   A pneumatic motor  2  is provided at the top of the body  5 . The pneumatic motor  2  has a rotor rotatable about its axis when it receives the compressed air from the compressed air chamber  4 . The rotor engages a planetary gear unit  3  to transmit the speed-reduced rotation to a rotary member  6 . The rotary member  6  causes a rotation in synchronism with the rotation of the rotor. 
   The rotary member  6  is in a cylindrical shape, and is roratably and directly supported by the body  5 . For example, an outer peripheral surface of the rotary member  6  is loosely fitted with the inner peripheral surface  55  of the body  5  without interposing a thrust bearing therebetween. The rotary member  6  includes a main rotary member  6 A ( FIG. 4 ) made from a plastic material, a sintered metal member  52 , and a washer  54  made from a metal such as steel or copper. As shown in  FIG. 4 , the main rotary member  6 A has a lower edge  50  formed with two grooves  51 . The sintered metal member  52  is porous, i.e., is formed with minute oil retaining holes. The sintered metal member  52  is fixed to the bottom surface  50 . To this effect, the sintered metal member  52  has two projections  53  each engageable with each groove  51 . The washer  54  is fixed to a bottom of the sintered metal member  52 . Because a major part of the rotary member  6  is made from the plastic material, rotational inertial force can be lower than that of a case where the rotary member is entirely made from a metal. For example, a density of aluminum is three times as high as a density of plastic material. This can avoid over-rotation of the rotary member  6  at a terminal phase of the screw driving operation in order to avoid excessive screwing operation. 
   The rotary member  6  has an inner peripheral surface formed with a pair of grooves  10  extending in an axial direction thereof. Within the rotary member  6 , a rotation slide member  7  is disposed. As shown in  FIG. 5 , the rotation slide member  7  has an upper portion from which a pair of projections  8  project radially outwardly and are slidingly engaged with the pair of grooves  10  for permitting the rotation slide member  7  to move in the axial direction relative to the rotary body  6 . The rotation slide member  7  defines an air shielding surface  14 . An entire portion of the rotation slide member  7  is made from an elastic material such as an urethane rubber. Even though the urethane rubber provides a frictional coefficient higher than that of an ordinary plastic material, the rotation slide member  7  can still provide a desirable axial sliding movement with respect to the rotary member  6  because the rotary member  6  is not made from a metal but is made from a plastic material. 
   A shaft  9  serving as an auxiliary piston extends in the longitudinal direction of the body  5 . The shaft  9  has an upper end portion connected to the rotation slide member  7 , an intermediate portion, and a lower portion. In the upper end portion and the intermediate portion, an air supply bore  38  extending in the axial direction of the shaft  9  and small diameter holes  37  extending in a radial direction thereof and in communication with the air supply bore  38  are formed for supplying a compressed air to a piston section  13  described later. 
   At the lower portion of the shaft  9 , a driver bit assembling section  40 , the piston section  13 , and a flange section  25  are provided. The driver bit assembling section  40  is disposed at the lower end portion of the shaft  9  for assembling a driver bit  11 . The piston section  13  is disposed as an outer peripheral section of the shaft  9  at a position immediately above the driver bit assembling section  40 . The piston section  13  has an outer peripheral surface provided with an O-ring  13 A. The flange section  25  is disposed as an outer peripheral section of the shaft  9  at a position below the piston section  13  for determining the termination of screw fastening operation. The flange section  25  has an outer diameter smaller than an outer diameter of the piston section  13 . 
   A cylinder  12  is disposed in the body  5  and extends in the axial direction of the shaft  9 . A main piston  21  is slidably disposed in the cylinder  12 . The main piston  21  is positioned below the rotation slide member  7  and is disposed to surround a part of the shaft  9 . That is, a lower part of the upper end portion, the intermediate portion, and the lower portion of the shaft  9  are surrounded by the main piston  21 . The main piston  21  has a hollow section  22  including a top end through which the shaft  9  extends, an upper hollow section, and a lower hollow section. An inner diameter of the upper hollow section is greater than an outer diameter of the shaft  9  and is smaller than an outer diameter of the piston section  13 . An inner diameter of the lower hollow section is greater than the inner diameter of the upper hollow section for allowing the piston section  13  to be in sliding engagement. That is, the O-ring  13 A is in sliding contact with the lower hollow section. Further, the flange section  25  has an outer diameter smaller than the inner diameter of the lower hollow section. Therefore, a minute annular space is defined between the flange section  25  and the lower hollow section. 
   An O-ring  45  in sliding contact with the inner peripheral surface of the cylinder  12  is assembled at a lower outer peripheral surface of the main piston  21 . Further, another O-ring  46  in sliding contact with the inner peripheral surface of the cylinder  12  is assembled at the outer peripheral surface and above the O-ring  45 . Piston holes  39  are formed in the main piston  21  at a position between the O-rings  45  and  46  for providing communication between an interior and exterior of the main piston  21 . 
   The rotation slide member  7  has a communication hole open at its upper surface, and the air supply bore  38  is in communication with an interior of the rotary member  6  through the communication hole. The small diameter holes  37  is adapted to communicate the air supply bore  38  with an inner space of the main piston  21 . 
   A plate section  15  is provided at an upper portion of the cylinder  12  made from a metal. The plate section  15  is adapted to permit the air shield surface  14  of the rotation slide member  7  to be brought into abutment therewith when the rotation slide member  7  is moved descent down by a predetermined distance. The plate section  15  is integral with the cylinder  12 . A vent hole  16  is formed below the plate section  15 . The vent hole  16  is in communication with an air inlet opening (not shown) of the pneumatic motor  2  through a compressed air passage (not shown). 
   The above-described O-ring  46  is located at a position between the piston hole  39  and the compressed air passage directed to the pneumatic motor  2  when the main piston  21  reaches its bottom dead center. In other words, the O-ring  46  prevents the compressed air from being supplied to the vent hole  16  through the air supply bore  38 , the small diameter holes  37  and the piston holes  39  after the main piston  21  reaches its bottom dead center. 
   A return chamber  20  is defined by a space between the lower portion of the body  5  and the outer peripheral surface of the cylinder  12 . The lower portion of the cylinder  12  is formed with compressed air flowage holes  23  for introducing compressed air into the return chamber  20 . A rubber ring  47  serving as a check valve is disposed over each outlet opening of the compressed air flowage holes  23  for preventing compressed air in the return chamber  20  to flow back into the cylinder  12 . At the lower portion of the cylinder  12 , a plurality of compressed air introduction holes  24  are formed at position below the compressed air flowage holes  23  for providing fluid communication between the return chamber  20  and the cylinder  12 . 
   A piston bumper  31  is provided at the lower portion of the cylinder  12 . A bottom surface of the main piston  21  and the flange section  25  of the shaft  9  bump against the piston bumper  31  when the main piston  21  and the shaft  9  reach their bottom dead centers. More specifically, as shown in  FIG. 6 , the piston bumper  31  is provided with an annular abutment projection  50  on which the bottom end of the main piston  21  will abuts. An outer diameter of the bottom end of the main piston  21  is slightly greater than an outer diameter of the abutment projection  50 . 
   A hole  5   a  is formed at the lowermost portion of the body  5  for allowing the driver bit  11  to pass therethrough. An inner diameter of the hole  5   a  is slightly greater than an outer diameter of the driver bit  11 , so that a minute space is defined therebetween. This minute space serves as an air discharge passage through which an air within the cylinder  12  and below the piston section  13  can be discharged to the atmosphere during downward stroke of the piston section  13 . 
   More specifically, in order to provide sufficient thrusting force or downward moving force of the piston section  13 , a sufficiently large volume of air must be smoothly discharged through the minute space. Therefore, the minute space must be sufficiently large so as to facilitate this air discharge. On the contrary, the minute space must be sufficiently small so as to maintain sufficiently high pressure in the cylinder space below the piston section  13  in order to move back the shaft  9  upwardly after completion of fastener driving. The latter high pressure is supplied from the return air chamber  20  into the cylinder space below the piston section  13  through the compressed air introduction holes  24 . Consequently, the area of the minute space is configured in an attempt to balance the conflicting requirements. 
   A nose portion  36  is provided to the lowermost portion of the body  5 . A magazine  17  is connected to the body  5 . The magazine  17  accommodates therein a plurality of screws arrayed side by side by an interlinking band (not shown). A screw feeder  19  is disposed in the magazine  17  and at a position adjacent to the nose portion  36  for automatically feeding a leading end screw of the screw array to the nose portion  36 . A push lever  26  in interlocking relation to the operation valve  30  is provided at a position below the screw feeder  19 . 
   Next, operation of the pneumatically operated screw driver thus constructed will be described. 
   In the screw driver, not only the operation valve  30  but also the push lever  26  are operated from the state shown in  FIG. 1  so as to start driving operation. In this case, screw fastening can be achieved by pulling the trigger lever  33  after the push lever  26  is pushed against a workpiece (not shown), or by pressing the push lever  26  against the workpiece while the trigger lever  33  is being pulled. 
   When the compressed air intake port  35  is connected to a compressor (not shown), the compressed air is introduced into the compressed air chamber  4  and the operation valve  30 . If the operation valve  30  is operated while the push lever  26  is pressed against the workpiece, the main valve  28  is opened, so that the compressed air is delivered into the rotary member  6  through the air passage (not shown). As a result, pneumatic pressure is applied to the upper surface of the main piston  21 . 
   Further, pneumatic pressure is also applied to the upper surface of the piston section  13  of the shaft  9  because the compressed air can pass through the air supply bore  38  and the small diameter holes  37 . Further, the compressed air leaked into a hollow space between the inner peripheral surface of the rotary member  6  and the outer peripheral surface of the main piston  21  is also applied to the upper surface of the piston section  13  through the piston holes  39  (see  FIG. 1 ). Thus, the main piston  21  and the shaft  9  are urged downward. 
   If the descent movement of the piston section  13 , i.e., the movement of the shaft  9  is decelerated due to the resistance incurred when the shaft  9  forcibly removes the screw  18  from the interlinking band, the main piston  21  catches up with the piston section  13  before the tip end of the screw  18  is driven into the workpiece. Consequently, the main piston  21  and the shaft  9  are integrally moved downwardly, so that the driver bit  11  drives the screw  18  into the workpiece. Incidentally, after the O-ring  46  of the main piston  21  starts sliding movement relative to the cylinder  12 , compressed air through the piston holes  39  will not be applied to the upper surface of the piston section  13  of the shaft  9 , because fluid passage from the piston holes  39  is blocked by the O-ring  46 . In the latter case, the compressed air through the air supply bore  38  and the small diameter holes  37  will be applied to the upper surface of the piston section  13 . 
   Immediately before the main piston  21  reaches its bottom dead center and when the O-ring  45  moves past the compressed air flowage hole  23 , the compressed air flowage hole  23  starts flowing of the compressed air into the return chamber  20  through the air supply bore  38 , the small diameter holes  37  and the piston holes  39 . 
   When the main piston  21  is positioned at a position shown in  FIG. 1 , the O-ring  45  blocks the fluid passage from the interior of the rotary member  6  to the air vent hole  16 . Therefore, compressed air supplied into the rotary member  6  cannot be delivered to the pneumatic motor  2 . On the other hand, compressed air supplied into the rotary member  6  is supplied to the pneumatic motor  2  through the air vent hole  16  once the O-ring  45  moves past the air vent hole  16  for starting rotation of the pneumatic motor  2 . It is unnecessary to rotate the pneumatic motor  2  at the initial stage. Instead, the rotation of the pneumatic motor  2  is started immediately before the driver bit  11  engages the grooves of the screw head. This can reduce consumption of the compressed air. The rotation of the pneumatic motor  2  is transmitted to the rotary member  6  and the rotation slide member  7  through the planetary gear unit  3 . 
   If the O-ring  46  has not reached the cylinder  12  in the downward movement of the main piston  21 , compressed air in the rotary member  6  is delivered to the air vent hole  16  by two routes. The first route is defined by the air supply bore  38 , the small diameter holes  37 , the piston holes  39 , and a gap between the outer peripheral surface of the main piston  21  and the inner peripheral surface of the cylinder  12 . The second route is defined by a gap between the rotary member  6  and the rotation slide member  7 , and the gap between the outer peripheral surface of the main piston  21  and the inner peripheral surface of the cylinder  12 . If the O-ring  46  reaches the cylinder  12 , the above-described second route is blocked by the O-ring  46 , and only the first route is effective for the delivery of the compressed air to the air vent hole  16 . Then if the O-ring  46  moves past the air vent hole  16 , the first route is blocked by the O-ring  46 , and only the second route is made effective for the delivery of the compressed air to the air vent hole  16 . 
   In the rotation phase of the rotary member  6 , since the main rotary member  6 A made from a plastic material and the metal member  52  are integrally rotated, no relative sliding movement occurs therebetween. Thus, heat generation of the rotary member  6  can be restrained. Further, since main rotary member  6 A made from the plastic material is loosely rotatably supported within the body  5  made from the metal, a bearing such as a thrust bearing can be dispensed with between the rotary member  6  and the body  5 . This leads to reduction in weight of the screw driver and provides stable depth of screw fastening. In other words, because the sliding relationship occurs between the plastic material and the metal, a problem of fuse-bonding can be avoided, the fuse-bonding may occur in case of the sliding relation between non-ferrous metals. Further, the rotary member  6  is only frictionally worn, which does not impart any surface injury to the opposing sliding member due to metallic wear particles released from the metal, since the main rotary member  6 A is made from the plastic material and since the opposing sliding member (body  5 ) does not release metallic wear particles because of difference in hardness between plastic material and the metal. Moreover, excessive heat generation does not occur, because the constant contact between the rotary member  6  and the body  5  does not occur, but the rotary member  6  is loosely supported within the body  5 . Moreover, because of the elimination of the bearing, a resultant outer diameter of the body  5  can be reduced to provide a compact screw driver. 
   As shown in  FIG. 2 , after the main piston  21  reaches its bottom dead center, the driver bit  11  continues descent movement only by the thrust of the auxiliary piston, i.e., the shaft  9 , so that the screw  18  can be screwed into the workpiece. In this case, since the bottom surface of the main piston  21 , i.e., an abutment end of the main piston  21  is in intimate contact with the piston bumper  31 , compressed air in the return chamber  20  cannot be entered into the lower space defined by the main piston  21  and the shaft  9 . Consequently, the thrust of the piston section  13  can be maintained to avoid accidental disengagement of the tip end of the driver bit  11  from the screw head groove due to shortage of the thrust. 
   In this case, because the difference in the outer diameter of between the bottom end of the main piston  21  and the annular abutment projection  50  is small so as to provide a sufficiently small pressure application area at the bottom end of the main piston  21  for returning the main piston toward its top dead center, the main piston  21  can be maintained at the bottom dead center position even if the pressure level in the return chamber  20  is increased at the terminal phase of the screw fastening operation as long as the pressure level in the rotary member  6  is still sufficient to maintain the main piston  21  to its bottom dead center. 
   When the screw  18  is fastened to a predetermined depth, the air shield surface  14  of the rotation slide member  7  abuts on the plate section  15  as shown in  FIG. 3  to stop further descent motion of the rotation slide member  7 . At the same time, the air communication between the rotary member  6  and the vent hole  16  will be blocked for stopping rotation of the pneumatic motor  2 , thereby completing the screw driving operation. Because the above-described first route has already been blocked by the O-ring  46 , it is only necessary to block the second route for stopping rotation of the pneumatic motor  2 . To this effect, the second route can be simply blocked by the abutment of the rotary slide member  7  onto the plate section  15 . Moreover, when the flange section  25  is seated on the bumper  31 , the shaft  9  cannot be any more moved to terminate the fastening operation. 
   Here, because the space between the hole  5   a  and the driver bit  11  is sufficiently small, a pressure in the cylinder  12  below the piston section  13  is gradually increased in accordance with the downward movement of the piston section  13 . This pressure increase resists downward movement of the piston section  13 . However, because the flange section  25  is disposed below the piston section  13  and the annular space is defined between the flange section  25  and the cylinder  12 , internal volume in the cylinder  12  and below the piston section  13  is sufficient in comparison with a case where no flange section is provided and a piston section is provided at the position of the flange section. Because the sufficiently large volume is provided, the degree of pressure increase in the volume can be moderated, which permits the piston section  13  to be smoothly moved downwardly even at the terminal phase of the fastening operation. 
   Further, since the rotation slide member  7  including the projections  8  and the shielding surface  14  is integrally molded with the elastic material, sealing performance relative to the plate section  15  can be improved to ensure the stop of the pneumatic motor  2 . Furthermore, the O-ring  46  is assembled at the outer peripheral surface of the main piston  21  at such a position between the piston hole  39  and the air vent hole  16  when the main piston  21  has reached the bottom dead center. Therefore, compressed air is supplied to the pneumatic motor  2  through the air vent hole  16  only through the gap between the rotation slide member  7  and the rotary member  6  (only through the second route) near a terminal phase of the screw driving operation. This ensures stop of the pneumatic motor  2  only by the abutment of the rotation slide member  7  against the plate section  15 . 
   If the operation valve  30  is released, compressed air in the rotary member  6  will be discharged to an atmosphere, and the compressed air in the return chamber  20  passes through the compressed air introduction hole  24  and is applied to the bottom face of the main piston  21  because the outer diameter of the bottom end of the main piston  21  is slightly greater than the outer diameter of the abutment projection  50 . 
   In accordance with the movement of the main piston  21 , air shielding between the main piston  21  and the piston bumper  31  becomes invalid, so that the compressed air from the return chamber  20  will be applied to the lower side of the piston section  13 . Therefore, the piston section  13  and the driver bit  11  are returned to their original positions when the internal pressure within the rotary member  6  becomes lowered. Simultaneously, a subsequent screw  18  is fed to a position in alignment with the driver bit  11  by the screw feeder  19 , and then the main piston  21  and the shaft  9  return to their initial positions. 
   A pneumatically operated screw driver according to a second embodiment is shown in  FIG. 7  wherein like parts and components are designated by the reference numerals added with “100” to the reference numerals of the corresponding parts in the first embodiment to avoid duplicating description. The second embodiment pertains to a modification described in U.S. Pat. No. 6,026,713 which is incorporated by reference. 
   In the second embodiment, a single piston  113  is provided instead of the combination of the main piston  21  and the auxiliary piston  9 . Further, similar to the first embodiment, a rotary member includes a plastic main rotary member  106 A, the sintered metal member  152 , and the washer  154  made from a metal such as steel or copper. The main rotary member  106 A has a lower edge formed with two grooves. The sintered metal member  152  is formed with minute oil retaining holes, and is fixed to the bottom surface. That is, the sintered metal member  152  has two projections each engageable with each groove. The washer  154  is fixed to a bottom of the sintered metal member  152 . 
   Further, a rotation slide member  107  is entirely made from an urethane rubber, and is equipped with an O-ring  160  on its outer cylindrical surface. The O-ring  160  is adapted to seal the upper end of the inner wall of a cylinder  112 . More specifically, the O-ring  160  prevents the compressed air within the cylinder  112  from being leaked into the air vent hole  116  at the time of completion of the screw fastening. 
   A shaft  109  has an upper end connected to the rotation slide member  107 . The shaft  109  has an enlarged lower portion having an inside space serving as a driver bit holder  140  for holding a driver bit  111 . The lowermost end of the enlarged lower portion of the shaft  109  serves as a piston  113 . A seal ring  113 A is provided on an outer cylindrical surface of the piston  113 . With this seal ring  113 A, the piston  113  is hermetically coupled with the inside wall of the cylinder  112 . The piston  113  is slidable in the axial (i.e., up-and-down) direction along the inside wall of the cylinder  112 . 
   A ventilation passage  107   a  extends across the rotation slide member  107  from the upper surface to the lower surface along the gap between the rotation slide member  107  and the shaft  109 . An O-ring  161  is provided at the lower end of the ventilation passage  107   a . The O-ring  161  acts as a one-way valve. Compressed air flowing manner is described in detail in the U.S. Pat. No. 6,026,713 which is incorporated by reference. 
   While the invention has been described in detail with reference to specific embodiments thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the invention. 
   For example, in order to avoid excessive wear of the projections  8 , the projections can be made by separate segments made from a metal such as steel and aluminum or high hardness plastic material. Even in the latter case, the grooves  10  of the rotary member  6  made from the plastic material does not cause frictional wearing, because the projections  8  is not in rotational sliding contact with the rotary member  6 , but is in axial sliding contact therewith whose sliding speed is excessively lower than that of the rotational sliding contact. 
   Further, in the depicted embodiment, the plate section  15  is provided integrally with the cylinder  12 . However, the plate section can be provided integrally with the body as long as the shielding surface  14  can be brought into abutment therewith. 
   Furthermore, in the depicted embodiment, the main rotary member  6 A is formed with recess  51  and the sintered metal member  52  is provided with projection  53 . However, the main rotary member can be provided with a projection and the sintered metal member  52  can be formed with a recess.