1.  Field of the Invention
The present invention relates to a sheet-fed printing press that is capable of selectively printing on a single side or both sides of a sheet, and more particularly, a cam-surface switching mechanism that is capable of selectively switching the cam surface, through which the driving of turn-over grippers on a sheet turn-over drum is controlled for switching to a single-side printing operation or a both-side printing operation. 
2.  Discussion of the Background
A sheet-fed printing press of the type is known in, for example, Japanese Examined Patent Publication (Kokoku) No.  SHO-56-2017. 
In the sheet-fed printing press according to the above-cited publication, a sheet transfer unit, as illustrated in FIG.  8, is provided between an upstream printing unit and a downstream printing unit so as to transfer sheets 1 from an impression cylinder (not shown) of the former to an impression cylinder 4 of the latter.  Aligned in sequence from the upstream to the downstream of the sheet transfer unit are a transfer drum (not shown), a storage drum 2 and a turn-over drum 3.  The sheets 1 printed at the upstream impression cylinder are transferred to the turn-over drum 3 via the transfer drum and the storage drum 2. 
The turn-over drum 3 includes turn-over grippers 7 adapted to grip the leading or trailing edge of each sheet 1.  The turn-over grippers 7 are constructed so that the sheet 1 is gripped through the associated operation of two gripping members 8 and 9 in each turn-over gripper 7.  The thus constructed turn-over grippers 7 are pivotable around the pivoting axis extending parallel to the rotational axis of the turn-over drum 3, and have different functions respectively for a single-side printing operation and a both-side printing operation. 
Specifically, in the single-side printing operation as illustrated in FIG.  9, the turn-over grippers 7 grip the leading edge of the oncoming sheet 1 at a tangential point A or a sheet receiving point between the storage drum 2 and the turn-over drum 3, enabling the turn-over drum 3 to receive the sheet 1 from the grippers 5 of the storage drum 2.  The turn-over grippers 7 then rotate along with the turn-over drum 3 with maintaining their sheet gripping states, and transfer the sheet 1 to the impression cylinder 4 at a tangential point B or a sheet delivery point between the turn-over drum 3 and the impression cylinder 4.  Then, the turn-over grippers 7 are pivoted through a predetermined angle according to the vectors 81 before reaching the tangential point A to be brought into a position as illustrated in FIG.  9, at which they grip the oncoming sheet 1. 
On the other hand, in the both-side printing operation as illustrated in FIG.  10, following to the grippers 5 of each set on the storage drum 2 passing the tangential point A with maintaining their sheet gripping states, a corresponding sheet suction device 6 subsequently reaches the tangential point A at which the turn-over grippers 7 grip the trailing edge of the sheet 1, enabling the turn-over drum 3 to receive the sheet 1 from the storage drum 2.  The grippers are then pivoted about the pivoting axis through about 180 degrees downstream with respect to the rotational direction of the turn-over drum 3 for the turning-over of the sheet 1, and then reach the tangential point B relative to the impression cylinder 4.  The grippers 7 then deliver the sheet 1, which has been turned over, to the impression cylinder 4 at this point, and are pivoted through about 180 degrees towards upstream with respect to the rotational direction of the turn-over drum 3 to again have the free ends facing upstream at the tangential point A relative to the storage drum 2. 
The turn-over grippers 7 performing the opening and closing actions, and the pivoting action each have the first gripping member 8 and the second gripping member 9, as described above.  Specifically, the first gripping member 8 is firmly secured to a gripper shaft 10 having the common pivoting axis around which the grippers are pivoted, while the second gripping member 9 is firmly secured to a gripper tube 11 disposed coaxially on the gripper shaft 10 for supportingly receiving the gripper shaft 10.  Gears 12 are respectively mounted on a first end of the gripper shaft 10 and a second end of the gripper tube 11 in meshing engagement with segment gears 13 disposed on the axially opposite sides of the turn-over drum 3. 
The segment gears 13 are respectively supported on rotational pins 14 of the turn-over drum 3, and include cam followers 15 for driving the grippers.  On the inner sides of a pair of flames 17 aligned at the axially opposite sides of the turn-over drum for supporting a drum shaft 16 of the turn-over drum 3 are respectively secured control cams 18 each having a torus shape with a hole defined therein through which the drum shaft 16 extends.  Each control cam 18 has an outer circumference forming cam surfaces being engageable with a corresponding cam follower 15.  Hence, the rotation of the turn-over drum 3 causes the cam followers 15 to respectively travel around the outer circumferences of the control cams 18, as sliding along the corresponding cam surfaces.  Thus, the circumferential travel of the cam followers on the cam surfaces causes the segment gears 13 to travel around the rotational pins 14, so that the gripper shaft 10 and the gripper tube 11 are rotated via the gears 12, and hence the first gripping members 8 and the second gripping members 9 are controlled independently of each other. 
Since the motion of the turn-over grippers 7 during the both-side printing operation is different from that during the single-side printing operation, the outer circumference of each control cam 18 forms two different cam surfaces arranged adjacent to each other along the axis of the turn-over drum 3.  Specifically, the cam surfaces on each control cam 18 are respectively for the single-side printing operation and the both-side printing operation.  Accordingly, the cam surfaces to be engaged with the corresponding cam followers 15 are selectively switched so as to adapt the motions of the first and second gripping members 8, 9 to the respective printing operations. 
The switching of each cam surface is performed by manually displacing a corresponding cam follower 15 along the rotational center thereof, or along the axis of the turn-over drum 3, as represented in chain double-dashed line of FIG.  12.  The same switching operation is performed at the opposite sides of the turn-over drum. 
The thus arranged sheet-fed printing press requires a manual switching operation for the switching of the cam surfaces.  To omit such a troublesome operation, attempts have been made to automatically perform the switching operation as disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No.  HEI-07-24998.  Specifically, this citation proposes the arrangement that, instead of the cam followers 15, the control cams 18 are displaced in the axial direction of the turn-over drum 3, which arrangement will be discussed in detail below with reference to FIG.  13. 
The description will be made mainly for the switching mechanism at the right-hand side of the turn-over drum, since the switching mechanism of the opposite side of the turn-over drum is identical in structure thereto. 
The drum shaft 16 of the turn-over drum 3 is supported by the flame 17 via a cylindrical bearing 19 whose part inwardly protrudes from the flame 17 and has an outer circumference on which the control cam 18 is loosely mounted in such a manner as to be displaceable in the axial direction of the turn-over drum 3.  An air cylinder 20 for the displacement of the control cam 18 is disposed on the outer side of the flame 17 to generate the driving force in the vertical direction of the air cylinder 20 or the direction orthogonal to the axial direction of the turn-over drum 3.  The thus generated driving force is then oriented in the axial direction of the turn-over drum 3 via the camming mechanism, enabling the control cam 18 to be displaced in the axial direction of the turn-over drum 3. 
Since the air cylinder 20 is disposed opposite to the control cam 18 with the flame 17 therebetween, the control cam 18 is provided with two switching rods 21 extending through the cylindrical bearing 19.  The two switching rods 21 are respectively and slidably disposed within guiding holes formed in the bearing 19, and have leading ends provided with switching cam followers 22.  A pair of sliders 23 are also provided to be vertically moved by the air cylinder 20 via a link mechanism, and each forms therein a camming recess 24 extending slantingly with respect to the vertical direction for engagingly receiving a corresponding cam follower 22. 
Accordingly, the vertical movement of the air cylinder 20 causes both the sliders 23 to vertically move through a predetermined distance via the link mechanism, which movement in turn causes the switching cam followers 22 to be displaced in the axial direction of the turn-over drum 3 towards the inner side or the outer side.  Accordingly, the axial displacement of the control cam 18 via the switching rods 21 is caused, so that the cam surface to be engaged with the cam followers 15 is switched.  Thus, the sheet-fed printing press can have the cam surfaces automatically switchable by the actuation of the air cylinder 20. 
However, there causes somewhat movement or play between the switching rods 21 and the walls of the cylindrical bearing defining the guiding holes.  This play may cause a slight displacement of the control cam 18 at the time of the completion of the switching operation.  Specifically, the switching cam followers 22 are adapted to move in the axial direction of the turn-over drum 3 through the engagement with the camming recesses 24, so that the switching rods 21 may be displaced in the direction substantially orthogonal to the axial direction of the turn-over drum 3, and hence the control cam 18 may be displaced in that direction.  Although the two switching rods 21 together act as a stopper for preventing the rotation of the control cam, the slight displacement of the switching rod within the guiding holes also causes the control cam to be slightly displaced in the circumferential direction of the bearing 19.  In addition, since the cam followers 22 are disposed on the ends of the switching rods 21 having the lengthwise axis, the switching rods each may be bent into an arc. 
In any case, the printing press of the conventional type as described above may cause the slight displacement of the control cam 18 due to the switching operation of the cam surfaces, which poses a problem of disrupting the smooth driving of the turn-over grippers 7, and hence a high precision registration. 
In consideration of the, above problem, it is an object of the present invention to provide a sheet-fed printing press that is capable of achieving a high precision registration by limiting even a slight displacement of the control cam at the time of the completion of the switching operation involving the movement of the control cam for adapting the printing press either to the single-side printing operation or the both-side printing operation. 
According to one aspect of the present invention, there is provided a sheet-fed printing press capable of printing on both sides of a sheet that includes a turn-over drum provided with turn-over grippers for gripping the sheet, control cams respectively disposed on the axially opposite sides of the turn-over drum in such a manner as to be displaceable in the axial direction of the turn-over drum, and cam followers for driving the turn-over grippers respectively disposed on the axially opposite ends of the turn-over drum.  The control cams each have an outer circumferential surface forming thereon a cam surface for the single-side printing operation and a cam surface for the both-side printing operation, both the cam surfaces on each of the control cams being arranged adjacent to each other in the axial direction of the turn-over drum.  The cam followers each are designed to travel around the outer circumference of a corresponding one of the control cams as sliding along one of the cam surfaces through the rotation of the turn-over drum, thereby controlling the motions of the turn-over grippers.  The control cams are displaced in the axial direction of the turn-over drum, thereby enabling the cam followers to switch the cam surface to be engaged for switching to the single-side printing operation or to the both-side printing operation. 
The printing press further includes stationary guide means for guiding the control cams in the axial direction of the turn-over drum, switching cam followers respectively provided on the control cams in such a manner as to protrude radially outwardly therefrom, sliders respectively forming therein camming recesses to be respectively engaged with the switching cam followers, and a slider driving means for reciprocatingly driving the sliders in such a manner as to enable the switching cam followers to reciprocatingly travel in the axial direction of the turn-over drum. 
Accordingly, the switching cam followers each travel from a first end to a second end of a corresponding lone of the camming recesses along the longitudinal axis thereof for switching to the both-side printing operation, and each travel from the second end to the first end of the corresponding one of the camming recesses along the longitudinal axis thereof for switching to the single-side printing operation.  The first and second ends along the longitudinal axis of each camming recess respectively have end wails adapted to press a corresponding one of the switching cam followers, so that the control cams are respectively biased against the stationary guide means in the direction substantially orthogonal to the axial direction of the turn-over drum at the time of completion of the switching operation. 
According to the printing press having the above arrangement, when the printing press is to be switched from the single-side printing operation to the both-side printing operation, the slider driving means drives the sliders so that the switching cam followers each travel from the first end to the second end within the camming recess of the corresponding slider, and hence are displaced in the axial direction of the turn-over drum.  The displacement of the switching cam followers 22 in the axial direction of the turn-over drum causes the control cams integrally arranged with the switching cam followers to be displaced in the axial direction of the turn-over drum, while being guided by the stationary guide means, and hence the cam surfaces for the both-side printing operation to be brought into a position at which the cam followers for driving the turn-over grippers are engageable therewith.  In the state that the switching operation has been completed, the switching cam followers each are pressed via the end wall of the second end of the corresponding camming recess.  The pressure applied to the switching cam followers causes the control cams to be respectively biased towards the stationary guide means in the direction substantially orthogonal to the axial direction of the turn-over drum, thereby reducing movement or play between the stationary guide means and the control cams. 
Likewise, when switching to the single-side printing operation, the switching cam followers each are pressed via the end wall of the first end of the corresponding camming recess along the longitudinal axis thereof. 
In the printing press having the above arrangement, the stationary guide means preferably includes guide members respectively disposed on the axially opposite sides of the turn-over drum to respectively provide abutments each having a longitudinal axis extending straight in the axial direction of the turn-over drum.  The abutments each are adapted to be engaged with a corresponding one of the switching cam followers so that the control cams can be guided in the axial direction of the turn-over drum.  The switching cam followers each are preferably adapted to be clamped between a corresponding one of the end walls of a corresponding one of the camming recesses and the abutment of a corresponding one of the guide members in the clamping direction substantially orthogonal to the axial direction of the turn-over drum at the time of completion of the switching operation. 
According to another aspect of the present invention, there is provided a sheet-fed printing press capable of printing on both sides of a sheet including a turn-over drum provided with turn-over grippers for gripping the sheet, control cams respectively disposed on the axially opposite sides of the turn-over drum in such a manner as to be displaceable in the axial direction of the turn-over drum, and cam followers for driving the turn-over grippers respectively disposed on the axially opposite ends of the turn-over drum.  The control cams each have an outer circumferential surface forming thereon a cam surface for the single-side printing operation and a cam surface for the both-side printing operation, both the cam surfaces on each of the control cams being arranged adjacent to each other in the axial direction of the turn-over drum.  The cam followers each are designed to travel around the outer circumference of a corresponding one of the control cams as sliding along one of the cam surfaces through the rotation of the turn-over drum, thereby controlling the motions of the turn-over grippers.  The control cams are displaced in the axial direction of the turn-over drum, thereby enabling the cam followers to switch the cam surface to be engaged for switching to the single-side printing operation or to the both-side printing operation. 
The printing press further includes switching cam followers respectively provided on the control cams in such a manner as to protrude radially outwardly therefrom, sliders respectively forming therein camming recesses to be respectively engaged with the switching cam followers, a slider driving means for reciprocatingly driving the sliders in such a manner as to enable the switching cam followers to reciprocatingly travel in the axial direction of the turn-over drum, and guide members respectively providing abutments each having a longitudinal axis extending straight in the axial direction of the turn-over drum.  The abutments each are adapted to be respectively engaged with a corresponding one of the switching cam followers so that the control cams can be guided in the axial direction of the turn-over drum. 
Accordingly, the switching cam followers each travel from a first end to a second end of a corresponding one of the camming recesses along the longitudinal axis thereof for switching to the both-side printing operation, and each travel from the second end to the first end of the corresponding one of the camming recesses along the longitudinal axis thereof for switching to the single-side printing operation.  The first and second ends along the longitudinal axis of each camming recess respectively have end walls adapted to press a corresponding one of the switching cam followers, so that the switching cam followers each are clamped between one of the end walls of a corresponding one of the camming recesses and a corresponding one of the guide members in the clamping direction substantially orthogonal to the axial direction of the turn-over drum. 
According to the above arrangement, the switching cam followers travel along the camming recesses of the sliders, while being guided along the abutments of the guide members.  Whereby, the driven point is matched to the guided point, so that the control cams can smoothly be displaced in the axial direction of the turn-over drum.  In addition, since the switching cam followers each are clamped between the end wall of the corresponding camming recess and the corresponding guide member, even a slight movement of each control cam can be limited.