Abstract:
A cam-controlled power differential gear transmission for a sheet acceleration system, which is formed of a pregripper having an acceleration course determinable by a control cam driven at a single speed, includes two compensating masses pivotably supported diametrically opposite one another, and respective control cams assigned to the compensating masses, respectively, for controlling the pivoting motion thereof.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a cam-controlled power differential gear for a sheet acceleration system, which is formed of a pregripper having an acceleration course determinable by a control cam driven at a single speed. 
     In sheet-fed rotary printing presses, it is customary for a sheet oriented on a feeding table to be gripped by a sheet acceleration system formed as a pregripper, and accelerated up to processing speed. After transferring the sheet to a sheet transport drum, the pregripper is decelerated down to a standstill and then is reaccelerated in the direction of the feeding table, with a subsequent deceleration down to a standstill at the feeding table. Because of the cyclical motion of the sheet acceleration system over a machine cycle, forces of inertia are generated which create torques acting upon the control cam; these torques are superimposed upon the torques operative in the drive system and thus cause torque fluctuations which, in the final analysis, cause registration errors and doubling or double impressions, respectively, which consequently lead to defective quality or a rejection of the printed products. The torque fluctuations also cause increased wear of the printing press. 
     From the published German Patent Document DE 41 09 824 A1, a cam-controlled power differential gear has become known heretofore which minimizes the aforedescribed problems. To that end, four compensating masses are provided, offset from one another by 90°, which engage with a common compensating cam. Rollers respectively assigned to one compensating mass pass per period, i.e., one reciprocating pivoting motion of the pregripper, through two identical motion segments without a resting phase at a sheet transfer location on the feeding table. The length of the resting phase is in fact determined solely by the reversal point of the pregripper on the feeding table. This very severely restricts the free choice of a pregripper motion principle. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a cam-controlled power differential gear for a sheet acceleration system wherein the principle of motion for the pregripper is freely selectable. 
     With the foregoing and other objects in view, there is provided, in accordance with the invention, a cam-controlled power differential gear transmission for a sheet acceleration system, which is formed of a pregripper having an acceleration course determinable by a control cam driven at a single speed, comprises two compensating masses pivotably supported diametrically opposite one another, and respective control cams assigned to the compensating masses, respectively, for controlling the pivoting motion thereof. 
     In accordance with another feature of the invention, the control cams for controlling the pivoting motion of the compensating masses are disposed on a common axis, and the differential gear transmission includes another control cam for generating a pivoting motion of the pregripper, the other control cam being disposed on an axis parallel to the common axis. 
     In accordance with a further feature of the invention, the control cams for controlling the pivoting motion of the compensating masses and another control cam for generating a rocking motion of the pregripper are disposed on a common axis. 
     In accordance with an added feature of the invention, one of the control cams is an inner control cam, and the other of the control cams is an outer control cam. 
     In accordance with an additional feature of the invention, the control cams for controlling the pivoting motion of the compensating masses are fixed to a machine frame, and the other control cam for generating a pivoting motion of the pregripper is disposed in a rotatably drivable manner. 
     In accordance with yet another feature of the invention, the pregripper has a resting phase on a feeding table amounting to a machine angle φ=approximately 60°. 
     In accordance with yet an added feature of the invention, the compensating masses are able to execute a pivoting motion even during the resting phase. 
     In accordance with a concomitant feature of the invention, the power differential gear transmission includes control rollers, respectively, assigned to the control cams, each of the control rollers having a roller lever, and each roller lever having an abutment for bracing against a compression spring. 
     An advantage of the invention is that by the free choice of the principle of motion of the pregrippers, the resting phase upon sheet transfer from the feeding table can be given such a length that a calm sheet transfer is possible. 
     If an angle of approximately 60° is provided, the resting phase of the pregripper on the feeding table is very long, thus assuring a calm sheet transfer. 
     Pivotally supported compensating masses are provided at locations diametrically opposite one another. This provision achieves good balancing of the power differential gear transmission. 
     A compression spring provided between control rollers for the pivoting motion of the compensating masses improves the contact of the control rollers with the various control cams to which they are assigned. 
     In the second exemplary embodiment, provision is made for the control cam effecting the oscillating motion of the pregripper to be disposed on a common axis of the control cams effecting the pivoting motion of the compensating masses. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a cam-controlled power differential gear for a sheet acceleration system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic front elevational view of a first exemplary embodiment of a power differential gear transmission according to the invention; 
     FIG. 2 is a diagrammatic side elevational view, partly in section, of FIG. 1; 
     FIG. 3 is a view like that of FIG. 1 of a second exemplary embodiment of the invention; and 
     FIG. 4 is a view like that of FIG. 2 of the second exemplary embodiment of the invention shown in FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings and, first, particularly to FIGS. 1 and 2 thereof, there is shown therein a pregripper  1  of a sheet-fed rotary printing press which accepts a sheet  2  from a feeding table  3 , accelerates it, and then transfers it to a sheet transport drum  4 . An oscillating motion of the pregripper  1  is initiated by a control cam  6 , which has a common rotary axis  7  with the sheet transport drum  4 . The control cam  6  cooperates with a control roller  8 , which is connected to the pregripper  1  by a four-bar linkage  9 . A gear wheel  11  of the sheet transport drum  4  meshes with a gear wheel  12  of a power differential gear transmission  13 . The gear wheel  12  is rotatably supported on an axis  17  firmly fixed in side frames  14  and  16  of the printing press. The gear wheel  12  has, in an outer region thereof, two diametrically opposed bearing points  18  and  19 , each for a respective compensating mass  21 ,  22 . Each compensating mass  21 ,  22  has a respective roller lever  26 ,  27  supporting a respective control roller  23 ,  24 . The control roller  23  of the compensating mass  21  is in rolling contact with an inner control cam  29 , and the control roller  24  of the compensating mass  22  is in rolling contact with an outer control cam  31 . The center points of the control cams are located on the axis  17 . Two abutments  32  and  33 , each mounted on the respective roller levers  26  and  27 , receive a compression spring  34 , which assures contact of the control rollers  23  and  24  with the respective control cams  29  and  31  thereof. 
     In a second exemplary embodiment shown in FIGS. 3 and 4, provision is made for a control cam  41  for effecting a rocking motion of the pregripper  1  to be disposed rotatably drivable on the axis  17  of the differential gear  13 . The control cam  41  is thus seated jointly on the axis  17  of the differential gear transmission  13  with the control cams  29  and  31 , which are fixedly disposed on the frame  14 ,  16 , and is in rolling contact with a control roller  42  disposed rotatably on the end of a roller lever  43 . The roller lever  43  is secured to the pregripper  1  and pivots together with the pregripper  1  about a bearing point  44  fixedly connected to the frame. 
     During one motion cycle of the pregripper  1 , the gear wheel  12  rotates once about its own axis  17 . The contours of the control cams  29  and  31 , respectively, via the respective control rollers  23  and  24  and the respective roller levers  26  and  27 , generate a pivoting motion of the respective compensating masses  21  and  22 . The contours are selected so that a pivoting motion of the compensating mass  21  with respect to its bearing point  18  executes the same pivoting motion as the compensating mass  22  with respect to its bearing point  19 . 
     The control rollers  23  and  24 , in the views of FIGS. 1 and 3, respectively, are shown located at the end of the resting phase of the pregripper  1  on the feeding table  3 . The resting phase amounts to approximately φ=60°. 
     The compensating masses  21  and  22 , in the resting phase of the pregripper  1 , experience a motion relative to the gear wheel  12  due to the cam disks  29  and  31 , the gear wheel  12  rotating at a constant angular speed. Because the center of gravity S 21 , S 22  of the respective compensating masses  21  and  22 , however, is eccentric to the respective pivot points  18  and  19  of the masses, the motion is such that the kinetic energy of the compensating masses  21  and  22 , which is composed of rotational and translational energy, does not change in the resting phase of the pregripper  1 , and consequently no driving moment about the gear-wheel axis  17  is generated. 
     The rotational energy, and the translational energy as well, do vary as a consequence of the rotary motion of the compensating masses  21  and  22 , however, with a different sign, i.e., +or −, so that the sum of the change and thus the drive moment has a zero value. 
     The transfer point Ü of the sheet  2  from the pregripper  1  to the downline sheet transport drum  4  is located on the control cams  29  and  31  at approximately α=80° after the end of the resting phase. 
     An arrow in FIGS. 1 and 3 indicates the direction of rotation of the gear wheel  12  and thus of the power differential gear transmission  13 .