The present invention relates to a printing switching apparatus, arranged in a sheet-fed rotary press with a reversing mechanism capable of performing both single-sided printing and perfecting printing, for adjusting circumferential phases of upstream and downstream cylinders with respect to a reversing cylinder in switching between single-sided printing and perfecting printing.
Various sheet-fed rotary presses each capable of performing single-sided printing and perfecting printing have been proposed and put into practice along with a variety of printing techniques. An example of such a rotary press is disclosed in Japanese Utility Model Publication No. 4-34034 in which a transfer cylinder, a double-diameter cylinder having a diameter twice that of the transfer cylinder, and a reversing cylinder are arranged between an upstream impression cylinder and a downstream impression cylinder along a paper convey direction. The sheet-fed rotary press disclosed in this prior art has a fixed gear fixed at the end shaft of the reversing cylinder, a rotary gear coaxial with the fixed gear and phase-adjustable in a circumferential direction, and a single-sided printing reference pin free to be inserted into pin holes formed in the gears. When single-sided printing is to be performed, the reference pin is inserted into the pin holes of the fixed and rotary gears, thereby matching the phases of the fixed gear and the rotary gear in single-sided printing. When perfecting printing is to be performed, the reference pin is removed from the pin hole of the fixed gear, and the rotary gear is phase-adjusted with respect to the fixed gear by an almost the longitudinal length of a paper sheet. Because of such a reference pin, a paper sheet does not fail to be gripped between the double-diameter cylinder and the reversing cylinder. Therefore, waste paper or damage to the machine can be prevented.
However, in the above-described conventional printing switching apparatus for switching between single-sided printing and perfecting printing, if, in switching from single-sided printing to perfecting printing, an operator performs the switching operation while leaving the reference pin unremoved, the machine may be damaged. In addition, in switching from perfecting printing to single-sided printing, if the operator does not confirm that the reference pin is properly inserted into the pin hole of the fixed gear, the following problem is posed. If the fixed gear is phase-shifted from the rotary gear, the machine may be damaged at the time of start of single-sided printing.