This invention relates to printing apparatus, particularly for highspeed rotary multicolour web printing as by the offset process in the production of newspapers and periodicals though the invention may have application to other types of single or multicolour web or sheet fed printing processes and apparatus.
It is known to provide printing apparatus having individual process units each driven mechanically independently by its own electric motor instead of coupling the units together by shafts, gearing etc to be driven in synchronism from a common motor. The individual motors are of a type which can be extremely accurately controlled electrically as to running speed and angular position of their drive output to provide synchronisation of the various units and accurate registration of their respective operations on the web or material being processed. This kind of drive arrangement is commonly referred to as xe2x80x9cshaftless drivexe2x80x9d and many examples are well known in the art such as in our GB 2149149A.
Although shaftless drive is successful in providing accurate and consistent results when the printing apparatus is running at its service speed, problems arise which have not hitherto been solved during changes in operating speed of the apparatus as a whole and, consequently, the linear speed of the web, due mainly to stretching or other distortion of the web consequent upon the speed change. This results in the operations carried out by the different units becoming out of register, for example registration of different colours in multicolour printing is lost giving rise to substantial wastage and loss of production time.
The apparatus has to be run up to speed on start-up and has to be run down again to close down giving wastage at the beginning and end of every print run.
The object of the invention is to provide printing apparatus having shaftless drive with all its consequent advantages and which is able to maintain accurate synchronisation and registration throughout substantial changes in overall running speed.
According to a first aspect of the invention there is provided drive means for printing apparatus of the kind comprising a plurality of process units each for effecting a respective repeat operation on web or sheet material fed along a feed path defined at least in part by said units; said drive means comprising an electrically powered rotary motor individual to each said unit or to one or more rotary elements within a unit, and control means operating to regulate the running speed and angular phasing of each said motor individually in relation to a predetermined master standard programmed in to the control means: characterised in that the control means further includes speed correction means for varying the relative phase angles of one or more of the motor drive outputs by a preselected factor proportionate to the overall running speed of the apparatus to maintain accurate registration relative to the material of the various operations carried out by said units throughout a substantial range of operating speeds.
The invention further resides in a method of operating printing apparatus incorporating shaftless drive including the steps of determining the extent of registration error caused by changes in overall operating speed taking place without any adjustment in relative speed of individual drive motors or changes in relative angular phasing of their drive outputs to provide an algorithm representing register error as a function of said operating speed, and applying that algorithm automatically to vary the relative phase angles of one or more of the motor drive outputs by a preselected factor proportionate to the overall running speed of the apparatus whereby registration is correctly maintained throughout a substantial range of operating speeds.