All of the multifarious machines that serve us in the process of placing words and images on paper, or doing things to papers already so decorated, have certain mechanisms in common. They have an input device or slot, an internal guide mechanism, a processing unit to do whatever it is that the machine does to the paper, and an output delivery. Some of them have external systems as well: in particular feed systems. This series of articles is all about the feed systems, internal and external, although the emphasis will be on external document feeders; in fact, unless otherwise stated, the discussion will be about external systems.
The internal guide mechanisms are interesting in their own right, but they function in a different environment and must serve different requirements. Except in crude brute-force machines such as shredders, the guide must maintain a precise speed of the paper through the machine and must keep the paper in a precise position ("registration") against the internal processing mechanisms. For machines such as laminators, the guide must also cope with unusual heat: in such cases there's a lowered requirement for registration accuracy (although it's still important) in favour of reliability of motion through the mechanism.
External document feeders perform a visible function. You load a pack of paper and watch it vanish into the machine, one at a time, either grandly at a rate of a few a minute or smoothly and swiftly in high-speed applications. An objection will occur to readers: you usually can't watch the paper flow in copiers. True; but for the purposes of the discussion these are considered to be external feeders, since they are a separate functional unit from the copier itself - and sometimes are physically apart from the copier.
There are all sorts of ways that feeders perform their function. Some are very simple roller drives, which start on demand and roll one piece of paper into the machine: obviously there's a neat timing problem to make sure that the roller stops before a second piece of paper is inadvertently fed in. Some use a "knife-blade" slide which picks one piece of paper at a time from the top of the pile with a spring-loaded or hydraulic mechanism to keep the top of the pile at the correct height; some systems use variable-pitch electric motors for this. Very high-speed machines may employ a vacuum feed: a single piece of paper is sucked from the pile and then delivered to the processing mechanism.
There is a great deal of good engineering and design that goes into these devices. These subjects are dealt with in the individual articles in this series.
Document Feeder Article Index