1. Field of the Invention
This invention relates to a sheet supply system, and more particularly to a sheet supply system which feeds out sheets stacked on a sheet support table one by one from the sheet support table.
2. Description of the Related Art
For example, in a stencil printer, a number of printing papers are supplied to a printing system one by one without lost feed (referring to malfunction that no printing paper is supplied) or multiple feed (referring to malfunction that a plurality of printing paper are supplied at one time).
In the stencil printer, a number of copies are often printed from one original, and accordingly a number of printing papers are stacked on a paper support table. In such a case, it is preferred that the uppermost printing paper in the stack on the support table be constantly positioned at a predetermined level (will be referred to as "the paper feed-out level", hereinbelow) so that a paper feed-out system can act on the uppermost printing paper constantly in an optimal manner. For this purpose, the paper support table is moved upward as the level of the uppermost printing paper in the stack remaining on the paper support table lowers.
FIG. 8 shows an example of a conventional paper supply system for a stencil printer. In FIG. 8, reference numeral 51 denotes a paper support table on which a stack of printing papers P is supported. The paper support table 51 is moved up and down by a table drive means (not shown). The printing papers P are fed out from the paper support table 51 one by one by a paper feed-out unit 53. The paper feed-out unit 53 comprises a pair of paper feed rollers (a scraper roller 54a and a pick-up roller 54b) which are mounted for rotation on one bracket and are rotated in synchronization with each other. The paper feed-out unit 53 is rotatable about a shaft 53a and is moved upward when the paper support table 51 is moved upward and the uppermost printing paper in the stack of the printing papers P pushes upward the scraper roller 54a. The shaft 53a is urged by a spring means (not shown) so that the contact pressure between the scraper roller 54a and the uppermost printing paper increases as the paper feed-out unit 53 is lifted higher.
A limit switch 52, which may be, for instance, a microswitch or a photo-interrupter, is disposed to be opened when the paper feed-out unit 53 is moved upward to a predetermined level. That is, when paper support table 51 is moved upward to push the paper feed-out unit 53 upward and the limit switch 52 is opened, the table drive means is de-energized and the paper support table 51 is stopped at the level. When the printing papers P on the support table 51 are fed out from the paper support table 51 and the level of the uppermost printing paper lowers, the limit switch 52 is closed again and the table drive means is energized to move upward the paper support table 51. In this manner, the uppermost printing paper is constantly kept at the level at which the limit switch 52 is opened, thereby keeping constant the contact pressure between the uppermost printing paper and the scraper roller 54a during paper supply operation of the paper supply system. The level of the uppermost printing paper at which the limit switch 52 is opened is the aforesaid "paper feed-out level".
However the conventional system is disadvantageous in that since the paper feed-out level is controlled by opening and closure of the limit switch 52, the accuracy in positioning the uppermost printing paper at the paper feed-out level is governed by the sensitivity of the limit switch 52, which fluctuates from switch to switch and changes with time. When the paper feed-out level fluctuates, the contact pressure between the uppermost printing paper and the scraper roller 54a fluctuates, which causes the aforesaid lost feed or the aforesaid multiple feed. Further, it takes a movement of the paper support table 51 through a certain distance to open and close the limit switch 52. The distance is generally larger than the thickness of one printing paper and is generally equal to triple of the thickness of the printing paper. Accordingly, the level of the uppermost printing paper is adjusted only once per three printing papers, which results in fluctuation in the contact pressure during supply of the three printing papers.
There are used various types of printing papers which are different in thickness, quality and the like. The contact pressure optimal to feed the printing papers one by one differs according to the thickness, quality and the like of the printing paper. Thus there has been proposed, as disclosed in Japanese Unexamined Patent Publication No. 8(1996)-259099, a paper supply system in which the contact pressure between the uppermost printing paper and the scraper roller (i.e., the paper feed-out level) can be changed. In the paper supply system, the position of the limit switch 52 relative to the paper feed-out unit 53 is changed in two steps by operation of a lever so that the contact pressure between the uppermost printing paper and the scraper roller 54a at the level of the uppermost printing paper at which the limit switch 52 is opened is changed in two steps. When thick printing papers are to be supplied, the contact pressure is increased and when thin printing papers are to be supplied, the contact pressure is reduced.
Also this system cannot be free from the aforesaid drawbacks inherent to the limit switch and since the paper feed-out level is changed by changing the position of the limit switch relative to the paper feed-out unit, an additional mechanism is required, which adds to the manufacturing cost of the system.
Further printing papers of different thicknesses can be sometimes stacked on the paper support table, and accordingly there is a demand for a paper supply system which can automatically change the paper feed-out level according to the thickness of the printing paper.