Sheet Feedinq Apparatus
This invention relates to a sheet feeding apparatus which is particularly, although not exciusively, useful In a xerographic copying machine or printer.
Where sheets are arranged in a stack it is conventional to feed them one at a time from the stack using a roller feed arrangement located In the feedhead of the apparatus. The sheets are stacked on a stack support, for example an elevating tray, movable between a lower, sheet loading, position in which sheets can be loaded on or unloaded from the tray, and an upper, sheet feeding, position in which the topmost sheet In the stack engages the feedhead so that sheets can be fed from the tray. It is difficult to feed the sheets one at a time since there is a tendency for sheets to feed together due to frictional forces between them.In order to alleviate this problem It Is weil known to use an arrangement of the kind known as a retard feeder in which the sheets are advanced into the nip between a driven sheet advancing member, suitably a roller, and a stationary friction member known as a retard member, which coact such that a first sheet in contact with the sheet advancing member is advanced through the nip by driving the advancing member, other sheets in the nip area being retarded by the stationary retard member
A known sheet retard feeder of this kind includes a nudger roller and a feed roller. in operation sheets are fed from a stack which is brought, by elevating the paper tray, into the feeding position.The top sheet in the stack is engaged by the nudger roller, which on rotation feeds the top sheet towards the nip formed between the cueed roller and a friction retard pacl Frictional forces and static electricity between the sheets of paoer In the stack may cause several sheets to move Into the nip together If several sheets of paper approach the nip together, the friction between the retard pad and the bottom sheet of those being fed is greater than thdr between two sheets he friction between the feed roller and the top sheet is also greater than the iricrion between two sheets The group of sheets being ed towards the nip will therefore tend to become staggered on the surface of the retard pad into the nip, until the lower sheet of the top two sheets is retained by the retard pad, while the topmost sheet is fed by the feed roller
A feedhead of the kind just described, containing the feed and nudger rollers, is typically mounted so that its rollers rest under the weight of the feedhead on the top sheet of a stack
In one known form of feedhead, the rollers are mounted in the central region of an elongate housing which extends the full width of the widest sheets to be fed The drive shaft for the feed roller is mounted In the housing, and is driven from 3 suitable drive System, usually powered by an eletr;;c motor, In the macnine containing the sheet feed apparatus This known kind of feedhead usually works well with sheet feeders assocated ivith the @@@ of sheet tra@s@ound r @e@ographic copiers containing @@ to 500 sheets @@ nowe@er this @nd of sheet feede@ @@se@ in situations where the sheets tend to cu@@ @@ a@ the edges @as might be encountered with certain types of paper in particular conditions of temperature or humidity) the sheet edges may push up on the extremities of the underside of the feedhead housing thereby pushing the feed and nudger rollers up out of contact with the stack and preventing sheet feeding. One way to overcome this problem is to put an upstanding rib in the central region of the tray base, thereby biasing the centre of the stack upwards. The problem is more likely to occur in a high capacity sheet feeder, which might typically contain 2000 or more sheets. The slightest tendency towards sheet curl is magnified over a large stack, and can produce a strong upward force at the edges of the stack. The use of a central rib in the tray base is virtually ineffective for a large sheet stack.
It is an object of the present invention to overcome this problem.
According to the present invention, there is provided a sheet feeding apparatus for feeding sheets from the top of a stack of sheets, comprising a nudger roller, and a feed roller mounted on a drive shaft within a support housing, the nudger roller being urged, in use, against the top sheet in the stack for feeding the top sheet off the stack towards the feed roller on rotation of the rollers, characterised in that the rollers and shaft are so shaped and arranged, and the underside of the housing is so shaped, as to provide space to accommodate upcurl of the edges of sheets in the stack.
A sheet feeding apparatus in accordance with the invention will now be descnbed, by way of exampie, with reference to the accompanying drawings, in which:
Figure 1 is a diagrammatic perspective view of a high capacity sheet feeder of the kind used to supply sheets to a xerographic copier or printer and incorporating the present invention;
Figure 2 is a diagrammatic cross section through the retard feeder of a feedhead incorporating the invention;
Figure 3 Is a perspective view of the feedhead incorporating the invention, with the feedhead inverted from its operating position so as to show its underside
Referring to Figure 1, a sheet feeding apparatus 10 includes a sheet stack container which comprises vertical walls 11 and 12, and an elevating tray base 13.A stack of sheets 14 is loaded onto the base 13, which then raises the stack until the top sheet engages a feed roller (not shown) mounted on the underside of a feedhead 15. The feedhead 15 is pivotally mounted in walls 11 and 12 so that the feed roller rests under the weight of the feedhead on the top sheet in the stack. On rotation of the feed roiler, the top sheet is fed out of the container in the direction
Indicated by arrow 16, i.e. generally horizontally in a direction parallel with the walls 11 and 12.
he fed sheet is then received by sheet feeding devices in a machine (not shown) in which the sheet feeder 10 is located, or to which it its attacned he machine which receives the feo sheet may be any vise!! known kind of printer or cooler. sucn as a xerographic printer or cooler vVhen It is desires to ioad the container, the base 13 is lowered to the bottom of vva;:s n '.2. ara exa.'r'pe n response to the opening of a cover or door of the container he stack is manoe,red : :rto OOSitiOfl on the base 3. the cimensons of 'he container being sucn that the stack is a loose fit between the walls 11 and 12. Once the stack has been properly loaded, and moved as far as it will go in the container in the direction of arrow 16, the base 13 may be raised using a motor (not shown) in well known fashion so as to bring the top of the stack into engagement with the feed roller beneath the feedhead 15. As the stack approaches the feedhead 15, its rearward edge (as viewed in Figure 1) encounters a pusher assembly 17.Two pusher members in the pusher assembly 17 urge the topmost part of the stack towards the front of the container as viewed in Figure 1, thereby registering the topmost sheets against a registration edge provided by the front wall 12.
Referring now to Figure 2, a nudger roller 20 and a feed roller 21 are mounted for rotation in the the housing of the feedhead 15. The nudger roller 20 moves the topmost sheets in stack 14 towards the feed roller 21, and the feed roller 21 co-operates with a retard pad 22 to feed the sheets one at a time to the right as viewed in Figure 2.
Referring now also to Figure 3, the feed head 15 Is pivotally mounted between the palls 11 and 12 by stub shafts like shaft 23. The nudger roller and feed roller are located forward (in the sheet feed direction 16) of the pivot axis of the feedhead, and rest under their own weight on the top of the sheet stack and the retard pad. A counterbalance weight, to regulate the downward force applied on the stack by the rollers, is provided within extension piece 24 to the rear of the feedhead. The feed roller 21 is driven by a shaft 25 which extends from the roller to an end portion 26 beyond one end of the feedhead. The end portion has a flat on it (so as to form a substantially D-shaped cross section) and is received in a coupling member (not shown) driven by a source of power such as an electric motor.The shaft 25 is contained within a channel in the houslng of the feedhead, and is of small diameter so as to intrude as little as possible Into the space below it (above it as viewed in Figure 3). The shaft is not enclosed towards its outer end, so that there is no enclosure to intrude into the space below it.
On the two sides of the rollers 20 and 21 (in the direction of their axes of rotatlon), the housing of the feedhead is formed Into two chambers, or hollow portions, 27 and 28 These charnbers are shaped to provide maximum depth ;n regions 29, 30 towards the extremities of the feedhead, and towards its rear(in the sheet feed direction 16). In use of the feedhead, regions 29, 30 are substantially horizontal, and accommodate any upcurl of the sheets at the edges of the stack. As can be seen from Figure 2, the front edge of the sheet stack is substantially as far forward as the dividing portion 31 of the feedhead housing between the two rollers.As a sheets fed forward by the nudger roller 20, it is deflected downwards by ramped portions 32 and 33 of the chambers 27 and 28, so that the leading edge of the fed sheet passes below the shaft 25, possibly contacting and sliding over the shaft 25 If there is any upcurl. The cross sectional shape of the ce:':::ngs of chambers 27 ancl 28 are best seen in the region of the intersection, indicated at 3z, between the substannally vertical end wall 35 of the feedhead and the regions 30 and 33 of the feedhead @ousing @@e wa@ 35 and @ counterpart 36 at the other end of the feedhead, provide sheet guides or@@e edges of sheets or maximum size being fed
Once a single sheet has been separated from the stack, of course, the sheet edges, even if tending to curl up, can be easily deflected into the plane of the desired sheet path.
Using the arrangement described, a considerable degree of sheet edge upcurl can be accommodated.