EP0099248A2

EP0099248A2 - Sheet stacker

Info

Publication number: EP0099248A2
Application number: EP83303969A
Authority: EP
Inventors: Robert William Hughes; Harvey Graham Martin
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1982-07-07
Filing date: 1983-07-07
Publication date: 1984-01-25
Also published as: EP0099248A3; JPS5926855A

Abstract

A sheet stacker in which the motion of sheets entering the stacker is damped by one or more rollers (5) arranged to lie on top of the stack being compiled on a support surface (1), which roller (5) is rotatable only in the direction corresponding to that which sheets enter the stacker. In this way sheets are prevented from bouncing back from an end registration stop (2) against which they are registered so as to improve registration of the stack.

Description

Sheet Stacker

This invention relates to sheet stackers and particularly to those in which sheets are collected on a support surface against a registration stop arranged in the path of sheets entering the stacker.
Sheet stackers are used in many situations where sheets are fed out for collection, for example in printing, photocopying or duplicating machines. The stacker, which is generally arranged to receive sheets from the exit rollers or chute of the machine, may take various forms. It may for example be a simple catch tray or it may include a stack support surface which can be elevated so as to keep the top of the stack being formed at a substantially constant height. Also it may take the form of a compiler station for a finishing apparatus in which sheets are compiled into sets and the sets further treated by being stapled or punched.
In order to register sheets entering the stacker, they are generally fed against a registration stop arranged in the sheet path. In order to achieve successful stadking the paper must come to rest with its leading edge against the registration stop. Without some form of control, some sheets will have too much energy when they strike the registration stop so that they will bounce back along the support surface. At the same time, it is necessary that sheets with lower energy not be prevented from reaching the registration stop. In order to overcome the tendency of sheets to bounce back from the registration stop plastic fingers have been provided which lie on the top of the stack. These, however, tend to provide resistance against the sheets travelling in the forward direction as well as in the reverse direction and can impede sheets with low energy.
According to the present invention damping means is provided which comprises a roller which contacts the upper surface of a sheet being stacked and is rotatable only in the direction corresponding to movement of a sheet towards the registration stop. Because the roller is able to rotate in the forward direction of the sheet it provides little resistance but by being prevented from rotating in the opposite direction it positively inhibits the reverse motion of the sheet.
The frictional resistance to sheet movement of the peripheral surface of the roller should be sufficient to inhibit sheet bounce in the expected range of sheet entry speeds and greater than the sheet to sheet resistance. It will be realised, however, that this resistance is dependent upon the downward force applied by the roller as well as the coefficient of function of its peripheral surface. For sheets travelling at high speeds it may be desirable that the roller exert a retardation force on sheets travelling towards the registration stop so as to remove some of the energy therefrom before they strike the registration stop. The coefficient of friction of the roller surface should be as high as possible and a figure of between 1.0 and 2.5 has been found acceptable in practice. For sheets travelling into the stacker at 1 metre/sec., it has been found that the force applied by the roller is suitably in the range of 8 grams to 12 grams, preferably 10 grams.
It has been found that sideways movement of the sheets is also reduced as they are stacked so improving lateral registration as well. Further, by arranging the roller so that the lead edge of a sheet is under it before its trail edge leaves the transport nip by which it is driven into the tray, the motion of the sheet is continuously controlled.
The reverse rotation of the roller may be prevented by any suitable mechanism such as a pawl and ratchet or a one-way clutch, e.g. a Torrington clutch.
Suitably the roller is mounted on an arm pivotally supported over the support surface and extends from it mounting downstream in the direction of sheet delivery. In a preferred form, two rollers arranged side-by-side are provided.
The support surface suitably slopes downwardly towards the registration stop and if desired the elevation of the support surface may be automatically adjustable in dependence upon the height of the stack.
In order that the invention may be more readily understood, reference will now be made to the accomanying drawings, in which:-

Figure 1 is a schematic side elevation of a sheet stacker according to the present invention,
Figures 2 and 3 are scrap views of the stacker shown in Figure 1 illustrating the manner of operation of the damping device,
Figures 4 and 5 are scrap views of a second embodiment of stacker according to the invention, and
Figure 6 is an end view of the damping device of the stacker shown in Figures 4 and 5,

Referring to Figures 1 - 3, there is shown a sheet stacker in which sheets are collected on a support surface against a registration stop 2 arranged across the path of sheets S which enter the stacker through nip rolls 3, 4 which serve to drive sheets into the stacker and may form part of the stacker or be the exit rolls of a photocopier or like machine in which sheets are processed.The support surface 1 may slope downwardly towards the registration stop.
In accordance with the invention the stacker includes sheet damping means comprising a roller 5 which is mounted on the end of an arm 6 which is pivotally mounted at 7 to a frame 8 so as to project from the mounting 7 downstream in the direction of sheet travel into the tray and arranged so that the roller overlies the support surface to contact the upper surface of a sheet being stacked. The arm 6 is freely pivotally mounted so that the roller 5 rests on the top of the stack already formed and the sheets entering the stack are driven underneath the roller 5. The roller 5 is freely rotatable on an axle 9 in the direction corresponding to movement of a sheet S towards the registration stop i.e. in the direction in which it enters the stacker. However the roller is prevented from rotating in the opposite direction so that a sheet S which strikes the end stop 2 is prevented from bouncing back and becoming mis-registered. This is illustrated in Figures 2 and 3.
In the embodiment illustrated in Figures 1 to 3 the roller 5 is prevented from rotating in the reverse direction by a one-way clutch, such as a Torrington clutch, embodied in the hub 10 of the roller. As shown in Figure 2 as the sheet travels forwards beneath the roller 5 in the direction of arrow A the clutch is disengaged and allows free rotation of the roller. When, however, the sheets attempt to move in the reverse direction B (Figure 3) the clutch locks and prevents rotation of the roller 5.
In order to provide sufficient frictional resistance to sheet movement to prevent bounce-back the roller 5 suitably has a tyre 5a made of natural or synthetic rubber. The coefficient of friction of its peripheral surface should desirably be between 1.0 and 2.5. Slippage between the sheet and the roller also depends to some extent on the downward force applied by the roller against the sheet. This force may be kept to a minimum in order to minimise the resistance to forward sheet movement and may be between 9 grams and 11 grams. In a preferred embodiment the tyre 5a is made of silicone elastomer having a coefficient of friction of 1.4 and applying a downward force of 10 grams.
In a second embodiment as shown in Figures 4 to 6, rotation of the roller 5 in the reverse direction is prevented by means of a pawl and ratchet device 11, 12. As shown, the ratchet 11 is mounted for rotation with the roller 5 and the pawl 12 is pivotally mounted on the arm 6 so as to engage the ratchet 11 and prevent rotation of the roller in the reverse direction (anticlockwise as shown) only. In the embodiment illustrated the roller 5 is formed by a tyre 5a of rubber or the like, e.g. silicone elastomer, which is a friction fit over the ratchet 11 itself.
In order to minimise the resistance to forward motion of the sheets by the roller it preferably has a low friction bearing. This is formed by the ratchet itself which is made of a low-friction material running on a small shaft diameter. A suitable material for the ratchet is Delrin.
While a single, centrally located, roller 5 may be adequate it is preferred that a pair of rollers mounted on separate . arms be provided in side-by-side relationship.
A damper according to this invention has been found also to reduce sideways movement of sheets as they are stacked because the tyres do not permit sideways motion and because the system is, as illustrated, set-up so that the sheets S are concurrently in the transport nips 3, 4 and under the roller or rollers 5 so that the paper is continuously under control.
Whilst specific embodiments of the present invention have been described above it will be understood that various modifications may be made to the specific details referred to herein without departing from the scope of the invention as defined in the appended claims. Thus, while specific means for preventing rotation of the rollers in the reverse direction have been described it will be understood that any suitable form of one-way mechanism may be employed. Further, the registration stop may be retractable so that a set of sheets compiled on the support surface can be ejected. Also, the stacker may incorporate a binding or other finishing device such as a stapler or stitcher. Instead of a pivotally mounted pawl, a leaf spring fixed to the damper arm may engage the ratchet to prevent the reverse rotation.
Further, while it has been mentioned that the resistance to sheet forward motion by the dampers should be kept to a minimum, where all the sheets are travelling at relatively high speeds, e.g. 1 metre/sec., the dampers may be used to reduce the forward speed of the sheets before they strike the end stop.

Claims

1. A sheet stacker including a roller (5) arranged to lie on top of a stack of sheets (S) being compiled, said roller (5) being rotatable only in the direction corresponding to that in which sheets enter the stacker.

2. A sheet stacker in which sheets are collected on a support surface (1) against a registration stop (2) arranged in the path of sheets (S) entering the stacker, including sheet damping means (5) arranged to overlie the support surface to contact the upper surface of a sheet being stacked, characterised in that said sheet damping means (5) comprises a roller rotatable only in the direction corresponding to movement of a sheet (S) towards the registration stop.

3. A sheet stacker according to claim 2, in which the roller (5) is mounted on an arm (6) pivotally supported over the support surface (1), the pivot mounting (7) of the arm (6) preferably being upstream in the direction of sheet entry of the roller (5).

4. A sheet stacker according to claim 2 or 3, in which the support surface (1) slopes downwardly towards said registration stop (2).

5. A sheet stacker according to any of claims 2 to 4, in which the elevation of the support surface (1) is automatically adjustable in dependence upon the height of the stack.

6. A sheet stacker according to any preceding claim, in which the roller (5) has a tyre (5a) of natural or synthetic rubber.

7. A sheet stacker according to any one of claims 1 to 6, including a pawl and ratchet (11,12) for preventing reverse rotation of said roller (5), said roller preferably comprising a tyre (5a) mounted on said ratchet.

8. A sheet stacker according to any of claims 1 to 6, including a one-way clutch, such as a Torrington clutch, for preventing reverse rotation of said roller (5).

9. A sheet stacker according to any preceding claim, in which the peripheral surface of the roller (5) has a coefficient of friction of between 1.0 and 2.5 µ, preferably 1.4 µ, and in which the roller (5) preferably applies a downward force against a sheet (S) beneath it of between 8 grams and 12 grams, preferably 10 grams.

10. A sheet stacker according to any preceding claim, including two said rollers (5) arranged in spaced side-by-side relation.