JPH09278258A - Sheet bundle stacking method and stacker - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely stack bundles of sheets on a sheet stacking tray without generating any jamming. SOLUTION: A stacker is provided with a sheet stacking tray 13 onto which a sheet bundle SB is conveyed, a sheet bundle upper face position detecting means which is brought into pressure contact with the upper face of the sheet bundle SB on the sheet stacking tray 13 so as to detect its upper face position, and a drive controlling unit controlling an action of a screw shaft driving motor so that the upper face position of the sheet bundle SB on the sheet stacking tray 13 is positioned in the predetermined position. The sheet bundle upper face position detecting means is provided with a detection lever 33 which is faced to the upper face of the sheet bundle SB on the sheet stacking tray 13 and is brought into pressure contact with the upper face of the sheet bundle SB by its tip part, a solenoid 37 driving the detection lever 33 in the facing direction toward the upper face of the sheet bundle SB on the sheet stacking tray 13, and a photoelectric sensor 38 detecting movement of the detection lever 33 to the predetermined position.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stacker in which a sheet bundle on a sheet carrying-out tray is moved in parallel with a sheet mounting surface of the sheet carrying-out tray and stacked on a sheet stacking tray.

[0002]

2. Description of the Related Art In office machines such as copiers and printers that process a large amount of documents, it is common practice to bundle these stapled documents and sort them into a sorter or stack them on a stacker. ing.

When stacking a sheet bundle on a stacker, conventionally, after carrying out a stapled sheet bundle, the next sheet bundle is placed on the sheet bundle stacked on this sheet stacking tray from the sheet unloading tray side. In order to smoothly push out the sheet stack tray, the sheet stack tray is lowered according to the thickness of the sheet stack, and the sheet stack is stacked on the sheet stack tray one after another.

As described above, in the case of intermittently lowering the sheet stacking tray, conventionally, the sheet stacking tray is lowered by a preset lowering stroke based on the thickness of the sheet bundle fed into the sheet stacking tray, or It has been practiced that a projector and a light receiver of a photoelectric sensor are opposed to each other in the width direction of the sheet stacking tray and the descending amount of the sheet stacking tray is controlled based on the detection information from the photoelectric sensor.

[0005]

In the case of a stacker in which the sheet stacking tray is mechanically lowered according to the thickness of the sheet bundle, the thickness of the sheet bundle is measured every time the thickness of the sheet bundle changes. It is necessary to reset the lowering amount of the loading tray. In addition, when the lowering amount of the sheet stacking tray is not appropriate, the sheet bundle newly fed to the sheet stacking tray side is caught by the sheet bundle on the sheet stacking tray and jamming occurs, or the sheet discharge tray side There is a problem that the step difference between the sheet stacking tray side and the sheet stacking tray side becomes too large and the sheet bundle cannot be sent out smoothly, and jamming occurs.

On the other hand, in a stacker in which a photoelectric sensor is used to control the descending amount of the sheet stacking tray, the top sheet of the sheet stack located at the top of the sheet stacking tray is floated up by static electricity or the like. Or if it is curled, the sheet stacking tray will be lowered more than necessary, and the step between the sheet carry-out tray side and the sheet stacking tray side becomes too large, and the sheet bundle cannot be smoothly sent out, and the sheet bundle is There was a problem that jamming occurred by sliding down.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a sheet bundle stacking method capable of reliably stacking a sheet bundle on a sheet stacking tray without causing jamming as described above.

Another object of the present invention is to provide a stacker capable of realizing such a sheet bundle stacking method.

[0009]

According to a first aspect of the present invention, a step of moving a sheet bundle on a sheet unloading tray in parallel with a sheet mounting surface of the sheet unloading tray and stacking the sheet on the sheet stacking tray, Pressing the tip of the detection dog against the upper surface of the sheet bundle located at the top of the sheet stacking tray, and until the position of the tip of the detection dog pressed against the upper surface of the sheet bundle reaches a predetermined position. And a step of lowering the sheet stacking tray.

According to the first aspect, even if the topmost sheet of the sheet bundle is floated up by static electricity or curled, the tip of the detection dog is pressed against the upper surface of the sheet bundle. As a result, the sheet stacking tray is lowered so that the upper surface of the sheet stack located at the top of the sheet stacking tray and the sheet unloading tray have a predetermined step.

According to a second aspect of the present invention, a sheet stacking tray on which a sheet bundle on the sheet carry-out tray is carried in parallel to the sheet mounting surface of the sheet carry-out tray, and the sheet stacking tray can be raised and lowered. Tray elevating means for supporting, elevating and lowering driving means for driving the tray elevating means to elevate and lower the sheet stacking tray, and pressing the upper surface of the sheet bundle located at the top of the sheet stacking tray to press the sheet bundle. Sheet bundle upper surface position detecting means for detecting the upper surface position, and the raising / lowering drive so that the upper surface position of the sheet bundle on the sheet stacking tray becomes a predetermined position based on the information from the sheet bundle upper surface position detecting means. Drive control means for controlling the operation of the means, and the sheet bundle upper surface detecting means faces the upper surface of the sheet bundle on the sheet stacking tray. A detection dog whose end is pressed against the upper surface of the sheet bundle, a dog driving unit which drives the detection dog in the direction opposite to the upper surface of the sheet bundle on the sheet stacking tray, and the detection by the dog driving means. The stacker is characterized by having a detection unit for detecting that the dog has moved to a predetermined position.

According to the second embodiment, when the sheet bundle on the sheet carry-out tray is carried into the sheet stacking tray,
The dog driving unit of the sheet bundle upper surface position detecting means operates to press the leading end portion of the detection dog against the upper surface of the sheet bundle on the sheet stack tray. In this state, the drive control unit operates the elevating drive unit until the detection unit detects via the detection dog that the upper surface of the uppermost sheet bundle on the sheet stack tray has descended to the predetermined position. , Lower the sheet stacking tray.

When the detection dog moves to a predetermined position, the lowering of the sheet stacking tray is stopped, and in this state, the sheet bundle on the sheet unloading tray is carried in again onto the sheet bundle stacked on the sheet stacking tray. It

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION In the sheet bundle stacking method according to the first embodiment of the present invention, when the leading edge of the detection dog is at a predetermined position, the upper surface of the sheet bundle located at the top of the sheet stack tray and the sheet unloading It is desirable to have a step between the tray and the sheet mounting surface. In this case, it is effective that the step is within a range of 5 to 8 mm that can prevent the sheet bundle from slipping off.

On the other hand, in the stacker according to the second aspect of the present invention, the detection dog of the sheet bundle upper surface position detecting means is
It may have a lever shape in which the base end is pivotally supported. Further, the dog driving unit of the sheet bundle upper surface position detecting means includes a biasing means for biasing the tip end portion of the detection dog toward one side in a direction facing the upper surface of the sheet bundle, and a biasing direction opposite to the biasing force by the biasing means. In addition, it may have a solenoid for driving the tip of the detection dog. Further, the detection unit may have a photoelectric sensor having a light emitter and a light receiver, and a shutter attached to the detection dog so as to be positioned between the light emitter and the light receiver of the photoelectric sensor, In this case, it is effective that the shutter is disengaged from between the light emitter and the light receiver when the detection dog moves to a predetermined position.

The solenoid moves the tip portion of the detection dog in the direction opposite to the upper surface of the sheet bundle against the biasing force of the biasing means. For example, when the urging means urges the tip end portion of the detection dog away from the upper surface of the sheet bundle, the tip end portion of the detection dog is pressed against the upper surface of the sheet bundle by energizing the solenoid. Conversely, when the biasing means biases the tip of the detection dog against the top surface of the sheet bundle, the solenoid is energized to move the tip of the detection dog away from the top surface of the sheet bundle. Let

In any case, when the tip of the detection dog moves to a predetermined position, the shutter is disengaged from between the light projector and the light receiver, and the detection signal from the photoelectric sensor is output to the drive control means. At this time, it is desirable that the upper surface of the sheet bundle located at the top of the sheet stacking tray and the sheet placing surface of the sheet unloading tray have a step, and in this case, the step is within a range of 5 to 8 mm. This is effective from the viewpoint of preventing the sheet bundle from slipping off, and the detection dog of the detection unit may be set in this way.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a sheet processing apparatus incorporating a stacker capable of realizing the sheet stacking method of the present invention will be described in detail below with reference to FIGS. Although the sheet processing apparatus in this embodiment is attached to a copying machine, it can be naturally installed in a printing apparatus other than the copying machine.

The planar shape, side surface shape and front surface shape of the sheet processing apparatus 10 in this embodiment are shown in order in FIGS. That is, the sheet processing apparatus 1 in this embodiment
Reference numeral 0 denotes a finisher 11 which is connected to the copying machine C and processes a sheet (not shown) sent from the copying machine C into a predetermined sheet bundle S _B , and a stacker which sequentially stacks the sheet bundle S _B processed by the finisher 11. 12 and a pusher 14 that carries out the sheet bundle S _B processed by the finisher 11 to the sheet stacking tray 13 of the stacker 12.
And is incorporated. Further, this sheet processing apparatus 10
In the lower part of the
2 and a drive control section 15 such as a pusher 14 are housed, and the sheet processing apparatus 10 is operated by an operator via an operation button group 17 on an operation panel 16 provided at the upper end thereof.

As shown in FIG. 6 which schematically shows the internal structure of the sheet processing apparatus 10 and FIG. 7 which shows the schematic structure of the upper end portion of the sheet processing apparatus 10, the copying machine C is discharged above the finisher 11. Buffer tray 1 as a sheet carrying-out tray of the present invention.
The sheet conveying unit 19 that feeds the sheet 8 is incorporated.
Further, the buffer tray 18 is formed on the upper end of the frame body 20 projecting from the side of the finisher 11, and the sheet mounting surface 21 thereof is an inclined surface that rises to the left in the figure. A stapler 22 that bundles a plurality of sheets stacked over the buffer tray 18 and the sheet stacking tray 13 is provided on the lower end side of the buffer tray 18, that is, the right end side in the drawing.

In the present embodiment, a plurality of sheets stacked over the buffer tray 18 and the sheet stacking tray 13 are processed into a sheet bundle S _B by the stapler 22. It is of course possible to process the sheet bundle S _B by the well-known means.

A lifting screw shaft 23 extending in the vertical direction is rotatably mounted in the frame body 20, and a screw shaft drive motor 24 for driving and rotating the lifting screw shaft 23 is attached to the lower end of the lifting screw shaft 23. It is connected. Further, the base end of the sheet stacking tray 13 of the stacker 12 is slidably fitted to the tray guide bar 25 whose upper and lower end portions are supported in the frame body 20 in parallel with the lifting screw shaft 23. The lifting screw shaft 23 described above is screwed into the base end portion of the sheet stacking tray 13. The sheet bundle loading surface 26 of the sheet stacking tray 13 is an inclined surface that is parallel to the sheet loading surface 21 of the buffer tray 18 and that is inclined upward to the left in the drawing.

Accordingly, by operating the screw shaft drive motor 24 to drive and rotate the elevating screw shaft 23, the sheet stacking tray 13 is moved up and down along the finisher 11 while being guided by the tray guide rod 25. .

A buffer tray 18 and a sheet stacking tray 13 are provided between the sheet conveying section 19 and the stapler 22.
A pusher 14 is provided for completely pushing out the sheet bundle S _B that straddles the sheet stack tray 13 toward the sheet stack tray 13 side. The pusher 14 in this embodiment has a sheet bundle S _{B at the} leading end.
A pair of push-out rods 27 that can come in contact with the lower end side of the sheet, and a pair of front and rear rod guide blocks 28 that slidably hold the push-out rods 27 in parallel with the sheet bundle loading surface 26 of the sheet stacking tray 13. Seat width direction (in Fig. 7,
A rod drive motor 29 for driving a pair of push rods 27 arranged in parallel along a direction (perpendicular to the paper surface), and a power transmission mechanism for transmitting the driving force of the rod drive motor 29 to the pair of push rods 27. 30 and.

The power transmission mechanism 30 in this embodiment is attached to a rod drive motor 29 and is attached to a pair of push rods 27.
Drive pinion (not shown), an idle gear (not shown) located between the drive pinion and the pair of push-out rods 27 and meshing with the drive pinion, and a pair of push-up rods 27, one of which is driven. A rack (not shown) that meshes with the pinion and the other meshes with the idle gear.

Then, the push-out rod 27 advances from the retracted position shown by the solid line in FIG. 7 to the push-out position shown by the chain double-dashed line by the rod drive motor 29, so that the sheet bundle S _B processed by the finisher 11 is buffered. Tray 1
The sheet 8 is completely pushed out to the sheet stacking tray 13 side.

A sheet placing surface 21 of the buffer tray 18 is provided above the buffer tray 18 and the sheet stacking tray 13 so that an operator does not accidentally contact the tip end portion of the pushing rod 27 protruding above the buffer tray 18. A protective cover 31 for covering is provided.

In this embodiment, as shown in FIG. 1 showing the schematic structure of the inside of the protective cover 31 and FIG. 2 in which the main part thereof is extracted and enlarged, the upper surface position of the sheet bundle according to the present invention is placed inside the protective cover 31. It incorporates detection means. That is, the sheet bundle upper surface position detecting means in the present embodiment is provided with a pivot 32 fixed horizontally to the front end side of the protective cover 31 via a bracket (not shown), and a base end of which is pivotally supported by the pivot 32. The detection lever 33 as a detection dog of the present invention, one end of which is connected to a lever connecting portion 34 provided at a base end portion of the detection lever 33, and the other end of which is connected to a front end portion of the protective cover 31. Detection lever 3
3 to the sheet stacking surface 2 of the sheet stacking tray 13
6, a tension spring 35 that biases counterclockwise around the pivot 32 in the drawing, and the detection lever 3
The solenoid 37 is attached to the protective cover 31 so as to be positioned on the opposite side of the tension spring 35 with the lever connecting portion 34 of No. 3 interposed therebetween, and the plunger 36 is connected to the lever connecting portion 34 of the detection lever 33, and the solenoid 37 not shown. Photoelectric sensor 38 in which a light portion and a light receiving portion are arranged with a gap therebetween
And a shutter 39 attached to the base end portion of the detection lever 33 so as to be positioned between the light emitting portion and the light receiving portion of the photoelectric sensor 38.

The shutter 39 is provided between the light emitting portion and the light receiving portion of the photoelectric sensor 38 when the solenoid 37 is driven from the standby position shown in FIG. 2 to rotate the detection lever 33 clockwise by a predetermined angle. The positional relationship is such that the light from the light projecting portion reaches the light receiving portion, and the light from the light projecting portion is blocked in other states.

Specifically, the rotation position of the detection lever 33 where the light from the light projecting portion reaches the light receiving portion is set at the sheet bundle S.
_{When B} is not present, the step between the sheet placing surface 21 of the buffer tray 18 and the sheet bundle placing surface 26 of the sheet stacking tray 13 of the stacker 12 is set to fall within a range of 5 to 8 mm. Further, when the sheet bundle S _B is present on the sheet stacking tray 13, the step between the upper surface of the uppermost sheet bundle S _B stacked on the sheet stacking tray 13 and the sheet placing surface 21 of the buffer tray 18 Is set to fit within the range of 5 to 8 mm. That is, when the solenoid 37 is driven to rotate the front end of the detection lever 33 to the sheet stacking tray 13 side, the step between the sheet stacking tray 13 side and the sheet mounting surface 21 of the buffer tray 18 is 5 to 8 mm. Thus, the screw shaft drive motor 24 operates to lower the sheet stacking tray 13.

When the above-mentioned step is less than 5 mm, the sheet bundle S straddling the buffer tray 18 and the sheet stacking tray 13 is formed.
_{When B} is sent to the sheet stacking tray 13 side, this sheet bundle S _B may be caught by the sheet bundle S _B stacked on the sheet stacking tray 13 and jamming may occur. On the contrary, when the above-mentioned step exceeds 8 mm, the step becomes too large and the sheet bundle S _B straddling the buffer tray 18 and the sheet stacking tray 13 cannot be smoothly delivered to the sheet stacking tray 13 side, and the sheet stacking is not possible. There is a risk that S _B will slip off.

[0032] In the present invention, since the front end portion of the detection lever 33 so that pressing on the upper surface of the sheet stack S _B, the upper surface of the sheet stack S _B is not damaged at the tip of the detection lever 33 As described above, it is also effective to form the tip end of the detection lever 33 with a rubber-like elastic body or to cover the surface with a soft material equivalent thereto. Further, in the present embodiment, the tip end of the detection lever 33 is urged in the direction away from the sheet stacking tray 13 by using the tension spring 35. On the contrary, the tip end of the detection lever 33 is moved toward the sheet stacking tray 13. The plunger 36 of the solenoid 37 may be driven in a direction in which the tip end of the detection lever 33 moves away from the sheet stacking tray 13 by applying a spring force that pushes the sheet stacking surface 26 side.

The drive control section 15 includes an operation button group 17
Based on a command from the operator via the operation of the copying machine C, the operation of the stapler 22 and the on / off control of the energization of the solenoid 37 are controlled in addition to the photoelectric sensor 38.
The operation of the screw shaft drive motor 24 is controlled on the basis of the detection signal from, and the forward / reverse rotation of the rod drive motor 29 is also controlled.

In the initial setting of the sheet processing apparatus 10 according to the present embodiment, the drive control section 15 energizes the solenoid 37 so that the tip of the detection lever 33 located at the retracted position resists the spring force of the tension spring 35. The tip of the detection lever 33 is pressed against the sheet stacking surface 26 of the sheet stacking tray 13 by rotating the sheet stacking tray 13 side.
Further, the drive control unit 15 operates the screw shaft drive motor 24 to start the descent of the sheet stack tray 13 at the rising end, and operates the screw shaft drive motor 24 until the photoelectric sensor 38 is turned on.

When the photoelectric sensor 38 is turned on, that is, when the level difference between the sheet placing surface 21 of the buffer tray 18 and the sheet bundle placing surface 26 of the sheet stacking tray 13 becomes 5 to 8 mm, the drive control unit 15 uses a screw. The shaft drive motor 24 is stopped, the solenoid 37 is de-energized, and the detection lever 33 is returned to its retracted position by the spring force of the tension spring 35.

In this state, the sheets are sent from the copying machine C to the sheet conveying section 19, and the sheets are stacked over the buffer tray 18 and the sheet stacking surface 26 of the sheet stacking tray 13 by a predetermined number. And the drive control unit 1
The stapler 22 is actuated based on the command from 5, and the plurality of sheets stacked over the buffer tray 18 and the sheet bundle mounting surface 26 of the sheet stacking tray 13 are combined into one sheet bundle S _B. Staple it. After completion of this stapling operation, the rod drive motor 29 of the pusher 14 is operated based on a command from the drive control unit 15 to move the push-out rod 27 in the retracted position forward to the push-out position, and the buffer tray 18 and the sheet stacking tray. The sheet bundle S _B straddling the sheet stacker 13 is completely pushed out onto the sheet stacking tray 13 of the stacker 12. At this time, the sheet stacking surface 26 of the sheet stacking tray 13 is the sheet stacking surface 2 of the buffer tray 18.
It is located 5 mm to 8 mm below 1 and the sheet bundle S _B is smoothly transferred.

When the push-out rod 27 advances to the push-out position, the drive control section 15 energizes the solenoid 37 so that the tip of the detection lever 33 in the retracted position resists the spring force of the tension spring 35. The tip of the detection lever 33 is pressed against the upper surface of the sheet stack S _B on the sheet stacking tray 13 by rotating the sheet stacking tray 13 side.
Then, in this state, the screw shaft drive motor 24 is operated to start the descent of the sheet stack tray 13, and the photoelectric sensor 38
The screw shaft drive motor 24 is operated until is turned on.

When the photoelectric sensor 38 is turned on, the drive controller 15 stops the screw shaft drive motor 24, releases the energization to the solenoid 37, returns the detection lever 33 to its retracted position, and further pushes the pusher 14. The rod drive motor 29 is operated in reverse to push the push rod 27 in the push position.
To the retracted position.

After that, the sheets are fed again from the copying machine C to the sheet conveying section 19, and the above-described work is repeated to stack the sheet bundle S _B on the sheet stacking tray 13 one after another.

As described above, by pressing the upper surface of the sheet bundle S _B located at the uppermost position of the sheet stacking tray 13 with the tip of the detection lever 33, the uppermost sheet of the sheet bundle S _B is electrostatically charged. Even when the sheet stack S 13 is lifted up or curled, it is possible to detect the regular thickness of the sheet bundle S _B and accurately set the descending amount of the sheet stacking tray 13.

[0041]

According to the sheet stacking method and the stacker of the present invention, the tip of the detection dog of the sheet bundle upper surface position detecting means is pressed against the upper surface of the sheet bundle carried onto the sheet stacking tray to detect the upper surface position. I decided to do so,
Even if the topmost sheet of this sheet bundle is lifted up or curled due to static electricity, it is possible to detect the top surface of the sheet bundle accurately and reliably, and to prevent the sheet from rising up to ensure normal operation. The thickness can be adjusted. By lowering the sheet stacking tray to a predetermined position based on this detection information, when the next sheet bundle is carried in, the sheet bundle stacked on the sheet stacking tray or the moving sheet bundle is You can stack smoothly without jamming.

Further, the sheet bundle upper surface position detecting means has a lever-shaped detection dog whose base end portion is rotatably supported, and one end portion of the detection dog which faces the upper surface of the sheet bundle. A biasing means for biasing to the side, a solenoid for driving the tip of the detection dog in a direction opposite to the biasing force of the biasing means, a photoelectric switch having a light emitter and a light receiver, and a light emitter and a light receiver of this photoelectric switch. And a shutter which is attached to the detection dog so that it can be located between the projector and the shutter, and has a shutter which is disengaged between the projector and the receiver when the detection dog moves to a predetermined position. It is possible to realize a stacker having the above at a low cost.

[Brief description of drawings]

FIG. 1 is a cutaway view showing a main part of an embodiment of a sheet processing apparatus incorporating a stacker capable of realizing a sheet bundle stacking method according to the present invention.

FIG. 2 is an extracted enlarged view of a portion of a sheet detecting unit in the embodiment shown in FIG.

FIG. 3 is a plan view of the embodiment shown in FIG. 1;

FIG. 4 is a side view of the embodiment shown in FIG.

5 is a front view of the embodiment shown in FIG.

FIG. 6 is a mechanism conceptual diagram showing a schematic structure of parts of a finisher and a stacker in the embodiment shown in FIG.

7 is a mechanism conceptual diagram showing a schematic structure of a pusher portion in the embodiment shown in FIG. 1. FIG.

[Explanation of symbols]

C Copier S _B Sheet bundle 10 Sheet processing device 11 Finisher 12 Stacker 13 Sheet stacking tray 14 Pusher 15 Drive control unit 16 Operation panel 17 Operation button group 18 Buffer tray 19 Sheet conveying unit 20 Frame body 21 Sheet placing surface 22 Stapler 23 Lifting screw shaft 24 Screw shaft drive motor 25 Tray guide bar 26 Sheet bundle mounting surface 27 Push bar 28 Bar guide block 29 Bar drive motor 30 Power transmission mechanism 31 Protective cover 32 Axis 33 Detection lever 34 Lever connection part 35 Tension spring 36 Plunger 37 Solenoid 38 Photoelectric Sensor 39 Shutter

Claims (10)

[Claims] 1. A step of stacking a sheet bundle on a sheet unloading tray on the sheet stacking tray by moving the sheet bundle on the sheet stacking tray of the sheet unloading tray, and the sheet bundle located at the top of the sheet stacking tray. The step of pressing the tip of the detection dog against the upper surface of the sheet stack, and the step of lowering the sheet stack tray until the position of the tip of the detection dog pressed against the top surface of the sheet bundle reaches a predetermined position. A method of stacking sheet bundles. 2. When the front end of the detection dog is at the predetermined position, the top surface of the sheet bundle located at the top of the sheet stacking tray and the sheet mounting surface of the sheet unloading tray have a step. The sheet bundle stacking method according to claim 1, wherein the sheet bundle stacking method is one. 3. The sheet bundle stacking method according to claim 2, wherein the step is in a range of 5 to 8 mm. 4. A sheet stacking tray into which a sheet bundle on the sheet carry-out tray is carried in parallel to a sheet mounting surface of the sheet carry-out tray, and tray elevating means for supporting the sheet stacking tray so as to be able to move up and down. Elevating and lowering driving means for driving the elevating and lowering means to elevate and lower the sheet stacking tray, and a sheet bundle for pressing the upper surface of the sheet bundle at the uppermost position of the sheet stacking tray to detect the upper surface position of the sheet bundle. Upper surface position detecting means, and drive control means for controlling the operation of the elevating and lowering means so that the upper surface position of the sheet bundle on the sheet stacking tray becomes a predetermined position based on information from the sheet bundle upper surface position detecting means. The sheet bundle upper surface position detecting means faces the upper surface of the sheet bundle on the sheet stacking tray and has a leading end portion above the sheet bundle. A detection dog that is pressed against, a dog drive unit that drives the detection dog in the direction opposite to the upper surface of the sheet bundle on the sheet stack tray, and that the detection dog has been moved to a predetermined position by the dog drive means. A stacker having a detection unit for detecting the. 5. The stacker according to claim 4, wherein the detection dog of the sheet bundle upper surface position detection means has a lever shape in which a base end portion is rotatably supported. 6. The dog driving portion of the sheet bundle upper surface position detecting means, and a biasing means for biasing the tip end portion of the detection dog to one side in a direction facing the upper surface of the sheet bundle, and the biasing means. The stacker according to claim 4 or 5, further comprising: a solenoid that drives a tip portion of the detection dog in a direction opposite to a biasing force of the means. 7. The detector includes a photoelectric sensor having a light emitter and a light receiver, and a shutter attached to the detection dog so as to be positioned between the light emitter and the light receiver of the photoelectric sensor. The stacker according to any one of claims 4 to 6, which is a stacker. 8. The stacker according to claim 7, wherein the shutter is disengaged from between the light emitter and the light receiver when the detection dog moves to a predetermined position. 9. When the front end portion of the detection dog moves to the predetermined position, a step is formed between the upper surface of the sheet bundle located at the uppermost position of the sheet stacking tray and the sheet mounting surface of the sheet unloading tray. The stacker according to any one of claims 4 to 8, which has one. 10. The stacker according to claim 9, wherein the step is in the range of 5 to 8 mm.