Pneumatic sheet feeder

A pneumatic sheet feeder includes a movable bottom plate which is disposed to form a constant angle with the uppermost sheet in a stack placed on a sheet receptacle, irrespective of the height of the stack. Sheets on the receptacle are attracted one by one by a negative pressure to be sequentially fed by a feed roller which is driven for rotation in an automatic manner.

BACKGROUND OF THE INVENTION 
The invention relates to a pneumatic sheet feeder. 
U.S. Pat. No. 3,964,740, for example, discloses a pneumatic sheet feeder 
which may be used in a copying machine or facsimile system. The feeder 
comprises a sheet receptacle on which a stack of sheets is placed, a 
vacuum casing located above the sheet receptacle at a location forwardly 
as viewed in the sheet feed direction and having an open bottom to define 
a vacuum chamber, a single movable bottom plate which is pivotally mounted 
on the vacuum casing on a horizontal axis at its rear end and including an 
air suction aperture or apertures adjacent its forward end and which is 
adapted to close the bottom opening of the vacuum chamber, and a sheet 
feed roller or rollers disposed within the vacuum chamber. A negative 
pressure is created within the vacuum chamber, and an uppermost sheet of 
the stack placed on the receptacle is held attracted to the underside of 
the bottom plate while the latter moves angularly to permit the feed 
roller or rollers to project through part of the openings to feed the 
sheet forwardly by friction. 
In a sheet feeder of this kind, the movable bottom plate usually comprises 
a single plate member pivotally mounted on a horizontal axis disposed at 
the rear end of the casing and extending in a direction transverse to the 
sheet receptacle. When the negative pressure is not created within the 
vacuum chamber, the front end of the bottom plate bears against the 
uppermost sheet of the stack by weight. Consequently, the movable bottom 
plate assumes a varying angle of inclination relative to the uppermost 
sheet as the number of sheets contained in the stack or the height thereof 
changes. When the negative pressure is created, there occurs an airflow 
which is directed through the air suction openings formed in the bottom 
plate into the vacuum chamber, and the uppermost sheet is attracted and 
held attracted against the lower surface of the bottom plate to close the 
openings, thus maintaining the vacuum chamber substantially air-tight to 
permit the negative pressure within the chamber to be increased. When the 
negative pressure increases above a given value, the bottom plate moves 
angularly in the upward direction about the pivotal axis, whereby the 
uppermost sheet is separated from the next lower sheet. In order to assure 
a stabilized sheet separation and feeding operation, it is desirable that 
the configuration of the space defined by the lower surface of the bottom 
plate and the uppermost sheet remains unchanged throughout, namely, from 
the time the initial sheet is fed until the last sheet in the stack is fed 
from the sheet receptacle. However, with pneumatic sheet feeders of the 
conventional design, the bottom plate will assume an increasing angle of 
inclination relative to the uppermost sheet in the stack as the height 
thereof decreases, resulting in a change in the configuration of the space 
defined by the bottom plate and the uppermost sheet. 
When the sheets are tightly stacked, or when there is no substantial air 
layer between adjacent sheets, more than one sheet may be attracted 
simultaneously, fouling the desired separation. If more than one sheet is 
fed in superposed relationship in a facsimile system, a jamming occurs, 
causing a failure of the system. Additionally, it will be appreciated that 
when an airflow occurs from the rear end of the feed rollers toward the 
air suction openings, the sheet separating air stream, that is, the flow 
of air around the front edge of the uppermost sheet into the space between 
the uppermost sheet and the next following sheet, is impeded, thus 
adversely influencing the sheet separation. In this manner, it will be 
appreciated that a close control of the sheet separation is required. 
When the sheet feeder of the type described is used with the input section 
of a facsimile system, the feed rollers of the feeder deliver an original 
to be transmitted into the input section. Thereupon, the feeding operation 
of the original is continued by an original feeder of the facsimile 
system, which feeds it into the processing station. It will be understood 
that it is desirable, for purpose of reducing the noise and the power 
dissipation, that the feed operation by the sheet feeder be interrupted 
when the sheet or the original can be fed by the original feeder. In this 
instance, if the original is elongate in the feed direction, when the feed 
operation of the feeder is interrupted, the downwardly moving bottom plate 
may clasp the sheet being fed, thus increasing a load on the original 
feeder and causing an adverse influence upon the read operation. In worst 
cases, the original may be damaged. 
In the sheet feeder disclosed in the above-mentioned U.S. Pat. No. 
3,964,740, the feed rollers are carried by the vacuum casing and project 
downwardly through the air suction openings formed in the bottom plate 
when the latter moves angularly through a given stroke in the upward 
direction. This disadvantageously prevents a stabilized sheet separation 
in that the front end of the uppermost sheet is kept from upward movement 
by the front end of the movable bottom plate. 
To avoid this difficulty, the present applicant has proposed a sheet feeder 
in U.S. application Ser. No. 825,389, now abandoned, in which the feed 
rollers are mounted on the bottom plate so as to press against the sheets 
on the receptacle. However, in this sheet feeder, the rotation is 
transmitted to the feed rollers by friction with a drive shaft which is 
arranged to be moved into sliding contact with the feed rollers when the 
feed rollers are displaced upwardly through a given stroke together with 
the bottom plate. This requires a biasing pressure of a relatively large 
magnitude between the feed rollers and the drive shaft in order to assure 
the stabilized rotation of the feed rollers. Consequently, a large 
proportion of the fan capacity which produces the negative pressure to 
lift the bottom plate upwardly must be reserved for use as the bias 
pressure. Hence, the fan becomes less effective in lifting the bottom 
plate, with a reduced holding effect of the feed roller. When the sheet 
holding effect of the feed roller is reduced, the uppermost sheet will be 
bent under the action of the negative pressure beyond a boundary defined 
by the feed rollers, and the bottom plate may begin to move upwardly 
before the uppermost sheet is held attracted to the lower surface of the 
bottom plate, thus resulting in an unstabilized sheet separation. In 
addition, when the feed rollers are mounted on the bottom plate, the 
overall weight of the latter increases, presenting a problem that an 
increased negative pressure is required to lift it upwardly. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a pneumatic sheet feeder which 
overcomes the difficulties of the conventional pneumatic sheet feeders by 
maintaining a space of uniform configuration defined by a movable bottom 
plate and an uppermost sheet irrespective of a change in the height of a 
stack of sheets placed on a sheet receptacle, thus achieving a uniform 
feed condition to assure a stabilized sheet separation and feeding 
operation. 
It is another object of the invention to provide an improved pneumatic 
sheet feeder capable of assuring a stabilized sheet separation even if the 
sheets are tightly stacked on the sheet receptacle. 
It is a further object of the invention to provide an improved pneumatic 
sheet feeder which may have its operation interrupted in the course of 
feeding a sheet from the original receptacle without clasping the sheet 
being fed to increase a mechanical load or damaging an original. 
It is another object of the invention to provide an improved pneumatic 
sheet feeder which blocks an airflow directed from rearwardly of the feed 
rollers toward the air suction openings, thereby assuring a stabilized 
sheet separation. 
It is still another object of the invention to provide an improved 
pneumatic sheet feeder which achieves a reliable sheet separation and 
feeding operation. 
It is a still further object of the invention to provide an improved 
pneumatic sheet feeder of the type having feed rollers mounted on a 
movable bottom plate in which the transmission of rotating force to the 
feed rollers is assured while simultaneously achieving a stabilized sheet 
separation and feeding operation. 
The above and other objects and features of the invention will become 
apparent from the following detailed description of embodiments thereof 
with reference to the drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to FIG. 1, there is shown a sheet feeder incorporating various 
features of the invention. As shown, the sheet feeder includes a sheet 
receptacle 1 having a generally flat upper surface 2 and on which sheets 
to be fed are placed. As shown in FIG. 2, the receptacle 1 is provided 
with a front barrier 3 which is slightly inclined from the vertical in 
order to reduce the resistance experienced during the sheet separation. 
The receptacle is also provided with vertical side plates 4, 5. Each side 
plate 4, 5 is provided with a fixture 6 which permits a detachable 
mounting of the entire sheet feeder shown in FIG. 1 on the input section 
of a facsimile system, for example, and is also formed with a notch 7 
which admits an airflow to enhance the separation effect. 
At its front end, each side plate has an upstanding bracket 4a, 5a (see 
FIG. 3) having respective support pins 8 or 9 mounted thereon and which 
are used to support a feed head casing 10 to be described later. The 
casing 10 is box-shaped and has an open bottom. A pair of fasteners 11, 12 
connect a pair of inner plates 13, 14 to the side walls of the casing 10, 
and forward portions of the inner plates are pivotally mounted on the 
support pins 8, 9 so that the casing is angularly movable between its 
operative position shown in FIG. 2, and its inoperative position in which 
it is moved away from the upper surface of the receptacle 1. The casing 10 
is rearwardly provided with an overhang 15 which may be engaged by hand to 
swing casing 10 in the direction of the arrow. A vertical partition 16 is 
provided between the inner side plates 13, 14 at a position corresponding 
to the barrier 3 when the casing 10 is in its operative position, dividing 
the inner space thereof into a forward chamber and a rear chamber. Fixedly 
mounted in the rear chamber is a vacuum casing 18 which defines a vacuum 
chamber 17 having an open bottom. An air suction device 22, including a 
casing 21 which contains an an electric motor 19 and a axial flow fan 20 
driven thereby, is disposed in the top of the casing 18. The fan 20 draws 
air into the vacuum chamber 17 through a vent hole 23 formed in the top 
wall of the casing 18, and discharges the air externally through an 
exhaust hole 24 formed in the top wall of the casing 10. It will be seen 
that the exhaust hole 24 is covered with an air permeable member 25 such 
as metal meshwork. 
The vacuum casing 18 carries a first pivot shaft 26 which extends virtually 
horizontally across the receptacle 1 along the rear edge of its bottom 
opening, and the rear end of a first movable bottom plate 27 is pivotally 
mounted on the shaft 26, whereby it is angularly movable about the axis of 
shaft 26. The bottom plate 27 has a length which is sufficient to close 
the rear half of the bottom opening. At its front end, the bottom plate 27 
carries a second pivot shaft 28 which extends horizontally across the 
receptacle 1, and the rear end of second movable bottom plate 29 is 
pivotally mounted on the shaft 28, whereby it is angularly movable about 
the axis of shaft 28. The front end of the second bottom plate 29 extends 
to the forward edge of the bottom opening. In this manner, the bottom 
opening of the vacuum casing 18 can be closed by the pair of lengthwise 
staggered movable bottom plates pivotally connected together by the shaft 
28. 
Referring to FIG. 4, in the embodiment shown, the lateral edges of the 
first movable bottom plate 27 as well as the front end and the lateral 
edges of the second movable bottom plate 29 are provided with folded, 
upstanding flanges 27a, 27b and 29a, 29b, 29c, respectively, which are 
disposed in surrounding relationship with both of the side walls and the 
front wall of the vacuum casing 18. The upstanding flanges are provided 
with flexible thin sheets 30 such as Mylar film to improve the air 
tightness between the folded pieces and the vacuum casing. As shown in 
FIGS. 2 and 5, the partition 16 is also provided with a flexible thin 
sheet 31 which depends downwardly therefrom outside the vertical folded 
flange 29a at the front end of the second movable bottom plate 29, thus 
improving the air tightness between the folded flange and the vacuum 
casing 18. Adjacent the front end, the bottom surface of the second 
movable bottom plate 29 is provided with a plurality of semi-spherical 
projections 61 which are spaced apart in a direction transverse to the 
sheet feed direction. Preferably, these projections 61 are located in a 
manner to correspond to the location of the feed rollers 33. 
As shown in FIGS. 3 and 4, the second movable bottom plate 29 is formed 
with a plurality of air suction openings 32 which are elongate along the 
direction of sheet feed and which are spaced apart across the width 
thereof. In the embodiment shown, plural feed rollers 33, formed of rubber 
or similar material, are disposed in alternate openings 32. These feed 
rollers 33 are fixedly carried by a roller support shaft 36 which is 
rotatably supported by a pair of bearing brackets 34, 35 mounted on the 
upper surface of the second bottom plate 29. The peripheries of these 
rollers partly project downwardly through the openings 32 so as to be 
placed in sliding contact with the upper surface 2 of the receptacle 1. 
Consequently, when the bottom plates 27, 29 are in their lower positions 
which they assume by weight, the front end of the bottom plate 27 and the 
rear end of the bottom plate 29 bear against the receptacle 1 while the 
bottom plate 29 is disposed at a small angle of inclination relative to 
the upper surface 2. It is preferred, for the purpose of assuring a sheet 
separation, that the feed rollers 33 be offset toward the rear end of the 
openings 32 or spaced from the front barrier 3, and the openings 32 are 
preferably tapered toward their front end 32a, as shown. 
As clearly indicated in FIG. 3, the shaft 36 carries a gear 37, which is 
adapted to mesh with a gear 38 disposed within the vacuum chamber 17 when 
the second bottom plate 29 moves upwardly through a given stroke. 
Preferably the gears 37, 38 have triangular teeth of a small module to 
permit a smooth engagement therebetween since they engage and disengage 
from each other as the second bottom plate 29 moves vertically. The gear 
38 is fixedly mounted on a shaft 39 which is rotatably supported between 
the inner side plates 13, 14. At its one end, the shaft 39 carries a 
timing belt pulley 40, which is drivingly connected with a timing belt 
pulley 42 associated with a feed roller drive motor 41 located within the 
casing 10 through a timing belt 43. It is to be noted that the gear 38 is 
driven counterclockwise as viewed in FIG. 2. 
The second bottom plate is moved vertically by an elevator mechanism. 
Specifically, the bottom plate 29 centrally carries a bracket 44. A link 
46 has its one end pivotally mounted on a third pivot shaft 45 located in 
the rear upward portion of the casing 18 and has its other end pivotally 
mounted on a fourth pivot shaft 47 carried by the bracket 44. The link 46 
cooperates with the first bottom plate 27 to form a parallelogram linkage, 
whereby the second bottom plate 29 can be translated bodily substantially 
in the vertical direction relative to the upper surface of the receptacle 
1. In this embodiment, the first bottom plate 27 centrally carries a 
bracket 48 carrying a pivot shaft 50 on which one end of a link 49 is 
mounted. The other end of the link 49 extends through an opening 51 (see 
FIG. 3) formed in the top wall of the casing 18 to project upwardly for 
pivotal connection at 55 with one end of a lever element 54 which is 
pivotally mounted at 53 on a bracket 52 which is in turn secured to the 
top surface of the casing 18. The other end of the lever element 54 
extends through an opening 155 (see FIG. 5) formed in the partition 16 
into the forward chamber defined within the head casing 10, and its free 
end carries a counterweight 56. The counterweight 56 acts through lever 
element 54 and link 49 to bias the first bottom plate 27 upward, thereby 
allowing the second bottom plate 29 which carries feed rollers 33, as well 
as the first bottom plate 27 to be raised with a relatively small force. 
In the example shown, the lever element 54 comprises a pair of elements 
54a, 54b which are connected in a telescopic manner and secured together 
by screws 57 to permit an adjustment of the overall length. A thin metal 
sheet 73 extending through the element 54b closes the opening 155 in order 
to prevent the negative pressure created within the vacuum chamber 17 from 
being disturbed. 
As shown in FIG. 16, the fan drive motor 19 associated with the air suction 
device 22 is directly connected with a start switch SW while the feed 
roller drive motor 41 is connected with the start switch SW through a 
delay circuit T. The delay circuit T may comprise a logic circuit of known 
form, including a CR or NAND circuit or may comprise a time limit relay, 
and has a time delay on the order of four to eight seconds. 
A flexible thin sheet 60 has its one end adhesively secured to the lower 
surface of the first bottom plate 27, and extends beyond the second pivot 
shaft 28 to a position short of feed rollers 33, with its free end 60a 
engaging the upper surface of a sheet on the receptacle 1 (see FIG. 5). 
The sheet 60 may have its one end secured to the rear wall 18a of the 
vacuum casing 18 as shown in FIG. 20. 
It will be seen in FIG. 2 that the front barrier 3 is formed with a guide 
plate 3a which is slightly offset from the upper end thereof and inclined 
toward the sheet feed direction. The guide plate 3a defines a sheet 
delivery passage 59 together with an upper guide plate 58 which is mounted 
to extend across the side plates 4, 5 and located above the guide plate 
3a. A rubber piece 62 has its one end secured to the barrier 3 and has a 
freely flexible upper end which projects above the guide plate 3a and into 
the passage 59. Another rubber piece 63 has its one end also secured to 
the guide plate 3a and extends forwardly within the passage 59, leaving a 
free end. 
As shown in FIG. 5, a bracket 64 is provided within the vacuum chamber 17, 
and a locking claw 65 is pivotally mounted on a pin 66 secured to the 
bracket. An engaging element 67 engageable with the claw 65 is mounted on 
the upper surface of the second bottom plate 29. The claw 65 is biased 
counterclockwise, as viewed in this Figure, by a torsion spring 68 and 
thus is normally located in a position engageable with the element 67. A 
ring 70 having unlock pins 69 is rotatably mounted on the shaft 39, and is 
connected with one end of a coiled spring 71 (see FIG. 11) which is 
disposed on the shaft 39. The spring 71 is normally coiled relatively 
tightly around the shaft 39, but is uncoiled by the forward or 
counterclockwise rotation of the shaft 39 when the ring 70 is prevented 
from rotating relative to the shaft 39, to thereby permit a free rotation 
of the shaft 39 relative to ring 70, thus forming so-called one-way 
clutch. The engaging element 67 includes a stop wall 67a which bears 
against an unlock pin 69 on the ring 70 to prevent a rotation of the ring 
in the up-position of the second bottom plate 29, while the locking claw 
65 includes an unlock pawl 65a which bears against an unlock pin 69. When 
the unlock pawl 65a is driven by an unlock pin 69, the locking claw 65 is 
moved into a position in which it is disengaged from the engaging element 
67. The function of the locking mechanism will be described later. 
The operation of the sheet feeder thus constructed will now be described 
with reference to FIGS. 5 and 6. Sheets S to be fed are placed in a stack 
in the receptacle 1 with their leading ends aligned by abutment against 
the front barrier 3, as shown in FIG. 5. The sheets can be located in 
place by raising the head casing 10. Where the number of sheets is small, 
they can be inserted in place without raising the casing. When the sheets 
are positioned as mentioned above, the feed rollers 33 bear against the 
upper surface of the uppermost sheet in the stack and are gently pressed 
thereagainst by their own weight and that of the second bottom plate 29. 
The pressure applied is determined by the balance between the weight of 
rollers 33, their support and second bottom plate 29 on one hand and the 
weight of the counterweight 56 on the other. It is to be noted that the 
magnitude of the pressure is critical in achieving the sheet separation, 
that is, attracting only the uppermost one of the sheets in the stack. 
When the start switch SW is closed under this condition, the motor 19 of 
the air suction device 22 is set in motion, whereby the axial flow fan 20 
begins to rotate. The feed roller drive motor 14 is not yet driven as a 
result of the delay circuit T (see FIG. 16). As the fan 20 rotates, the 
air is drawn into the vacuum chamber 17 through the air suction openings 
32, whereby a suction is applied to a portion of the uppermost sheet S in 
the stack which is located forwardly of the point of contact between the 
rollers 33 and the sheet. It will be appreciated that, if the air is 
withdrawn through a portion of the openings 32 located rearwardly of the 
feed rollers 33, the airflow which serves the sheet separation, namely, 
the flow of air entering the space created between the uppermost sheet and 
the next following sheet around the leading edge of the uppermost sheet, 
will be reduced, and the air pressure then prevailing above the uppermost 
sheet acts to hold back the sheet to prevent an upward movement of movable 
bottom plate 27, 29, thus exerting adverse influences upon the sheet 
separation. However, these influences are prevented by the provision of 
the flexible sheet 60. The sheet 60 also prevents the admission of the air 
through the junction between the first and second bottom plates 27, 29. As 
the fan 20 increases its angular velocity, the uppermost sheet begins to 
be drawn up, gradually blocking the openings 32 until it is flat against 
the lower surface of the second bottom plate 29 to thereby completely 
block the openings 32. The front end of the sheet will be corrugated by 
abutment against the projections 61 as shown in FIG. 7. It will be 
understood that, if a second sheet is drawn or attracted together with the 
uppermost sheet, the corrugations are effective to create layers of air 
between the two sheets in the valley regions inasmuch as the lower sheet 
is not corrugated by virtue of the rigidity of of the paper material. As a 
consequence, the lower sheet will fall down onto the stack or the 
receptacle by its own weight, thus preventing a double sheet feeding. 
When the openings 32 are blocked by the sheet, the vacuum chamber 17 is 
substantially enclosed to permit the negative pressure therein to be 
increased, and such negative pressure is effective to raise the second 
bottom plate 29. As a result of the parallelogram linkage formed by the 
first bottom plate 27 and link 46, the second bottom plate 29 moves while 
maintaining a substantially horizontal position. When the bottom plate 29 
moves through a given stroke upwardly while maintaining the sheet S 
attracted to its bottom surface, the gear 37 moves into meshing engagement 
with the gear 38. After several seconds have passed, the motor 41 is 
energized to initiate the rotation of the gear 38, which is transmitted to 
the gear 37. As a consequence, the feed rollers 33 are driven for 
clockwise rotation, feeding the sheet which is held attracted forwardly, 
namely, to the left as viewed in the drawings, by friction. Since there is 
a time interval on the order of several seconds after the movable bottom 
plate is raised until the feeding operation of the sheet is initiated, 
there is sufficient time for any sheet or sheets other than the uppermost 
one which may have been drawn upwardly together with the uppermost one to 
fall down by weight, thus effectively preventing a plurality of sheets 
from being simultaneously fed. The double sheet feeding is also prevented 
by the provision of rubber pieces 62 and 63 which the sheets have to ride 
past in sliding relationship. Specifically, if two sheets are fed in 
superimposition, the lower sheet will be retarded by sliding friction with 
the rubber pieces 62, 63, and hence cannot be simultaneously fed with the 
upper sheet. 
As shown in FIG. 8, when the shaft 39 is driven for rotation in the up 
position of the bottom plate 29, the stop pawl 67a engages the unlock pin 
69 on the ring 70, preventing the rotation of the ring. However, this does 
not interfere with the rotation of the shaft 39 by virtue of the 
functioning of the coiled spring 71. 
The sheet delivered by the sheet feeder is fed into a facsimile system by 
sheet feed means thereof, not shown, and hence the feeding operation by 
the feed rollers 33 may be interrupted. Thus, the motors 19, 41 are 
deenergized at this time to stop the rotation of the feed rollers 33. 
Thereupon the negative pressure within the vacuum chamber 17 collapses and 
the second bottom plate 29 moves down by weight. However, in the course of 
the downward movement, the element 67 engages the locking claw 65 as shown 
in FIG. 9 to maintain the bottom plate 29 in an intermediate position. 
This maintains an opening of given magnitude between the front barrier 3 
and the front edge 29a of the second bottom plate 29, allowing a continued 
feeding of the sheet without imposing a load from the bottom plate 
thereon. Since the motor 41 is now deenergized, the shaft 39 ceases to 
rotate. As a consequence, if the stop pawl 67a no longer prevents the 
rotation of the ring 70, the locking claw 65 cannot be moved by the unlock 
pin 69 to its unlock position. When a read complete signal is fed from the 
reader of the facsimile system, for example, to initiate the feeding 
operation of a next following sheet, the shaft 39 is again set in rotation 
simultaneously, whereby the locking claw 65 is moved by the unlock pin 69 
to its unlock position shown in phantom line in FIG. 10 to thereby move 
the element 67 away from the locking claw 65. Then, the second bottom 
plate 29 falls down into abutment against the next sheet by its own 
weight, initiating the sheet separation. The stroke through which the 
bottom plate 29 moves down at this time will be increased by an amount 
corresponding to the thickness of one sheet which has been fed. However, 
because the downward movement takes place by its translational movement 
achieved with the function of the parallelogram linkage, there occurs no 
change in the angle of inclination of the second bottom plate 29 relative 
to the upper surface of the sheet, thus maintaining the initial favorable 
condition independently of the number of sheets on the receptacle 1. This 
assures a stabilized sheet separation and feeding operation. 
FIG. 12 shows another form of locking element. It is to be noted that 
corresponding parts are designated by like numerals as used in FIGS. 8 to 
11. In this instance, the locking claw 65 is adapted to be operated by a 
solenoid unit 72. The timing at which the second bottom plate 29 is locked 
or unlocked can be freely chosen by controlling the energization of the 
solenoid unit 72. 
FIG. 17 shows a further form of locking mechanism which maintains the 
bottom plate 29 in its up position. It is to be noted that corresponding 
parts are designated by like numerals as used in FIGS. 1 to 12. In this 
instance, the locking claw 65 is rotatably mounted on the shaft 66 in a 
freely detentable manner. Specifically, the claw 65 can be stopped at any 
desired angular position relative to the bracket 64. The element 67 
comprises an anchorage 74 secured to the upper surface of the bottom plate 
29 and carrying a pin 75, and a movable member 76 is pivotally mounted on 
the pin 75. The movable member 76 includes a pawl 76a adapted to engage 
the locking claw 65, a stop pawl 76b engageable with an unlock pin 69 on 
the ring 70, and a stepped pin 77. The movable member 76 is angularly 
movable about the pin 75 through an angle of 10.degree. when the large 
diameter portion 77a of the stepped pin 77 bears against horizontal and 
vertical end faces of an L-shaped notch 74a formed in the free end of the 
anchorage 74, and normally assumes a position which is indicated in 
phantom line, by its own weight. The stepped pin 77 also has a small 
diameter portion 77b which can abut against the rear end face of the 
unlock pawl 65a which is integral with the locking claw 65, and is 
effective to move the locking claw 65 from its unlocked position, shown in 
phantom line, to its locked position, shown in solid line, as the movable 
member 76 angularly moves from its phantom line to its solid line 
position. 
With this arrangement, when the bottom plate 29 assumes its up position 
owing to the negative pressure created within the vacuum chamber 17, an 
unlock pin 69 bears against the stop pawl 76b to raise the movable member 
76 to a position shown in solid line, and consequently the locking claw 65 
is moved to its locked position shown in solid line, by the small diameter 
portion 77b of the stepped pin 77. Upon completion of the feeding 
operation by the sheet feeder, the shaft 39 ceases to rotate and the 
negative pressure within the vacuum chamber collapses, whereby the bottom 
plate 29 has its pawl 76a engaged with the locking claw 65 to be 
maintained in its intermediate position. Subsequently when the next sheet 
separation and feeding operation is initiated, an unlock pawl 65a is 
driven by the unlock pin 69 as the shaft 39 begins to rotate, whereby the 
locking claw 65 rotates clockwise about the shaft 66 to its unlock 
position, thus disengaging the pawl 76a from the locking claw 65 to permit 
the bottom plate 29 to move down by weight. Hence, during the sheet 
separation process, the unlock pawl 65a cannot be engaged by an unlock pin 
69 on the rotating ring 70, preventing the generation of percussion sound 
produced upon the abutment between a pawl 65a and the pin 69. The 
formation of the element 67 in two parts 74, 76 avoids an interference of 
a pin 69 with the upward movement of the bottom plate 29. Specifically, 
the difficulty that the pawl 67a may abut against a pin 69 as the bottom 
plate 29 is raised (see FIG. 9) is avoided by dividing the element 67 into 
two parts, thus ensuring the upward movement of the bottom plate 29. 
FIGS. 18 and 19 show still another from of locking mechanism. Again it is 
to be noted that corresponding parts are designated by numerals as used in 
FIGS. 1 to 12. In this instance, the locking claw 65, having its integral 
unlock pawl 65a, is biased counterclockwise, as viewed in these Figures, 
as in the initially mentioned embodiment. However, the claw 65 is formed 
with a V-groove 65b in its hub. The anchorage 74 of the element 67 carries 
pin 75 on which the movable member 76, formed with stop pawl 76b, is 
pivotally mounted. The bracket 64 carries a pin 79 on which a holding 
lever 80 is pivotally mounted. The holding lever 80 is biased 
counterclockwise by a torsion spring 81, and is formed with a V-shaped end 
80a which is adapted to engage the V-groove 65b to maintain the locking 
claw 65 in its unlock position whenever the latter has moved to such 
position. 
When the movable bottom plate 29 is raised to its up position by the 
negative pressure created within the vacuum chamber 17 as shown in FIG. 
18, an unlock pin 69 bears against the stop pawl 76b and the V-shaped end 
80a is disengaged from the groove 65b, whereby the locking claw 65 is in 
its locked position. When the sheet feeding operation by the sheet feeder 
is completed under this condition, the negative pressure collapses and the 
shaft 39 ceases to rotate. The bottom plate 29 is maintained in its 
intermediate position shown in phantom line in which the pawl 74b of the 
anchorage 74 engages portion 65c of the locking claw 65. A rocking motion 
of the movable member 76 engaged by an unlock pin 69 is prevented as a 
result of the engagement between its one side 76c with the pawl 74b. When 
the next sheet separation and feeding operation is initiated and the shaft 
39 begins to rotate, the unlock pawl 65a is driven by an unlock pin 69 as 
shown in FIG. 19, thus rotating the locking claw 65 clockwise about the 
shaft 66. As a consequence, the element 67 is disengaged from this claw, 
allowing the bottom plate 29 to move down by weight. At this time, the 
claw 65 is maintained in its unlock position as a result of the engagement 
between the V-groove 65b and the end 80a of the holding lever 80. The 
locking claw is maintained in this position until the bottom plate 29 is 
raised to a position such that the element 67 displaces the end 80b of the 
holding lever 80. In this manner, the abutment between the unlock pawl 65a 
and an unlock pin 69 during the sheet separation is again avoided. 
In the forgoing description, it has been stated that the unlock pin 69 
abuts against the stop pawl 76b and the V-shaped protrusion 80a is 
disengaged from the V-groove 65b (see FIG. 18) when the bottom plate 29 is 
raised as a result of the negative pressure within the vacuum chamber. 
However, depending on the positional relationship between the unlock pin 
69 and the bottom plate 29 as its is raised, the end 80a may engage the 
groove 65b as shown in FIG. 19. Specifically, when the bottom plate 29 is 
raised under the condition shown in FIG. 19, the element 67 intially 
drives the end 80b of the holding lever 80 to disengage the end 80a from 
the groove 65b as shown in FIG. 18. However, as an unlock pin 69 rotates 
the pawl 65a, they are again engaged with each other as shown in FIG. 19, 
rocking the locking claw 65 to its unlock position. When the negative 
pressure created within the vacuum chamber is released and the bottom 
plate 29 moves down, the pawl 74b of the element 67 displaces the end 80b 
now from above, disengaging the end 80a from the groove 65b to rock the 
locking claw 65 to its locked position as shown in FIG. 18. 
In the above description, the movable bottom plate against which a sheet is 
held attracted comprises a pair of bottom plate portions. However, a 
single movable bottom plate may be used to achieve the intended object 
provided it can be maintained at a given angle of inclination with respect 
to the uppermost sheet irrespective of the height of the sheet stack. 
Referring to FIGS. 13 and 14, there is shown a vacuum casing 180 which 
defines a vacuum chamber 170 having a bottom opening which is closed by a 
single movable bottom plate 290. The bottom plate 290 is connected with 
the casing 180 through a linkage including a plurality of links. Referring 
to FIG. 13, a link 181 has its one end 181a pivotally connected with the 
casing 180 and its other end 181b loosely fitted into an elongate slot 
290a formed in the rear end of the bottom plate 290. Another link 182 has 
its one end 182a pivotally connected with the front end of the bottom 
plate 290 and its other end 182b loosely fitted into an elongate slot 180a 
formed in the rear end of the casing 180. Both links are pivotally 
interconnected by a pin 185. FIG. 14 shows a pair of parallel links 183, 
184 which have their opposite ends pivotally connected with the vacuum 
casing 180 and the bottom plate 290, respectively, and a four-link linkage 
is formed by links 183, 184, bracket 290b and bracket 180b of the casing. 
The linkage constitutes an elevator mechanism for moving the bottom plate 
290 between a down position shown in phantom line and an up position shown 
in solid line in accordance with the presence or absence of the negative 
pressure created within the vacuum chamber 170, while maintaining a 
horizontal position. As a consequence, the space defined by the bottom 
plate and the uppermost sheet does not change in configuration if the 
height of the stack of sheets placed on a sheet receptacle, which is 
located below the bottom plate, changes. In other words, it is assured 
that the bottom plate is oriented at a constant angle of inclination 
relative to the uppermost sheet in the stack irrespective of the number of 
sheets therein. The botton plate shown in FIGS. 13 and 14 is slanted in a 
direction perpendicular to the direction of sheet advance, but it may be 
formed with a flat sheet holding surface as shown at 291a of a movable 
bottom plate 291 (see FIG. 15). In this instance, however, it is preferred 
that it is mounted on the vacuum casing with a slight inclination relative 
to the sheet receptacle (refer to the second bottom plate 29 shown in FIG. 
5). 
FIG. 21 shows another embodiment of the pneumatic sheet feeder according to 
the invention. It is to be understood that corresponding parts are 
designated by like numerals as used in FIGS. 1 to 20. The vacuum chamber 
17 is closed by a single movable bottom plate generally shown by numeral 
100. The bottom plate 100 is pivotally mounted on a pin 101 at the rear 
end of the chamber 17, and is biased to move upward by a spring 102 
extending between the vacuum casing and the bottom plate. Feed rollers 33 
are driven frictionally by a drive shaft 103 against which they are 
brought into sliding contact as the bottom plate 100 is raised to a given 
level. In this embodiment, the resilience of spring 102 reduces the 
effective weight of the movable bottom plate 100, which can therefore be 
raised with a negative pressure of a relatively small magnitude. A thin 
flexible sheet 60 is applied against the bottom surface of the bottom 
plate 100 for the same purpose as mentioned above. Again projections 61 
are formed on the front end of the bottom plate to produce corrugations in 
the uppermost sheet attracted thereto, thus preventing a double sheet 
feeding. 
While specific embodiments of the invention have been shown and described 
in detail to illustrate the application of the principles of the 
invention, it will be understood that the invention may be embodied 
otherwise without departing from such principles.