Image forming apparatus

An image forming apparatus which is able to stably perform paper feeding without double feeding the printing medium, and is also able to prevent a quality of the printing medium from being damaged is provided. The image forming apparatus includes a pick-up roller configured to pick up the printing medium, and a driving source configured to transmit a driving force to the pick-up roller, wherein the pick-up roller is provided to be raised and then to be returned to its original location when picking up the printing medium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of Korean Patent Application No. 10-2015-0116724, filed on Aug. 19, 2015 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

BACKGROUND

Embodiments of the present invention relate to an image forming apparatus which performs a stable paper feeding operation.

2. Description of the Related Art

An image forming apparatus is an apparatus for printing an image on a printing medium, and includes printers, copiers, facsimiles and all-in-devices in which functions thereof are integrated.

An image forming apparatus adopting an electrophotography method scans light on a photoreceptor charged with a predetermined potential, forms an electrostatic latent image on a surface of the photoreceptor, and then supplies a toner to the electrostatic latent image to form a visible image. The visible image formed on the photoreceptor is directly transferred to a printing medium or is transferred to the printing medium via an intermediate transfer body, and the visible image transferred to the printing medium is fixed to the printing medium while passing through a fixing unit.

The image forming apparatus may include a paper pick-up unit for picking up paper loaded in a cassette or a tray one sheet at a time, and a paper feeder for conveying the picked-up paper to a paper conveying route provided inside the image forming apparatus. Two or more sheets of paper may sometimes be picked-up at one time due to a frictional force of the paper or the like and be conveyed to the paper conveying route by the paper pick-up unit to then be conveyed to an inside of the paper conveying route. In the image forming apparatus, a jam due to double feeding the paper may occur, and various other problems may also occur.

Recently, a retarding pick-up structure, in which a torque limiter and a retard roller connected to the torque limiter to be rotated in a normal or reverse direction according to the torque limiter are used, is provided to prevent a double feed of paper. The torque limiter has a predetermined critical torque value, is rotated in the normal direction when a paper conveying frictional force is greater than the critical torque value, and is rotated in the reverse direction when the paper conveying frictional force is less than the critical torque value. The retard roller is installed at a lower side of the paper conveying route and prevents a double feed of paper.

SUMMARY

Therefore, it is an aspect of the present invention to provide an image forming apparatus which prevents a double feed of paper and also prevents damage to a printing medium.

In accordance with an aspect of the present invention, an image forming apparatus includes a cassette configured to store a printing medium, a pick-up roller configured to pick up the printing medium stored in the cassette, a feed roller located in front of the pick-up roller and configured to convey the printing medium picked up by the pick-up roller, a first shaft connected to a driving source and at which the feed roller is installed, a second shaft connected to the first shaft to receive a driving force and at which the pick-up roller is installed, and a swing member at which the first shaft and second shaft are installed and which is provided to be rotated about the first shaft.

A third shaft may be further provided at the swing member, and the third shaft may be connected to the first shaft to receive the driving force.

A first gear may be provided at the first shaft, and a second gear engaged with the first gear may be provided at the third shaft.

The second gear may revolve on the first gear such that one side of the swing member may be rotated about the first shaft.

According to an embodiment, when one side of the swing member is rotated about the first shaft in a counterclockwise direction, the pick-up roller may be raised, and when one side of the swing member is rotated about the first shaft in a clockwise direction, the pick-up roller may be lowered.

A feed clutch may be provided so as to selectively transmit the driving force to the second shaft.

According to an embodiment, when the feed clutch is in an ON state, the driving force may be transmitted to the second shaft, and the printing medium may be picked up by the pick-up roller.

A rod clutch may be provided so as to selectively transmit the driving force to the third shaft.

According to an embodiment, when the rod clutch is in an ON state, the driving force may be transmitted to the third shaft and one side of the swing member may be provided to be rotatable about the first shaft.

The image forming apparatus may further include an elastic member which presses the swing member so that the pick-up roller presses the printing medium.

DETAILED DESCRIPTION

FIG. 1is a perspective view of an image forming apparatus according to an embodiment, andFIG. 2is a view schematically illustrating a configuration of the image forming apparatus according to an embodiment.

As illustrated inFIGS. 1 and 2, an image forming apparatus1includes a main body10, a paper feeder20for storing and feeding a printing medium S, a developing unit30for forming an image on the printing medium S which is supplied by the paper feeder20, a toner unit40for supplying toner to the developing unit30, an optical scanner50for forming an electrostatic latent image on a photoreceptor32of the developing unit30, a fixing unit60for fixing a toner image transferred to the printing medium S on the printing medium S, and a discharging unit70for discharging the printing medium S, on which image forming is completed, to an outside of the main body10.

The paper feeder20is configured to store and feed the printing medium S. The paper feeder20may be provided at a lower portion of the main body10to supply the printing medium S toward the developing unit30.

The paper feeder20may include a cassette21storing the printing medium S and coupled to the main body10to be opened and closed, and a conveying member80configured to pick up the printing medium S stored in the cassette21one sheet at a time and to convey the picked-up printing medium S toward the developing unit30.

A knock-up plate23may be provided in the cassette21with one end rotatably coupled thereto and the other end supported by a pressing spring22to guide the loaded printing medium S toward the conveying member80.

The conveying member80may include a pick-up roller27for picking up the printing medium S loaded in the knock-up plate23one sheet at a time, and a feed roller28for conveying the printing medium S picked up by the pick-up roller27toward the developing unit30.

The developing unit30includes a housing31forming an exterior thereof, the photoreceptor32rotatably coupled inside the housing31to form the electrostatic latent image, agitating screws33aand33bfor agitating the toner supplied from the toner unit40, a developing roller34for supplying the toner agitated by the agitating screws33aand33bto the photoreceptor32, and a charging member35for charging the photoreceptor32.

The toner supplied from the toner unit40may be introduced into the housing31, and agitated and conveyed by the agitating screws33aand33btoward one side of the housing31, and the agitated and conveyed toner is supplied to the photoreceptor32by the developing roller34to form a visible image.

The photoreceptor32may be in contact with a transfer roller14, and forms a transfer nip N1so that the toner supplied to the photoreceptor32to form the visible image is transferred to the printing medium S. The transfer roller14may be rotatably disposed inside the main body10.

The toner unit40may be coupled to the developing unit30. The toner unit40is configured to accommodate and store the toner for forming an image on the printing medium S, and to supply the toner to the developing unit30when an image forming operation is performed.

The optical scanner50scans light including image information on the photoreceptor32to form the electrostatic latent image on the photoreceptor32.

The fixing unit60includes a housing62and a heating member64and a pressing member66which are rotatably disposed inside the housing62.

The printing medium S to which a toner image is transferred passes between the heating member64and the pressing member66, and at this time, the toner image may be fixed to the printing medium S by heat and pressure.

The heating member64may be engaged and rotated with the pressing member66, forms a fixing nip N2with the pressing member66, is heated by a heat source68, and transfers heat to the printing medium S passing through the fixing nip N2. The heating member64may be configured with a heating roller which receives power from a driving source (not shown) to be rotated. The heat source68for applying the heat to the printing medium S to which the toner is transferred may be disposed inside the heating member64. A halogen lamp may be used as the heat source68, and the heat source68may also be variously configured with a heating wire, an induction heater, or the like.

The pressing member66may be disposed to be in contact with an outer circumferential surface of the heating member64and form the fixing nip N2between the heating member64and the pressing member66. The heating member64may be configured with a pressing roller which receives power from a driving source (not shown) to be rotated.

The discharging unit70includes a first discharge roller71and a second discharge roller72which are engaged with each other, and discharges the printing medium S which passed through the fixing unit60to the outside of the main body10.

An image scanning unit11for scanning a document S1may be located at an upper portion of the housing10. The image scanning unit11includes a document supply tray111on which the document S1to be scanned is loaded, a pick-up roller112for picking up the document S1loaded on the document supply tray111and loading the document S1into a conveying route12, and a feed roller113for conveying the picked-up document. The document S1conveyed by the feed roller113may be scanned by a scanner module110, and then may be guided to a discharging portion (not shown) located at a lower side of the document supply tray111.

The printing medium S may be loaded in the cassette21provided inside the housing10, and picked up and conveyed one sheet at a time by the pick-up roller27. The document S1loaded on the document supply tray111provided at a side of the image scanning unit11may be picked up and conveyed one sheet at a time by the pick-up roller112. The printing medium S loaded in the cassette21and the document S1loaded on the document supply tray111are picked up and supplied one sheet at a time by the pick-up rollers27and112, and a double feed may occur by the pick-up rollers27and112applying a pressure to the printing medium S or the document S1or a friction force between the pick-up rollers27and112and the printing medium S or the document S1.

A structure for preventing a double feed of the printing medium S or the document S1may be provided at a side of the paper feeder20or the image scanning unit11.

An exemplary paper feeder20which prevents a double feed of the printing medium S is described. A double-feed preventing structure provided at a side of the cassette21may also be similarly applied to a side of the pick-up roller112located at the side of the image scanning unit11.

FIG. 3is a side view illustrating a shape of the paper feeder according to an embodiment, andFIG. 4is a view illustrating the shape of the paper feeder according to an embodiment viewed from above.

Referring toFIGS. 3 and 4, the paper feeder20according to an embodiment may include the pick-up roller27for picking up the printing medium S one sheet at a time, the feed roller28for conveying the printing medium S picked up by the pick-up roller27toward the developing unit30(referring toFIG. 2), and a driving source800for transmitting a driving force to the pick-up roller27and the feed roller28. The driving force of the driving source800may be transmitted to the pick-up roller27and the feed roller28through a driving force transmitting unit801. The driving force transmitting unit801may be realized as various structures such as a gear connection type and a pulley and belt type. An exemplary structure in which the driving force transmitting unit801is a gear connection type is described.

The feed roller28may be installed at a first shaft810that is rotated by receiving the driving force from the driving source800. The first shaft810may receive the driving force from the driving source800through a gear connection to be rotated.

The pick-up roller27may be located to be spaced apart from the feed roller28. The pick-up roller27may be installed at a second shaft270spaced apart from the first shaft810by a predetermined gap. The pick-up roller27may be rotated by being connected to a side of the feed roller28and receiving the driving force.

The first shaft810may have a first gear281, and the second shaft270may have a second gear271. The first gear281and the second gear271may be engaged with a third gear291. When the first shaft810is rotated by receiving the driving force, the driving force may be transmitted to the second gear271through the first gear281and the third gear291. When the driving force is transmitted to the second gear271, the second shaft270is rotated, and the pick-up roller27installed at the second shaft270may be rotated with the second shaft270.

A one-way clutch282may be installed at the first shaft810. Assuming that a rotating direction when the pick-up roller27picks up the printing medium S is defined as rotation in a clockwise direction B, the pick-up roller27may be rotated in the clockwise direction B when the first gear281is rotated in the clockwise direction B.

The one-way clutch282may enable the first gear281to be rotated with the first shaft810in one direction only when the first shaft810is rotated in the one direction. When the first shaft810is rotated in the clockwise direction B by the one-way clutch282, a rotating force is not transmitted to the first gear281. Therefore, the pick-up roller27may be provided to be rotated to pick up the printing medium S and to convey the printing medium S toward the feed roller28. At this time, a conveying direction of the printing medium S may be referred to as P.

The first shaft810and the second shaft270may be rotatably installed at a swing member81. A third shaft811may be installed at one side of the swing member81. The swing member81may be provided to be moved with the third shaft811. The third shaft811may be connected to the first shaft810to receive the driving force, and may revolve on an eighth gear88installed at the first shaft810which will be described later. For example, the third shaft811may be gear-connected to the first shaft810. A connection method between the third shaft811and the first shaft810is not limited to a gear connection method.

Based on the first shaft810, the third shaft811may be provided at the one side of the swing member81, and the second shaft270may be provided at the other side thereof. When the third shaft811revolves on the eighth gear88in the clockwise direction B, the other side of the swing member81is lowered, and when the third shaft811revolves in a counterclockwise direction A, the other side of the swing member81is raised.

When the other side of the swing member81is raised, the pick-up roller27installed at the other side of the swing member81may also be raised. When the pick-up roller27is raised, it may be spaced apart from the printing medium S, or a force which presses the printing medium S when the printing medium S is picked up may be reduced.

After the printing medium S is picked up, the third shaft811may revolve in the clockwise direction B, the other side of the swing member81is lowered, and the pick-up roller27may be returned to its original position. A printing medium picking-up operation according to an operation of the swing member81will be described later.

The printing medium S is loaded on the knock-up plate23supported by the pressing spring22. Since the knock-up plate23is elastically supported by the pressing spring22, as the printing medium S loaded on the knock-up plate23is consumed, the knock-up plate23is raised and a printing medium St located at an uppermost portion may be located at a location at which the pick-up roller27may pick it up.

An elastic member802may be installed at the swing member81. The elastic member802may provide an elastic force to the swing member81so that the pick-up roller27approaches the printing medium S. Due to the elastic force of the elastic member802and the elastic force of the pressing spring22which supports the knock-up plate23, the pick-up roller27may pick up the printing medium S one sheet at a time by pressing the printing medium S with an appropriate pressure.

The first shaft810may be connected to the driving source800by the driving force transmitting unit801. The driving force transmitting unit801may be connected to the first shaft810by a gear connection. A fourth gear83engaged with a gear portion801aprovided at the driving force transmitting unit801may be provided at the first shaft810.

A feed clutch82may be provided at the first shaft810. The feed clutch82enables the driving force transmitted to the fourth gear83to be selectively transmitted to the first shaft810.

When the feed clutch82is in an ON state, the driving force of the driving source800may be transmitted to the first shaft810through the driving force transmitting unit801and the fourth gear83. When the first shaft810is rotated in the clockwise direction B by the driving force, the driving force is transmitted toward the second shaft270through the first gear281, the third gear291, and the second gear271so that the pick-up roller27may be rotated in the clockwise direction B.

When the feed clutch82in an OFF state, the driving force of the driving source800is not transmitted to the fourth gear83. Therefore, when the feed clutch82in the OFF state, the first shaft810is not rotated, the driving force is not transmitted to the pick-up roller27, and thus the pick-up roller27is not rotated.

The driving force transmitted to the first shaft810may be transmitted to the third shaft811through a gear connection. A fifth gear85may be provided at the first shaft810, and a sixth gear86engaged with the fifth gear85may be provided at the third shaft811. When the first shaft810is rotated, the driving force may be transmitted through the fifth gear85and the sixth gear86.

A seventh gear87may be further provided at the third shaft811, and an eighth gear88engaged with the seventh gear87may be further provided at the first shaft810. The eighth gear88may be provided to not move even when the first shaft810is rotated. When the first shaft810is rotated, a rotating force thereof may be transmitted to the seventh gear87through the fifth gear85and the sixth gear86. The seventh gear87may revolve on the eighth gear88by receiving the rotating force. When the seventh gear87revolves on the eighth gear88, the swing member81installed at the third shaft811may be moved with the third shaft811.

For example, when the first shaft810is rotated in the clockwise direction B, the third shaft811may revolve on the eighth gear88in the counterclockwise direction A. When the third shaft811revolves in the counterclockwise direction A, the one side of the swing member81is raised, and the other side thereof is lowered. When the other side of the swing member81is lowered, the pick-up roller27may approach the printing medium S by being lowered.

Conversely, when the first shaft810is rotated in the counterclockwise direction A, the third shaft811may revolve in the clockwise direction B. When the third shaft811revolves in the clockwise direction B, the other side of the swing member81is raised and the pick-up roller27may be spaced apart from the printing medium S or a force applied by the pick-up roller27to the printing medium S may be reduced.

A rod clutch84may be provided at the first shaft810. The driving force of the first shaft810may be selectively transmitted to the third shaft811by the rod clutch84.

When the rod clutch84is in an ON state, the driving force of the driving source800may be transmitted to the third shaft811through the fifth gear85of the first shaft810and the sixth gear86. When the rod clutch84is in an OFF state, the driving force of the first shaft810is not transmitted to the third shaft811even when the first shaft810is rotated. Therefore, the third shaft811is not rotated, and the swing member81and the pick-up roller27are not moved.

A torque limiter89may be provided at the third shaft811. The torque limiter89may serve as a stopper. When a magnitude of the driving force input to the seventh gear87is a predetermined value or more, the torque limiter89may block the driving force from being transmitted to the third shaft811.

A gear ratio among the fifth gear85, the sixth gear86, the seventh gear87, and the eighth gear88may be appropriately set so that the seventh gear87revolves on the eighth gear88with a proper force and a proper speed according to rotation of the first shaft810. For example, the sixth gear86may be provided to have a greater dimension than that of the fifth gear85, the seventh gear87may be provided to have a smaller dimension than that of the sixth gear86, and the eighth gear88may be provided to have a greater dimension than that of the seventh gear87.

FIG. 5is a view illustrating a partial shape of the paper feeder according to an embodiment, andFIG. 6is a view illustrating an operating state of the paper feeder according to an embodiment.

Referring toFIGS. 5 and 6, in a preparation stage before the printing medium picking-up operation, the pick-up roller27according to an embodiment may be in contact with the printing medium S. When a command for the printing medium picking-up operation is generated by a controller (not shown), the pick-up roller27may pick up the printing medium S one sheet at a time by being rotated in the clockwise direction B by a driving force transmitted from the driving source800. At this point, when the feed clutch82is turned on, the driving force of the driving source800may be transmitted through the driving force transmitting unit801, the first shaft810, the first gear281, the third gear291, and the second gear271.

When the friction force between the pick-up roller27and the printing medium S is appropriate, the printing medium S may be picked up one sheet at a time by the pick-up roller27. Assuming the friction force between the pick-up roller27and the printing medium S when the printing medium S may be picked up one sheet at a time by the pick-up roller27as a reference value, a double feed of the printing medium S may occur when the friction force between the pick-up roller27and the printing medium S is greater than the reference value.

In a case of a special printing medium S having a special coating layer or a concavo-convex portion formed on a surface thereof, the printing medium S may be damaged by a pressing force applied by the pick-up roller27to the printing medium S.

According to an embodiment of the present invention, when the printing medium S is picked up, the pick-up roller27may be raised for a predetermined time and may be spaced apart from the printing medium S, or the pressing force applied by the pick-up roller27to the printing medium S may be reduced. The force of the driving source800may drive the seventh gear87may revolve along the eighth gear88in the counterclockwise direction A. When the seventh gear87revolves along the eighth gear88in the counterclockwise direction A, the other side of the swing member81at which the pick-up roller27is located may be raised. When the other side of the swing member81is raised, the pick-up roller27may be spaced apart from the printing medium S, or the pressing force applied by the pick-up roller27to the printing medium S may be reduced.

By raising the pick-up roller27for the predetermined time, a double feed of the printing medium S or damage to the surface of the special printing medium S may be prevented.

When the seventh gear87revolves along the eighth gear88in the clockwise direction B by the driving force of the driving source800, the other side of the swing member81is lowered, and the pick-up roller27may be returned to its original position by approaching the printing medium S. When the predetermined time passes, the pick-up roller27may be returned to its original location at which the printing medium S may be picked up.

FIG. 7is a view illustrating a state in which a sensor is provided at the paper feeder according to an embodiment, andFIG. 8is a schematic view illustrating a configuration of the paper feeder having the sensor.

Referring toFIGS. 7 and 8, sensors201and202may be provided at the paper feeder20according to an embodiment and may sense the printing medium S. The sensors201and202may sense the printing medium S, and may transmit a sensed result to the controller. The controller may use the sensed result to raise or lower the pick-up roller27.

Assuming that a direction in which the printing medium S is picked up and moved by the pick-up roller27is toward a front direction, the feed roller28is located in front of the pick-up roller27. The feed roller28may include a first feed roller28aand a second feed roller28bthat is spaced apart from the first feed roller28aand located in front of the first feed roller28a.

A retard roller29may be provided under the feed roller28. The retard roller29may be provided to be rotated in a direction opposite to a rotating direction of the feed roller28, or may be provided as an idle roller which is not rotated. A double feed of the printing medium S passing through the feed roller28may be prevented by the retard roller29. A first retard roller29amay be provided under the first feed roller28a, and a second retard roller29bmay be provided under the second feed roller28b.

One or a plurality of the sensors201and202may be provided. For example, the sensors201and202may include a first sensor201and a second sensor202. The first sensor201may be located between the first feed roller28aand the second feed roller28b. The second sensor202may be located in front of the second feed roller28b. Locations and the number of sensors201and202are not limited to the above description.

The controller may control the pick-up roller27to be raised or lowered using the sensed result from the sensors201and202. For example, when the printing medium S is picked up by the pick-up roller27, moved toward the first feed roller28a, and the printing medium S is sensed by the first sensor201, the controller may control the pick-up roller27to be raised.

When the printing medium S is moved toward the second feed roller28bby the first feed roller28aand then sensed by the second sensor202, the controller may control the pick-up roller27to be lowered. The pick-up roller27may be lowered and thus may be returned to its original location.

FIG. 9is a view illustrating a paper feeder according to an embodiment.

Referring toFIG. 9, a torque limiter89aprovided at a paper feeder20aaccording to another embodiment may be installed at the first shaft810. Similar to the torque limiter89according to the embodiment illustrated inFIG. 4, the torque limiter89aillustrated inFIG. 9may serve as a stopper.

The fifth gear85and the eighth gear88may be provided at the first shaft810, and the sixth gear86, which is engaged with the fifth gear85, and the seventh gear87, which is engaged with the eighth gear88, are provided at the third shaft811. A driving force transmitted to the first shaft810through the driving force transmitting unit801may be transmitted to the eighth gear88through the fifth gear85, the sixth gear86, and the seventh gear87.

The torque limiter89ais installed at the first shaft810, and when the magnitude of the driving force transmitted from the seventh gear87to the eighth gear88is a predetermined value or more, the revolving of the seventh gear87on the eighth gear88may be hindered.

When the rod clutch84is in the ON state and the magnitude of the driving force transmitted to the seventh gear87is the predetermined value or less, the seventh gear87may revolve on the eighth gear88to move the swing member81. The pick-up roller27may be raised or lowered according to a revolving direction of the seventh gear87.

FIG. 10is view illustrating a paper feeder according to still another embodiment.

Referring toFIG. 10, in a paper feeder20baccording to still another embodiment, the rod clutch84may not be provided at the first shaft810. The paper feeder20bmay further include a fourth shaft812which is located to be spaced apart from the first shaft810. A rod clutch84amay be provided at the fourth shaft812. The fourth shaft812may receive a driving force from the driving source800by being connected to the driving force transmitting unit801.

A ninth gear812aselectively engaged with the fifth gear85provided at the first shaft810may be provided at the fourth shaft812. The ninth gear812amay be provided to be moved along the fourth shaft812.

A tenth gear812bengaged with the fourth gear83provided at the first shaft810may be further provided at the fourth shaft812. The tenth gear812bmay connect the fourth gear83with the driving force transmitting unit801. The driving force of the driving source800may be transmitted to the first shaft810through the driving force transmitting unit801, the tenth gear812b, and the fourth gear83. The first shaft810may be rotated by the driving force received through the tenth gear812b. The driving force transmitted to the first shaft810may be transmitted to the pick-up roller27through the first gear281, the third gear291, and the second gear271. The pick-up roller27receiving the driving force may be rotated and may pick up the printing medium S.

When the rod clutch84ais in the ON state, the swing member81may be moved by receiving the driving force. When the rod clutch84ais in the ON state, the ninth gear812amay be engaged with the fifth gear85. When the ninth gear812ais engaged with the fifth gear85, the driving force of the driving source800may be transmitted to the fifth gear85. The driving force transmitted to the fifth gear85may be transmitted to the third shaft811through the sixth gear86. By the driving force being transmitted to the third shaft811, the seventh gear87may move the swing member81while being rotated about the eighth gear88in a clockwise direction or counterclockwise direction. Accordingly, the pick-up roller27may be raised or lowered.

When the rod clutch84ais in the OFF state, the ninth gear812amay be spaced apart from the fifth gear85by being moved along the fourth shaft812. Since the ninth gear812aand the fifth gear85are not engaged with each other, the driving force is not transmitted to the fifth gear85and is also not transmitted to the third shaft811. When the rod clutch84ais in the OFF state, a raising or lowering of the pick-up roller27is not performed.

FIG. 11is a view illustrating a paper feeder according to yet another embodiment.

Referring toFIG. 11, in a paper feeder20caccording to yet another embodiment, the torque limiter89amay not be provided at the third shaft811. The paper feeder20cmay include a fifth shaft813which is located to be spaced apart from the third shaft811. The torque limiter89amay be installed at the fifth shaft813.

The fifth shaft813may have the sixth gear86which receives a driving force and the seventh gear87provided to receive the driving force from the sixth gear86and to revolve on the eighth gear88. The torque limiter89amay be provided at the fifth shaft813and may serve as a stopper. When a magnitude of the driving force input to the sixth gear86is a predetermined value or more, the torque limiter89amay enable the driving force not to be transmitted to the fifth shaft813.

A connection gear90engaged with the fifth gear85and the sixth gear86may be provided at the third shaft811. When the magnitude of the driving force input to the sixth gear86through the fifth gear85and the connection gear90is the predetermined value or less, the driving force may be transmitted to the eighth gear88through the seventh gear87. When the driving force is transmitted to the eighth gear88, the third shaft811may be rotated.

A first cam91may be provided at the third shaft811. A second cam92, which may be pressed by the first cam91, may be provided at the swing member81. The first cam91may press the second cam92due to a rotation of the third shaft811, and thus the other side of the swing member81may be lowered or raised. Accordingly, the pick-up roller27located at the other side of the swing member81may be lowered or raised.

A location of the torque limiter, the rod clutch, or the like is not limited to the above description and the present invention may be realized in various structures.

FIGS. 12A-12Eare graphs illustrating operations of the paper feeder in a separating mode, andFIGS. 13A-130are graphs illustrating operations of the pick-up roller of the paper feeder in the separating mode.

FIGS. 12A-12EandFIGS. 13A-130are graphs of an embodiment in which the pick-up roller27is spaced apart from the printing medium S by the swing member81when the pick-up roller27picks up the printing medium S.

FIG. 12Ais a graph illustrating an ON/OFF operation of the driving source800over time.FIG. 12Bis a graph illustrating an ON/OFF operation of the feed clutch82over time.FIG. 12Cis a graph illustrating an ON/OFF operation of the rod clutch84over time.FIG. 12Dis a graph illustrating whether the printing medium S is sensed by the first sensor201over time.FIG. 12Eis a graph illustrating whether the printing medium S is sensed by the second sensor202over time.

FIG. 13Ais a graph illustrating a magnitude of force applied by the pick-up roller27to the printing medium S when the feed clutch82is turned on.FIG. 13Bis a graph illustrating a magnitude of a force which is applied by the swing member81to raise the pick-up roller27.FIG. 130is a graph illustrating a sum of the force at the pick-up roller27.

When the driving source800is turned on, a driving force may be transmitted to the first shaft810by the driving force transmitting unit801. After the driving source800is turned on, the controller may control the feed clutch82to be turned on. A time point at which the feed clutch82is turned on may be referred to as t1. When the feed clutch82is turned on, the pick-up roller27may be rotated by the driving force transmitted to the first shaft810. The pick-up roller27may pick up the printing medium S while being rotated.

At this point, a force applied by the pick-up roller27to the printing medium S may be +F1.

The printing medium S may be picked up by the pick-up roller27and then conveyed toward the first feed roller28a. When the printing medium S is sensed by the first sensor201, the controller may control the rod clutch84to be turned on. A time point at which the first sensor201starts to sense the printing medium S may be referred to as t2. The rod clutch84may be turned on at the same time as the printing medium S is sensed by the first sensor201. Therefore, a time point at which the rod clutch84is turned on may be referred to as t2.

When the rod clutch84is turned on, the driving force may be transmitted by the swing member81and the pick-up roller27may be raised. At this point, a force which raises the pick-up roller27is −F2. In −F2, “−” indicates a force in a direction opposite to a direction of the force when the pick-up roller27presses the printing medium S.

When the rod clutch84is in the ON state, a sum of the force at the pick-up roller27may be −F3. −F3is a sum of +F1and −F2. Since the force which raises the pick-up roller27is greater than the force applied by the pick-up roller27to the printing medium S, the sum of the force at the pick-up roller27is −F3. Since the sum of the force at the pick-up roller27has a negative value, the pick-up roller27is in a spaced apart state from the printing medium S.

A time point at which the printing medium S is conveyed by the first feed roller28aand the second feed roller28band at which the second sensor202starts to sense the printing medium S may be referred to as t3. After the printing medium S is sensed by the second sensor202, the rod clutch82may be turned off. When the rod clutch82is turned off, the swing member81may be returned to its original location by the elastic force of the elastic member802. The pick-up roller27may be lowered with the swing member81and may come into contact with the printing medium S. A time point at which the rod clutch82is turned off may be referred to as t4.

At this point, the force which raises the pick-up roller27may be 0, and the force applied by the pick-up roller27to the printing medium S may be +F1again. The pick-up roller27may be spaced apart from the printing medium S by being raised for a time of t4−t1.

When the printing medium S is not sensed any more by the second sensor202, the controller may control the driving source800to be turned off. A time point at which the printing medium S is finally sensed by the second sensor202may be referred to as t5, and a time point at which the driving source800is turned off may be referred to as t6.

As described above, after the printing medium S is picked up by the pick-up roller27, the pick-up roller27is spaced apart from the printing medium S for a predetermined time (t4−t1) before the entire printing medium S passes through the pick-up roller27, and thus a double feed of the printing medium S may be effectively prevented.

FIGS. 14A-14Eare graphs illustrating operations of the paper feeder in a decompression mode, andFIGS. 15A-15Care graphs illustrating operations of the pick-up roller of the paper feeder in the decompression mode.

FIGS. 14A-14EandFIGS. 15A-15Care graphs illustrating a case in which a magnitude of the force applied by the pick-up roller27to the printing medium S is reduced when the pick-up roller27picks up the printing medium S.

FIG. 14Ais a graph illustrating the ON/OFF operation of the driving source800over time.FIG. 14Bis a graph illustrating the ON/OFF operation of the feed clutch82over time.FIG. 14Cis a graph illustrating the ON/OFF operation of the rod clutch84overtime.FIG. 14Dis a graph illustrating whether the printing medium S is sensed by the first sensor201over time.FIG. 14Eis a graph illustrating whether the printing medium S is sensed by the second sensor202over time.

FIG. 15Ais a graph illustrating the magnitude of the force applied by the pick-up roller27to the printing medium S when the feed clutch82is turned on.FIG. 15Bis a graph illustrating the magnitude of the force applied by the swing member81to raise the pick-up roller27.FIG. 150is a graph illustrating the sum of the force at the pick-up roller27.

When the driving source800is turned on, a driving force may be transmitted to the first shaft810by the driving force transmitting unit801. After the driving source800is turned on, the controller may control the feed clutch82to be turned on. A time point at which the feed clutch82is turned on may be referred to as t1. When the feed clutch82is turned on, the printing medium S is picked up by the pick-up roller27, and the force applied by the pick-up roller27to the printing medium S may be +F1.

When the feed clutch82is turned on, the rod clutch84may also be turned on. The rod clutch84may be turned on simultaneously with the feed clutch82, or may be turned on before the feed clutch82is turned on. When the rod clutch84is in the ON state, the force which raises the pick-up roller27is −F4. F4may have a value smaller than F1. Therefore, the sum of the force at the pick-up roller27may be +F5. In +F5, “+” indicates that a force that the pick-up roller27presses the printing medium S with has a magnitude of F5.

The printing medium S may be pressed and picked up with the magnitude of F5, which is smaller than F1, by the pick-up roller27.

When the printing medium S is sensed by the first sensor201, the rod clutch84may be turned off. A time point at which the rod clutch84is turned off may be referred to as t3. When the rod clutch84is turned off, the pick-up roller27may be lowered by the elastic force of the elastic member802, and may pick up the printing medium S while pressing the printing medium S with the force of +F1.

A case in which the rod clutch84is turned off when the printing medium S is sensed by the first sensor201(t3) has been described. However, the time point at which the rod clutch84is turned off may be later than t3.

Accordingly, since the force applied to the printing medium S is reduced when the pick-up roller27picks up the printing medium S, the printing medium S may be prevented from being damaged when the special printing medium S having a coating layer or a concavo-convex portion is picked up.

The control method of the paper feeder inFIGS. 12A-12EtoFIGS. 15A-15Care just examples, and the paper feeder of the present invention may be controlled in various ways.

The image forming apparatus according to an embodiment can stably supply the printing medium without double feeding the printing medium, and also can prevent the printing medium from being damaged.

Although a few embodiments of the present invention have been shown and described, it should be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.