This application claims the benefit of Korean Application No.  2002-12988, filed Mar. 11, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
1.  Field of the Invention
The present invention relates to an electrophotographic image forming apparatus having a duplex function, and more particularly, to a drive mechanism for an electrophotographic printer capable of operating an image forming apparatus using a low capacity motor and efficiently having a duplex function. 
2.  Description of the Related Art
An electrophotographic image forming apparatus is an apparatus, such as a printer or copier, that prints by forming an electrostatic latent image on a photosensitive body using a laser, developing this image using a developing agent, and transferring the image to an image transfer body.  The present invention will be explained by referring to a laser printer, however, the present invention may be applied to any electrophotographic image forming apparatus. 
Referring to FIG.  1, an electrophotographic image forming apparatus having a duplex printing function has a paper feeding unit, a developing unit, a fixing unit, a delivery unit and a duplex printing unit.  This apparatus also includes a drive mechanism, which may be the known drive mechanism of FIG.  2, or the drive mechanism according to an embodiment of the present invention, as shown in FIGS.  3 and 4. 
The paper feeding unit supplies an image transfer body (such as a sheet of paper) and includes a paper feeding cassette 1, a feeding unit transfer roller 5, and a registration roller 7.  Also, the paper-feeding unit can include a manual paper feeding tray 13 that can supply various types of image transfer bodies. 
The paper feeding cassette 1 stacks a large number of the image transfer bodies and has a pickup roller 3 at the front end thereof to supply the stacked image transfer bodies one by one.  The feeding unit transfer roller 5 conveys the image transfer body supplied by the pickup roller 3 to the registration roller 7.  The registration roller 7 aligns the front end of the image transfer body conveyed from the feeding unit transfer roller 5 and supplies the image transfer body to the developing unit.  The manual paper feeding tray 13 supplies various types of image transfer bodies such as OHP film, an envelope or a labeling sheet, and conveys the image transfer body stacked on the manual paper feeding tray 13 to the registration roller 7, using a multipurpose pickup roller 11 installed at the front end thereof. 
The developing unit forms an image on the image transfer body according to the printing content transferred from a computer, and includes a laser scan unit 20, a developing device 19, an OPC drum 15, and a developing unit transfer roller 17. 
The laser scan unit 20 is installed above the OPC drum 15 and scans a laser beam corresponding to the printing content according to a command from an image forming apparatus controller (not shown) on the OPC drum 15. 
The OPC drum 15 has a surface on which an electrostatic latent image is formed according to the laser beam and is installed above the developing unit transfer roller 17.  The electrostatic latent image is developed as a visible image by the developing agent supplied by the developing device 19 as the OPC drum 15 rotates, and the developed visible image is transferred to the image transfer body by the developing unit transfer roller 17.  The developing device 19 has the developing agent and develops an electrostatic latent image to a visible image by supplying the developing agent to the OPC drum 15. 
The fixing unit fixes an image, transferred by the developing unit to the image transfer body, on the image transfer body using heat and pressure, and has a fixing roller 21 and a pressure roller 23. 
A delivery unit discharges the image transfer body from the fixing unit when printing is completed and has a first delivery roller 25, a second delivery roller 27, a third delivery roller 31 and a delivery convey guide 33. 
In the case of duplex printing, a duplex printing unit returns a one-side printed image transfer body to the transfer roller 5.  Then the image transfer body returned to the transfer roller 5 passes the developing unit and the fixing unit to print on the other side.  The duplex printing unit has a first duplex roller 35, a first duplex convey guide 37, a second duplex convey guide 39, and a second duplex roller 41 to return the image transfer body.  The first duplex roller 35 is installed in contact with the second delivery roller 27, with an idle roller 29 therebetween. 
A drive mechanism driving each structural unit of the electrophotographic image forming apparatus as described above will now be explained.  The drive mechanism is installed at one side of each structural unit. 
Referring to FIG.  2, a conventional drive mechanism of the electrophotographic image forming apparatus capable of duplex printing of FIG.  1 has a first driving part and a second driving part. 
The first driving part drives all described structural units except the third delivery roller 31 and has a first driving motor 50, a first gear train 51, and a second gear train 71. 
The first gear train 51 transmits the power of the first driving motor 50 to the pickup roller 3, the transfer roller 5, and the registration roller 7 and has an intermediate gear train 55 and a pickup roller gear train 59.  If the manual paper feeding tray 13 is attached, the first gear train 51 additionally has a multipurpose pickup roller gear train 69 for driving the manual paper feeding tray 13. 
A gear 52 at one end of the intermediate gear train 55 is engaged with a pinion 50xe2x80x2 of the first driving motor 50.  An intervening gear 54 at the other end of the intermediate gear train 55 is engaged with a registration gear 63 coaxially connected with the registration roller 7 and a transfer roller gear train 61 coaxially connected with the transfer roller 5.  A gear 56 at one end of the pickup roller gear train 59 is engaged with the intermediate gear train 55 and the other end includes a pickup roller gear 58 coaxially connected with the pickup roller 3.  One end of the multipurpose pickup roller gear train 69 is a reverse roller gear 65 engaged with the transfer roller gear train 61 and the other end includes a multipurpose pickup roller gear 67 coaxially connected with the multipurpose pickup roller 11.  The reverse roller gear 65 is coaxially connected with a reverse roller 9 conveying the image transfer body together with the feeding unit transfer roller 5. 
The second gear train 71 transmits the power of the first driving motor 50 to the OPC drum 15, the fixing roller 21, the first delivery roller 25, the first duplex roller gear 35, and the second duplex roller 41, and has an OPC drum gear train 75, a fixing roller gear train and a second duplex roller gear train 93 having gears 91 and 92. 
The OPC drum gear train 75 has a transmission gear 72 at one end, which is engaged with the pinion 50xe2x80x2 of the first driving motor 50, and an OPC drum gear 74 at the other end, which is coaxially connected with the OPC drum 15.  The fixing roller gear train includes sub-fixing roller gear train 80, a first delivery roller gear train 85 and a first duplex roller gear train 89.  The sub-fixing roller gear train 80 has a gear 77 at one end, which is engaged with the transmission gear 72 of the OPC drum gear train 75, and a gear at the other end, which includes a fixing roller gear 79 coaxially connected with the fixing roller 21.  A gear 82 at one end of the first fixing roller gear train 85 is engaged with the fixing roller gear 79, and the other end includes a first delivery roller gear 84, coaxially connected with the first delivery roller 25.  A gear 86 at one end of the first duplex roller gear train 89 is engaged with a gear of the first delivery roller gear train 85, and the other end includes a first duplex roller gear 88 coaxially connected with the first duplex roller 35. 
The second driving part is for driving the third delivery roller 31 and includes a second driving motor 95 and a third delivery roller gear train 99. 
A gear 96 at one end of the third delivery roller gear train 99 is engaged with a pinion 95xe2x80x2 of the second driving motor 95 and a gear at the other end includes a third delivery roller gear 98 coaxially connected with the third delivery roller 31. 
Hereinbelow, the operation of the conventional drive mechanism will be described. 
When a printing order is received from the computer, the controller of the image forming apparatus drives the first driving motor 50 and the second driving motor 95. 
If the first driving motor 50 rotates, the gear 52 at one end of the intermediate gear train 55 engaged with the pinion 50xe2x80x2 of the first driving motor 50 is rotated and the intervening gear 54 at the other end is rotated by a connecting mediate gear. 
If the intervening gear 54 rotates, a gear 56 at one end of the pickup roller gear train 59 engaged with the intervening gear 54, transfer roller gear 61 and the registration roller gear 63 rotates.  If the gear 56 at one end of the pickup roller gear train 59 rotates, the pickup roller gear 58 rotates accordingly.  If the pickup roller gear 58 rotates, the pickup roller 3 coaxially connected with the pickup roller gear 58 rotates and the image transfer body stacked on the paper feeding cassette 1 is separated and conveyed to the feeding unit transfer roller 5.  As the transfer roller gear train 61 and the registration roller gear 63 rotate in association with the rotation of the intervening gear 54, the feeding unit transfer roller 5 and the registration roller 7 coaxially connected with these gears rotate.  Therefore, the image transfer body conveyed from the pickup roller 3 is supplied to the developing unit passing the feeding unit transfer roller 5 and the registration roller 7. 
Meanwhile, if the paper feeding unit has the manual paper feeding tray 13, the multipurpose pickup roller gear train 69 is engaged with the transfer roller gear 61.  Accordingly, the rotation of the first driving motor 50 rotates the multipurpose pickup roller gear train 67 through the reverse roller gear 65 engaged with the transfer roller gear train 61.  If the multipurpose pickup roller gear 67 rotates, the multipurpose pickup roller 11 coaxially connected thereto rotates, and therefore the image transfer body stacked on the manual paper feeding tray 13 is separately supplied to the registration roller 7.  The controller of the image forming apparatus selects which image transfer body to supply using a sensor (not shown) when the image transfer body is stacked on both the paper feeding cassette 1 and the manual paper feeding tray 13. 
In addition, if the first driving motor 50 rotates, the transmission gear 72 of the OPC drum gear train 75 engaged with the pinion 50xe2x80x2 of the first driving motor 50 rotates.  If the transmission gear 72 rotates, the OPC drum gear 74, which is the other end gear of the OPC drum gear train 75, rotates by the associated gear train.  If the OPC drum gear 74 rotates, the OPC drum 15 coaxially connected to the OPC drum gear 74 rotates.  If the OPC drum 15 rotates, an electrostatic latent image generated on the surface of the OPC drum 15 is developed as a visible image by a developing device and transferred to the image transfer body between the OPC drum 15 and the developing unit transfer roller 17 by the registration roller 7. 
Additionally, if the transmission gear 72 rotates, the sub-fixing roller gear train 80 engaged with the transfer gear 72 also transmits the rotation.  Accordingly, the other end gear of the sub-fixing roller gear train 80 (the fixing roller gear 79) rotates and the fixing roller 21 coaxially connected to the fixing roller gear 79 also rotates.  If the fixing roller 21 rotates, the image transfer body which has passed the OPC drum 15 passes between the fixing roller 21 and the pressure roller 23 installed below the fixing roller 21.  At this point, the image transferred to the image transfer body is fixed by heat and pressure. 
Furthermore, if the fixing roller gear 79 rotates, the first delivery roller gear train 85 engaged with the fixing roller gear 79 rotates and also the first duplex roller gear train 89 engaged with a gear of the first delivery roller gear train 85 rotates.  Then, the first delivery roller gear 84 installed at the other end of the first delivery roller gear train 85 and the first duplex roller gear 88 installed at the other end of the first duplex roller gear train 89 also rotate.  If the first duplex roller gear 88 rotates, the second delivery roller gear (not shown) also rotates by the idle gear (not shown).  Therefore, since the first delivery roller 25 and the second delivery roller 27 each coaxially connected with the first delivery roller gear 84 and the second delivery roller gear also rotate, the image transfer body which has passed the fixing roller 21 is conveyed to the third delivery roller 31 along the delivery convey guide 33 by the driving force of the first and second delivery rollers 25, 27. 
The third delivery roller 31 rotates by the third delivery roller gear train 99 engaged with the second driving motor 95.  The second driving motor 95 is driven simultaneously with the first driving motor 50 or may be driven by the controller of the image forming apparatus before the image transfer body enters the third delivery roller 31. 
Hence, if the second driving motor 95 rotates, a gear at one end of the third delivery roller gear train 99 engaged with the pinion 95xe2x80x2 of the second driving motor 95 rotates and the third delivery roller gear 98 at the other end rotates by the associated gear train.  Then the third delivery roller 31 coaxially connected with the third delivery roller gear 98 rotates and consequently the image transfer body conveyed from the second delivery roller 27 is discharged to the outside. 
In the case of duplex printing, before the end of the image transfer body passes the third delivery roller 31, the third delivery roller 31 rotates in reverse and has the front page printed image transfer body enter the duplex printing unit.  The third delivery roller 31 is rotated in reverse by the reverse rotation of the second driving motor 95 when the end of the image transfer body passes the third delivery roller 31, exits the delivery convey guide 33 and rotates upward by its own rigidity, thereby being able to enter the duplex printing unit. 
In the duplex printing unit, the image transfer body enters the first duplex convey guide 37 via the first duplex roller 35, rotating due to the first driving motor 50.  After passing the first duplex convey guide 37, the image transfer body is conveyed to the feeding unit transfer roller 5 by the second duplex convey guide 39 and the second duplex roller 41.  The second duplex roller 41 also rotates in the direction to which the image transfer body is conveyed to the feeding unit transfer roller 5 by the first driving motor 50.  The front page printed image transfer body conveyed to the feeding unit transfer roller 5 is delivered to the outside by the third delivery roller 31 with the other side being printed as it undergoes the same process described above. 
Therefore, the drive mechanism of the conventional electrophotographic image forming apparatus is able to print the front side of the image transfer body by unidirectionally rotating the first driving motor 50 and the second driving motor 95.  Also, in the case of duplex printing, the back side can be printed by rotating the second driving motor 95 in reverse at a certain point and the front page printed image transfer body is then conveyed to the duplex printing unit. 
However, since the conventional drive mechanism has the first driving motor 95 set to drive the pickup roller 3, the transfer roller, the multipurpose pickup roller, the registration roller 7, the OPC drum 15, the fixing roller 21, the first and second delivery rollers 25, 27, and the first and second duplex rollers 35, 39, the first driving motor 95 drives 90xcx9c95% of the total load of the image forming apparatus.  Hence, the first driving motor 95 requires a large capacity and the manufacturing cost is high because large capacity motors are expensive. 
In addition, because the fixing roller 21 continuously rotates from the point the image transfer body is supplied by the pickup roller 3, abrasion of the fixing roller 21 increases due to high heat and pressure, and image staining, such as offset, or electronic instability may occur due to the increase in static electricity. 
Accordingly, it is an object of the present invention to overcome the above problems of the conventional drive mechanism. 
It is another object of the present invention to provide a drive mechanism of an electrophotographic image forming apparatus that is inexpensive to produce by balancing the load of the first driving motor and the second driving motor, and using a low capacity and inexpensive motor for the first driving motor. 
It is still another object of the present invention to provide an electrophotographic image forming apparatus having high efficiency during duplex printing by improving the transfer speed of the image transfer body during the duplex printing. 
It is yet another object of the present invention to provide a drive mechanism of an electrophotographic image forming apparatus having improved lifespan of the fixing roller by rotating this element only when necessary. 
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
The foregoing and other objects of the present invention are achieved by providing a drive mechanism of an electrophotographic image forming apparatus to form an image on an image transfer body, including a first driving part, including a conveyor to convey the image transfer body, an image forming apparatus to form the image on the image transfer body, a first driving motor to drive the conveyor and the image forming apparatus, and a first power transmitter to transmit power generated in the first driving motor to the conveyor and the image forming apparatus; and a second driving part, including a fixing device to fix the formed image on the image transfer body, a delivery roller to discharge the image transfer body having the image fixed thereon, a conveying roller to convey the image transfer body for duplex printing, a second driving motor to drive the fixing device, the delivery roller, and the conveying roller, and a second power transmitter to transmit power generated in the second driving motor to the fixing device, the delivery roller, and the conveying roller. 
Furthermore, the second power transmitter may include a unidirectional power transmitting unit to prevent power of the second driving motor from being transmitted to the fixing device and the delivery roller when the second driving motor rotates in the direction in which the image transfer body is conveyed during duplex printing.  The unidirectional power transmitter may be a ratchet gear. 
Additionally, the second driving motor may have a rotation speed in a duplex printing direction set to be higher than a rotation speed in the opposite direction, and to be between 1.5 and 2 times higher than the rotation speed in the opposite direction. 
The foregoing and other objects of the present invention may also be achieved by providing a drive mechanism of an electrophotographic image forming apparatus to form an image on an image transfer body, the drive mechanism including a first driving part, including a conveyor to convey the image transfer body, an image-forming device to form the image on the image transfer body, a first driving motor to drive the conveyor and the image forming apparatus, and rotating unidirectionally, and a first power transmitter to transmit power generated in the first driving motor to the conveyor and the image forming apparatus; and a second driving part, including a fixing device to fix the image on the image transfer body, a delivery roller to discharge the image transfer body having the image fixed thereon, a conveying roller to convey the image transfer body for duplex printing, a second driving motor to drive the fixing device, the delivery roller, and the conveying roller and rotating bidirectionally, and a second power transmitter to transmit power generated in the second driving motor to the fixing device, the delivery roller, and the conveying roller; wherein the second power transmitter includes a unidirectional power transmitting unit to prevent power of the second driving motor from being transmitted to the fixing device and the delivery roller when the second driving motor rotates in a duplex printing direction in which the image transfer body is transferred. 
The unidirectional power transmitter may be a ratchet gear. 
The second driving motor may have rotation speed in a duplex printing direction which is higher than a rotation speed in the opposite direction, and may be between 1.5 and 2 times higher than the rotation speed in the opposite direction. 
The foregoing and other objects of the present invention may also be achieved by providing an electrophotographic image forming apparatus including a drive mechanism to form an image on a plurality of image transfer bodies, the drive mechanism including a first driving motor including a pinion; a paper feeding cassette; a pickup roller; a conveying roller; a registration roller; first and second duplex rollers; a fixing roller; first and third delivery rollers; an OPC (Organic Photo Conductor) drum; a first gear train, engaged with the pinion of the first driving motor to transmit power of the first driving motor to the pickup roller to thereby separate and convey the image transfer bodies stacked in the paper feeding cassette one by one to the conveying roller, which conveys the image transfer body conveyed by the pickup roller in a printing direction, and to the registration roller aligning and conveying the image transfer body conveyed from the conveying roller; a second gear train, engaged with the pinion of the first driving motor transmitting the power of the first driving motor to the OPC drum, transmitting an image to a front page of the image transfer body conveyed from the registration roller and to the second duplex roller conveying the front page transmitted image transfer body to the conveying roller; a second driving motor including a pinion; a third gear train, engaged with the pinion of the second driving motor, transmitting power of the second driving motor to the fixing roller fixing the transmitted image, and to the first delivery roller delivering the image transfer body conveyed from the fixing roller; and a fourth gear train, engaged with the pinion of the second driving motor, transmitting the power of the second driving motor to the third delivery roller, delivering the image transfer body conveyed from the first delivery roller to an outside. 
The first gear train may include a registration gear coaxially connected with the registration roller; an intermediate gear train including a first mediate gear at a first end thereof engaged with the pinion of the first driving motor, and an intervening gear at a second end thereof and engaged with the registration gear; a transfer roller gear engaged with the intervening gear of the intermediate gear train and coaxially connected with the transfer roller; and a pickup roller gear train including a pickup roller gear having a gear at a first end thereof engaged with the intervening gear of the intermediate gear train and a second end coaxially connected with the pickup roller. 
The second gear train may include an OPC drum gear train including an OPC gear having a second mediate gear at a first end thereof engaged with the pinion of the first driving motor and a second end coaxially connected with the OPC drum; and a second duplex roller gear train including a second duplex roller gear having a gear at a first end thereof engaged with the second mediate gear and a second end coaxially connected with the second duplex roller. 
The third gear train may include a first delivery roller gear train including a first delivery roller gear having a third mediate gear at a first end thereof engaged with the pinion of the second driving motor and a second end coaxially connected with the first delivery roller; and a fixing roller gear train including a fixing roller gear having a gear at a first end thereof engaged with the third mediate gear of the first delivery roller gear train and a second end coaxially connected with the fixing roller. 
The fourth gear train may include a third delivery roller gear train including a third delivery roller gear having a fourth mediate gear at a first end thereof engaged with the pinion of the second driving motor and a second end coaxially connected with the third delivery roller; and a first duplex roller gear train including a first duplex roller gear having a first end engaged with the fourth mediate gear and a second end coaxially connected with the first duplex roller. 
A unidirectional power transmitting unit may be interposed between the pinion of the second driving motor and the third gear train to selectively transmit the unidirectional rotation of the second driving motor to the third gear train.  The unidirectional power transmitting unit may be either a ratchet gear or a one-way clutch.  Additionally, the unidirectional power transmitting unit may transmit power to the third gear train when the second driving motor rotates the fixing roller in the printing direction and does not transmit power when the second driving motor rotates in the opposite direction. 
The drive mechanism of an electrophotographic image forming apparatus may have a multipurpose pickup roller gear train engaged with the transfer roller gear to drive a multipurpose pickup roller.  One end of the multipurpose pickup roller gear train is a reverse rotation roller gear that is engaged with the transfer roller gear and coaxially connected to a reverse roller, and the other end is a multi-purpose pickup roller gear coaxially connected to the multi-purpose pickup roller gear.