Sheet feeder and image forming apparatus

1. A sheet feeder comprising:

a feed roller that includes a feed roller shaft, the feed roller being capable of feeding a sheet;

a frame that supports the feed roller shaft;

a helical gear attached to the feed roller shaft;

a motor that drives the feed roller shaft via the helical gear;

a disc attached to the feed roller shaft;

a sensor body that detects a rotation of the disc;

a pressing member that presses the feed roller shaft, to which both the disc and the helical gear are attached, in a thrust direction of the feed roller shaft; and

a bearing shaft that is mounted to the frame and rotatably supports the feed roller shaft, wherein the pressing member presses the feed roller shaft such that a part of the helical gear contacts with one end of the bearing shaft.

2. The sheet feeder according to claim 1 ,

wherein the pressing member is a coil spring disposed coaxially with the feed roller shaft.

3. The sheet feeder according to claim 2 , wherein the disc passes through a slit in the sensor body.

4. The sheet feeder according to claim 2 , wherein the feed roller shaft penetrates through the coil spring.

5. The sheet feeder according to claim 2 , wherein the motor comprises: a drive shaft; and a drive helical gear that is attached to the drive shaft and meshes with the helical gear, and wherein when the motor rotates in a normal direction to feed the sheet, a thrust force produced by the drive helical gear acts upon the feed roller shaft to separate the helical gear from the feed roller shaft.

6. The sheet feeder according to claim 2 , wherein the pressing member presses the feed roller shaft with a force greater than a thrust force applied to the feed roller shaft.

7. The sheet feeder according to claim 2 , wherein the sensor body is attached to the frame.

8. The sheet feeder according to claim 2 , wherein the frame is disposed between the helical gear and the pressing member.

9. The sheet feeder according to claim 1 , wherein the pressing member contacts with another end of the bearing shaft.

10. The sheet feeder according to claim 1 ,

wherein the feed roller shaft further comprises a first fixing member provided on an outer circumference thereof, wherein the bearing shaft further comprises a second fixing member provided on an end thereof, and wherein the pressing member is interposed between the first fixing member and the second fixing member.

11. The sheet feeder according to claim 1 ,

wherein the frame is disposed between the disc and the pressing member.

12. A sheet feeder comprising:

a feed roller that includes a feed roller shaft, the feed roller being capable of feeding a sheet;

a frame that supports the feed roller shaft;

a helical gear attached to the feed roller shaft;

a motor that drives the feed roller shaft via the helical gear;

a disc attached to the feed roller shaft;

a sensor body that detects a rotation of the disc; and

a pressing member that presses the feed roller shaft, to which both the disc and the helical gear are attached, in a thrust direction of the feed roller shaft;

wherein the pressing member is a coil spring disposed coaxially with the feed roller shaft,

wherein the motor comprises: a drive shaft; and a drive helical gear that is attached to the drive shaft and meshes with the helical gear,

wherein when the motor rotates in a normal direction to feed the sheet, a thrust force produced by the drive helical gear acts upon the feed roller shaft to separate the helical gear from the feed roller shaft, and

wherein the drive shaft is pressed in a direction so as to be pulled into the motor, and wherein a direction of a reaction force, reactive to the thrust force, acted upon the feed roller shaft is in a same direction as the drive shaft is pressed in.

13. An image forming apparatus comprising:

a sheet feed unit that feeds stacked sheets one at a time;

an image forming unit that forms an image on a sheet fed from the sheet feed unit, the image forming unit comprising a sheet feeder comprising:

a feed roller that includes a feed roller shaft, the feed roller being capable of feeding a sheet;

a frame that supports the feed roller shaft;

a helical gear attached to the feed roller shaft;

a motor that drives the feed roller shaft via the helical gear;

a disc attached to the feed roller shaft;

a sensor body that detects a rotation of the disc;

a pressing member that presses the feed roller shaft, to which both the disc and the helical gear are attached, in a thrust direction of the feed roller shaft; and

a bearing shaft that is mounted to the frame and rotatably supports the feed roller shaft; and

a sheet ejector that ejects the sheet on which the image is formed;

wherein the pressing member is a coil spring disposed coaxially with the feed roller shaft, and wherein the pressing member presses the feed roller shaft such that a part of the helical gear contacts with one end of the bearing shaft.