Abstract:
A paper feed for a printer includes a frame, a platen rotatably mounted on the frame, a friction roller support juxtaposed to the platen, a spring mounted on the frame and having a first operable condition biasing the friction roller support toward the platen with a first biasing force and having a second operable condition biasing the friction roller support toward the platen with a second biasing force less than the first biasing force, and a cam rotatably mounted on the frame and operable between first and second operable positions to effect disposition of the spring in said first and second operating conditions.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a paper feed device for a printer, for example, provided with both a tractor feed mechanism and a friction feed mechanism serving as paper feed means. 
     In conventional printers equipped with a tractor feed mechanism and a friction feed mechanism, there is also provided a mechanism for changing the resilient contacting force of a friction roller with a platen in accordance with the mode of paper feed. With this changing mechanism, the friction roller is forced to resiliently contact the platen strongly in a friction feed mode and weakly in a tractor feed mode. For example, Japanese Patent Application No. 60-243323, filed by the present applicant, discloses a mechanism for changing the resilient contacting force of a friction roller with a platen, wherein there is disclosed a cam having a complicated shape, a cam follower, several levers, and a biasing spring. 
     Such a mechanism is used to control the frictional force between a recording paper and the platen for a tractor feed mode and a friction feed mode. However, this conventional arrangement is complicated in structure, requires a long assembly time, and can hardly set the frictional force between the recording paper and the platen to a proper level in accordance with the mode of paper feed. 
     Accordingly, it is an object of the present invention to make it easy to set the frictional force between a recording paper and a platen to a proper level for a tractor feed mode and for a friction feed mode using a simplified structure, and to improve the efficiency of assembly. 
     SUMMARY OF THE INVENTION 
     To achieve the foregoing object, a paper feed device according to the present invention comprises a friction roller in resilient contact with the outer surface of a platen, a movably mounted supporting member disposed at the outer surface of the platen for rotatably supporting the friction roller, a biasing spring for urging the friction roller toward the platen via the supporting member, the biasing spring having tongue portions connected to leg portions of the supporting member and the portion of the spring which excludes the tongue portions is contactable with contacting portions of the supporting member, and rotatable cam means are provided which have a first rotational position engageable with the biasing spring to separate the contacting portions of the supporting member from the biasing spring, and a second rotational position disengageable from the biasing spring to allow the biasing spring to contact the contacting portions. 
     At the first rotational position, the cam means separates the contacting portions of the supporting member from the biasing spring so that the friction roller is pressed against the platen by the weaker spring force of only the tongue portions. At the second rotational position, the cam means brings the contacting portions of the supporting member into contact with the biasing spring so that the friction roller is pressed against the platen by the stronger spring force of the whole biasing spring. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings show an embodiment of the present invention in which: 
     FIG. 1 is a front view in a friction feed mode; 
     FIG. 2 is a front view of a rotating mechanism for the cam means; 
     FIG. 3 is an enlarged front view showing the switching between the friction feed mode and a tractor feed mode; and 
     FIG. 4 is an enlarged plan view of a biasing spring. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will now be described with reference to the drawings. 
     As shown in FIG. 1, a platen 1 is secured to a shaft 2 whose both end portions are rotatably supported on opposite side plates 3 (shown in FIG. 2) which are spaced from each other. Friction rollers 4 are held in resilient contact with a lower portion of the outer surface of the platen 1. A supporting member 5 is movable in a direction facing the outer surface of the platen 1. The friction rollers 4 are rotatably supported on the supporting member 5. The upper surface of the supporting member 5 is formed with U-shaped grooves 5a for rotatably receiving the shafts 4a of the friction rollers 4. 
     The bottom surface of the supporting member 5 has leg portions 5b which are connected with a biasing spring 6, hereinafter described in greater detail, and contacting portions 5c which are contactable with the biasing spring 6. The supporting member 5 is formed with a recess portion 5d through which a cam means 7 passes. The length of the leg portions 5b is longer than the length of the contacting portions 5c. 
     The biasing spring 6 is a leaf spring which urges the friction rollers 4 toward the platen 1 via the supporting member 5 Specifically, as shown in FIG. 4, the biasing spring 6 is formed with four U-shaped slits 6a, each of which forms a tongue portion 6b. A distal end portion of each tongue portion 6b has a coupling hole 6c fitable with the leg portion 5b. The biasing spring 6 has engaging holes 6d, 6e at both of its end portions so that by engaging these engaging holes 6d, 6e with protrusions 8a 8b of a baseplate 8, the biasing spring 6 is supported in place. The contacting portions 5c are adapted to contact surface portions except for the tongue portions 6b, of the biasing spring 6. 
     The cam means 7 extends across and over the biasing spring 6 (in the top-to-bottom direction in FIG. 4) and passes through a recess portion 5d of the supporting member 5. The cam means 7 is shaped like a rod and is rotatably supported by the side plates 3 (shown in FIG. 2) via shafts 7c formed at both of its end portions. The cam means 7 is formed with a first cam surface 7a and a second cam surface 7b. The first cam surface 7a is a flat cam surface spaced further from the center of rotation of the cam means 7, and is operable to engage the biasing spring 6 to separate the contacting portions 5c of the supporting member 5 from the biasing spring 6 when the cam means 7 is at a first rotational position. The second cam surface 7b is another flat cam surface whose distance from the center of rotation of the cam means 7 is less than that of the first cam surface 7a, and disengages from the biasing spring 6 to allow the biasing spring 6 to contact the contacting portions 5c when the cam means 7 is at a second rotational position. 
     A print head 9 is mounted on a carriage not shown in the drawings in confronting relation to the platen 1, and a tractor device 10 is provided on the opposite side of the print head 9. The tractor device 10 is relatively non-rotatably but axially shiftably fitted on a shaft 11 whose both end portions are rotatably supported by the side plates 3. Reference symbol P designates a recording paper. 
     FIG. 2 shows a mechanism for rotating the cam means 7 for switching between a friction feed mode and a tractor feed mode. Specifically, a distal end portion of the shaft 2 of the platen 1 and a distal end portion of the shaft 7c of the cam means 7 project outwardly from the side plate 3. The projecting end of the shaft 2 swingably supports an operation lever 12. One end portion of the operation lever 12 has an operating portion 12a which can be operated from outside, and the other end portion is formed with a sector gear 12b. The projection end of the shaft 7c of the cam means 7 has a pinion 13 secured thereto which meshes with the sector gear 12b. 
     In the friction feed mode, the operation lever 12 is at the solid line position in FIG. 2 and the cam means 7 is at its second rotational position. In this state, the second cam surface 7b of the cam means 7 is spaced from the biasing spring 6 so that they are out of engagement. Therefore, as illustrated by the solid lines in FIGS. 1 and 3, the contacting portions 5c are in contact with the biasing spring 6, and the spring force of the biasing spring 6 is applied via the supporting member 5 to the friction rollers 4. That is, upon receiving the strong spring force of the whole biasing spring 6, the friction rollers 4 strongly press the recording paper P against the platen 1, thereby attaining a friction feed operation. 
     If the tractor feed mode is selected, the operating portion 12a is controlled to swing the operation lever 12 counterclockwise to the chain line position in FIG. 2. In response to this swinging, the pinion 13 is rotated clockwise via the sector gear 12b, the cam means 7 is also rotated clockwise to the first rotational position, and the first cam surface 7a comes into engagement with the biasing spring 6 as illustrated by chain lines in FIG. 3. Since the biasing spring 6 is pushed down and deflected by the first cam surface 7a, the contacting portions 5c separate from the biasing spring 6. As a result, the supporting member 5 is pushed by the spring force of only the tongue portions 6b of the biasing spring 6, so that the friction rollers 4 contact resiliently with the platen 1 with a weak force. That is, since the spring force of the tongue portions 6b is weaker than that of the whole biasing spring 6, the friction rollers 4 in the tractor feed mode resiliently press the recording paper P against the platen 1 with a weaker spring force than in the case of the friction feed mode so that the recording paper P is fed with a moderate tensile force. 
     Although in the illustrated embodiment the length of the leg portions 5b is longer than the length of the contacting portions 5c and the tongue portions 6b for receiving the leg portions 5b lie in the same plane as that of the biasing spring 6, the length of the contacting portions 5c may be made longer than the length of the leg portions 5b, and in this case, the tongue portions 6b are received by the leg portions 5b after being bent toward the supporting member 5. 
     As described above, according to the paper feed device for a printer of the present invention, the pressing force of the friction rollers against the platen is changed in response to the switching between the tractor feed mode and the friction feed mode; therefore, the frictional force between the recording paper and the platen can be set to a proper level depending on the mode of paper feed. Further, the device can be readily assembled by stretching the biasing spring, and successively mounting the supporting member and the friction rollers. Further, the cost of the device can be reduced because the parts, such as biasing spring and supporting member, are simplified in configuration, the number of parts is small, and the structure of the device is simple. 
     Although the present invention has been described through specific terms, it should be noted here that the described embodiment is not necessarily exclusive and that various changes and modifications may be imparted thereto without departing from the scope of the invention which is limited solely by the appended claim.