Background

[0001] Some end users may have a tendency to pull on sheets as they advance through the printer. Pulling on the advancing sheet can exert a torque on a roller, which can be transmitted to a motor through a gear train. Such manually applied torque to the roller can have a negative impact on the wear life of the gear train and/or the motor.

Brief description of the Drawings

[0002] Examples are described in further detail below, with reference to the drawings, in which:

[0003] Figures la and lb are diagrams of an example gear train assembly;

[0004] Figure 2 is a side elevation view of an example gear train assembly;

[0005] Figure 3 is a side elevation view of the gear train assembly of Figure 2, showing a different position;

[0006] Figure 4 is a top perspective view of a printer sheet advancement mechanism including the gear train assembly according to some examples;

[0007] Figure 5 is a plan view of the printer sheet advancement mechanism of Figure 4;

[0008] Figure 6 is a schematic diagram of an internal ratchet mechanism of a gear of the gear train of Figures 2 and 3;7 is a block diagram to illustrate that the gear train assembly and printer sheet advancement mechanism may form part of a printer subsystem and the printer subsystem may form part of a printer; and

[0009] Figure 8 is a flowchart of an example method of mitigating wear on a printer gear train. Detailed Description

[0010] Described examples relate generally to printer gear train assemblies. Such assemblies may be employed in sheet advancement mechanisms, for example in printers.

[0011] Some examples relate to a printer gear train assembly that comprises a gear train to engage with a drive gear of a drive motor and to transmit movement to drive a printer sheet roller in a first direction to advance a sheet. There is also a movable carrier coupled to at least one gear of the gear train that is movable to disengage the gear from the drive gear in response to an external torque applied to rotate the roller in the first direction.

[0012] Some examples relate to a printer comprising the described printer gear train assembly.

[0013] Some examples relate to a printer sheet advancement mechanism that comprise a roller to advance a sheet by rotating in a first direction. A gear train to engage with a drive gear of a motor and to transmit movement to the roller. There is also a movable carrier coupled to at least one gear of the gear train that is movable to disengage the gear from the drive gear in response to an external torque applied to rotate the roller in the first direction.

[0014] Some of the described examples can have the effect of reducing wear on the gear train due to pulling of the sheet by a user before it is properly released from the roller. In one example, this effect can be achieved by using the torque externally applied (by the user pulling on a sheet that is frictionally retained against the roller) to the roller to disengage the gear train from the drive gear. The disengagement is effected by the use of a movable carrier to which is coupled a gear of the gear train that is positioned to engage with the drive motor. This disengagement means that the gear train is not subjected to stresses and wear associated with the gear train being effectively caught between the externally applied torque and the oppositely directed holding torque of the stepper motor. This disengagement is effected by pulling of the sheet in the same direction as the sheet is normally advanced by the roller.

[0015] Referring to Figure 1, an example gear train assembly 100 is illustrated. The gear train assembly 100 comprises a gear train 110 and a movable carrier 140. The gear train 110 to engages with a drive gear 135 associated with a drive motor (335, Figure 4). The gear train 110 also transmits movement to drive a roller 105 in a first direction to advance a sheet in direction M. With this arrangement, movement of the drive gear 135 is transmitted through the gear train 110 to cause movement of the roller 105. In response to external torque TEXT applied to the roller 105 to cause the roller to rotate in the first direction, for example, as a result of a user pulling on the sheet before it is properly released from the roller 105, torque is transmitted from the roller 105 to the gear train 110 which disengages from the drive gear 135. In particular and as best illustrated in Figs. 2 and 3, a first gear 125 of the gear train 110 engages with the drive gear 135 and a second gear 115 of the gear train 110 to transmit movement to drive the roller 105. Thus, in response to external torque applied to the roller 105 to cause the roller to rotate in the first direction, torque is transmitted from the roller 105 to the second gear 115 of the gear train 110, and the moveable carrier 140, which is coupled to the first gear 125 and the second gear 115, allows the first gear 125 to disengage from the drive gear 135. Once the first gear 125 is disengaged from the drive gear 135, the drive gear 135 no longer causes movement of the first gear 125 or the roller 105.

[0016] Referring now to Figures 2 and 3, example gear train assembly 100 is shown and described in further detail. Gear train assembly 100 comprises a frame 102 on which is mounted or carried the gear train 110. The gear train 110 is positioned to transmit torque from the motor drive gear 135 to the roller 105, so that rotation of the drive gear 135 causes rotation of printer sheet roller 105. The gear train 110 thus rotationally couples the drive gear 135 with the roller 105. The roller 105 has a roller gear 107 that rotates about an axis defined by an axle 108 of roller 105. The roller gear 107 is engaged by a gear of the gear train 110 in order to cause rotation of roller 105. The drive gear 135 is rotated in a direction that, via gear train 110, causes the roller 105 to advance a sheet of material. The sheet of material can be a paper sheet or other printable material, for example. Such as a paper sheet or other printable sheet material is advanced in one direction, which may be a direction to output the sheet from a printer 600 (Figure 6) or other system or subsystem 500 (Figure 6) in which the gear train assembly 100 may be disposed.

[0017] The gear train 110 comprises one gear 115 to engage with the roller gear 107 and another gear 125 to engage with the drive gear 135. A further gear 120 may form part of the gear train 110 and may be positioned intermediate the two gears 115, 125 within the gear train 110. For spatial efficiency, each of gears 115, 120 and 125 may be a composite gear (i.e. a gear having more than one set of gear teeth) with a large root diameter gear portion and an adjacent and co-axial small root diameter gear portion. In this way, the gears 115, 120 and 125 can be arranged in series, with a small root diameter gear portion of a first composite gear engaging with a large root diameter gear portion of a second composite gear and the small root diameter gear portion of the second composite gear in turn engages with a large root diameter gear portion of a third composite gear and the small root diameter gear portion of the third composite gear drives the trailer gear 107. However, gears 115, 120 and 125 may be replaced by a series of non-composite gears or instead of those three gears, a series of composite and non-composite gears may be employed. For purposes of the present description, the term "gear" is intended to include a composite gear and a non-composite gear.

[0018] The gears shown by way of example in the Figures 2 to 5 are circular gears. However, in other examples (not shown), other gear configurations may be employed to form the gear train and to allow movement by the drive gear of the drive motor (335, Figs 4 &5) to cause rotation of the printer sheet roller 105.

[0019] The gear train assembly 100 further comprises a carrier 140 that is moveable relative to the frame 102 and other components of the gear train assembly 100 that are fixed relative to the frame 102. For example, the drive gear 135, the roller 105 and roller gear 107 are positionally fixed relative to the frame 102. The carrier 140 is coupled to gear 115 and has an axis of rotation coinciding (co-axial) with an axis of rotation of the gear 115. The carrier 140 and gear 115 may be coupled to each other by a co-axial rotational coupling 142 that may also serve as an axle about which the gear 115 can rotate.

[0020] The rotational coupling 142 may allow the gear 115 to freely rotate relative to the carrier 140 in a drive direction of the gear train in order to allow transmission of power and torque from the drive gear 135 to the roller 105. Additionally, the rotational coupling 142 may be a one-way rotation mechanism and may hinder rotation of the gear 115 relative to the carrier 140 in an opposite direction to the drive direction. Rotation of the gear 115 in the drive direction, as indicated by arrow 208 in Figure 3, tends to cause rotation of the carrier 140 in the same direction as the gear 115. The rotational coupling 142 may include, for example, an internal ratchet mechanism or a variable friction mechanism that provides minimal friction in the drive direction of rotation but increased friction in the other direction. An example of an internal ratchet mechanism is illustrated in Figure 6. The internal ratchet mechanism 600 may comprise a ratchet wheel 602 including teeth 604 disposed about a circumference of the ratchet wheel 602. The ratchet mechanism 600 may further comprise an arm 606 having a first end 608 attached to the ratchet wheel 602 and a second end 610 extending beyond a perimeter of the ratchet wheel 602. A pawl 612 may be disposed at the second end of the arm 606. The arm 606 may engage with the teeth 604 of the ratchet wheel 602. As the ratchet wheel 602 of the rotational coupling 142 and the gear 115 rotate in the drive direction, the pawl 612 is slidably released from engaging with a tooth 604 of the ratchet wheel 602 and sidably engaged with a neighbouring tooth 604. By engaging with a tooth 604 of the ratchet wheel 602, the pawl 612 acts against movement of the ratchet wheel 602 in an anti -drive direction, thereby hindering rotation of the gear 115 relative to the carrier 140 in an opposite direction to the drive direction. As shown in Figures 2 and 3, each of gears 115, 120 and 125 in gear train 110 are coupled to the carrier 140. Gears 120 and 125 are mounted or otherwise coupled to the carrier 140 to have moveable axes of rotation. For example, the axis of rotation of gear 115 is not moveable relative to the frame 102. Thus the carrier 140 has a fixed end that it is coupled to the gear 115 for coaxial rotation and an opposite free end positioned close to the drive gear 135. In this way, with the gear 125 being coupled to carrier 140 at the carrier free end to freely rotate about an axis defined by a rotational coupling or short axle 146, the carrier 140 can position gear 125 (as part of gear train 110) to engage with the drive gear 135. Gear 120 may be coupled to the carrier 140 to freely rotate about an axis defined by a rotational coupling or short axle 144.

[0021] Carrier 140 may be formed as a plate having a plate body 141 and apertures formed therein for the coupling of gears 115, 120 and 125 (via rotational couplings 142, 144 and 146) of the gear train 110 to the plate body 141. The plate body 141 may have a bend or kink 149 formed therein in order to suitably position the gears of the gear train 110 (such as gear 120, 125) relative to each other in parallel planes of rotation. Additionally, the plate body 141 may have a window or aperture 148 formed therein and optionally co-located with part of the bend or kink 149 so that a gear, such as gear 120, can rotate through the space defined by the window aperture 148 without interfering with material of the plate body 141 that might otherwise get in the way of rotation of the gear teeth. Further, the plate body 141 may be generally flat and planar, except for the bend or kink 149, and may define a shape (as seen in side elevation) suitable for coupling gears of the gear train 110 for rotation. In the examples shown in Figs 2 and 3, the shape of the plate body 141 in side elevation may be analogous to an elbow shape or inverted V shape. Alternatively, the plate body 141 may adopt other suitable shapes. [0022] The gear train assembly 100 may further comprise a bias mechanism 130, which in the illustrated examples may comprise a spring. The bias mechanism 130 is arranged to be anchored to the frame and to bias the carrier 140 in to a position such that the gear 125 can engage with and be driven by the drive gear 135. Where the bias mechanism 130 comprises a spring, for example, the bias mechanism 130 may be coupled to the carrier 140 at one end 132 of the bias mechanism 130, for example by a hook at end 132 being received in an aperture of the plate body 141. The bias mechanism 130 may be coupled to a frame projection 103 of the frame 102 at an opposite end 131 of the bias mechanism 130, for example by a hook at end 131 being received in an aperture of the frame projection 103. The bias mechanism exerts a light returning force, for example in the order of about 0.35N, to return the carrier 140 to the drive gear engagement position. The bias mechanism may have a spring constant, k, between about 6.5 N/m and about 20 N/m, for example.

[0023] Referring also to Figures 4 and 5, a sheet advancement mechanism 300 is described according to further examples. The sheet advancement mechanism 300 includes the gear train assembly 100 and also comprises the motor 335, drive gear 135, a sheet in-feed ramp 310, a bias plate 320 and a further frame component 302 to assist in securing the axle 108 of the roller 105 in a fixed position.

[0024] The motor 335 is fixed in relation to the frame 102 by a fixation component 137, such as a screw received through the frame 102 and a suitable threaded aperture in the motor housing. The motor receives power via a power input 337 to drive the drive gear 135. The power input 337 may be an electrical power input or a mechanical power input.

[0025] As is shown best in Figure 4, the roller 105 is arranged to receive a sheet, such as a paper sheet, for advancement in the direction indicated by arrow 305. In order to promote advancement of the sheet in such a direction, the roller rotates in a direction indicated by arrow 308, which is counter-clockwise when viewing it end-on in Figures 2 and 3. The sheet is advanced by frictional engagement with the roller 105, which pushes the sheet in a curved path in between the roller 105, the in- feed ramp 310 and the bias plate 320. The bias plate 320 holds the sheet against the roller 105 in order to promote frictional engagement of the roller 105 with the sheet and thereby advancement of the sheet. The sheet is thus advanced upward in between the roller 105 and the bias plate 320 as seen in Figure 4 and in a direction out of the page, as seen in Figure 5. [0026] When the sheet is still entrained and frictionally engaged between the bias plate 320 and the roller 105 and it is pulled upward by a user, this pulling force tends to cause the roller to rotate in the direction of the arrow 308, since the roller 105, roller gear 107 and axle 108 are coupled together and rotate synchronously. This rotation is in-turn applied by the roller gear 107 to the gear 115, tending to rotate the gear 115 in a clock- wise direction, which is the same as the drive direction 208. When the gear 115 rotates in the clockwise direction under torque applied by the roller gear 107, the gear 115 drives the rotation of gear 120 and in doing so, applies a moment across the distance between the axles 142 and 144 and directed downwardly at axle 144. Since the gear 120 coupled to carrier 140 at axle 144 is in practice not rotationally frictionless, a small moment is effectively applied by the clockwise rotation of the gear 115 to the carrier 140 at axle 144, which can displace the engagement gear 125 of the gear train 110 a sufficient distance away from the drive gear 135, as shown in Figure 3, that the engagement gear 125 disengages from the drive gear 135. The applied moment can be at least somewhat proportional to the pulling force applied by the user to the sheet. Once the pulling force is absent, the moment is no longer applied by the gear 115 to the carrier 140 and the bias mechanism 130 tends to return the carrier 140 to the engagement position, as shown in Figure 2, in which the engagement gear 125 engages with drive gear 135 and can be driven thereby.

[0027] It can be observed that when the sheet is pulled by a user, the torque direction applied from the roller gear 107 to the output gear 115 and from the output gear 115 to the intermediate gear 120 is the opposite of the torque direction applied between those gears when the motor 335 is driving the drive gear 135 to drive the gear train 110 to rotate roller in direction 308, even if in each case the sheet is moving in the same direction 305. When the motor 335 is driving the sheet forward in direction 305 (by causing roller to rotate in direction 308), the torque applied by the drive gear 135 to the input (engagement) gear 125 tends to keep the gear train closed, which is directionally aligned with the recall direction of the bias mechanism 130 (.i.e. the contraction direction of the spring, which is opposite to the disengagement direction of the gear train 110).

[0028] Figure 7 is a block diagram to illustrate that the gear train assembly 100 and printer sheet advancement mechanism 300 may form part of a printer subsystem 500, which may form part of a printer 700. Described examples may have application to continuous sheet printers where the sheet material is spooled in a roll within or near the printer body, so that in order for the user to take a portion of the sheet material (in the form of a printed sheet) from the printer, it is manually pulled and torn by the user, for example against a serrated anvil.

[0029] Figure 8 is a flowchart of a method 800 of mitigating wear on a printer gear train. The method 800 includes at 810 positioning the gear train 110 to be driven by the drive gear 135 of the printer stepper motor 335. In this example, the gear train 110 comprises a first gear 125 to engage with the drive gear 135 and a second gear 115 to engage with a printer sheet roller 105 to drive the printer sheet roller 105 in a first direction 308 to advance a sheet. Movement of the first gear 125 is coupled to movement of the second gear 115. The method 800 includes at 820 allowing disengagement of the first gear 125 from the drive gear 135 in response to an external torque applied to rotate the printer sheet roller in the first direction 308.

[0030] Variations and/or modifications may be made to the above-described examples, without departing from the broad general scope of the present disclosure. The present examples are, therefore, to be considered in all respects as illustrative and not restrictive.