JP2014122081A - Recording device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress a length of a gap downstream from an inlet in a feed direction from becoming small depending on dimension accuracy for component processing.SOLUTION: A recording device includes: a hopper 3 provided to be rockable, and loading thereon a sheet P; a feed roller 2 provided rotatably downstream of the hopper 3 in a feed direction, and feeding the sheet P; a medium guide surface 30 opposed to an outer circumferential surface 21c of the feed roller 2 with a gap B given therebetween; a separation roller 14 provided downstream of the medium guide surface 30 in the feed direction, and separating the sheet P in cooperation with the feed roller 2; and a recording unit recording characters or an image on the sheet P separated by the feed roller 2 and the separation roller 14. A smallest length between the outer circumferential surface 21c of the feed roller 2 and the medium guide surface 30 in the gap B increases as the feed roller 2 rotates during one rotation of the feed roller 2.

Description

  The present invention relates to a recording apparatus.

Some ink jet printers as an example of a recording apparatus include a mechanism that separates a plurality of sheets placed in a stacked state one by one and feeds them to the recording unit side. For example, Patent Document 1 has a hopper 80 that swings in swing directions D10 and D11 with a swing shaft 81 in FIG. 8A as a fulcrum, and an outer peripheral surface 84 that has the same distance C from the rotary shaft J10 over the outer periphery. In addition, a substantially D-shaped feeding roller 83 that rotates in the rotation direction D12 and a retard roller 85 that contacts and follows the feeding roller 83 are provided. A guide surface 82 that can face the outer peripheral surface 84 of the feed roller 83 is formed between the hopper 80 and the nip position N between the feed roller 83 and the retard roller 85. A gap G is formed between the two.
When the hopper 80 is swung in the swing direction D11, a plurality of upper sheets P11 among the plurality of sheets P10 placed in a stacked state enter the gap G. The plurality of sheets P11 that have entered the gap G are stopped on the upstream side in the feeding direction E from the nip position N between the feeding roller 83 and the retard roller 85. When the feeding roller 83 in FIG. 8A further rotates in the rotation direction D12, the uppermost sheet P12 is fed in the feeding direction by the feeding roller 83 and the retard roller 85 as shown in FIG. 8B. E is fed and separated.

JP 2005-112696 A

However, depending on the dimensional accuracy of component processing, the position of the rotation axis J10 may vary as the feed roller 83 rotates, or the distance C between the outer peripheral surface 84 and the rotation axis J10 may not be formed equally over the outer periphery. Therefore, when the feed roller 83 in FIG. 8A is rotated in the rotation direction D12, the distance L11 of the gap G at the rotation position of the feed roller 83 in FIG. 8B is the feed in FIG. It becomes shorter than the distance L10 of the gap G at the rotation position of the roller 83. That is, as the feed roller 83 rotates, the distance between the outer peripheral surface 84 and the guide surface 82 in the gap G may be shortened.
As a result, the force with which the plurality of sheets P11 that have entered the gap G are pressed by the outer peripheral surface 84 and the guide surface 82 increases as the feed roller 83 rotates. As a result, a plurality of sheets of the plurality of sheets P11 that have entered the gap G are conveyed downstream from the nip position N between the feeding roller 83 and the retard roller 85 in the feeding direction E, and are double-fed. There is a problem that the driving rotation of the feeding roller 83 stops.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples.

  [Application Example 1] A hopper provided so as to be able to swing, a hopper on which a recording medium is placed, a feeding roller which is rotatably provided downstream in the feeding direction of the hopper and feeds the recording medium, and the feeding A medium guide surface facing the outer peripheral surface of the roller with a gap; a separation roller provided downstream of the medium guide surface in the feeding direction; and separating the recording medium in cooperation with the feeding roller; A recording unit that records on the recording medium separated by the feeding roller and the separation roller, and the outer peripheral surface of the feeding roller and the medium guide in the gap while the feeding roller rotates once. A recording apparatus, wherein a shortest distance to a surface becomes longer as the feeding roller rotates.

  According to this application example, the shortest distance between the outer peripheral surface of the feeding roller and the medium guide surface in the gap becomes longer as the feeding roller rotates while the feeding roller makes one rotation. As a result, in the gap where the outer peripheral surface of the feed roller and the medium guide surface face each other, the outer peripheral surface of the feed roller and the medium guide surface face each other as the feed roller rotates, depending on the dimensional accuracy of part processing. It can suppress that the distance of the clearance gap to be shortened. Therefore, it is possible to suppress an increase in the force that the paper that has entered the gap is pressed by the outer peripheral surface of the feeding roller and the medium guide surface with the rotation of the feeding roller. Thus, it is possible to prevent the recording medium from being double-fed, the driving rotation of the feeding roller from being stopped, and the printer from being stopped.

  Application Example 2 The outer peripheral surface viewed from the axial direction of the rotating shaft of the feeding roller forms an arc, and the rotating shaft is located on the opposite side of the tip of the outer peripheral surface from the tip of the outer peripheral surface. The recording apparatus described above, wherein the recording apparatus is provided eccentrically.

  According to this application example, in the gap where the outer peripheral surface of the feed roller and the medium guide surface face each other, depending on the dimensional accuracy of the part processing, the outer peripheral surface of the feed roller and the medium guide are accompanied by the rotation of the feed roller. It can suppress that the distance of the clearance gap which the surface opposes becomes short. Therefore, it is possible to suppress an increase in the force that the paper that has entered the gap is pressed by the outer peripheral surface of the feeding roller and the medium guide surface with the rotation of the feeding roller. Thus, it is possible to prevent the recording medium from being double-fed, the driving rotation of the feeding roller from stopping, and the printer from stopping.

  Application Example 3 After the outer peripheral surface of the feeding roller viewed from the axial direction of the rotating shaft is connected to the first outer peripheral surface forming the arc and the first outer peripheral surface, and is opposed later And a second outer peripheral surface whose distance from the center of the arc becomes shorter as it is positioned closer to the end.

  According to this application example, it is possible to suppress the distance between the gap between the outer peripheral surface of the feeding roller and the medium guide surface from being shortened. In addition, the distance that the recording medium is rotated by the feeding roller and the separation roller is increased. Therefore, it is possible to suppress an increase in the force that the paper that has entered the gap is pressed by the outer peripheral surface of the feeding roller and the medium guide surface with the rotation of the feeding roller. Thus, it is possible to prevent the recording medium from being double-fed, the driving rotation of the feeding roller from being stopped, and the printer from being stopped.

FIG. Sectional drawing of a printer. (A) is a figure which shows the part which isolate | separates a paper, (b) is a figure which shows the part in which the outer peripheral surface of a feed roller opposes a medium guide surface. (A) is a figure which shows the part which isolate | separates a paper, (b) is a figure which shows the part in which the outer peripheral surface of a feed roller opposes a medium guide surface. The figure which shows the outer peripheral surface in a feed roller, and the medium guide surface facing an outer peripheral surface. The figure which shows the outer peripheral surface in a feed roller, and the medium guide surface facing an outer peripheral surface. The figure which shows the feeding roller seen from the main scanning direction. (A) is a diagram showing a paper separation mechanism provided in a conventional recording apparatus, and (b) shows a gap formed between a feeding roller and a medium guide surface in a conventional paper separation mechanism. Figure.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1)
FIG. 1 is a perspective view of a printer 1 which is an example of a recording apparatus. FIG. 2 is a cross-sectional view of the printer 1 as viewed from the main scanning direction Y. On the back side of the printer 1 (upstream side in the sub-scanning direction X), a hopper 3 and a paper support 7 on which paper (not shown) as a recording medium is placed are provided. The hopper 3 is provided with a paper edge restricting section 5 that slides in the main scanning direction Y and restricts the position of the edge of the paper.

  A feeding roller 2 is provided at the center in the main scanning direction Y on the downstream side in the feeding direction (sub-scanning direction X) of the hopper 3. The separation roller 14 provided below the feeding roller 2 in FIG. 2 in the vertical direction Z is urged toward the feeding roller 2 by a spring member (not shown), and is multi-feeded in cooperation with the feeding roller 2. Separator paper is separated.

  The separation roller 14 rotates following the feeding roller 2 when one sheet is nipped at the nip position between the separation roller 14 and the feeding roller 2, but two or more sheets are double-fed. Is not driven to rotate with the feed roller 2. For this reason, the uppermost sheet in contact with the feeding roller 2 is fed downstream in the feeding direction, and sheets other than the uppermost sheet in contact with the feeding roller 2 are not fed downstream in the feeding direction. As a result, it is possible to separate the double-fed sheets.

  Paper feed levers 8 are provided on both sides of the feed roller 2 in FIG. 1 in the main scanning direction Y, and the paper return lever 8 rotates to feed paper other than the uppermost paper that contacts the feed roller 2. Push the missing paper back upstream in the feeding direction.

  A carriage 11 that is fixed to an endless belt (not shown) that is rotationally driven by a carriage motor 9 and that reciprocates in the main scanning direction Y is provided on the downstream side of the feeding roller 2 in the feeding direction. The carriage 11 carries an ink cartridge 10 containing ink, and a recording head 15 shown in FIG. 2 for discharging ink is provided at the lower end in the vertical direction Z of the carriage 11.

  A sheet of paper fed by the feeding roller 2 and the separation roller 14 is moved along the support unit 13 by a conveyance driving roller 16 and a conveyance driven roller 12 that rotates in contact with the conveyance driving roller 16. It is conveyed in the sub-scanning direction X.

  Ink is ejected from the recording head 15 that reciprocates in the main scanning direction Y onto the paper transported in the sub-scanning direction X, and characters and images are recorded on the paper. The recorded paper is discharged downstream in the sub-scanning direction X by the discharge driving roller 17 and the discharge driven roller 18 that is driven in contact with the discharge driving roller 17.

  A recording unit 19 is configured including the carriage 11, the recording head 15, the support unit 13, the conveyance driving roller 16, and the conveyance driven roller 12.

  FIG. 3A is a diagram showing a portion for separating the paper P placed on the hopper 3, and is a diagram seen from the main scanning direction Y. The hopper 3 is provided with a rotation fulcrum (not shown) on the upper left side of the drawing, and is provided so as to be swingable in the arrow directions D1 and D2 by the cam 6 of FIG. It is done.

  The feeding roller 2 is a substantially D-shaped roller when viewed from the side. The feed roller 2 has a friction layer 21 in which an outer peripheral surface 21c that can be in contact with the separation roller 14 and can form a nip portion is formed of rubber, and a circle A indicated by a broken line extending from the outer peripheral surface 21c. And a flat portion 21d.

  The center angle R1 is an angle opened around the center S1 of the arc. The friction layer 21 is formed in the range of the central angle R1. The distance K1 from the arc center S1 to the outer peripheral surface 21c is equal over the range of the center angle R1. In the present embodiment, the center angle R1 is 180 degrees.

  A medium guide surface 30 arranged with a gap B is provided at a position facing the outer peripheral surface 21c. When the feeding roller 2 makes one rotation in the rotation direction D3, the side of the outer peripheral surface 21c that faces the medium guide surface 30 first is the front end 21a, and the side that faces the medium guide surface 30 after the front end 21a is the rear end 21b. It is. FIG. 3A is a diagram showing a state in the initial stage of rotation when the tip 21 a side of the outer peripheral surface 21 c of the friction layer 21 faces the medium guide surface 30.

  When the hopper 3 swings in the arrow direction D1, the lower end portion of the paper P placed in a stacked state on the hopper 3 is connected to the medium guide surface 30 and is formed on the upstream guide surface 4 ( (See FIG. 1) is pushed upward in the vertical direction Z. For this reason, a plurality of sheets P at the upper part of the sheets P placed on the hopper 3 are separated and enter the gap B.

  As described above, the plurality of sheets P that have entered the gap B are separated along the downstream guide surface 31 by the cooperative operation of the feeding roller 2 and the separation roller 14. It is fed downstream in the direction.

  FIG. 4A is a diagram illustrating a state in the later stage of rotation when the rear end 21 b side of the outer peripheral surface 21 c of the friction layer 21 faces the medium guide surface 30. When the feeding roller 2 in FIG. 3A rotates in the rotation direction D3, the outer peripheral surface 21c on the rear end 21b side faces the medium guide surface 30 as shown in FIG. 4A. The hopper 3 is in a posture of swinging in the arrow direction D2 side.

  FIG. 3B is an enlarged view of a portion of the outer peripheral surface 21 c in the initial stage of rotation shown in FIG. 3A where the tip 21 a side faces the medium guide surface 30. FIG. 4B is an enlarged view of a portion where the rear end 21 b side of the outer peripheral surface 21 c in the latter stage of rotation in FIG. 4A is opposed to the medium guide surface 30.

  In the present embodiment, the rotation axis J1 of the feeding roller 2 in FIG. 3A is eccentric to the opposite side of the tip 21a by a distance H from the center S1 of the arc. Accordingly, the shortest distance L2 in the gap B between the outer peripheral surface 21c and the medium guide surface 30 in the later stage of rotation in FIG. 4A is the gap between the outer peripheral surface 21c in the initial rotation and the medium guide surface 30 in FIG. It becomes longer than the shortest distance L1 in B.

  That is, with the rotation of the feed roller 2 until the feed roller 2 rotates in the rotation direction D3 from the initial rotation state of FIG. 3A to the late rotation state of FIG. 4A. The shortest distance in the gap B between the outer peripheral surface 21c and the medium guide surface 30 becomes longer.

  As described above, the printer 1 described in the present embodiment is provided so as to be swingable, and is provided so as to be rotatable downstream of the hopper 3 in which the paper P is placed, and in the feeding direction of the hopper 3, and downstream of the paper P in the feeding direction. A feed roller 2 that feeds the sheet, a medium guide surface 30 that is opposed to the outer peripheral surface 21c of the feed roller 2 with a gap B, and a feed roller 2 that is provided downstream of the medium guide surface 30 in the feed direction. A separation roller 14 that cooperates to separate the paper P and a recording unit 19 that records on the paper P separated by the feeding roller 2 and the separation roller 14 are provided.

  The outer peripheral surface 21c of the feeding roller 2 viewed from the axial direction of the rotation axis J1 forms an arc, and the rotation axis J1 is provided eccentrically from the center S1 of the arc on the opposite side to the tip 21a facing the tip of the outer peripheral surface 21c. It is done.

  With such a configuration, the shortest distance between the outer peripheral surface 21c of the feeding roller 2 and the medium guide surface 30 in the gap B becomes longer as the feeding roller 2 rotates while the feeding roller 2 makes one rotation. .

  As a result, in the gap B where the outer peripheral surface 21c of the feed roller 2 and the medium guide surface 30 face each other, depending on the dimensional accuracy of component processing, the feed roller 2 is rotated along with the rotation of the feed roller 2 in the rotation direction D3. It can suppress that the distance of the clearance gap B which the outer peripheral surface 21c and the medium guide surface 30 oppose becomes short. Therefore, it is possible to suppress an increase in the force that the paper P that has entered the gap B is pressed by the outer peripheral surface 21c of the feeding roller 2 and the medium guiding surface 30 as the feeding roller 2 rotates. It is possible to prevent the printer from stopping due to double feeding by the feeding roller 2 and the separation roller 14 or stopping the rotation of the feeding roller 2.

(Embodiment 2)
In the second embodiment, a description will be given of a feeding roller in which an outer peripheral surface that can come into contact with a separation roller is formed in a range of a central angle of 180 degrees or more. FIGS. 5A, 5 </ b> B, and 6 are views viewed from the main scanning direction Y, and are diagrams illustrating the feeding roller 2 a and the medium guide surface 30 at the respective rotation angles.

  An outer peripheral surface 22d of the friction layer 22 of the feeding roller 2a is connected to the first outer peripheral surface 22d1 formed in the range of the central angle R1 and the first outer peripheral surface 22d1, and is formed in the range of the central angle R2. The second outer peripheral surface 22d2. The boundary position 22b indicates the boundary position between the first outer peripheral surface 22d1 and the second outer peripheral surface 22d2.

  The first outer peripheral surface 22d1 viewed from the axial direction of the rotation axis J2 forms an arc from the tip 22a to the boundary position 22b. The distance K1 from the arc center S2 to the first outer peripheral surface 22d1 is equal over the range of the center angle R1. In the present embodiment, the center angle R1 is 180 degrees.

  The distance K2 from the center S2 of the arc is such that the second outer peripheral surface 22d2 viewed from the axial direction of the rotation axis J2 is positioned closer to the rear end 22c facing the rear end 22a after the feed roller 2a rotates once. Becomes shorter.

  FIG. 5A is a diagram illustrating an initial state of rotation when the tip 22 a side of the outer peripheral surface 22 d of the friction layer 22 faces the medium guide surface 30. FIG. 5B is a diagram illustrating a state when the boundary position 22 b on the outer peripheral surface 22 d of the friction layer 22 faces the medium guide surface 30.

  The rotation axis J2 in FIG. 5A is provided so as to be rotatable eccentrically on the opposite side to the tip 22a by a distance H (see FIG. 5B) from the center S2 of the arc. As a result, the shortest distance L4 in the gap B between the outer peripheral surface 22d and the medium guide surface 30 when the boundary position 22b in FIG. 1 is longer than the shortest distance L3 in the gap B between the outer peripheral surface 22d1 and the medium guide surface 30.

  That is, the feeding roller 2a is rotated until the boundary position 22b in FIG. 5B is opposed to the medium guide surface 30 by the rotation of the feeding roller 2a in the rotation direction D3 from the initial rotation state in FIG. With the rotation, the distance in the gap B between the first outer peripheral surface 22d1 and the medium guide surface 30 becomes longer.

  FIG. 6 is a diagram illustrating a state in the later stage of rotation when the feeding roller 2a of FIG. 5B further rotates in the rotation direction D3 and the second outer peripheral surface 22d2 of the friction layer 22 faces the medium guide surface 30. It is. A circle F indicated by a broken line is an extension of the arc of the first outer peripheral surface 22d1. If the outer peripheral surface is formed at the position of the circle F obtained by extending the arc of the first outer peripheral surface 22d1, the rotation axis J2 is eccentric from the center S2 of the arc to the side opposite to the tip 22a. When the roller further rotates in the rotation direction D3 from the state in which the boundary position 22b in FIG. 5B faces the medium guide surface 30, the outer peripheral surface formed at the position of the circle F approaches the medium guide surface 30 side. End up.

  That is, in the feeding roller in which the outer peripheral surface is formed at the position of the circle F obtained by extending the arc of the first outer peripheral surface 22d1, the boundary position 22b in FIG. When rotating in the rotation direction D3, the distance between the outer peripheral surface formed at the position of the circle F and the medium guiding surface 30 is shortened.

  In the present embodiment, as described above, the second outer peripheral surface 22d2 is formed such that the distance K2 (see FIG. 5A) from the arc center S2 is shorter as the position is closer to the rear end 22c.

  Accordingly, the distance L5 in the gap B between the second outer peripheral surface 22d2 and the medium guide surface 30 in the latter rotation stage in FIG. 6 is the outer peripheral surface when the boundary position 22b in FIG. It becomes longer than the shortest distance L4 in the gap B between 22d and the medium guide surface 30. That is, even when the feeding roller 2a further rotates in the rotation direction D3 from the state of FIG. 5B to the state of FIG. 6, the shortest distance between the second outer peripheral surface 22d2 and the medium guide surface 30 becomes long. .

  As described above, the outer peripheral surface 22d viewed from the axial direction of the rotation axis J2 of the feeding roller 2a in the present embodiment is connected to the first outer peripheral surface 22d1 that forms an arc, and further to the first outer peripheral surface 22d1. The second outer peripheral surface 22d2 is configured such that the distance K2 from the arc center S2 becomes shorter as the position is closer to the rear end 22c. Thereby, after the feed roller 2a rotates by a certain amount, it is possible to prevent the distance between the gap between the outer peripheral surface 22d of the feed roller and the medium guide surface from being shortened, and the rotation direction D3 of the outer peripheral surface 22d. Therefore, the distance of rotation while the paper P is sandwiched between the feeding roller 2a and the separation roller 14 is increased.

  Therefore, even in a range where the second outer peripheral surface 22d2 faces the medium guide surface 30 due to the rotation of the feeding roller 2a, the paper P that has entered the gap B may rotate the feeding roller 2a depending on the dimensional accuracy of component processing. Accordingly, it is possible to suppress the distance of the gap B where the second outer peripheral surface 22d2 of the feeding roller 2a and the medium guide surface 30 face each other from being shortened. Therefore, it is possible to suppress an increase in the force that the sheet P that has entered the gap B is pressed by the second outer peripheral surface 22d2 of the feeding roller 2a and the medium guiding surface 30 with the rotation of the feeding roller 2a. Therefore, it is possible to suppress the double feeding by the feeding roller 2a and the separation roller 14, or the stop of the printer from stopping by stopping the driving rotation of the feeding roller 2a. Note that other configurations of the printer of the present embodiment are the same as those of the printer 1 described in the first embodiment.

(Embodiment 3)
In the present embodiment, a description will be given of a feeding roller that is formed such that the distance from the rotation axis becomes shorter from the front end to the rear end of the outer peripheral surface. FIG. 7 is a diagram illustrating the feeding roller 2 c as viewed from the main scanning direction Y.

  A circle H indicated by a broken line is a circle having a radius W as a straight line connecting the tip 23a and the rotation axis J3 on the outer peripheral surface 23c on which the friction layer 23 is formed. The distance K3 between the outer peripheral surface 23c and the rotation axis J3 becomes shorter as the feed roller 2c is positioned at the rear end 23b facing the rear end 23a during one rotation.

  As a result, the shortest distance L6 between the outer peripheral surface 23c of the feeding roller 2c and the medium guide surface 30 in the gap B becomes longer with the rotation of the feeding roller 2c while the feeding roller 2c rotates once.

  According to this configuration, in the gap B where the outer peripheral surface 23c of the feed roller 2c and the medium guide surface 30 face each other, depending on the dimensional accuracy of component processing, the feed roller 2c rotates in the rotation direction D3. It can suppress that the distance of the clearance gap B where the outer peripheral surface 23c of the feed roller 2c and the medium guide surface 30 face each other is shortened. Therefore, it is possible to suppress an increase in the force that the paper P that has entered the gap B is pressed by the outer peripheral surface 23c of the feed roller 2c and the medium guide surface 30 with the rotation of the feed roller 2c. It is possible to suppress the double feed by the feed roller 2c and the separation roller 14, or the stop of the printer from stopping due to the drive rotation of the feed roller 2c being stopped.

  Other configurations of the printer of the present embodiment are the same as those of the printer 1 described in the first embodiment. In the first to third embodiments, the printer including the recording head 15 that ejects ink has been described. However, the present invention can also be applied to an electrophotographic recording apparatus including a photoconductor.

  DESCRIPTION OF SYMBOLS 1 ... Printer, 2, 2a, 2c ... Feed roller, 3 ... Hopper, 9 ... Carriage motor, 11 ... Carriage, 12 ... Conveyance driven roller, 13 ... Support part, 14 ... Separation roller, 15 ... Recording head, 16 ... Transport drive roller, 17 ... discharge drive roller, 18 ... discharge driven roller, 19 ... recording unit, 21a, 22a, 23a ... tip, 21b, 22c, 23b ... rear end, 21c, 22d, 23c ... outer peripheral surface, 30 ... medium Guide surface, J1, J2, J3 ... rotation axis, K1, K2, K3 ... distance, L1-L6 ... shortest distance, P ... paper, S1, S2 ... center of arc.

Claims (3)

A hopper provided so as to be able to swing and for placing a recording medium;
A feed roller that is rotatably provided downstream in the feed direction of the hopper and feeds the recording medium;
A medium guide surface facing the outer peripheral surface of the feeding roller with a gap,
A separation roller that is provided downstream of the medium guide surface in the feeding direction and separates the recording medium in cooperation with the feeding roller;
A recording unit for recording on the recording medium separated by the feeding roller and the separation roller,
The recording is characterized in that the shortest distance between the outer peripheral surface of the feeding roller and the medium guide surface in the gap increases with the rotation of the feeding roller while the feeding roller makes one rotation. apparatus. The recording apparatus according to claim 1,
The outer peripheral surface viewed from the axial direction of the rotation axis of the feeding roller forms an arc, and the rotation shaft is eccentrically provided on the opposite side of the tip of the outer peripheral surface facing the tip from the center of the arc. A recording apparatus. The recording apparatus according to claim 2,
The outer peripheral surface of the feeding roller viewed from the axial direction of the rotating shaft is connected to the first outer peripheral surface forming the arc and the first outer peripheral surface, and the rear end side facing the rear end from the front end. And a second outer peripheral surface in which the distance from the center of the arc becomes shorter as it is located at the center.