Patent Publication Number: US-2003230847-A1

Title: Sheet feed roller

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to an improved sheet feed roller for feeding various types of sheets in imaging machines, such as printers, copying machines or facsimile machines, so as to print information onto the sheet or read out information from the sheet.  
       [0003] 2. Description of Related Art  
       [0004] Sheet feed rollers are generally classified into two types. A first type makes use of rollers having an outer peripheral surface with a high friction coefficient. In this instance, the sheet is sandwiched between the feed roller and a pinch roller and transferred primarily by friction force. The feeding of the sheet relies upon unstable friction force, and it is often difficult to achieve a sufficient feeding accuracy. For overcoming such difficulty associated with the feed rollers of the first type and improving the feeding accuracy of the sheet, there has been proposed a second type wherein the outer peripheral surface of the roller is provided with a plurality of microscopic spikes that can be pierced into the sheet. In this instance, the sheet is positively transferred under engagement of the spikes and corresponding microscopic recesses formed in the sheet by the spikes. The latter type of sheet feed rollers are disclosed, for example, in JP 08-310703A, JP 10-109777A, JP 10-203675A, JP 10-236683A, JP 2000-159377A, JP 2000-159378A and JP 2000-159379A.  
       [0005] However, it has been found by the inventors that even the sheet feed rollers with the microscopic spikes may not realize a satisfactory feeding accuracy, depending upon the material of the sheet to be transferred, or the pressure under which the spikes are pierced into the sheet. Here, the feeding accuracy is typically represented by the difference between the desired feeding distance and the actual feeding distance, per unit rotation of the feed roller.  
       [0006] The inventors conducted thorough research and investigations to seek measures for improving the feeding accuracy of the feed rollers, and found the mechanism whereby unsatisfactory feeding accuracy occurs, as follows. That is to say, the feeding accuracy of the feed roller provided with the microscopic spikes is degraded by fluctuation of the piercing depth of the spikes, which occurs depending upon the material of the sheet to be fed and/or the pressure for urging the feed roller against the sheet. When the piercing depth of the spike is insufficient, there occurs fluctuation of the sheet feeding between the spike and the recess formed in the sheet by the spike. Fluctuation of the piercing depth of the spike also causes fluctuation of the sheet feeding. On the other hand, when a required piercing depth of the spike is achieved by a sufficient urging force of the roller against the sheet, in an attempt to avoid occurrence of fluctuation of the sheet feeding, the sheet feeding radius changes depending upon the piercing depth and inevitably causes fluctuation of the sheet feeding. The above-mentioned mechanism will be more fully explained below with reference to FIGS.  1 ( a ),  1 ( b ) and  1 ( c ) and FIG. 2.  
       [0007]FIG. 1( a ) is a sectional view of a sheet feed roller  10 , wherein the projection  11  is in the form of a spike  12  having a height H, and the spike  12  is pierced into the sheet  21 A under an urging force F 1 . It is assumed that the sheet  21 A is relatively hard, and the piercing depth of the spike  12  into the sheet  21 A is D 1  and the feeding radius of the sheet  21 A is R 1 . In this instance, the piercing depth D 1  of the spike  12  is insufficient so that the spike  12  moves as shown by imaginary line  12 ′, without being synchronized with the recess  22  in the sheet  21 A, thereby causing fluctuation in sheet feeding. The actual feeding distance of the feed roller  10  deviates from the desired feeding distance and reduced by an amount corresponding to the fluctuation of the sheet feeding.  
       [0008] When the urging force F 1  is increased so as to increase the piercing depth D 1  from the state shown in FIG. 1( a ), it is possible to decrease fluctuation in sheet feeding. As shown in FIG. 1( b ), an optimum piercing depth D 0  is achieved under an increased urging force F 0 , with which the fluctuation in sheet feeding is decreased to a negligible level. In this instance, the sheet feeding radius R 0  is a predetermined, optimum value and the slipping rate between the feed roller and the sheet is substantially zero so that a predetermined sheet feeding distance 2πR 0  is achieved for each rotation of the feed roller.  
       [0009] In this way, it is possible to achieve a predetermined feeding distance 2πR 0  under an increased urging force F 0 , insofar as a relatively hard sheet  21 A is concerned. When, however, a relatively soft sheet  21 B is to be fed by the feed roller under the same urging force F 0 , there arises a tendency that the predetermined sheet feeding distance 2πR 0  is not achieved. Thus, as shown in FIG. 1( c ), when the spike  12  of the feed roller  10  is pierced into the relatively soft sheet  21 B, the piercing depth D 2  is larger than the optimum depth D 0  since the sheet  21 B exhibits a relatively small piercing resistance. In this instance, because the distance between the center axis of the feed roller and the tip end of the spike  12  is not changed, the sheet feeding radius R 2  is smaller than the predetermined value R 0 . Therefore, when a relatively soft sheet  21 B is to be fed under an increased urging force F 0  that is made optimum for feeding a relatively hard sheet  21 A without noticeable fluctuation in sheet feeding, the sheet feeding distance per unit rotation of the feed roll is decreased to 2πR 2  that is smaller than the predetermined distance 2πR 0 .  
       [0010] In order to eliminate the above-mentioned problems, it is necessary to achieve a constant piercing depth D 0  irrespective of the hardness of the sheet. To this end, there may be used a feed roller  30  as shown in FIG. 2, wherein microscopic projections in the form of spikes  32  having a triangular section are provided on the outer surface  30 A of the roller  30 . In this instance, as with the case of the spikes  12  shown in FIG. 1( b ), the spikes  32  under the same urging force F 0  are not only pierced into a relatively hard sheet  21 A with the desired piercing depth D 0 , but also pierced into a relatively soft sheet  21 B with a piercing depth that is not increased beyond the desired depth D 0 , due to a contact of the lower surface of the sheet  21 B with the outer surface  30 A of the roller  30 .  
       [0011] However, the feed roller  30  of the type shown in FIG. 2 is not easy to produce efficiently and at reasonable cost, since it would be necessary either to subject the outer surface  30 A of the roller  30  to grinding or the like machining so as to remove materials and thereby leave the microscopic spikes  32  on the outer surface  30 A, or to join separately prepared microscopic spikes  32  to the flat outer surface  30 A of the roller  30 .  
       [0012] An alternative method for producing the feed roller  30  of the type shown in FIG. 2 is disclosed in the patent documents cited above, wherein a round rod is subjected to roll forming so that the material at the outer surface of the rod is raised to form the spikes. While such a method makes it possible to produce the feed roller  30  efficiently and at low cost, there arises a problem that even when it is desired to form a microscopic spike  32  having exactly triangular section and height D 0 , limitations imposed on the production technology make it inevitable that a spike  32  having a somewhat flared root portion is formed. As a result, it is still impossible to maintain substantially constant the piercing depth of the spike  32  as it is pierced into a relatively soft sheet, since the piercing depth depends on the hardness of the sheet and the piercing resistance of the spike at its flared root portion. Therefore, the problem of fluctuation in sheet feeding radius or sheet feeding distance remains unsolved.  
       SUMMARY OF THE INVENTION  
       [0013] It is therefore an object of the present invention to eliminate the problems of the prior art mentioned above, and to provide an improved sheet feed roller capable of achieving a highly precise sheet feeding distance without fluctuations, irrespective of the hardness of the sheet, and suitable for production at high manufacturing productivity and at low cost.  
       [0014] To this end, according to the present invention, there is provided a sheet feed roller having an outer peripheral surface, said outer peripheral surface including at least one feed surface region that extends at least locally in an axial direction of the roller and over an entire circumference of the roller, said feed surface being provided with a plurality of projections, said plurality of projections being comprised of microscopic spikes that can be pierced into the sheet, and stoppers for limiting a piercing depth of the spikes in the sheet.  
       [0015] With the sheet feed roller according to the present invention, since the projections on the feed surface of the feed roller are comprised of microscopic spikes that can be pierced into the sheet, and stoppers for limiting a piercing depth of the spikes in the sheet, it is always possible to maintain the optimum piercing depth of the spikes by the stoppers even when the hardness of the sheet changes from time to time. In this way, the desired sheet feeding radius or distance can be maintained without causing fluctuations, thereby realizing a highly precise sheet feeding.  
       [0016] In the case of a sheet feed roller for ink jet printers, for example, due to limitations in machine design, the total urging force applied to the sheet feed roller in use is made relatively low. In order to achieve an optimum piercing depth for each spike, it is sometimes necessary to reduce the number of the spikes that are simultaneously in engagement with the sheet. Thus, it is preferred that each of the projections comprises one of the spike and the stopper. In this instance, it is possible to reduce the number of the spikes that are simultaneously in engagement with the sheet since projection comprising the stoppers may be arranged adjacent to the projections comprising the spikes, and the distance between the adjacent spikes can be increased. Such an arrangement may also be advantageous when the distance between the adjacent projections cannot be readily reduced due to roll forming conditions or the like.  
       [0017] Alternatively, each of the projections comprising the spikes may further comprise the stopper. In this instance, it is possible to ensure that each spike can be pierced into the sheet by a constant, optimum piercing depth since the stopper of the projection limit the radial position of the sheet relative to the feed roller.  
       [0018] It is preferred that each of the projections is arranged in that region of the feed surface, which is defined by first helices extending in parallel with each other on the outer peripheral surface, and second helices extending in parallel with each other on the outer peripheral surface, wherein the second helices are crossed with the first helices. Here, the term “helices” signifies helical lines that extend in the axial direction of a cylindrical body, along the outer peripheral surface thereof. In this instance, it is possible to efficiently form the projections by a roll forming device comprising a first roll forming die for forming grooves on the outer surface of the feed roller so as to extend along the first helices, and a second roll forming die for forming grooves on the outer surface of the feed roller so as to extend along the second helices, thereby minimizing the production cost of the sheet feed roller.  
       [0019] In the arrangement described above, at least one projection comprising the spike may be arranged alternately with at least one projection comprising the stopper, along the first or second helices. Such an arrangement of the spikes and the stoppers makes it readily possible to ensure that each spike can be pierced into the sheet by a constant, optimum-piercing depth.  
       [0020] It is alternatively preferred that each of the projections is arranged in that region of the feed surface, which is defined by generatrices extending along the outer peripheral surface in parallel with an axial direction of the roller, and circumferential lines extending in parallel with each other on the outer peripheral surface, wherein the circumferential lines are crossed with the generatrices. Here, the term “generatrices” signifies straight lines that, in the case of a cylindrical body, extend axially along the outer peripheral surface of the cylindrical body. In this instance also, it is possible to efficiently form the projections by a roll forming device comprising a first roll forming die for forming grooves on the outer surface of the feed roller so as to extend along the circumferential lines, and a second roll forming die for forming grooves on the outer surface of the feed roller so as to extend along the circumferential lines, thereby minimizing the production cost of the sheet feed roller.  
       [0021] In the arrangement described above, at least one projection comprising the spike may be arranged alternately with at least one projection comprising the stopper, along the generatrices or the circumferential lines. Such an arrangement of the spikes and the stoppers makes it readily possible to ensure that each spike can be pierced into the sheet by a constant, optimum piercing depth.  
       [0022] It is further preferred that the stopper has a flat surface that is substantially at right angles to a radial direction of the roller. The flat surface of the stopper serves to positively maintain the desired optimum piercing depth of the spikes, in highly accurate manner 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0023] The present invention will be more fully explained below with reference to some preferred embodiment shown in the accompanying drawings.  
     [0024] FIGS.  1 ( a ) to  1 ( c ) are sectional views showing a conventional sheet feed roller.  
     [0025]FIG. 2 is a sectional view showing another conventional sheet feed roller.  
     [0026]FIG. 3( a ) is a perspective view showing a sheet feed roller according to a first embodiment of the present invention, and FIG. 3( b ) is a perspective view showing a modification thereof.  
     [0027]FIG. 4 is a developed view of the sheet feeding surface region of the feed roller shown in FIG. 3( a ) or  3 ( b ).  
     [0028]FIG. 5 is a sectional view corresponding to section  5 - 5  in FIG. 4, but showing the feed roller in use.  
     [0029]FIG. 6 is a view showing the arrangement of a roll forming device for forming the feed roller according to the first embodiment.  
     [0030]FIG. 7 is a perspective view showing a first die of the roll-forming device shown in FIG. 6.  
     [0031]FIG. 8 is a perspective view showing a second die of the roll-forming device shown in FIG. 6.  
     [0032]FIG. 9 is a perspective view showing a sheet feed roller according to a second embodiment of the present invention.  
     [0033]FIG. 10 is a developed view of the sheet feeding surface region of the feed roller shown in FIG. 9.  
     [0034]FIG. 11 is a view showing the arrangement of a roll forming device for forming the feed roller according to the second embodiment.  
     [0035]FIG. 12 is a perspective view showing a first die of the roll-forming device shown in FIG. 11.  
     [0036]FIG. 13 is a perspective view showing a second die of the roll-forming device shown in FIG. 11.  
     [0037]FIG. 14 is a perspective view showing a sheet feed roller according to a third embodiment of the present invention.  
     [0038]FIG. 15 is a developed view of the sheet feeding surface region of the feed roller shown in FIG. 14.  
     [0039]FIG. 16 is a sectional view corresponding to section  16 - 16  in FIG. 15, but showing the feed roller in use.  
     [0040]FIG. 17 is a view showing the arrangement of a roll forming device for forming the feed roller according to the third embodiment.  
     [0041]FIG. 18 is a perspective view showing a die of the roll-forming device shown in FIG. 17.  
     [0042]FIG. 19 is a graph showing the deviation of sheet feeding distances obtained by performance tests under various surface pressure conditions.  
     [0043]FIG. 20 is a developed view similar to FIG. 4, showing the sheet feeding surface region of the modified feed rollel  
     [0044]FIG. 21 is a graph showing the deviation of sheet feeding distances obtained by further performance tests under various surface pressure conditions. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0045] A first embodiment of the present invention is shown in FIG. 3( a ), wherein the sheet feed roller is generally denoted by reference numeral  101 . The feed roller  101  may be suitably used in an imaging machine, such as printers, copying machines or facsimile machines, for feeding a sheet S on which image information is printed. The feed roller  101  includes a cylindrical roller body  102  provided on both axial ends with shaft end portions  103  for rotatably supporting the feed roller  101  in the imaging machine. The feed roller  101  has an outer peripheral surface provided with at least one feed surface region  104  that extends at least locally in an axial direction of the roller  101  and over an entire circumference thereof. In the embodiment shown in FIG. 3( a ), there are provided three feed surface regions  104  that are spaced from each other in the longitudinal direction of the feed roller  101 .  
     [0046] A modification is shown in FIG. 3( b ), wherein the sheet feed roller is generally denoted by reference numeral  101 A includes a cylindrical roller body  102 A provided on both axial ends with shaft end portions  103 A for rotatably supporting the feed roller  101 A in the imaging machine. The feed roller  101 A differs from that shown in FIG. 3( b ) essentially in that the outer surface of the roller  101 A as a whole constitutes a feed surface region  104 A.  
     [0047] The sheet feed rollers  101 ,  101 A are rotatably mounted in the imaging machine with their feed surface regions  104 ,  104 A in pressure contact with pinch rollers PR so that the sheet S sandwiched between the feed surface regions  104 ,  104 A and the pinch rollers PR is highly accurately fed toward the downstream side of the feed roller  101 ,  101 A.  
     [0048] As mentioned above, FIG. 4 is a developed view of the sheet feeding surface region  104 ,  104 A of the feed roller  101 ,  101 A shown in FIG. 3( a ) or  3 ( b ), and FIG. 5 is a sectional view corresponding to section  5 - 5  in FIG. 4, but showing the feed roller  101 ,  101 A in use. It can be seen that the entire feed surface region  104  of the roller  101  is comprised of a number of diamond-shaped microscopic areas defined by a plurality of first helices L 1  that are in parallel with each other, and a plurality of second helices L 2  that are also in parallel with each other but arranged so that they are crossed with the first helices L 1 . Each of such microscopic areas is provided with a microscopic first projection  105  or a microscopic second projection  106 , which are combined with each other such that the first and second projections  105  and  106  are arranged alternately with each other along the first helices L 1 , and either the same first projections  105  or the same second projections  106  are arranged continuously along the second helices L 2 .  
     [0049] In FIG. 4, reference character R denotes a direction parallel to the circumferential direction of the sheet feeding region  104 , and reference character W denotes a direction parallel to the center axis of the roller  101 . The first helices L 1  are oriented so as to form an angle of 45° with reference to the axial direction W of the roller  101  and spaced from each other by a pitch P 1 . The second helices L 2  are oriented so as to form an angle of −45° with reference to the axial direction W of the roller  101  and spaced from each other by the same pitch P 1 . Thus, as shown in FIG. 5, the projections  105  and  106  are arranged alternately with each other in the axial direction W of the roller  101  so as to be spaced from each other by a pitch P 2 . It is to be noted, however, that the directions and the pitches of the first and second helices L 1 , L 2  are not limited to those of the embodiments shown in FIG. 4.  
     [0050] The first projection  105  is in the form of a microscopic pyramid having a height H 1 . The first projection  105  has a spike  105 A at it top portion, which can be pierced into the sheet S so as to feed the sheet, and four facets  105 B,  105 C of which two facets  105 B are opposed to the first helices L 1  and the other two facets  105 C are opposed to the second helices L 2 . The first projection  105  forms an angle θ between opposite edges of each facet  105 B,  105 C. The second projection  106  is in the form of a frustum of a microscopic pyramid having a height H 2  that is lower than the height H 1  of the first projection  105 . The second projection  106  has a flat top surface  106 A, which can be brought into engagement with the surface of the sheet S as a stopper for limiting the piercing depth D of the spike  105 A of the adjacent first projection  105 , and four facets  106 B,  106 C, of which two facets  106 B are opposed to the first helices L 1  and the other two facets  106 C are opposed to the second helices L 2 . The second projection  106  forms an angle θ between opposite edges of each facet  106 B,  106 C.  
     [0051] It is preferred that the opposite facets  105 B or  105 C of the first projection  105  form an angle φ that is within a range of 30° to 60°. It is to be noted that the angle φ between the opposite facets  105 B or  105 C of the first projection  105  is slightly different from the angle θ between the opposite edges of each facet  105 B,  105 C. If the angle φ between the opposite facets  105 B or  105 C of the first projection  105  is larger than 60°, there may be instances wherein a sufficient piercing depth D for a relatively hard sheet cannot be achieved, thereby causing slipping of the sheet while it is being fed. On the other hand, if the angle φ is smaller than 30°, there may be instances wherein the mechanical strength of the spike  105 A is insufficient, thereby degrading the durability of the feed roller.  
     [0052] It is also preferred that the piercing depth D of the first projection  105  is within a range of 10 μm to 40 μm. If the piercing depth D is smaller than 10 μm, there may be instances wherein slipping of the sheet occurs due to insufficient piercing depth. On the other hand, if the piercing depth D is larger than 40 μm, there may be instances wherein the surface of the sheet S cannot be properly supported by the stoppers  106 A of adjacent second projections  106  particularly when the sheet S is relatively hard, thereby causing fluctuation in the sheet feeding radius depending upon the hardness of the sheet S.  
     [0053] As for the first projection  105  having a spike  105 A to be pierced into the sheet S, although the tip of the spike  105 A may be sharp from the viewpoint of piercing function, it is often preferred from the viewpoint of manufacturing technology that the spike  105 A is in the form of a frustum of a pyramid. In this instance, it is preferred that the spike has a top surface with a surface area not greater than 400 μm 2 , more preferably not greater than 100 μm 2 , and more preferably not greater than 50 μm 2 . As for the second projection  106 , while it is desirable for the top surface  106 A to have as large a surface area as possible, from the viewpoint of the stopper function, it is often preferred from the viewpoint of manufacturing technology that the tip surface  106 A has a surface area within a range of 160-3600 μm 2 , more preferably 400-2500 μm 2 . Furthermore, although the first projection  105  in the illustrate embodiment is in the form of a pyramid, it may be in the form of a cone provided that it has a spike that can be pierced into the sheet S. Also, although the second projection  106  in the illustrate embodiment is in the form of a frustum of a pyramid, it may be in the form of a frustum of a cone, provided that it has a top surface serving as a stopper for limiting the piercing depth D of the first projection  105 .  
     [0054] When the roller body  102  is comprised of a metal material, the sheet feeding surface region  104  can be advantageously formed by a pair of roll forming dies by a roll forming process to be described hereinafter. FIG. 6 shows the arrangement of a roll forming device comprising a first die  120  and a second die  121  for forming the feed roller according to the embodiment of FIG. 3( a ), and FIGS. 7 and 8 are perspective views showing the first die  120  and the second die  121 , respectively.  
     [0055] The first roll forming die  120  has an outer surface provided with ridges  120 A that are arranged at a constant distance over the entire periphery thereof. Neighboring ridges  120 A are spaced from each other with a groove  120 B therebetween, wherein the groove  120 B is of triangular cross-section. Each ridge  120 A is of a trapezoidal cross-section, and has a cutting surface  120 C on its top, for forming grooves along the first helices L 1  on the sheet feeding surface region  104 . Similarly, the second roll forming die  121  has an outer surface provided with ridges  121 A that are arranged at a constant distance over the entire periphery thereof. Neighboring ridges  121 A are spaced from each other alternately with a groove  121 B and another groove  121 C therebetween, wherein the groove  121 B is of triangular cross-section and the groove  120 C is of trapezoidal cross-section. Thus, for example, a groove  121 B with triangular cross-section is arranged between the first and the second ridges  121 A,  121 A, and a groove  121 C with trapezoidal cross-section is arranged between the second and the third ridges  121 A,  121 A, and such an arrangement of the ridges and the grooves is repeated in the circumferential direction of the second roll forming die. Here also, each ridge  121 A of the second die  121  is substantially of trapezoidal cross-section, and has a cutting surface  121 D on its top, for forming grooves along the second helices L 2  on the sheet feeding surface region  104 .  
     [0056] When the sheet feeding surface regions  104  are to be formed on a roller body  102 , the roller body  102  is clamped between the first and second roll forming dies  120 ,  121 , which are arranged with their respective center axes in parallel with each other. By urging the roller body  102  against the first and second roll forming dies  120 ,  121  under a predetermined working pressure, and rotating these dies  120 ,  121 , it is possible to form the desired feed surface region  104  on the roller body  102 .  
     [0057] As seen in exploded views, the angle formed between the ridge  120 A and the center axis of the first roll forming die  120  is the same as the angle (e.g., +45°) between the first helices L 1  on the sheet feeding surface region  104  and the center axis of the feed roller  101 . Similarly, the angle formed between the ridge  121 A and the center axis of the second roll forming die  121  is the same as the angle (e.g., −45°) between the second helices L 2  on the sheet feeding surface region  104  and the center axis of the feed roller  101 .  
     [0058] With such an arrangement of the roll forming device, the facets  105 B,  106 B of the projections  105 ,  106  opposed to the first helices L 1  are formed by the wall surfaces  120 D of the triangular grooves  120 B in the first die  120 , the facets  105 C of the projections  105  opposed to the second helices L 2  are formed by the wall surfaces  121 E of the triangular grooves  121 B in the second die  121 , the facets  106 C opposed to the second helices L 2  are formed by the wall surfaces  121 F of the trapezoidal grooves  121 C in the second die  121 , and the stoppers  106 A of the second projections  106  are formed by the bottom surfaces  121 G of the trapezoidal grooves  121 C of the second die  121 .  
     [0059] In the roll forming device shown in FIGS.  6  to  8 , all of the grooves in the first roll forming die  120  are comprised of triangular grooves  120 B. It is however possible to arrange one or more trapezoidal grooves between neighboring triangular grooves  120 B. As for the second roll forming die  121 , it is likewise possible to arrange two or more trapezoidal grooves  121 C between neighboring triangular grooves  121 B. By appropriately selecting the number of the trapezoidal grooves provided for the roll forming dies  120 ,  121 , it is possible to realize a desired arrangement of the microscopic projections  105 ,  106  on the sheet feeding surface region  104  wherein the number of the stoppers  106 A is optimized for each spike  105 A.  
     [0060] As mentioned above, the first and second dies  120 ,  121  of the roll forming device shown in FIGS.  6 - 8  are arranged with their respective center axes in parallel with each other, as mentioned above. The roller body  102  is oriented in parallel with the dies  120 ,  121  and urged against the dies  120 ,  121  under a predetermined working pressure, while the dies  120 ,  121  are rotated. In this instance, it is possible to form the sheet feeding surface region  104  on the roller body  102  without causing an axial movement of the roller body  102  relative to the first and second dies  120 ,  121 , provided that the width of the sheet feeding surface region  104  on the roller body  102  as seen in the axial direction is the same as the width of the dies  120 ,  121 . This type of roll forming method is known as infeed roll forming process.  
     [0061] When such an infeed roll forming process is applied to formation of the sheet feeding surface region  104 A of the sheet feed roller  101 A shown in FIG. 3( b ), which extends over the entire length of the roller body  102 A, the roll forming dies  120 ,  121  must have a large width corresponding to the axial length of the sheet feeding surface region  104 A, thereby making it difficult to achieve an uniform roll forming over the entire length of the roller body  102 A. In order to eliminate such difficulty, it is preferred to carry out a thru-feed roll forming process wherein the roll forming device has a slightly different arrangement in that the center axis of the first die  120  is inclined relative to the center axis of the roller body  102 B by a predetermined angle, and the center axis of the second die  121  is oppositely inclined relative to the center axis of the roller body  102 B by the same angle of the opposite sign, without causing intersection of the center axes of the dies  120 ,  121  with the center axis of the roller body  102 B. In this instance, the roller body  102  is urged against the dies  120 ,  121  under a predetermined working pressure, while the first and second roll forming dies  120 ,  121  are rotated, so as to feed the roller body  102  axially relative to the dies  120 ,  121  under a predetermined speed, and thereby form the feed surface region  104  uniformly over the roller body  102 .  
     [0062] A second embodiment of the present invention is shown in FIG. 9, wherein the sheet feed roller is generally denoted by reference numeral  131 . The feed roller  131  includes a cylindrical roller body  132  provided on both axial ends with shaft end portions  133  for rotatably supporting the feed roller  131  in the imaging machine. The feed roller  131  has an outer peripheral surface provided with three sheet feeding surface regions  134  that are spaced from each other axially and extend over an entire circumference of the roller  131 .  
     [0063] The sheet feed roller  131  is rotatably mounted in the imaging machine with its sheet feeding surface regions  134  in pressure contact with pinch rollers PR so that the sheet S sandwiched between the feed surface regions  134  and the pinch rollers PR is highly accurately fed toward the downstream side of the feed roller  131 .  
     [0064] As mentioned above, FIG. 10 is a developed view of the sheet feeding surface region  134  of the feed roller  131  shown in FIG. 9, wherein reference character R denotes a direction parallel to the circumferential direction of the sheet feed roller  131 , and reference character W denotes a direction parallel to the center axis of the feed roller  131 . It can be seen that each feed surface region  134  of the roller  131  is comprised of a number of rectangular microscopic areas defined by a plurality of circumferential lines L 3  that extend over the sheet feeding surface region  134 , and a plurality of generatrices L 4  that extends axially over the sheet feeding surface region  134 . Each of such microscopic areas is provided with a microscopic first projection  135  or a microscopic second projection  136 , which are combined with each other such that the first and second projections  135  and  136  are arranged alternately with each other along the generatrices L 4 , and either the same first projections  135  or the same second projections  136  are arranged continuously along the circumferential lines L 3 .  
     [0065] The first projection  135  is in the form of a microscopic pyramid having a spike  135 A at it top portion, which can be pierced into the sheet S so as to feed the sheet, and four facets  135 B,  135 C of which two facets  135 B are opposed to the circumferential lines L 3  and the other two facets  135 C are opposed to the generatrices L 4 . The second projection  136  is in the form of a frustum of a microscopic pyramid having a flat top surface  136 A, which can be brought into engagement with the surface of the sheet S as a stopper for limiting the piercing depth D of the spike  135 A of the adjacent first projection  135 , and four facets  136 B,  136 C, of which two facets  136 B are opposed to the circumferential lines L 3  and the other two facets  136 C are opposed to the generatrices L 4 .  
     [0066] When the roller body  132  is comprised of a metal material, the sheet feeding surface region  134  can be advantageously formed by a pair of roll forming dies by a roll forming process to be described hereinafter. FIG. 11 shows the arrangement of a roll forming device comprising a first die  140  and a second die  141  for forming the feed roller according to the embodiment of FIG. 9, and FIGS. 12 and 13 are perspective views showing the first die  140  and the second die  141 , respectively.  
     [0067] The first roll forming die  140  has an outer surface provided with ridges  140 A that are arranged at a constant distance over the entire periphery thereof. Each ridge  140 A has a flat top surface  140 D for forming grooves in the sheet feeding surface region  134  so as to extend along the. Neighboring ridges  140 A are spaced from each other alternately with a groove  140 B or another groove  140 C therebetween, wherein the groove  140 B is of triangular cross-section and the groove  140 C is of trapezoidal cross-section. Each ridge  140 A is of trapezoidal cross-section, and has a cutting surface  140 D on its top, for forming grooves along the circumferential lines L 3  on the sheet feeding surface region  134 . Similarly, the second roll forming die  141  has an outer surface provided with ridges  141 A that are arranged at a constant distance over the entire periphery thereof. Neighboring ridges  141 A are spaced from each other alternately with a groove  141 B, which is of triangular cross-section. Here also, each ridge  141 A of the second die  141  is substantially of trapezoidal cross-section, and has a cutting surface  141 C on its top, for forming grooves along the generatrices L 4  on the sheet feeding surface region  134 .  
     [0068] With such an arrangement of the roll forming device, the facets  135 C,  136 C of the projections  135 ,  136  opposed to the generatrices L 4  are formed by the wall surfaces  141 D of the triangular grooves  141 B of the second die  141 , the facet  135 B of the projection  135  opposed to the circumferential lines L 3  are formed by the wall surfaces  140 E of the triangular grooves  140 B of the first die  140 , the facets  136 B of the projection  136  opposed to the circumferential lines L 3  are formed by the wall surfaces  140 F of the trapezoidal grooves  140 C of the first die  140 , and the stoppers  136 A of the projection  136  are formed by the bottom surfaces  140 G of the trapezoidal grooves  140 C of the first die  140 .  
     [0069] In the roll forming device shown in FIGS.  10 - 12 , it is possible to arrange two or more trapezoidal grooves  140 C between neighboring triangular grooves  140 B. By appropriately selecting the number of the trapezoidal grooves provided for the roll forming dies  140 ,  141 , it is possible to realize a desired arrangement of the microscopic projections  135 ,  136  on the sheet feeding surface region  134  wherein the number of the stoppers  136 A is optimized for each spike  135 A.  
     [0070] A third embodiment of the present invention is shown in FIG. 14, wherein the sheet feed roller is generally denoted by reference numeral  151 . The feed roller  151  includes a cylindrical roller body  152  provided on both axial ends with shaft end portions  153  for rotatably supporting the feed roller  151  in the imaging machine. The feed roller  151  has an outer peripheral surface provided with three sheet feeding surface regions  134  that are spaced from each other axially and extend over an entire circumference of the roller  151 .  
     [0071] The sheet feed roller  151  is rotatably mounted in the imaging machine with its sheet feeding surface regions  154  in pressure contact with pinch rollers PR so that the sheet S sandwiched between the feed surface regions  154  and the pinch rollers PR is highly accurately fed toward the downstream side of the feed roller  151 .  
     [0072] As mentioned above, FIG. 15 is a developed view of the sheet feeding surface region  154  of the feed roller  151  shown in FIG. 14, and FIG. 16 is a sectional view corresponding to section  16 - 16  in FIG. 15, but showing the feed roller  151  in use. It can be seen that the entire feed surface region  154  of the roller  151  is comprised of a number of diamond-shaped microscopic areas defined by a plurality of first helices L 5  that are in parallel with each other, and a plurality of second helices L 6  that are also in parallel with each other but arranged so that they are crossed with the first helices L 5 . Each of such microscopic areas is provided with a microscopic projection  155 .  
     [0073] In FIG. 15, reference character R denotes a direction parallel to the circumferential direction of the sheet feeding region  154 , and reference character W denotes a direction parallel to the center axis of the roller  151 . The first helices L 5  are oriented so as to form an angle of 45° with reference to the axial direction W of the roller  151  and spaced from each other by a pitch P 1 . The second helices L 6  are oriented so as to form an angle of −45° with reference to the axial direction W of the roller  151  and spaced from each other by the same pitch P 1 . Thus, as shown in FIG. 14, the projections  155  are aligned in the axial direction W of the roller  151  so as to be spaced from each other by a pitch P 2 . It is to be noted, however, that the directions and the pitches of the first and second helices L 5 , L 6  are not limited to those of the embodiments shown in FIG. 15.  
     [0074] The projection  155  includes a lower portion in the form of a frustum of pyramid, and an upper portion in the form of a pyramid, wherein the bottom surface of the upper portion is smaller than the top surface of the lower portion. The upper portion forms a spike  155 A that can be pierced into the sheet S so as to feed the sheet. The top surface of the lower portion can be brought into engagement with the surface of the sheet S as a stopper for limiting the piercing depth D of the spike  155 A. The lower portion of the projection  155  has four facets  155 C that are opposed to the first or second helices L 5 , L 6 . Similarly, the upper portion of the projection  155  has four facets  155 D that are opposed to the first or second helices L 5 , L 6 . The first projection  155  forms an angle θ between opposite edges of each facet  155 C,  155 D. The projection  155  has a height H 1 , and the lower portion has a height H 2 .  
     [0075] When the roller body  152  is comprised of a metal material, the sheet feeding surface region  154  can be advantageously formed by a pair of roll forming dies by a roll forming process to be described hereinafter. FIG. 17 shows the arrangement of a roll forming device comprising a first die  160  and a second die  161  for forming the feed roller according to the embodiment of FIG. 14, and FIGS.  18  and  19  are perspective views showing the first die  160  and the second die  161 , respectively.  
     [0076] The first roll forming die  160  each has an outer surface provided with ridges  160 A that are arranged at a constant distance over the entire periphery thereof. Neighboring ridges  160 A are spaced from each other with a groove  160 B therebetween, wherein the groove  160 B is of stepped cross-section defined by a trapezoidal portion and a triangular portion. Each ridge  160 A is of a trapezoidal cross-section, and has a cutting surface  160 C on its top, for forming grooves along the first helices L 5  on the sheet feeding surface region  154 . Similarly, the second roll forming die  161  has an outer surface provided with ridges  161 A that are arranged at a constant distance over the entire periphery thereof. Neighboring ridges  161 A are spaced from each other with a groove  161 B therebetween, wherein the groove  161 B is of stepped cross-section defined by a trapezoidal portion and a triangular portion. Each ridge  161 A is of trapezoidal cross-section, and has a cutting surface  161 C on its top, for forming grooves along the second helices L 6  on the sheet feeding surface region  154 .  
     [0077] As seen in exploded views, the angle formed between the ridge  160 A and the center axis of the first roll forming die  160  is the same as the angle (e.g., +45°) between the first helices L 5  on the sheet feeding surface region  154  and the center axis of the feed roller  151 . Similarly, the angle formed between the ridge  161 A and the center axis of the second roll forming die  161  is the same as the angle (e.g., −45°) between the second helices L 6  on the sheet feeding surface region  154  and the center axis of the feed roller  151 .  
     [0078] With such an arrangement of the roll forming device, the facets  155 C of the lower portion of the projection  155 , which are opposed to the first helices L 5 , are formed by the wall surfaces  160 E,  161 E at the trapezoidal portions of the triangular groves  160 B,  161 B, the facets  155 D of the upper portion of the projection  155 , which are opposed to the second helices L 6 , are formed by the wall surfaces  160 F,  161 F at the triangular portions of the groves  160 B,  161 B, and the stoppers  155 B of the projections  155  are formed by the bottom surfaces  160 D,  161 D of the trapezoidal portions of the grooves  160 B,  161 B.  
     [0079] Performance Tests  
     [0080] In order to confirm functional advantages of the sheet feed roller according to the present invention, performance tests were conducted as follows. First of all, a sheet feed roller  101  shown in FIG. 3( a ) was used as Example 1, to measure the sheet feeding distance under various surface pressures of the pinch rollers PR. Another sheet feed roller was used as Control, wherein the projections  106  having the stoppers  106 A were replaced by the projections  105  having the spikes  105 A, to measure the sheet feeding distance under various surface pressures of the pinch rollers PR. The result of the tests is shown in FIG. 19.  
     [0081] In the next place, a sheet feed roller similar to that shown in FIG. 3( a ) was used as Example 2, to measure the sheet feeding distance under various surface pressures of the pinch rollers PR. In this instance, as shown in FIG. 20, the sheet feeding surface  104 X of the feed roller as Example 2 has an arrangement of the projections  105 ,  106  wherein two projections  106 X having stoppers are arranged between two neighboring projections  105 X having spikes. The sheet feed roller used as Control was the same as that described above. The result of the tests is shown in FIG. 21.  
     [0082] It can be seen that FIGS. 19 and 21 are graphs wherein the abscissa indicates the theoretical feeding distance that can be obtained by multiplying the moving angle of the feed roller with the nominal diameter of the feed roller, and the ordinate indicates the deviation of the feeding distance, i.e., the difference between the theoretical feeding distance and the actual feeding distance. It is noted that the surface pressure of the pinch rollers is per 1 mm in the axial direction of the feed roller. The sheets used for the performance tests were coated sheets for ink jet printers.  
     [0083] In the sheet feed rollers used in the performance tests as Examples 1, 2 and Control, the angle of the first helices L 1  relative to the center axis of the roller is +45°, the angle of the second helices L 2  relative to the center axis of the roller is −45°, the distance between the neighboring helices is 0.35 mm, and the projections with the spikes or stoppers are in the form of pyramid of which the opposite facets form an angle of 50° relative to each other. The dimensions (μm) of the projections are as shown in the following Table.  
                                               Projections   Items   Example 1   Example 2   Control                  Projection   Height   75   80   75       with spike   Top surface   10 × 10   7 × 7   10 × 10       Projection   Height   62   49   —       with stopper   Top surface    8 × 20   40 × 40   —                  
 
     [0084] It will be appreciated from FIGS,  19  and  21  that the sheet feed rollers of Examples 1 and 2 according to the present invention exhibit larger average sheet feeding distance due to suppressed fluctuation in sheet feeding, and stable feeding distance due to fluctuations relative to the surface pressure of the pinch rollers.  
     [0085] With the sheet feed roller according to the present invention, since the projections on the feed surface of the feed roller are comprised of microscopic spikes that can be pierced into the sheet, and stoppers for limiting a piercing depth of the spikes in the sheet, it is always possible to maintain the optimum piercing depth of the spikes by the stoppers even when the hardness of the sheet changes from time to time. In this way, the desired sheet feeding radius or distance can be maintained without causing fluctuations, thereby realizing a highly precise sheet feeding.  
     [0086] While the present invention has been described above with reference to some preferred embodiments, various modifications or variations may be made without departing from the scope of the invention as defined by the appended claims.