Abstract:
An image forming device includes a roll tensioner adapted to hold a medium roll rotatably mounted thereon for providing a printable medium strip to the imaging device. The roll tensioner comprises a torsion mechanism exerting torsion to the printable medium strip that tends to pull back the printable medium strip and, thus, to prevent slack of the printable medium strip, the roll tensioner further including a torsion control mechanism to prevent the torsion from getting too large to stop the rotation of the printable medium roll, the roll tensioner also including a clutch mechanism to prevent damage to the torsion mechanism.

Description:
This application is based upon provisional patent application Ser. No. 60/106,895 which was filed in the United States Patent and Trademark Office on Nov. 3, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to an image forming device, and more particularly to a roll holder/tensioner adapted to be incorporated into a printer for holding a printable medium roll and for providing tension on the printable medium strip to prevent looping, jamming, or other printing problems associated with a slack printable medium moving in the image forming device. 
     BACKGROUND OF THE INVENTION 
     All conventional image forming devices are designed to form images on printable media that are fed into the image forming devices either by external feeding devices coupled to the image forming devices or by certain internal feeding mechanisms incorporated into the image forming devices. Ordinarily, the printable media has at least one side of its surface suitable for forming information-carrying images thereon. However, many commercially available printable media have printable surfaces on both sides. There are many types of printable media available in the market, such as regular papers, labels, or thermal paper, etc. Thus, every conventional image forming device is adapted to receive at least one type of printable medium for printing purposes. In addition, many different types of image forming devices, e.g., printers, fax machines, or copiers, etc., are also available in the market, and the methods these image forming devices use to impart images on the printable media are often very different. For example, some image forming devices need ribbons to transfer images to the printable medium, while others use a direct thermal transfer method to form images on the printable medium. 
     Moreover, even one type of commercially available printable media may have many different forms. For instance, the printable media may come in the form of separate sheets, or it may be in the form of a continuous paper strip. Consequently, different types of feeding mechanisms are specially designed and are adapted to be incorporated into selected printers for respectively feeding these different forms of printable media into the printers. Separate sheets of plain paper are undeniably still the predominant printable medium type that is used in today&#39;s business applications. There is, however, an ongoing need to have a printable medium in the form of a strip wrapped into a printable medium roll. Typically, this printable medium roll, such as a label roll, has the printable medium strip, such as a label strip, winding around a cardboard support tube, or a similarly shaped tube made by other suitable materials. In the case of a label roll, the label roll has serially arranged labels positioned on the label strip, and each label of the label strip has a printable surface on a front side and an adhesive back side attached to a continuous protective backing of the label strip. The protective backing of the label strip generally has a treated glossy surface attached to the adhesive side of the labels permitting the labels to be easily peeled off from the protective backing. The labels are then, together with the protective backing strip, wound around the support tube to form the label roll. 
     Most conventional printers have relatively simple holding mechanisms incorporated therein for holding label rolls. Typically, a conventional printer includes either a cylindrically shaped roll holder transversely positioned across the printer or, alternatively, two ear-like spool ends positioned at corresponding opposite inner sides of the printer. As a result, the label strip in the printer normally moves in a forward direction, which is perpendicular to the axis of the label roll, toward a front side of the printer. The roll holder, or the pair of spool ends, of the conventional printer function to hold the label roll in order to facilitate the label strip to be unwound and moved toward a print head of the printer. Each conventional printer has a print head, and the print head is the part of the printer that converts electrical signals into images formed on the printable medium, such as the label strip. Additionally, a platen is rotatably mounted within the printer and is adapted to press the label strip tightly against the print head for receiving the images. The platen is coupled to a motor for rotation in order to move the label strip through the printer. In some conventional printers, their cylindrically shaped roll holders (and/or the ear-like spool ends, do not themselves rotate during operation of the printers. They work only as a stationary roll holder support for the printable media. In other printers, the roller holders or the spool ends are rotatable. 
     Most conventional printers, and other image forming devices that use printable medium rolls, often experience a common problem of loose media. The loose media problem happens when the printable medium strip traveling within the printer becomes loose before and/or after being fed through the print head of the printer. There are different reasons that may cause the loose media problem in conventional printers, but it is almost impossible to predict when or how often this loose media problem will happen to any particular printer. The loose media problem frequently causes printing errors, such as skid printing or double printing, on the printable medium. It may possibly jam the conventional printer as well. 
     The loose media problem generally occurs when various parts of the printable medium strip travel through the conventional printer at slightly different speeds. The speed differences experienced by different parts of the medium strip are normally due to the inability of the conventional printer to move the printable medium strip at a constant speed throughout the printer. For instance, the platen of the conventional printer may move the label strip at a first speed that is slower than, albeit maybe slightly, a second speed traveled at by the label strip moved by the roll holder. As a result, a section of the label strip between roll holder and the platen may become loose. 
     In addition, a user may often need to move forward and/or back up the printable medium strip in the printer in order to adjust the position of the printable medium strip within the printer. Moving forward or backing up the printable medium strip in the printer is ordinarily achieved by rotating, manually or automatically by a motor of the printer, the platen of the printer. After the platen has been rotated to adjust for a proper printing position of a particular label on the label strip, the label roll will also need to be adjusted accordingly to maintain the tightness of the label strip between the label roll and the print head. Otherwise, the label strip will become loose and may cause many printing problems, such as a sudden jerk or a jump, when the next printing job begins. A built-in mechanism of the printer is therefore needed to maintain tightness of the label strip, or of any other type of printable media used, within the printer to prevent the loose media problem. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a tension mechanism incorporated into an image forming device in order to maintain a proper tension level in a printable medium strip, such as a label strip, thereby to prevent the loose media problem of the medium strip commonly experienced by many conventional printers. This object is met by providing a roll tensioner incorporated into a printer according to the present invention, as indicated in the claims appended thereto. 
     Accordingly, a preferred embodiment of the present invention provides the roll tensioner incorporated into the printer for holding a printable medium roll having the medium strip wound thereon. The roll tensioner has an internal tension mechanism adapted to constantly maintain a proper tension level on a portion of the printable medium strip, which extends from out of the medium roll toward the print head of the printer. According to the present invention, the tension level on that portion of the medium strip will be properly maintained both during unwinding or rewinding of the medium roll, whether automatically driven by a motor of the printer or manually driven by other means. 
     The foregoing and additional features and advantages of this present invention will become apparent byway of non-limitative examples shown in the accompanying drawings and detailed descriptions that follow. In the figures and written description, numerals indicate the various features of the invention, like numerals referring to like features throughout for both the drawing figures and the written description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a printer that incorporates a roll tensioner according to the present invention. 
     FIG. 2 shows an isometric view of the roll tensioner of the present invention. 
     FIG. 3 a - 3   c  show the roll tensioner inserted into a medium roll to be mounted on the printer. 
     FIG. 3 bb  is a cross-sectional view taken along section line  3   bb — 3   bb  of FIG. 3 b.    
     FIG. 3 cc  is a cross-sectional view taken along section line  3   cc — 3   cc  of FIG. 3 c.    
     FIG. 4 is a cross-sectional view taken along section line  4 — 4  of FIG. 2 excluding the retainers of FIG.  2 . 
     FIG. 5 shows a retainer of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a roll tensioner  10  according to the present invention incorporated into a printer  1  depicted in an open loading position. In FIG. 1, the roll tensioner  10  is positioned inside the printer  1  near a back end. In a preferred embodiment of the invention, the printer  1  includes a support frame  64  having a pair of roller slots  60 ,  62  (FIG. 1) respectively located at opposite side ends of the support frame  64  and near the back end of the printer  1 . Roll tensioner  10  is coupled to the support frame  64  by insertion into the roller slots  60 ,  62 . Roll tensioner  10  is in a generally stepped-cylindrical shape with a diameter of approximately 1 inch at a center main portion and is suitable to hold a printable medium roll, such as a label roll  100 , mounted thereon. In the preferred embodiment, the roll tensioner  10  is also directionally sensitive and will not work properly if it is incorrectly installed in the printer  1 , as will be explained in further details. Accordingly, roll tensioner  10  has a pair of differently shaped end caps respectively located at opposite ends (left and right) to help a user determine which end of the roll tensioner  10  should mesh with which roller slot ( 60  or  62 ) of support frame  64  of printer  1 . Roller slots  60 ,  62  also have different matching shapes respectively to house the correspondingly shaped end caps of roll tensioner  10  (FIG.  1 ). 
     In the preferred embodiment, the roll tensioner  10  also includes a pair of retainers  102 ,  104  respectively coupled to the roller tensioner  10  at opposite sides (left and right), as shown in FIGS. 1 and 2. The retainers  102 ,  104 , however, can be dispensed with in other alternative embodiments of the present invention. The retainers  102  and  104  are adopted for the purpose of securing a lateral position of the printable medium roll mounted on the roller tensioner  10 , thereby the printable medium roll will not move laterally during the operation once mounted on the printer  1 . Referring to FIG. 2, the roll tensioner  10  comprises a tube sleeve  11  having two sets of surface notches  66   a ,  66   b  positioned at opposite sides on the surface of the tube sleeve  11 . The tube sleeve  11  is approximately 9.13 inches long, and each set of the notches  66   a  or  66   b  is approximately 2.4 inches long respectively extending from the opposite ends toward the center of the tube sleeve  11 . Scale indicators positioned adjacent to both sets of the notches  66   a ,  66   b  may also be provided for indicative purposes. The scale indicators may be of alphabetical letters or of numerical numbers, but they should be in a same alphanumeric set at both sides and, preferably, should ascend the alphanumeric order from the center to the opposite ends of the tube sleeve  11  for easy reading. Additional sets of notches (not shown) may also be included and are respectively positioned on the surface at approximately diametrically opposite sides to the sets of notches  66   a ,  66   b.    
     Referring to FIG. 5, the retainers  102 ,  104  are of generally rhomboidal shape but with smoothly round angles at each apex. In the preferred embodiment, each retainer  102  or  104  is approximately 5 inches long and approximately 2.25 inches wide. Each of the retainers  102 ,  104  has a generally round center hole such as center hole  112  (FIG. 5) having a diameter slightly larger than 1 inch. The center holes of the retainers  102 ,  104  have detent-snap-like notches  116 ,  118  on their respective innerwalls of the center holes. As a result, the retainers  102 ,  104  are adapted to be snapped onto the surface notches  66   a ,  66   b  of the roll tensioner  10  at both ends, as shown in FIG.  2 . The retainers  102 ,  104  are adapted to trap the label roll  100  on the roll tensioner  10  between the retainers  102 ,  104 . Therefore, if the retainers  102 ,  104  tightly confine the label roll  100  in between, the label roll  100  will not move laterally along the roll tensioner  10  when the label roll  100  rotates during the operation of the printer  1 , thereby helping to prevent misalignment of the label strip in the printer  1 . In addition, the retainers  102 ,  104  are also adapted to center the label roll  100  on roll tensioner  10 . The scale indicators on roll tensioner  10  will indicate the positions of the retainers  102 ,  104  respectively once they are mounted on the roll tensioner  10 . Thus, the user may easily center the label roll  100  on the roll tensioner  10  by adjusting the respective positions of the retainers  102 ,  104  that are indicated by the scale indicators. As a result, although the retainers  102 ,  104  may be optional to the present invention as mentioned, they are particularly useful when a narrow printable medium roll is used for printer  1 . 
     Referring to FIG. 2, roll tensioner  10  further includes two holding springs  106 ,  108  respectively positioned on the surface center of the tube sleeve  11  at diametrically opposite sides. Each holding spring  106  or  108  is placed on a corresponding recess of the surface of the tube sleeve  11 , and a center portion of the holding spring  106  or  108  protrudes slightly above the surface of the tube sleeve  11  to hold the support tube of the label roll  100 . The center portions of the respective holding springs  106 ,  108  are adapted to be pressed downward slightly toward the surface of the tube sleeve  11 . In the preferred embodiment, each holding spring  106 ,  108  is basically a piece of curved metal plate and is securely mounted on tube sleeve  11  (see FIG. 4) of roll tensioner  10  by a screw such as screw  110  (FIG. 2 ). The holding springs  106 ,  108  are of approximately 1.25 inches in bent length respectively. In an alternative embodiment, only one holding spring is provided to the present invention, as compared to the preferred embodiment which has two holding springs. In yet other alternative embodiments, other suitable elastic means may be used in lieu of the holding springs  106 ,  108  so long as they serve a similar purpose, i.e., holding the printable medium roll. 
     FIGS. 3 a - 3   c  show the roll tensioner  10  being inserted into the label roll  100  for mounting on the printer  1 . In FIG. 3 b , the roll tensioner  10  is inserted into the label roll  100  correctly, while the roll tensioner  10  is inserted incorrectly in FIG. 3 c . As shown in FIGS. 3 bb  and  3   cc , the roll tensioner  10  has first and second stepped end caps  12  and  44  respectively of different outer periphery shapes. Both of the first and second end caps  12 ,  14  are approximately 1.1 inches long and are located at opposite ends of the roll tensioner  10  respectively. In FIG. 4, the first stepped end cap  12  has first and second sections  13 ,  14  and a collar  15 , wherein the first section  13  has an outer diameter, of approximately 0.5 inch. The collar  15  has an outer diameter of approximately 0.9 inch, that is larger than that of the second section  14 , and an inner diameter of approximately 0.6 inch. Second section  14  has a hexagonal cross section (FIG. 3 bb ) with a flat to flat distance of approximately 0.7 inch. First section  13  is tubular shaped while collar  15  is of ring shape. Second stepped end cap  44  also has first and second sections  45 , 46  and a collar  47 . Both the first and second sections  45 ,  46  are of cylindrical tube shape wherein the first section  45  has a similar diameter as the diameter of the first section  13  of the first stepped end cap  12 . Second section  46  has an outer diameter of approximately 0.7 inch (FIG. 3 cc ), which is larger than that of first section  45 . The collar  47  is of round shape and has a diameter size similar to the diameter size of the collar  15 , which is larger than that of second section  46 . 
     FIG. 4 shows a cross sectional view of the roll tensioner  10 . The roll tensioner  10  includes a metal shaft  20  encircled within the tube sleeve  11 . The shaft  20  is approximately 11.5 inches long and has a diameter of approximately {fraction (5/16)} inch. The length of the shaft  20  is slightly longer than a combinational length of the tube sleeve  11  and both end caps  12  and  44 . Thus, the shaft  20  extends through and slightly outward of both stepped end caps  12  and  44  when mounted. The shaft  20  is coupled to a slip mechanism  23  (FIG.  4 ), which will be elaborated further in the following paragraphs, and is adapted to be rotated by the slip mechanism  23  in a rotational direction, opposite to a relative rotational direction of the tube sleeve  11 . Further, a torsional mechanism  21  (FIG. 4) is coupled to slip mechanism  23  and has elastic means to impart tension on label roll  100 . In the preferred embodiment, the elastic means shall not be permitted to rotate in a rewinding rotation to prevent the elastic means from being damaged. Both the stepped end caps  12  and  44  have cylindrical channels therein to allow the shaft  20  to pass through. The tube sleeve  11  and both the end caps  12  and  44  are made of plastic materials in the preferred embodiment, but any other suitable materials may be used to manufacture the same. 
     As mentioned, in contrast to the hexagonal shape of second section  14  of first stepped end cap  12 , second section  46  of second stepped end cap  44  is of to round shape. The shape difference between the first and second sections  14  and  46  is particularly useful because it prevents a user from accidentally inserting roll tensioner  10  into the printer in a wrong orientation, opposite of that shown in FIG.  1 . As mentioned above, printer  1  has respective roller slots  60  and  62  on opposite side ends of the printer  1  for housing end caps  12  and  44 , respectively, of roll tensioner  10 . Roller slot  60  has a matching hexagonal shape to receive section  14  (FIG.  1 ), and roller slot  62  has a matching round shape to receive section  46  (FIG.  1 ). By giving different shapes to end caps  12  and  44 , and the respective matching shapes of roller slots  60  and  62 , a user will, therefore, not make mistakes in installing roll tensioner  10  into printer  1  and, thus, ensure that roll tensioner  10  will function properly during operation. In an alternative embodiment, the shapes of both sections  14 ,  46  could be exchanged, so long as the respective roller slots  60 ,  62  will also change their matching shapes as well. In yet another embodiment, sections  14 ,  46  could have shapes other than hexagonal and round. But the matching shapes of the roller slots  60 ,  62  shall also be changed accordingly. 
     Furthermore, once end caps  12  and  44  are inserted into roller slots  60  and  62 , respectively, end cap  12  does not rotate during the operation of printer  1 . As shown in FIG. 4, end caps  12  and  44  are not rotationally coupled to tube sleeve  11 . During operation, the label roll  100  will be unwound to feed labels into the printer  1 . The label roll  100  is tightly held by the metal holding springs  106  and  108  securely coupled to the surface of the tube sleeve  11 . Therefore, when the label roll  100  rotates, it will pull the tube sleeve  11  to rotate accordingly. Additionally, inside the sections  14  and  46  of the respective stepped end caps  12  and  44 , there are respective first and second needle roller bearings  16  and  48  coupled to respective inner walls of the end caps  12  and  44  and encircling the shaft  20 . The needle roller bearings  16  and  48  are used to allow low frictional rotation to the shaft  20  and may be obtained from any of a number of standard needle bearing manufacturers. 
     As noted, shaft  20  is coupled to slip mechanism  23  inside tube sleeve  11 . Torsional mechanism  21  comprises a torsion spring  18  and collar  15  and is coupled to slip mechanism  23  (FIG.  4 ). Slip mechanism  23  has two functional portions. A first portion of slip mechanism  23  prevents torsion spring  18  from achieving torsion in a coil-unwinding rotational direction and a second portion of slip mechanism  23  prevents torsion spring  18  from winding past a threshold value of torsion in a coil winding rotational  10  direction. 
     The first portion of the slip mechanism  23  that protects the torsion spring  18  from gaining torsion in the coil-unwinding rotational direction includes a metal sleeve  34  and a one way slip clutch  36 . In FIG. 4, the torsion spring  18  is positioned inside the tube sleeve  11 , next to the first stepped end cap  12 , and encircles the shaft  20 . In the preferred embodiment, the torsion spring  18  is approximately 1.02 inches long (excluding the bent, extending straight coil parts at both ends). The torsion spring  18  is made up of a coil with a first end of the coil at the right side of the torsion spring  18  bent approximately 90 degrees for correct insertion into a small hole (not shown) of the collar  15  of the first end cap  12 . A second end, opposite to the first, of the coil is also bent  90  degrees to facilitate insertion into a similar hole (not shown) in the right end face of the metal sleeve  34 , which is next to the torsion spring  18  at the left side. In the preferred embodiment, a plastic sleeve (not shown) is enclosed within the torsion spring  18  to encircle the shaft  20 . The plastic sleeve has a lateral length of approximately 1.0 inches and prevents the torsion spring  18  from collapsing or rubbing against the shaft  20  during rotation or winding with torsion. In an alternative embodiment, no plastic sleeve is provided to the present invention. 
     The metal sleeve  34  is situated next to the torsion spring  18  at the left side and toward the center portion of the shaft  20 . The metal sleeve  34  is approximately 0.427 inches long and has an outer diameter of approximately 0.67 inches and an inner diameter of approximately 0.47 of an inch. The metal sleeve  34  encircles the one way slip clutch  36  with an interference fit that prevents rotation between the metal sleeve  34  and an outer ring of the one way slip clutch  36 . The one way slip clutch  36  encircles the shaft  20  and is approximately the same length as the metal sleeve  34 . The one way slip clutch  36  coupled together with the metal sleeve  34  rotate freely in a first rotational direction (counterclockwise relative to the tube sleeve  11  when seen inward from the first end cap  12 ) causing the torsion spring  18  to rotate freely without allowing torsion to increase in the rotational direction that would unwind the torsion spring&#39;s coils. But, the metal sleeve  34  and the one way slip clutch  36  resist rotation in an opposite rotational direction (clockwise relative to the tube sleeve  11 ). Thus, they would cause the torsion in the torsion spring  18  to increase when the tube sleeve  11  and shaft  20  rotate counterclockwise and the first end cap  12  is not allowed to rotate relatively. 
     The torsion of the torsion spring  18  cannot be increased indefinitely. Otherwise, it would eventually halt the shaft  20  and the tube sleeve  11  and, thus, the label roll  100 , from rotating. Therefore, the roll tensioner  10  requires the slip mechanism  23  to additionally prevent the torsion of the torsion spring  18  from being increased over a predetermined threshold value, as well as to prevent the torsion spring  18  from accumulating torsion in the coil-unwinding direction. The slip mechanism  23  of the present invention is designed to maintain a roughly constant torsion of the torsion spring  18  at the threshold value when this torsion has reached the threshold value. The metal sleeve  34 , the one way slip clutch  36 , first and second locking collars  38  and  32 , and the shaft  20  then cease rotating when the threshold value is reached, as the tube sleeve  11 , tube internal rib support bearings  42 ,  26 , an anti-rotation washer  28 , a compression spring  22 , a flat washer  24  and first and second felt bushings  40 ,  30  continue to rotate counterclockwise coupled to the inner diameter of the media roll support tube through holding the springs  106  and  108 . 
     The second portion of slip mechanism  23  that limits torsion of the torsion spring  18  approximately to the threshold value includes first and second locking collars  38 ,  32 , first and second felt bushings  40 ,  30 , the compression spring  22 , the anti-rotation washer  28 , and the flat washer  24 . The first and second locking collars  38 ,  32  are adapted to rotate with the shaft  20  at a precise axial separation along the shaft  20  with set-screws or other similar clamping means so that the compression spring  22  places both felt bushings  40 ,  30  under compression constrained by the axial separation of internal tube ribs  35 ,  25  within the tube sleeve  11 . This compression loop starts with the first tube sleeve internal rib  35  and continues with the first felt bushing  40  being pressed against its right side by the first locking collar  38 , which is affixed to shaft  20 . The second locking collar  32  is also affixed to shaft  20  at such a dimension as to properly compress the compression spring  22  between the stack of the anti-rotation washer  28  and the left side of the internal rib  25  of the tube sleeve  11  to achieve slip torsion at the threshold value between the right face of the first felt bushing  40  and the first locking collar  38  as well as between the right face of second felt bushing  30  and the left face of the anti-rotation washer  28 . In other embodiments of the invention, other faces and stacks of parts could be used instead of those mentioned. As generally depicted in FIG. 4, slip mechanism  23  comprises first and second locking collary  38 ,  32 , anti-rotation washer  28 , first and second felt bushings  40 ,  30 , compression spring  22 , flat washer  24 , one way slip clutch  36 , metal sleeve  34  and internal tube ribs  25 ,  35 . 
     When the torsion of the torsion spring  18  increases, but before it reaches the threshold value, the friction between the first felt bushing  40  and the first collar  38 , and the similar friction between the second felt bushing  30  and the anti-rotation washer  28 , do not allow slip and, thus, transmit counterclockwise rotation from the tube sleeve  11  to the shaft  20  through the first and second locking collars  38 ,  32 . Tabs on the anti-rotation washer  28  at diametrically opposed locations (180 degrees apart) fit into matching notches of the tube sleeve  11  to transmit the rotation from the tube sleeve  11  to the anti-rotation washer  28  and then, through friction face coupling, to the second felt bushing  30 . Thereafter, the rotation is transmitted by a higher friction coupling from the second felt bushing  30  to the second locking collar  32  and then to the shaft  20 . Similarly, friction between the right face of the first internal support rib  35  and the left face of the first felt bushing  40  transmit rotation from the right face of the first felt bushing  40  to the left face of the first locking collar  38  and to the shaft  20 . The shaft  20 , in turn, then transmits clockwise rotation to the slip clutch  36  and, through the interference fit, to the metal sleeve  34  and finally to the left end of the torsion spring  18  from the 90 degree bent end of the spring inserted into the hole in the right end face of the metal sleeve  34 . When the shaft  20  rotates, the end caps  12  and  44  are kept from being rotated by needle bearings  16  and  48 . 
     Once the torsion spring  18  has reached the threshold torsion value, slip occurs thereafter between the right face of the second felt bushing  30  and the left face of the anti-rotation washer  28  as well as between the right face of the first felt bushing  40  and the left face of the first locking collar  38 . From this time, and thereafter with continued counterclockwise rotation of tube sleeve  11 , as shown by rotational arrow  200  in FIG. 2, only the tube sleeve  11 , the anti-rotation washer  28 , the compression spring  22 , the flat washer  24  and the first felt bushing  40  continue to rotate counterclockwise with the printable medium roll. The torsion spring  18 , the shaft  20 , the locking collars  38 ,  32 , the second felt bushing  30 , the slip clutch  36 , and the metal sleeve  34  remain rotationally still at the amount of torsion for slip to occur between the combination of the right face of the first felt bushing  40  and left face of the first locking collar  38  as well as between the right face of the second felt bushing  30  and the left face of the anti-rotation washer  28 . This rotational separation is eased by allowed slip between the shaft  20  and the internal tube rib support bearings  26 ,  42  which also rotate with the tube sleeve  11  but do not allow rotation to be transmitted to the shaft  20 . 
     The first felt bushing  40  has a ring shape and has an inner diameter similar to an outside diameter of the shaft  20  to allow the shaft  20  to pass through. The first felt bushing  40  is a widely available standard part and persons skilled in the art could find a suitable felt bushing for the present invention from a variety of manufacturers. Operation of this portion of the invention is described in the following example, when the tube sleeve  11  starts to rotate counterclockwise to provide the label strip to the printer  1 , the shaft  20  rotates counterclockwise accordingly. Thus, the left end of the torsion spring  18 , which has the left bent coil inserted into the metal sleeve  34 , will rotate counterclockwise because the shaft  20  forces the metal sleeve  34  and the first locking collar  38  to rotate counterclockwise. However, the right end of the torsion spring  18 , which has the right bent coil inserted into collar  15 , will not rotate since end cap  12  is inserted into slot  60 , which has the hexagonal shape to prevent end cap  12  from rotating. As a result, torsion will be built up in torsion spring  18  until it reaches the threshold value. Therefore, the torsion spring  18 , together with the slip clutch  36 , the metal sleeve  34 , and the shaft  20  will “slip”, i.e., rotate clockwise relative to the tube sleeve  11 , to maintain the torsion of the torsion spring  18  at approximately the threshold value, as long as the tube sleeve  11  and, thus, the shaft  20  and the torsion spring  18  will continuously rotate during operation. The threshold value is named as the first felt bushing  40  friction threshold value. When the printer  1  stops, there is still remaining torsion in the torsion spring  18 . This remaining torsion will exert a torque force on the label roll  100  to pull back the label strip of the label roll  100  and will prevent the label strip from becoming slack. 
     Also, the one way slip clutch  36  and the first needle bearing  16  together work to prevent the torsion spring  18  from being unwound when the tube sleeve  11  rotates in the clockwise direction (and thus the torsion spring  18  rotates in the counterclockwise direction relative to the tube sleeve  11 ). As noted, the first needle bearing  16  rotates freely within the first end cap  12  in both rotational directions. When the tube sleeve  11  rotates clockwise, e.g., when a user backs up the label strip, the shaft  20  rotates clockwise accordingly. Both the slip clutch  36  and the torsion spring  18  will not rotate, but, in a sense, they rotate counterclockwise relative to the tube sleeve  11  and the shaft  20 . The first needle bearing  16 , however, will rotate clockwise relative to the shaft  20  to release any tension built up in the torsion spring  18 . Thereby, the one way slip clutch  36  and the first needle bearing  16  protect the torsion spring  18  from being damaged. The one way slip clutch  36  is available from many manufacturers and persons skilled in the art may find many forms of it from the market that are suitable to be used as the one way slip clutch  36  of the present invention. 
     The first bearing  42  is of round shape with two extrusions positioned 180° apart from each other on the circumference of the first bearing  42 . The first locking collar  38 , the first felt bushing  40 , and the first bearing  42  are generally in ring shape to allow the shaft  20  to pass through. In the preferred embodiment, the first bearing  42  has an outer diameter of approximately 0.44 inch and an inner diameter of approximately 0.32 inch, and each of the extrusions of the first bearing  42  is approximately 0.15 inch wide, approximately 0.04 inch thick, and extends approximately 0.08 inch outward from the outer peripheral rim of the first bearing  42 . The first bearing  42  sits on the first circle rib  35  of the inner surface of the tube sleeve  11 . The two extrusions of the first bearing  42  are inserted into respective recesses of the first internal support rib  35  to press against the first internal support rib  35 , and they cause the first bearing  42  to act as a stop to prevent the first bearing  42  from moving toward the center of the tube sleeve  11 . By this configuration, the first bearing  42 , the first felt bushing  40 , and the first locking collar  38  act together to keep the metal sleeve  34  and the slip clutch  36  near the torsion spring  18 . 
     On the end of the tube sleeve  11  next to the second end cap  44 , there is the second locking collar  32  encircling the shaft  20 . The second locking collar  32  is serially coupled to, toward the center of the tube sleeve  11 , the second felt bushing  30  and the anti-rotation washer  28 . Again, the second locking collar  32  is in tubular shape, and the second felt bushing  30  and the anti-rotation washer  28  are generally in ring shape to allow the shaft  20  to pass through. The anti-rotation washer  28  as previously stated also has two extrusions located at the outer circumference of the anti-rotation washer  28  and spaced 180° apart from each other. The anti-rotation washer  28  has an outer diameter of approximately 0.7 inch, an inner diameter of approximately 0.33 inch, and a thickness of approximately 0.04 inch. Each of the two extrusions of the anti-rotation washer  28  is approximately 0.18 inch wide, 0.04 inch thick, and extends approximately 0.09 inch outward of the outer peripheral rim of the first washer  28  respectively. The two extrusions of the anti-rotation washer  28  respectively sit on two washer recesses located at the inner surface of the tube sleeve  11 . Each of the two washer recesses has sufficient room to allow slight lateral axial movements of the anti-rotation washer  28 . In this way, the extrusions do not interfere with the slip mechanism  23  compression loop. 
     Further toward the center of the tube sleeve  11 , the compression spring  22  (approximately 2.5 inches long) tightly pushes against the anti-rotation washer  28  at a first end and against the flat washer  24  at a second end, opposite to the first end. The compression spring  22  is approximately 2.5 inches long prior to being compressed, and it has an outer diameter of approximately 0.72 inch. The flat washer  24  pushes, in response to the push of the compression spring  22 , against the second inner support rib  25  and the second bearing  26 . The flat washer  24  is of ring shape and has an outer diameter of approximately 0.7 inch and an inner diameter of approximately 0.32 inch to allow the shaft  20  to pass through. The second bearing  26  has a similar form and size of the first bearing  42 , and it sits on the second inner support rib  25  of the tube sleeve  11 . Similarly, two extrusions of the second bearing  26  are inserted in respective recesses of the second inner support rib  25 . The two extrusions of the second bearing  26  press against the second inner support rib  25  and they also act as a stop to prevent the second bearing  26  from moving toward the center of the tube sleeve  11 . As a result, compression spring  22  is trapped between the anti-rotation washer and the flat washer  28 ,  24 . In one embodiment, the anti-rotation washer and the flat washer  28 ,  24  are made of metal materials, such as aluminum materials, and the first and second bearings  42  and  26  are made of plastic materials. However, any other materials suitable to manufacture the washers and bearings according to the present invention may be used. 
     As mentioned, the compression spring  22  presses the anti-rotation washer  28  and, thus, the second felt bushing  30  and the second locking collar  32 . In response to a pressing force from the compression spring  22 , the second locking collar  32  is fixed to the shaft  20  and aids in providing the threshold torsion to the tube sleeve  11 . The compression spring  22  establishes the above-mentioned felt bushing friction threshold value of the torsion in the torsion spring  18 , which causes the slip mechanism  23  to slip at the threshold torsion. Therefore, by choosing a proper compression spring  22 , which is widely available in the market, a manufacturer of the printer  1  may characterize how much of the torsion is needed until the shaft  20  will slip in an opposite rotational direction relative to that of the tube sleeve  11  during operation. 
     When the printer  1  starts printing labels, a platen of the printer  1  will starting pulling the label strip of the label roll  100  into the printer  1 . Since the label roll  100  is tightly mounted on the tube sleeve  11  by the holding springs  106  and  108 , the tube sleeve  11  of the roll tensioner  10  will be pulled to start rotating in a first rotational direction (counterclockwise if viewed from the right side of the printer  1 ) to feed labels toward the print head. One end of the torsion spring  18  is engaged in the collar  15  of the first end cap  12 , which does not rotate during operation. The tube sleeve  11  thus rotates against the first end cap  12  and urges the torsion spring  18  to wind up. This causes the torsion to be built up in the torsion spring  18 . When the torsion is continually building up, the torsion spring  18  urges the locking collars  38 ,  32  to rotate the shaft  20 . After being engaged by the locking collars  38 ,  32 , the shaft  20  will still not slip and will also rotate unless the torsion provided by the torsion spring  18  is greater than or equal to the felt bushing friction threshold value. Once the torsion reaches the threshold value, the torsion spring  18  slips in a second rotational direction, opposite to the first, and maintains the torsion in the torsion spring  18  at an approximately constant value. When the printer  1  stops printing, there is still remaining torsion in the torsion spring  18  to provide tension on a label strip and to prevent slack of the label strip. The remaining torsion also allows a user to back up the label strip to some extent, i.e., until the torsion spring  18  releases all torsion stored in it, without causing slack of the label strip. 
     From the foregoing, it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For example, the roll tensioner may be used in any number of imaging devices to prevent slack of an image medium used in the imaging devices. The dimensions of various parts of the invention may be changed to fit into different imaging devices of different sizes. Various compression springs and torsion springs available in the market may also be adopted by a person skilled in the art to provide a suitable torsion of the roll tensioner for any specific imaging device according to the present invention. Furthermore, the manufacturing materials of various parts of the invention may also be changed.