Automatically-adjusting web media tensioning mechanism

An automatically-adjusting tensioning mechanism for use in a roll-fed web media transport system, the tensioning mechanism adding tension to the web media, comprising a bracket assembly being adapted to freely pivot around a pivot axis, and first and second tensioning shoe having curved surfaces attached to the bracket assembly. The web media feeds through the tensioning mechanism in an S-shaped media path where the web media is wrapped around the first and second tensioning shoes. The pivot angle of the bracket assembly automatically adjusts in response to differences in a coefficient of friction between the web media and the tensioning shoes such that the tension in the web media has a reduced level of variability relative to configurations where the bracket assembly is held in a fixed position.

CROSS-REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly assigned, co-pending U.S. Patent Application Serial No. 13/456,296 , entitled: “Method for automatically-adjusting web media tension”, by Turner et al., which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention generally relates to a digital printing system for web media, and more particularly to a web media tensioning mechanism that adjusts responsive to changes in characteristics of the web media.

BACKGROUND OF THE INVENTION

Continuous web printing allows economical, high-speed, high-volume print reproduction. In this type of printing, a continuous web of paper or other print media material is fed past one or more printing subsystems that form images by applying one or more colorants onto the print media surface. With this type of printing system, finely controlled dots of ink are rapidly and accurately propelled from the printhead onto the surface of a moving print media, with the web of print media often coursing past the printhead at speeds measured in hundreds of feet per minute. During printing, variable amounts of ink may be applied to different portions of the rapidly moving print media web, with drying mechanisms typically employed after each printhead or bank of printheads. Variability in ink or other liquid amounts and types or variability in drying times can cause print media stiffness and tension characteristics to vary dynamically for different types of print media, contributing to the overall complexity of print media handling and print media dot registration.

In some prior art web printing systems, such as the KODAK VERSAMARK VT3000 Printing System, the web media is slack when it enters the printing system and an “S-wrap” tensioning mechanism is used to add tension to the web media in preparation for feeding the web media into the rest of the system. S-wrap tensioning mechanisms provide an S-shaped media path where the web media is pulled across curved surfaces of tensioning shoes. Friction between the web media and the tensioning shoes introduce a tension into the web media.

The amount of tension introduced into the web media by an S-wrap tensioning mechanism will be a function of the coefficient of friction between the web media and the tensioning shoes. As a result, the amount of tension provided in a particular configuration can vary widely depending on the factors such as characteristics of the web media, operating speed and environmental conditions. Therefore, it is commonly necessary to manually adjust the geometry of the S-wrap tensioning mechanism (for example, by adjusting a wrap angle) to tune the system performance in accordance with the variation in these factors. Such manual adjustments can be time-consuming, and can be prone to operator error.

U.S. Patent Application Publication 2009/0101686 to Lane, entitled “Web processing apparatus,” discloses a web tensioning assembly configured to balance the tension across the width of a web. With this arrangement, the tension in the web media before and after the tensioning assembly will be the same. Therefore it is incompatible with applications where tension needs to be added to a slack web media.

U.S. Patent Application Publication 2011/0077115 to Dunn, entitled “System and method for belt tensioning,” discloses a method for adding tension to a belt which involves using a spring to apply a force to a tensioning roller. This configuration provides a controlled amount of tension throughout a closed belt, but cannot be used to add tension to a slack web media.

There remains a need for a tensioning mechanism for adding tension to a slack web that provides a consistent level of tension independent of varying media and environmental characteristics.

SUMMARY OF THE INVENTION

The present invention represents an automatically-adjusting tensioning mechanism for use in a roll-fed web media transport system, the tensioning mechanism adding tension to the web media, the web media having a width, comprising:

a bracket assembly mounted to a frame, the bracket assembly being adapted to freely pivot around a pivot axis through a range of pivot angles, the pivot axis being oriented in a direction across the width of the web media;

a first tensioning shoe extending in a lengthwise direction across the width of the web media and having a first curved surface, the first tensioning shoe being attached to the bracket assembly; and

a second tensioning shoe extending in a lengthwise direction across the width of the web media and having a second curved surface, the second tensioning shoe being attached to the bracket assembly at a fixed distance from the first tensioning shoe;

wherein the web media feeds through the automatically-adjusting tensioning mechanism in an S-shaped media path where the web media is wrapped around the first curved surface of the first tensioning shoe and is wrapped around the second curved surface of the second tensioning shoe such that a frictional drag resulting from friction between the web media and the first and second tensioning shoes provides a tension in the web media as it exits the automatically-adjusting tensioning mechanism, the web media being in contact with the first curved surface for a first contact distance and being in contact with the second curved surface for a second contact distance;

and wherein the pivot angle of the bracket assembly automatically adjusts in response to differences in a coefficient of friction between the web media and the first and second tensioning shoes such that the tension in the web media as it exits the automatically-adjusting tensioning mechanism has a reduced level of variability as a function of the coefficient of friction relative to configurations where the bracket assembly is held in a fixed position.

This invention has the advantage that it provides adequate pre-tensioning of the web media independent of the frictional characteristics of the web media without the need for manual reconfiguration.

It has the additional advantage that the tensioning mechanism automatically and passively adjusts to correct for variations in the friction coefficient in real time during a printing process.

It has the further advantage that the tensioning mechanism is more robust and less prone to human errors that may be introduced with prior art tensioning mechanisms that require manual reconfiguration.

DETAILED DESCRIPTION OF THE INVENTION

The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.

The invention is inclusive of combinations of the embodiments described herein. References to “a particular embodiment” and the like refer to features that are present in at least one embodiment of the invention. Separate references to “an embodiment” or “particular embodiments” or the like do not necessarily refer to the same embodiment or embodiments; however, such embodiments are not mutually exclusive, unless so indicated or as are readily apparent to one of skill in the art. The use of singular or plural in referring to the “method” or “methods” and the like is not limiting. It should be noted that, unless otherwise explicitly noted or required by context, the word “or” is used in this disclosure in a non-exclusive sense.

The apparatus and method of the present invention are well suited for roll-fed web media transport systems. In a preferred embodiment, the roll-fed web media transport system is part of a roll-fed printing system that applies colorant (e.g., ink) to a web of continuously moving print media. In some embodiments, the printing system is a non-contact printing system that provide for the application of ink or other colorant onto web media. In such systems a printhead selectively moistens at least some portion of the media as it moves through the printing system, but without the need to make contact with the print media. While the present invention will be described within the context of a roll-fed printing system, it will be obvious to one skilled in the art that it could also be used for other types of systems that include a roll-fed web media transport system. For example, the present invention can be used in a roll-fed coating system that coats one or more layers of material onto a web of continuously moving substrate.

In the context of the present invention, the terms “web media” or “continuous web of media” are interchangeable and relate to a media (e.g., a print media) that is in the form of a continuous strip of media as it passes through the web media transport system from an entrance to an exit thereof. The continuous web media serves as the receiving medium to which one or more colorants (e.g., inks or tonors), or other coating liquids are applied. This is distinguished from various types of “continuous webs” or “belts” that are actually transport system components (as compared to the print receiving media) which are typically used to transport a cut sheet medium in an electrophotographic or other printing system. The terms “upstream” and “downstream” are terms of art referring to relative positions along the transport path of a moving web; points on the web move from upstream to downstream.

Additionally, as described herein, the example embodiments of the present invention provide a printing system or printing system components typically used in inkjet printing systems. However, many other applications are emerging which use inkjet printheads to emit liquids (other than inks) that need to be finely metered and deposited with high spatial precision. As such, as described herein, the terms “liquid,” “ink,” “print,” and “printing” refer to any material that can be ejected by the liquid ejector, the liquid ejection system, or the liquid ejection system components described below.

Kinematic web handling is provided not only within each module of the system described below, but also at the interconnections between modules, as the continuously moving web medium passes from one module to another. Unlike a number of conventional continuous web imaging systems, the apparatus described below does not require a slack loop between modules, but typically uses a slack loop only for media that has been just removed from the supply roll at the input end. Removing the need for a slack loop between modules or within a module allows the addition of a module at any position along the continuously moving web, taking advantage of the automatically-adjusting and self-correcting design of media path components. As part of this adaptation, techniques have been developed to enable the moving web media to maintain proper tension in a “passive” manner.

Referring to the schematic side view ofFIG. 1, there is shown a digital printing system10for continuous web printing according to one example embodiment of the invention. A first module20and a second module40are provided for guiding continuous web media60that originates from a source roller12. Following an initial slack loop52, the web media60that is fed from source roller12is then directed through digital printing system10, past one or more printheads16and supporting components of the digital printing system10. Module20has a support structure28that includes a cross-track positioning mechanism22for positioning the continuously moving web media60in the cross-track direction, that is, orthogonal to the direction of travel and in the plane of travel. In one embodiment, the cross-track positioning mechanism22is an edge guide for registering an edge of the moving web media60. A tensioning mechanism24, affixed to the support structure28of module20, includes structure that pretensions the web media60. In accordance with the present invention, the tensioning mechanism24is automatically adjusting to provide a substantially constant amount of tension of the web media60independent of the characteristics of the web media60. Additional details of the tensioning mechanism24will be described later with reference toFIG. 4and following.

The second module40, positioned downstream from the first module20along the path of the web media60, also has a support structure48, similar to the support structure28for module20. Affixed to one or both of the support structures28and48is a kinematic connection mechanism that maintains the kinematic dynamics of the continuous web of web media60in traveling from the module20into the module40. Also affixed to one or both of the support structures28and48are one or more angular constraint structures26for setting an angular trajectory of the web media60.

Printing system10optionally includes a turnover mechanism30that is configured to turn the media60over, flipping it backside-up in order to allow printing on the reverse side as the web media60as it travels through module40. When printing is complete, the web media60leaves the digital printing system10and travels to a media receiving unit, in this case a take-up roller18. A roll of printed media is then formed, rewound from the printed web media60. The printing system10can include a number of other components, including, for example, dryers14and additional print heads (e.g., for different colored inks), as will be described in more detail below. Other examples of digital printing system components include web cleaners, web tension sensors, or quality control sensors.

Referring to the schematic side view ofFIG. 2, an enlarged view of a portion of the printing system10ofFIG. 1is shown and includes the web media60routing path through the modules20and40. Within both modules20and40, in a print zone54, a printhead16is followed by a dryer14. Optionally, the digital printing system10can also include other components within either or both of the modules20and40. Examples of these types of system components include components for inspection of the print media, for example, components to monitor and control print quality.

Table 1 identifies the lettered components used for web media transport and shown inFIG. 2. An edge guide A is provided in which the web media60is pushed laterally so that an edge of the web media60contacts a stop. The slack web entering the edge guide A allows the web media60to be shifted laterally without interference and without being over constrained. An S-wrap tensioning mechanism24provides curved surfaces over which the web media60slides during transport. As the web media60, for example, an inkjet paper, is pulled over the curved surfaces of the tensioning mechanism24, the friction of the web media60across these surfaces produces tension in the web media60feeding into roller B. As will be discussed below, in accordance with the present invention, the tensioning mechanism24is automatically adjusting to provide a substantially constant amount of tension of the web media60independent of the characteristics of the web media60.

The first angular constraint is provided by in-feed drive roller B. This is a fixed roller that cooperates with a drive roller in the turnover section TB and with out-feed drive roller N in module40in order to move the web media60through the printing system with suitable tension in the direction of movement or travel in the web media60(generally from left to right as shown inFIG. 2). The tension provided by the preceding tensioning mechanism24serves to hold the paper against the in-feed drive roller B so that a nip roller is not required at the drive roller. Angular constraints at subsequent locations downstream along the web are often provided by rollers that are gimbaled so as not to impose an angular constraint on the next downstream web span.

The media transport system of the example embodiment shown inFIG. 2includes other components. The edge guide A at the beginning of the web media path provides lateral constraint for registering the continuous web media60. However, given this lateral constraint and the following angular constraint, the lateral constraint for subsequent web spans can be fixed. In one example embodiment, a gentle additional force is applied along the cross-track direction as an aid for urging the web media60edge against the edge guide A. This force is often referred to as a nesting force as the force helps cause the edge of the web media60to nest alongside the edge guide A. A suitable edge guide is described in commonly-assigned U.S. Patent Application Publication 2011/0129278, published on Jun. 2, 2011, entitled “Edge guide for media transport system”, by Muir et al., the disclosure of which is incorporated by reference herein in its entirety.

In one example embodiment of the present invention, cross track position of the print media is center justified as it enters the media operating zone. This is done at transport element E either by a passive centering web guide (for example, by a web guide such as is described in commonly-assigned U.S. Pat. No. 5,360,152 entitled “Web guidance mechanism for automatically centering a web during movement of the web along a curved path” by Matoushek, the disclosure of which is incorporated by reference herein in its entirety) or by an active centering web guide (for example, by a servo-caster with gimbaled roller (i.e., a steered angular constraint with hinge), as is described in commonly-assigned U.S. patent application Ser. No. 13/292,117, the disclosure of which is incorporated by reference herein in its entirety). Fixed rollers F and L precede printhead(s)16in the first module20and the second module40, respectively, providing the desired angular constraint to the web in each print zone54. These rollers provide a suitable location for mounting an encoder for monitoring the motion of the web media60through the printing system10. Under printheads16, the web media60is supported by fixed non-rotating supports32, for example, brush bars. Alternatively, fixed rollers can support the paper under the printheads, if the print media has minimal wrap around the rollers. Supports32provide minimal constraint to the web.

Printhead16prints in response to supplied print data on the web media60in the span between roller F and G, which includes the media operation zone. Water-based inks add moisture to the print media, which can cause the print media to expand, especially in the cross-track direction. The added moisture also lowers the stiffness of the print media. Dryer14following the printhead16dries the ink, typically by a directing heat and a flow of air at the print media. The dryer drives moisture out of the print media, causing the print media to shrink and its stiffness to change. These changes to the print media in the media operation zone can cause the print media to drift in the cross-track direction as it passes through the media operation zone. The width of the print media as it leaves the media operation zone can also differ from the width of the print media as it entered the media operation zone. To accommodate these effects, one example embodiment of the present invention includes a servo-caster with gimbaled roller G (i.e., a steered angular constraint with hinge) to center justify the print media as it leaves the media operation zone. Because of the relative length to width ratio of the web media60in the segment between rollers F and G, the continuous web media60in that segment is considered to be non-stiff, showing some degree of compliance in the cross-track direction. As a result, the additional constraint provided by the steered angular constraint can be included without over constraining that web segment.

A similar configuration is used in the second module40. Accordingly, in one example embodiment of the present invention servo-caster with gimbaled roller M (a steered angular constraint with hinge) is included to center justify the web media60as it leaves the media operation zone. Roller K includes either a passive web centering guide (for example, the centering guide of U.S. Pat. No. 5,360,152) or an active mechanism such as a servo-caster with gimbaled roller (a steered angular constraint with hinge) to center justify the print media as it enters the media operation zone.

The angular orientation of the web media60in the print zone containing one or more printheads and possibly one or more dryers is controlled by a roller placed immediately before or immediately after the print zone. This is critical for ensuring registration of the images printed from multiple printheads16. It is also critical that the web not be over constrained in the print zones54. As a result of the transit time of the ink drops from the printhead16to the web media60that can result from variations in spacing of the printhead to the web media60from one side of the printhead to the other, it is desirable to orient the printheads16parallel to the web media60. To maintain the uniformity of the spacing between the printheads16and the web media60, constraint relieving rollers placed at one end of the print zones54are preferably not free to pivot in a manner that will alter the spacing between printheads16and the web media60. Therefore, the castered roller following the print zone should preferably not include a gimbal pivot. However, the use of non-rotating supports32under the media60in the print zone as shown inFIG. 2can be used to eliminate this design restriction.

Another example embodiment of a printing system10shown schematically inFIG. 3has a considerably longer print path than that shown inFIG. 2where a plurality of printheads16are provided in each of a first printhead module72and a second printhead module78. The plurality of printheads16can be used to print different ink colors (e.g., cyan, magenta, yellow and black) to enable the printing of color images. The print path shown inFIG. 3provides the same overall sequence of angular constraints as theFIG. 2configuration, with the same overall series of gimbaled, castered, and fixed rollers. Table 2 lists the arrangement of media transport components used with the system ofFIG. 3for one example embodiment of the invention. Non-rotating supports32, for example, brush bars, shown between rollers rollers F and G and between rollers L and M inFIG. 3, include non-rotating surfaces and thus apply no lateral or angular constraint forces. In accordance with the present invention, tensioning mechanism24automatically adjusts to reduce variability in the tension of the web media60as well be described below.

For the embodiments shown inFIG. 2andFIG. 3, the pacing drive component of the printing system10is the turnover module TB. Turnover module TB is conventional and has been described in commonly-assigned U.S. Patent Application Publication 2011/0128337, entitled “Media transport system for non-contact printing”, by Muir et al., the disclosure of which is incorporated by reference herein in its entirety.

Load cells are provided in order to sense web tension at one or more points in the system. In the embodiments shown inFIG. 2(Table 1) andFIG. 3(Table 2), load cells are provided at gimbaled rollers D and J. Control logic for the respective printing system10monitors load cell signals at each location and, in response, makes any needed adjustment in motor torque in order to maintain the proper level of tension throughout the system. There are two tension-setting mechanisms, one preceding and one following turnover module TB, which cooperate with the tensioning mechanism24to control the tension in the web media60as it moves through the printing system10. On the input side, load cell signals at roller D indicate tension of the web preceding turnover module TB; similarly, load cell signals at roller J indicate web tension on the output side, between turnover module TB and take-up roll18(not shown inFIG. 3). Control logic for the appropriate in- and out-feed driver rollers at B and N, respectively, can be provided by an external computer or processor, not shown in figures of this application. Optionally, an on-board control logic processor90, such as a dedicated microprocessor or other logic circuit, is provided for maintaining control of web tension within each tension-setting mechanism and for controlling other machine operation and operator interface functions. As described, the tension in a module preceding the turn bar and a module following the turnover module TB can be independently controlled relative to each other further enhancing the flexibility of the printing system. In this example embodiment, the drive motor is included in the turnover module TB. In other example embodiments, the drive motor need not be included in a turnover mechanism. Instead, the drive motor can be appropriately located along the web path so that tension within one module can be independently controlled relative to tension in another module.

The configuration shown inFIGS. 1 and 2were described as including two modules20and40with each module providing a complete printing apparatus. However, the “modular” concept need not be restricted to apply to complete printers. Instead, the configuration ofFIG. 3can be considered as including as many as seven modules, as described below.

An entrance module70is the first module in sequence, following the media supply roll, as was shown earlier with reference toFIG. 1. Entrance module70provides the edge guide A that positions the web media60in the cross-track direction and includes the S-wrap tensioning mechanism24. In the embodiment ofFIG. 3, entrance module70also provides the in-feed drive roller B that cooperates with the tensioning mechanism24and other downstream drive rollers to maintain suitable tension along the web, media60as noted earlier. Rollers C, D, and E are also part of entrance module70in theFIG. 3embodiment. Transport roller E preferably includes either a passive centering web guide (for example, by a web guide such as is described in the aforementioned commonly-assigned U.S. Pat. No. 5,360,152) or a servo-caster with gimbaled roller (i.e., a steered angular constraint with hinge) in order to center justify the print media as it enters the media operation zone. The first printhead module72accepts the web media60from entrance module70, with the given edge constraint, and applies an angular constraint with fixed roller F. A series of stationary fixed non-rotating supports32, for example, brush bars or, optionally, minimum-wrap rollers then transport the web along past a first series of printheads16with their supporting dryers14and other components. Here, because of the considerable web length in the web segment beyond the angular constraint provided by roller F (that is, the distance between rollers F and G), that segment can exhibit flexibility in the cross track direction which is an additional degree of freedom that may need be constrained. As such, in one example embodiment of the present invention roller G is a servo-caster with gimbaled roller (i.e., a steered angular constraint with hinge).

An end feed module74provides an angular constraint to the incoming web media60from printhead module72by means of gimbaled roller H. Turnover module TB accepts the incoming media60from end feed module74and provides an angular constraint with its drive roller, as described above. Optionally, digital printing system10can also include other components within any of the modules described above. Examples of these types of system components include components for inspection of the print media, for example, components to monitor and control print quality.

A forward feed module76provides a web span corresponding to each of its gimbaled rollers J and K. These rollers again provide angular constraint only. The lateral constraint for web spans in module76is obtained from the edge of the incoming web media60itself Roller K includes either a lateral constraint (for example, an additional edge guide like the one included at roller A) or a servo-caster with gimbaled roller (i.e, a steered angular constraint with hinge) in order to maintain the cross-track position of the web media60.

A second printhead module78accepts the web media60from forward feed module76, with the given edge constraint, and applies an angular constraint with fixed roller L. A series of stationary fixed non-rotating supports32, for example, brush bars or, optionally, minimum-wrap rollers then feed the web along past a second series of printheads16with their supporting dryers and other components, while providing little or no lateral constraint on the print media. In one example embodiment of the present invention, roller M is a servo-caster with gimbaled roller (i.e., a steered angular constraint with hinge) to center justify the web media60as it leaves the media operation zone that is located between rollers L and M. Here again, because of considerable web length in the web segment (that is, extending the distance between rollers L and M), that segment can exhibit flexibility in the cross track direction which is an additional degree of freedom enabling the use of the steered angular constraint without over constraining the print media in that span.

An out-feed module80provides an out-feed drive roller N that serves as angular constraint for the incoming web and cooperates with other drive rollers and sensors along the web media path that maintain the desired web speed and tension. Optional rollers O and P (not shown inFIG. 3) may also be provided for directing the printed web media60to an external accumulator or take-up roll.

Each module in this sequence provides a support structure and an input and an output interface for kinematic connection with upstream or downstream modules. With the exception of the first module in sequence, which provides the edge guide at A, each module utilizes one edge of the incoming web media60as its “given” lateral constraint. The module then provides the needed angular constraint for the incoming media60in order to provide the needed exact constraint or kinematic connection of the web media transport. It can be seen from this example that a number of modules can be linked together using the apparatus and methods of the present invention. For example, an additional module could alternately be added between any other of these modules in order to provide a useful function for the printing process.

When multiple modules are used, as was described with reference to the embodiment shown inFIG. 3, it is important that the system have a master drive roller that is in control of web transport speed. Multiple drive rollers can be used and can help to provide proper tension in the web transport (x) direction, such as by applying suitable levels of torque, for example. In one embodiment, the turnover TB module drive roller acts as the master drive roller. The in-feed drive roller B in entrance module70(or, referring toFIG. 2, module20) adjusts its torque according to a load sensing mechanism or load cell that senses web tension between the drive and in-feed rollers. Similarly, out-feed drive roller N can be controlled in order to maintain a desired web tension within printhead module78(or, referring toFIG. 2, module40).

As noted earlier, slack loops are not required between or within the modules described with reference toFIG. 3. Slack loops can be appropriate, however, where the continuous web is initially fed from a supply roll or as it is re-wound onto a take-up roll, as was described with reference to the printing system10shown inFIG. 1.

FIG. 4shows a schematic diagram of an automatically-adjusting tensioning mechanism24according to an exemplary embodiment of the present invention. The tensioning mechanism24includes a first tensioning shoe102and a second tensioning shoe104, which are attached to a bracket assembly including a pair of bracket plates106A and106B. The tensioning shoes102and104extend in a lengthwise direction across the width of the web media60(not shown inFIG. 4), and have curved surfaces over which the web media60slides. Friction between the web media60and the tensioning shoes102and104imparts a drag force on the web media60, thereby producing a corresponding tension. In a preferred embodiment, the tensioning shoes102and104are hollow cylinders, having cylindrical surfaces. In other embodiments, the tensioning shoes102and104can use other types of curved surfaces, such as elliptical or parabolic curves. The tensioning shoes102and104only need to be curved around the portion of the surface which comes in contact with the web media60.

The bracket assembly (i.e. bracket plates106A and106B), is mounted to a frame100, and is adapted to freely pivot around a pivot axis108through a range of pivot angles. The pivot axis108is oriented in a direction across the width of the web media60(not shown inFIG. 4), the pivot axis being perpendicular to the direction of travel of the web media60. The bracket assembly is mounted to the frame100using bracket mounting plates112A and112B, to which the bracket plates106A and106B are connected using freely rotating connections as will be described in more detail with respect toFIGS. 5A and 5B. While the bracket assembly illustrated inFIG. 4is comprised to two bracket plates106A and106B, it will be obvious to one skilled in the art that other types of bracket assemblies can be used in accordance with the present invention. For example, in some embodiments only a single bracket plate is used on one end of the tensioning shoes102and104. In other embodiments the bracket assembly may also include other components, such as cross-members that connect the bracket plates106A and106B.

In some embodiments, the tensioning mechanism24can also include other optional components such as an upper brush bar110and a lower brush bar111as shown inFIG. 4. These brush bars provide surfaces over which the web media60may ride depending on the pivot angle of the bracket assembly. Optionally, an upper stop114and a lower stop (not visible inFIG. 4) can be provided to limit the rotation of the bracket assembly to a defined range of pivot angles. The upper stop114limits the rotation of the bracket assembly in a counter-clockwise direction, and the lower stop116limits the rotation of the bracket assembly in a clockwise direction.

FIGS. 5A and 5Bare schematic diagrams showing additional details of the bracket assembly in the tensioning mechanism24ofFIG. 4. The bracket plate106A is connected to the bracket mounting plate112A using a flange bearing122, which freely rotates around the pivot axis108within a hole in the bracket mounting plate112A. A shoulder screw120is inserted through a hole in the center of the flange bearing122, and is used to attach the flange bearing to the bracket plate106A. In the illustrated configuration the pivot axis108passes through the center of the bracket plate106A. As will be discussed later, in other configurations, the pivot axis108may be positioned off center toward one end or the other of the bracket plate106A. In some embodiments, a series of holes may be provided in the bracket plate106A so that the bracket assembly can be reconfigured as desired.

FIGS. 6A and 6Bshow schematic side view diagrams of the tensioning mechanism24ofFIG. 4at two different pivot angles. In the illustrated configuration, the pivot axis108is centered with respect to the bracket plate106A.

In theFIG. 6Adiagram, the bracket plate106A is rotated in a counter-clockwise direction to its limiting pivot angle where it comes in contact with the lower stop116, thereby preventing further rotation. At this position, the web media60is in contact with the tensioning shoes102and104for a contact distance corresponding to total wrap angle of 326.4°.

In theFIG. 6Bdiagram, the bracket plate106A is rotated in a clockwise direction to its limiting pivot angle where it comes in contact with the upper stop114, thereby preventing further rotation. At this position, the web media60is in contact with the tensioning shoes102and104for a contact distance corresponding to total wrap angle of 110.2°. Since the contact distance in this case is much lower than that shown inFIG. 6A, the drag force placed on the web media60will be correspondingly lower. Consequently, the tension in the web media60will also be correspondingly lower.

In accordance with the present invention, the pivot angle of the bracket assembly is allowed to freely adjust to provide a passive and automatic adjustment of the tension in the web media60. As will be discussed in more detail later, the result is that the tension in the web media as it exits the automatically-adjusting tensioning mechanism has a reduced level of variability as a function of the coefficient of friction between the web media and the tensioning shoes102and104relative to configurations where the bracket assembly is held in a fixed position.

In the embodiment illustrated inFIGS. 6A and 6B, the web media60follows an S-shaped media path where the web media60feeds down into the tensioning mechanism24from the top and passes by the upper brush bar before being wrapped around the lower surface of the first tensioning bar102. It then wraps over the top surface of the second tensioning bar104and exits out the lower side of the tensioning mechanism24. It will be obvious to one skilled in the art that in other embodiments the tensioning mechanism can be configured to use different media paths. For example, in some embodiments the web media60can feed up into the tensioning mechanism24from below and wrap around the top surface of the first tensioning bar102and the lower surface of the second tensioning bar104before exiting out the upper side of the tensioning mechanism24.

FIGS. 7A and 7Bshow schematic side view diagrams of a second configuration of the tensioning mechanism24ofFIG. 4at two different pivot angles. These figures are similar to those shown inFIGS. 6A and 6Bexcept that in this configuration, the pivot axis108is off-center with respect to the bracket plate106A, being closer to the second tensioning shoe104than to the first tensioning shoe102. In theFIG. 7Aposition, the web media60is in contact with the tensioning shoes102and104for a contact distance corresponding to total wrap angle of 326.4°, which is the same as that shown inFIG. 6A. In theFIG. 7Bposition, the web media60is in contact with the first tensioning shoes102and104for a contact distance corresponding to total wrap angle of 110.2°, which is slightly less than that shown in theFIG. 6Bconfiguration. As will be discussed with reference toFIG. 8, the use of an off-center pivot axis is one method for achieving a torque imbalance, which is desirable in many embodiments.

In accordance with the embodiments ofFIGS. 6A-6Band7A-7B, the tensioning mechanism24should be configured such that the first tensioning shoe102imparts a downward force on the web media as it passes under the bottom of it and the second tensioning bar104imparts an upward force on the web media as it passes over the top of it. In a preferred embodiment this is accomplished by creating a torque imbalance in the tensioning mechanism24. (Note that if a different S-shaped path is used other than that illustrated inFIGS. 6A-6Band7A-7B, the torque imbalance should be arranged to provide a downward force on the tensioning shoe102or104that the web media60passes under and an upward force on the tensioning shoe102or104that the web media60passes over.)

FIG. 8is a diagram illustrating a number of ways that a torque imbalance can be provided according to embodiments of the present invention. The main components of the tensioning mechanism24include the tensioning shoes102and104and the bracket assembly. The bracket assembly is adapted to pivot around the pivot axis108. When the bracket assembly is in a horizontal position, as shown inFIG. 8, there will be a counter-clockwise torque component produced by gravity acting on the first tensioning shoe102(and the portion of the bracket assembly to the left of the pivot axis). Likewise, there will be a clockwise torque component produced by gravity acting on the second tensioning shoe104(and the portion of the bracket assembly to the right of the pivot axis).

The counter-clockwise torque component τ1will be given by:
τ1=W1×R1(1)
where W1is the weight of the left-side components (i.e., the first tensioning shoe102and the portion of the bracket assembly to the left of the pivot axis), and R1is the radius to the center of mass for the left-side components. Similarly, the clockwise torque component τ2will be given by:
τ2=W2×R2(2)
where W2is the weight of the right-side components (i.e., the second tensioning shoe104and the portion of the bracket assembly to the right of the pivot axis), and R2is the radius to the center of mass for the right-side components.

The torque imbalance Δτ will be given by the difference between the counter-clockwise torque component τ1and the clockwise torque component τ2:
Δτ=τ1−τ2=(W1×R1)−(W2×R2).  (3)

From this equation it can be seen that there are several different ways that the components can be arranged to provide the torque imbalance. In some embodiments the pivot axis108can be position off center relative to the bracket plate106A so that R1>R2. This will cause τ1>τ2so that Δτ>0. In other embodiments, additional weight can be added to the left-side components so that W1>W2. Once again, this will cause τ1>τ2so that Δτ>0. In some embodiments, both the weights and the radiuses can be non-equal so that both effects combine to provide the torque imbalance.

There are a number of ways that additional weight can be added to the left-side components to provide the desired torque imbalance. In a preferred embodiment, a weight of the first tensioning shoe102is adjusted to be larger than a weight of the second tensioning shoe104. One way to accomplish this is illustrated inFIG. 9, which illustrates a configuration where the first tensioning shoe102is a hollow cylinder128having end caps130. One or more masses132are affixed to the end caps130before they are attached to the hollow cylinder128using screws134to provide a larger weight relative to the second tensioning shoe104. With this approach an arbitrary amount of weight can be added by controlling the size and number of the masses132. In other embodiments, the weight of the first tensioning shoe102can be adjusted by other means such as changing the thickness of the hollow cylinder128, making the first tensioning shoe102from a solid cylinder, or adjusting the material from which the first tensioning shoe102. In other embodiments, additional weight can be added in proximity to the first tensioning shoe102without changing the weight of the first tensioning shoe102itself (e.g., by affixing a weight to one or both of the bracket plates106A and106B).

In some embodiments, it can be beneficial to form a series of fine grooves138(e.g., 40 grooves/inch) into the surface of the tensioning shoes102and104as illustrated in the inset136inFIG. 9. The grooves have the advantage that they prevent air entrapment between the web media60and the tensioning shoes102and104. (Air entrapment can result in a reduced drag force since the web media60will be floating over air rather than contacting the tensioning shoes102and104.) In a preferred embodiments, the grooves138are oriented around the tensioning shoes102and104in line with the direction of movement for the web media60. In practice, there is sometimes an advantage to orient them at an angle so that they form spirals around the tensioning shoes102and104. This can reduce the likelihood of marking the web media60, and also can be advantaged relative to manufacturing the grooves on a lathe mechanism.

The total amount of torque imbalance that is provided in the tensioning mechanism24will determine the amount of tension that is introduced into the web media60. In an application where the tensioning mechanism is used in the printing system10with 20 inch wide web media60, it has been found that providing a total tension in the range of 20-40 lb is desirable. In other applications, the preferred tension may be higher or lower.

There are a number of factors which should be considered when determining the preferred method to provide the torque imbalance. The use of an off-center pivot axis108has the advantage that less weight is required to create the same torque imbalance. However, it has the disadvantage that it requires a larger space for the tensioning mechanism24to accommodate the larger swing radius. Therefore, for applications where there is a tight space constraint, it is preferable to use a centered pivot axis108, and to provide the torque imbalance by the addition of weight to the first tensioning shoe102.

In some embodiments, the torque imbalance can be provided (or supplemented) using other means. For example, an external weight can be attached to the bracket assembly using a cable, or a spring can be connected between the bracket assembly and the frame100that provides a torque on the bracket assembly in a direction that opposes the torque applied by the tension in the web media60. An example embodiment where the torque imbalance is provided by an external weight or spring force will be discussed later with respect toFIGS. 12A-12B.

FIGS. 10A and 10Billustrate the automatic adjustment of the pivot angle in the tensioning mechanism24to provide a reduced variability in the tension of the web media60.FIG. 10Ashows an initial state of the tensioning mechanism24where it is positioned at an initial pivot angle150. In this orientation the web media60contacts the first tensioning shoe102through an initial first shoe contact distance140and contacts the second tensioning shoe104through an initial second shoe contact distance142. In some embodiments, the web media60is received into the tensioning mechanism24in a slack state having a negligible level of tension (e.g., if the tensioning mechanism24is positioned following a slack loop in the printing system10.) In other embodiments, there may be some level of tension in the web media before it passes through the tensioning mechanism24.

Friction between the web media60and the tensioning shoes102and104as the media is pulled through the tensioning mechanism24produces a drag force and consequently provides a tension in the web media60. The magnitude of the drag force will be a function of the coefficient of friction between the web media60and the tensioning shoes102and104. There are a variety of different factors that will affect the coefficient of friction including the physical characteristics of the web media60(e.g., width, thickness, stiffness, glossiness, texture and chemical composition) and the physical tensioning shoes102and104(e.g., glossiness, texture, chemical composition of the tensioning shoes102and104, temperature, as well as any coatings that are applied intentionally or contamination that is picked up over time as the web media60rubs on the tensioning shoes). It will also be affected by other factors such as the speed that the web media60is being pulled through the tensioning mechanism24and the environmental characteristics (e.g., temperature and humidity). In some embodiments, the web media60may be treated by applying a chemical substance to the surface of the web media60before it enters the tensioning mechanism24(e.g., a conditioning pre-treatment, or ink applied at an earlier point in a printing process), which can also affect the coefficient of friction. Some of these factors can change gradually over time even if the same type of web media60is being used (e.g., environmental characteristics, changes in the physical characteristics of the tensioning shoes102and104due to wear, heating, burnishing or contamination that build up on the surface). Others of these factors may change when operating conditions (e.g., web speed) are changed, a pre-treatment process is initiated, or a new type of web media60is loaded into the roll-fed web media transport system.

Let us assume that the tensioning mechanism24inFIG. 10Ais initially operating in a steady state condition at the initial pivot angle150. In this steady state condition, the torques on the tensioning mechanism24are balanced such that the clockwise and counter-clockwise torques are the same. The counter-clockwise torque is provided by the torque imbalance of the tensioning shoe24that was discussed relative toFIG. 8. The clockwise torque originates from the tension in the web media60, which results from the frictional drag force produced as the web media60is pulled through the tensioning mechanism24.

If the coefficient of friction between the web media60and the tensioning shoes102and104now increases for some reason (e.g., changing environmental characteristics, different type of web media60, or different web speed), this will increase the drag force and thereby will increase the tension in the web media60. As a result, the clockwise torque on the tensioning mechanism24will increase, and the torques will no longer be balanced, thereby disturbing the steady state condition. This will cause the tensioning mechanism24to rotate in a clockwise direction. As the tensioning mechanism24rotates, the contact distance between the web media60and the tensioning shoes102and104will decrease, this will cause the drag force to be reduced, and will consequently reduce the clockwise torque. (The counter-clockwise torque will also change to some degree due to the change in lever arm resulting from the change in the angle between the gravitational force and the orientation of the tensioning mechanism.) The tensioning mechanism24will continue to rotate until it reaches a new steady state condition where the torques are once again balanced.

FIG. 10Bshows an adjusted state of the tensioning mechanism24inFIG. 10Awhere it has reached a new steady state at an adjusted pivot angle152. In this orientation the web media60contacts the first tensioning shoe102through an adjusted first shoe contact distance144and contacts the second tensioning shoe104through an adjusted second shoe contact distance146. In the new steady state, the tension in the web media60has been reduced to a value at or near the original tension when the system was operating in the initial steady state condition.

FIG. 11shows a table comparing the variability in the tension of the web media60provided with an automatically-adjusting tensioning mechanism24in accordance with the present invention to that of a conventional fixed S-wrap tensioning mechanism (having tensioning shoes positioned to provide a 0° “pivot angle.” Four different types of web media60were compared, one of which was tested with and without a coating applied as a pre-treatment.

With the conventional fixed S-wrap tensioning mechanism, the tension produced in the web media60varies over the range of 10-198 lbs. Web media #2has the highest coefficient of friction, and therefore produces the highest tension. (This tension was so high that it actually resulted in the media breaking during the test.) Web media #4has the lowest coefficient of friction, and accordingly produces the lowest tension. The application of the pre-treatment coating to web media #3significantly lowers the coefficient of friction, and consequently lowers the tension provided by the conventional S-wrap tensioning mechanism. This range of tensions would be too large to provide acceptable system performance for many of the media types (the tension should preferably be in the range of 15-40 lbs). A costly and time consuming manual reconfiguration of the S-wrap tensioning mechanism would therefore be required to determine an acceptable operating position for the S-wrap tensioning mechanism each time the web media is changed.

In accordance with the present invention, the pivot angle of the automatically-adjusting tensioning mechanism24automatically adjusts to the characteristics of the different web media types inFIG. 11, thereby providing a reduced variability in the tension of the web media60. The range of tensions in this case is reduced to the range 16-33 lbs. Aside from web media #2, the range of tensions was even smaller (16-26 lbs). The tension of web media #2was somewhat higher because the pivot angle of the automatically-adjusting tensioning mechanism24had reached its maximum pivot angle (80°), and therefore could not pivot any more to further reduce the tension. The range of tensions provided by the automatically-adjusting tensioning mechanism24is more than a 10× improvement relative to the fixed S-wrap tensioning mechanism, and is within the range of acceptable tensions to achieve satisfactory system performance for a typical printing system10. Consequently, using the automatically-adjusting tensioning mechanism24of the present invention is effective to eliminate the costly and time-consuming manual reconfiguration process required using the conventional S-wrap tensioning mechanism.

It has been found that when the printing system10is initially started up, it typically takes some initial period of time until the system reaches a steady state condition. During this initial period of time, the tension in the web media60can vary significantly when using a conventional S-wrap tensioning mechanism as the various the characteristics of the various system components change (e.g., due to heating). This can significantly complicate the process of manually adjusting the configuration of the conventional S-wrap tensioning mechanism, and can sometimes result in significant frustration for the system operators. However, in accordance with the present invention, the automatically-adjusting tensioning mechanism24will continuously and passively adjust to account for the changing system characteristics without the need for any manual operator interaction.

FIGS. 12A and 12Billustrate an alternate embodiment where the torque imbalance for the tensioning mechanism24is provided by an external weight or spring. In this embodiment, a cable160is attached to the tensioning shoe102at a connection point162. The cable160is wrapped around the tensioning shoe102and around a pulley164. A force W is exerted on the cable160by an external weight (not shown) hanging from the cable or by a spring (not shown) attached to the frame100(not shown in these figures). The magnitude of the force W will determine the tension provided in the web media60. If a spring is used to provide the force W, preferably a constant force spring should be used so that the tension in the web media60will also be constant.

In a preferred embodiment, the position of the pulley164will be symmetric with the position of the roller B relative to the axis of symmetry166, which passes vertically through the pivot axis108. This arrangement has the advantage that as the tensioning mechanism24rotates around the pivot axis108(e.g., to the position inFIG. 12B), the lever arm corresponding to the tension in the web media60will vary in the same way that the lever arm corresponding to the tension in the cable160varies. As a result, the variation in the tension added to the web media60will be minimized. In some embodiments, the cable160and external force W can be arranged in alternate geometries to accommodate the space available, and to avoid interference between the cable160, the pulley164and other components, such as the roller B.

As shown inFIGS. 13A and 13B, in other embodiments, the cable160can be attached to the tensioning shoe104and can be used to provide an upward force that opposes the force from tension in the web media60. As with the embodiment ofFIGS. 12A-12B, a force W is exerted on the cable160by an external weight or a spring (not shown). In the arrangement ofFIGS. 13A-13B, the position of the pulley164will preferably be symmetric with the position of the roller B relative to a horizontal axis of symmetry168that passes through the pivot axis108. In this way, as the tensioning mechanism24rotates around the pivot axis108(e.g., from the position inFIG. 13Ato the position inFIG. 13B), the lever arm corresponding to the tension in the web media60will vary in the approximately same way that the lever arm corresponding to the tension in the cable160varies.

PARTS LIST