Patent Application
20110268489
The present invention relates generally to back tensioning assemblies for controlling web tension on roll unwinding systems such as roll fed printers. More specifically, the present invention relates to an improved back tensioning assembly for a roll fed printer that insures consistent tensioning of the paper web regardless of the amount of paper remaining on the roll.
It is known that bulk quantities of paper or plastic sheeting or web materials can economically be distributed in the form of rolls. Further, rolled webs also facilitate efficiency when using bulk quantities of these materials in various machines such as printers. When dispensing such materials the paper or plastic web is pulled to unwind it from the roll. In some cases equipment is utilized to unwind the material from the roll while in other applications the material is unwound manually by pulling on the material. As an example, a paper web that is used in roll fed printing equipment is supplied on rolls that are mounted to a supply end of a bulk-printing machine and drawn into the printer by a traction device.
The rolls are generally rotatably supported on a mounting apparatus or in a cradle that facilitates unwinding of the roll and supply of the paper web to the printer. Often the rolls are mounted onto a mandrel assembly that is in turn supported on a mounting frame. As the paper is drawn from the roll, the roll rotates freely about the mandrel. This prior art mandrel assembly, however, does not provide the ability to apply tension to the paper roll as it unwinds, except for whatever rotational friction is generated between the mandrel and the roll. In the absence of tension, paper backlash may occur when the traction drive is stopped to cut the paper r to align the next print job. Excess backlash can separate the paper from the printer or cause the paper to jerk beneath the print head producing an unsatisfactory product. In other words, the roll of material can keep turning even after the printer has suddenly stopped pulling the web forward, which causes the material to lose tension and slacken resulting in extra material hanging loosely from the roll. Then, when the web is quickly pulled forward again, the slack is taken out before the roll begins turning, causing the material to rip or jerk. This is particularly problematic in computer printers and other apparatus where the webs are pulled from the roll intermittently such that the web movement starts, the paper roll accelerates, and when the web movement stops the roll must decelerate. During deceleration, the roll tends to continue turning and can result in the spewing of the paper between the roll and the printer feed devices.
Various techniques have been proposed in the prior art to counteract roll overspin and maintain back tension forces on the paper. These have involved pinch rollers that engage the paper between the roll and the infeeding device or weights that drag on the roll so as to establish fictional forces between the roll and a roller on which the roll rests. Other devices employ a tension roller that rests on a rotating arm against the outer surface of the paper. The problem in this arrangement is that as the roll unwinds the relative angle of the arm changes causing variation in the tension applied to the roll as well as changes in the feed characteristics and angle of the web being fed. Other solutions have included the provision of a frictional clutch at the central spindle or mandrel to which the roll is mounted. The difficulty in these arrangements is that the force of the paper back tension is equal to the torque of the clutch divided by the radius of the roll. Accordingly, as the roll unwinds the back tension applied to the web increases as the diameter of the roll decreases.
Accordingly, there is a need for an apparatus that can apply a uniform back tension force to a media roll. There is a further need for an apparatus that can maintain constant back tension on a roll of media being unwound regardless of the radius of the roll or the material remaining on the roll. There is still a further need for an apparatus that applies linear back tensioning force to a web of material that remains constant despite the rate of feed or the media or the amount of media remaining on the roll.
In this regard, the general purpose of the present invention, which will be described subsequently in greater detail, is to provide an improved back tensioning system for roll fed media such as paper web media for a bulk printer. Back tension is the resistance applied by the feeder system to the printer in order to control tracking. When a roll of media unwinds and a torque is applied about the axis of the unwinding roll to generate back tension, the force increases as the size of the roll decreases. This increase in force is due to the fact that the back tension force is equal to the frictional torque applied by the roller divided by the radius of the media roll causing an increase in force as the radius decreases due to the payout of the web. In contrast, the present invention system controls back tension by applying a constant resistance directly to the outer surface of the unwind roll of media. A spring loaded arm keeps a roller in direct contact with the outer diameter of the unwind roll. The roller is coupled to a slip clutch that exerts a constant force against the roll of media. Since this system does not apply a torque about the axis of rotation, as was the case in the prior art, the back tension does not vary with unwind roll size. As a result, linear back tension is provided in the present invention in a manner that critically improves web handling and the printing process. As a result, the problem is eliminated by applying resistance directly to the outer diameter of the roll of media.
It is therefore an object of the present invention to provide an apparatus that can apply a uniform back tension force to a media roll. It is a further object of the present invention to provide an apparatus that can maintain constant back tension on a roll of media being unwound regardless of the radius of the roll or the material remaining on the roll. It is still a further object of the present invention to provide an apparatus that applies linear back tensioning force to a web of material that remains constant despite the rate of feed or the media or the amount of media remaining on the roll.
These together with other objects of the invention, along with various features of novelty that characterize the invention, are pointed out with particularity in the claims annexed hereto and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated a preferred embodiment of the invention.
In the drawings which illustrate the best mode presently contemplated for carrying out the present invention:
Now referring to the drawings, an improved back tensioning system for roll fed media is shown and described in the figures. A back tensioning system as disclosed herein is described generally in the context of a roll fed media system for a device such as a bulk printer. However one skilled in the art can appreciate that the disclosure herein may be applied equally well to any roll fed material in any context including paper feed stock for printing or any other manufacturing process, label feedstock for printing and/or labeling applications and plastic sheeting materials for printing and/or packaging.
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The roller 18 is rotatably supported at the first end of the support shaft 14. The roller 18 is of any suitable material that will exert frictional force against the outer surface of the media roll. In the preferred embodiment the outer surface of the roller is urethane, although any other suitable material would also fall within the scope of the invention. The roller 18 is rotatably supported on a shaft and is preferably coupled to a slip clutch 24 that exerts a constant resistance force against the roller rotation, which in turn exerts a constant force against the rotation of the roll of media. The force exerted by the slip clutch 24 may be constant or may be adjustable to allow a user to properly set and calibrate the tension of the overall system.
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To assist in maintaining constant force against the outer surface of the unwind roll it is preferred that the back tensioning system have a central axis that extends through the support shaft and along the axis of the linear motion of the support shaft. It is further preferred that the central axis be offset relative to the axis of rotation of the roll of media. Still further it is preferred that eh central axis be offset relative to the axis of rotation of the media in a direction that is opposite the rotation of the roll. This prevents the dynamic force of the rotation of the roll from overcoming the spring force that extends the roller forward and effectively reducing the back tension placed on the roll due to slippage or skipping.
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It can therefore be seen that the present invention provides an apparatus that can apply a uniform back tension force on a roll of media being unwound regardless of the radius of the roll or the material remaining on the roll. Further, the present invention provides an apparatus that applies linear back tensioning force to a web of material that remains constant despite the rate of feed or the media or the amount of media remaining on the roll. For these reasons, the instant invention is believed to represent a significant advancement in the art, which has substantial commercial merit.
While there is shown and described herein certain specific structure embodying the invention, it will be manifest to those skilled in the art that various modifications and rearrangements of the parts may be made without departing from the spirit and scope of the underlying inventive concept and that the same is not limited to the particular forms herein shown and described except insofar as indicated by the scope of the appended claims.
