Background of the Invention * The invention relates generally to a method and apparatus for driving thin materials, and in partic¬ ular to a method and apparatus for precisely control¬ ling the movement of film materials between a pair of rollers in an electronic imaging system.

In a high resolution system where film material is advanced for exposure to a laser beam one line at a time, it is important to precisely control the advancement of the film. When advancing the film between two rollers, if the film is misaligned by even 1/8000 of an inch, the recorded image may display unwanted artifacts and be flawed. Therefore, it is important to ensure that the film is driven uniformly along its entire length.

Various combinations of driven and pinch rollers have been used for driving film materials. The construction of such rollers can include covering one or both rollers with a rubber layer to grab the film. When the rubber covering is very soft, it advanta¬ geously grasps the film; however, the rubber can pull or stretch, causing the film to move unevenly, especially in the driven direction. When a hard rubber is used, the rubber may not grip the film firmly enough and slippage may occur. Other roller combinations pair a soft rubber-coated driven roller with a hard rubber-coated non-driven roller; but in such a compromise the film tends to curl around the ^* " soft rubber roller rather than properly exiting from

' between the roller pair. Pairing a metallic surfaced non-driven roller with a hard rubber-coated driven

roller proved more successful; but friction between the rollers and the film was often insufficient, and some slippage can occur. Furthermore, in any of the above combinations, minute differences in the outside radius from one end of a roller to the other can cause the film to skew when driven.

Other prior solutions to the problem attempt to align the rollers by applying equal or separate pressure to the non-driven roller in order to control the radius of the driven roller. This solution has met with some success but results in a mechanically elaborate and expensive system.

A primary object of the invention therefore is to construct a roller system wherein one roller is urged against another roller in a manner which precisely controls the location of the film, produces a precise drive control, eliminates skew, and is mechanically simple to build.

Summary of the Invention

The invention relates to an apparatus and method for passing film material between two rollers wherein a non-driven roller is urged against a driven roller in a manner that causes one roller to compress in a precise and controlled manner as the material passes between the rollers.

In a preferred embodiment of the invention, the driven roller is the width of the film or sheet to be driven through the roller pair, while the non-driven roller is longer than the width of the material being driven. The driven roller has a metal core and an outer surface made of a medium hardness rubber

(between 45-70 durometers). Bearings, having precision surfaces, are attached at either end of the

shaft of the driven roller. The non-driven roller is made of a hard material, preferably metal, and extends beyond the width of the film on both ends to contact the precision alignment bearing surfaces and thus create a precise alignment surface relative to the driven roller.

At each end of the non-driven roller shaft is a spring which connects to the non-driven roller and urges the non-driven roller against the driven roller bearing surface. Pressure from the spring ensures that the non-driven roller stays in contact with the driven roller bearing surface, and when film is inserted between the two rollers, the rubber of the driven roller is caused to compress under the spring force.

In the illustrated embodiment, the rubber cover¬ ing of the driven roller is approximately 1/8 inch thick, but the thickness can vary as long as the rubber is thick enough to be compressed when the film passes between the rollers in their operative driving position. The effective radius of the driven roller will be determined, mainly, by the diameter of the bearing surfaces (which is constant and very precise) and by the thickness of the film material inserted between the two rollers (the thickness variations are minute). In this way, any fluctuation in the diameter of the uncompressed driven roller rubber surface will automatically be compensated for by the compression of the rubber by the film.

In another embodiment of the invention, precision wheels, attached to or a part of, or integral with the driven roller shaft, can be used in place of bearings. The wheels would be coaxial with the driven roller shaft and engage the bottom roller in the same manner

as the bearings described above. Other alignment mechanisms can be attached to the shaft of the driven roller or be otherwise incorporated in the driven roller, for the same purpose.

For loading film between the rollers, the non- driven roller of the invention is attached at its ends to respective pivoting arms, each of which is attached to a spring that urges the non-driven roller against the precision alignment surfaces of the driven roller. One arm is engaged by a cam-lever mechanism which pivots the non-driven roller away from the driven roller to allow insertion of the film therebetween.

Brief Description of the Drawings

Other objects, features, and advantages of the invention will be apparent from the following descrip¬ tion of the invention together with the drawings in which:

Figure 1 is a schematic representation of a typical application of the invention;

Figure 2 is a front elevation view showing the roller assembly of a preferred embodiment of the invention;

Figure 3 is a front elevation view showing the roller assembly of the preferred embodiment of the invention with film material therebetween;

Figure 4 is a cross-sectional view along lines AA of Figure 2;

Figure 5 is a cross-sectional view along lines BB of Figure 3.

Figure 6 is a side elevation view showing the roller assembly with the pivoting arm mechanism; and

Figure 7 is a front elevation view the roller assembly according to another particular embodiment of the invention.

Description of a Preferred Embodiment

Referring to Figure 1, in a typical application of the invention, a film recording system 3 has a supply cassette 5a and a take-up cassette 5b for a film material 24. The film material 24 passes around a driven roller 6 which has a rubber coating 22 to grab and drive the film material 24. The driven roller 6 drives the film primarily at a nip formed between the roller and a non-driven roller 12. The film is exposed, for example by a laser scanning mechanism 7 along an image line 5c as the film passes the line 5c (which is traced by the laser scanning system) .

Referring to Figure 2, a roller system 4 advances a film material between a driven roller 6, driven by a motor 8 which engages its shaft 10, and a non-driven roller 12 which is urged into contact with precision alignment surfaces 13 of bearings 14 constrained at each end of the driven roller shaft 10. Non-driven roller 12 is a precision machined metallic cylinder which is urged against the bearing surfaces 13 of roller bearings 14 by springs 16 attached to a pivoting arm 26 (shown in Figure 6) at spring posts 20.

The driven roller 6 has a medium hardness rubber coating 22 of between 45-70 durometers, approximately 1/8 inch thick, covering a metallic cylindrical core 23.

Referring to Figure 3, as the motor 8 rotates the shaft 10 of driven roller 6, a film material 24, inserted between the two rollers, is grabbed by the rubber coating 22 of driven roller 6 and causes the non-driven roller 12 to rotate in a direction opposite

that of the driven roller. Rollers 6 and 12 are so spaced from each other that the film material, as it moves between the rollers, causes the coating 22 to compress. As roller 6 is driven, the film advances between the rollers.

Thus, referring to Figure 4, which is cross- sectional view taken along line AA in Figure 2, the two rollers 6 and 12 are urged against each other so that the hard surface of roller 12 rests against the precision surfaces 13 of bearings 14. Thus, the interaction of the surfaces 13 and the hard, non- driven roller 12 control the spacing between the non- driven roller and the rubber surface of the driven roller 6. Since the surface of driven roller 6 may have small imperfections because it is composed of a rubber surface coating, the actual spacing between the two rollers may vary at their closest points of contact. Preferably, however, according to the preferred embodiment of the invention, the two rollers are just touching when in this film-free operative position.

Referring to Figure 5, when the film material 24 is inserted between the rollers, and the non-driven roller 12 is thereafter urged toward and abuts the precision surfaces 13, the film material is pushed or urged into the softer rubber coating 22 of roller 6, compresses that rubber coating, and accordingly is grabbed by it in a driving relationship as the driven roller 6 rotates. Importantly, since the thickness of the film material is a closely controlled value, and since the spacing between the axis of the driven roller 6 and the precision surface of the non-driven roller 12 is tightly controlled by the tolerances placed on the non-driven roller and the precision with

which the precision alignment surfaces 13 can be obtained, the effective driving radius R (between the shaft axis 25 and the film) of the driving roller 6 is tightly controlled across its entire length, thereby precisely and accurately driving the film material through the nip between the rollers. The compression of the rubber coating 22 is illustrated in the cross- sectional view along line B-B taken in Figure 3 and illustrated in Figure 5.

Referring to Figure 6, the mechanism for separat¬ ing the rollers 6 and 12 for loading the film material is shown. Non-driven roller 12 is attached at either end of its shaft 18 to a pivoting arm 26. Each arm 26 pivots around pivot pins 28. A cam 30, rotated by a lever 32, has an interfering relationship with arm 26 as the cam rotates (as lever 32 rotates); and the pivoting arm 26, in response, moves or pivots roller 12 away from driven roller 6 for loading or inserting film between the rollers. As the pivoting arms 26 are moved downward (in the illustrated embodiment), springs 16 extend. After the film material is inserted between the rollers, the cam 30 is rotated to allow pivoting arm 26 to swing upward into a closed, operative position, where roller 12 is urged against bearing surface 15, with the rubber coating 22 of driven roller 6 being compressed by the action of roller 12 bearing against the film material.

It should be apparent to those of ordinary skilled in the art that other methods for precisely controlling the spacing between the driven and non- driven rollers can be employed. Thus, referring to Figure 7, the precision surfaces 13 can be integral with the rubber coated roller surface 22 and can be integrally attached to and be part of the shaft 10.

An alternative construction could provide for a single driven roller of constant nominal diameter wherein the ends of the roller contain the hard precision align¬ ment surfaces while the interior of the roller has relatively softer rubber surface. It is also impor¬ tant to note that the rubber surface should not be either too hard or too soft since to make it too hard prevents the compression needed to properly grab and move the film through the nip between the rollers, while to make it too soft can cause, as noted above, an undesirable flexibility and uncontrollable movement of the rubber, much like a spongy material, even under compression. Finally, other materials having the desired coefficients of friction and hardness could be substituted for the materials described in the illustrated embodiment.

Additions, subtractions, deletions, and other modifications of the claimed invention will be appar¬ ent to those practiced in the art and are within the scope of the following claims.