Unscrambling conveyor

Abstract

An unscrambling conveyor includes two side-by-side sets of rollers arranged in a herringbone pattern on either side of a central axis. The rollers of the two sets are positioned at different angles with respect to the axis and the drive speeds for the rollers are chosen to provide equal and opposite forces in the lateral direction and unequal but similarly directed forces in the axial direction. The sets of rollers are divided into zones having progressively increasing roller speeds in the downstream direction with the zones staggered with respect to one another on either side of the central axis. The lateral and axial forces drive articles on the conveyor toward and away from the center axis depending on the roller speeds in the staggered zones and in the downstream direction while imparting a twist to the articles. The successive zones of increasing speed stretch a randomly arranged mass of articles into a single file line. Two unscrambling conveyors may be positioned side-by-side to form a flow splitting-unscrambling conveyor.

Description

This invention relates to conveyors, and more particularly to power driven roller conveyors used to unscramble randomly arranged cartons by aligning the cartons in a single file and orienting the cartons along their long axis.

BACKGROUND OF THE INVENTION

A variety of unscrambler or singulation type conveyor systems are presently known in the art. Such conveyors generally perform the function of arranging a randomly oriented mass or flow of articles into a single file oriented stream to facilitate processing by downstream conveyors and associated equipment. Typically such conveyors involve the use of rollers which are skewed to effect the forcing of the articles against a side guiding rail as in U.S. Pat. No. 3,866,739. Further, in order to jostle such articles into the desired orientation, the side guiding rail may be provided with an abrasive surface to encourage the rotation and jostling of articles into a single file orientation, as in U.S. Pat. No. 4,039,074. Such side rails may also be formed as offset segments or even randomly adjustable offset segments adjusted so that for some average mix of article configurations the segments provide the desired unscrambling as in U.S. Pat. No. 4,284,186. Such jostling and mixing of cartons takes place over a series of rollers which have segmented drives of increasing velocity to open up the spaces between successive articles and allow articles traveling side by side to be manipulated into single file. Some segmented drives also utilize varying roller velocities oriented to encourage separation of articles and promote movement into the desired single file arrangement as in U.S. Pat. No. 4,039,074 mentioned above.

As such systems are forced to operate at higher speeds and increased throughput capacity certain problems appear with the existing technology. For example, forcing an article to ride along a guide rail which is coated with an abrasive substance can cause the surface of the article to become abraded, and in the case of a carton with printing, an identification label or other information on the exterior, such printing or information will be deteriorated by the abrading action causing a degradation at either the information or the esthetic appearance of the printing or exterior finish of the carton or article. Such abrading also causes a build-up of debris, dust and the like in the area of the unscrambler apparatus which is not desirable. In the case of segmented adjustable guide rails, such rails are purposely arranged to provide a maximum jostling and mixing action to the articles flowing by, and at high speeds, the impact produced by these rails can cause damage to the articles and to the contents thereof. The rails also contribute to the creation of a jam condition by restricting lateral article travel. It would accordingly be desirable to provide an unscrambling conveyor which would decrease the damage to the articles, cartons, surfaces and the contents thereof as they are manipulated by the conveyor into a single file.

SUMMARY AND OBJECTS OF THE INVENTION

An unscrambler conveyor comprises a herringbone pattern of rollers in which the angle of the rollers on one side of the pattern is different than the angle on the other. By proper adjustment of the roller speed, the force vectors produced by the rollers can be made equal and opposite in the lateral direction and unequal but in the same direction in the axial direction. The force vectors will cause an article to be driven to the center of the herringbone pattern and to travel down the axis of the conveyor while twisting about its own axis. Zones of increasing speed in the downstream direction of the conveyor aid in unscrambling a mass of articles into a single file and may cause the formation of gaps between adjacent articles. The zones are staggered to create unequal force vectors in the lateral direction causing articles to move away from the center axis while traveling down the conveyor. In an alternate embodiment, two unscrambler conveyors are placed side by side to form a combined flow splitter-unscrambler conveyor.

It is accordingly an object of the invention to provide an unscrambling conveyor which minimizes the damage to articles, cartons, their surfaces and contents as such items pass through the conveyor.

It is another object of the invention to provide an unscrambling conveyor which will be operable at higher speeds and throughput capacities than known devices.

It is another object of the invention to provide an unscrambling conveyor which provides the required mixing action of articles to achieve the descrambling process without requiting such articles to contact, rub along or be impacted by any fixed guide, surface or obstruction as it passes through the conveyor.

It is another object of the invention to provide an unscrambling conveyor which has an omni-directional mixing action rather than the limited mixing action which can be achieved by present adjustable segmented rails, paddles or other flow disturbing devices.

It is yet another object of the invention to provide an unscrambling conveyor that will urge articles of elongated shape to assume a position where the long axis of the article is parallel to the direction of travel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an unscrambler conveyor according to the invention.

FIGS. 2-4 are vector diagrams showing the forces acting on an article on the unscrambler conveyor of FIG. 1.

FIG. 5 is an plan view of an alternate embodiment of the unscrambler conveyor of FIG. 1.

FIGS. 6-10 are vector diagrams showing the forces acting on an article on the unscrambler conveyor of FIG. 5.

FIG. 11 is a plan view of a combined flow splitter unscrambler conveyor according to a further alternate embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawing figures there is shown in FIG. 1 an unscrambling conveyor generally designated by the reference numeral 10. The unscrambling conveyor 10 is shown in overhead or plan view and comprises two sets, 12 and 14 of skewed elongated rollers 15 arranged in a herringbone pattern along the length of the conveyor 10. The conveyor is used to transport articles from left to right and the herringbone pattern of the rollers 15 will cause the articles to propagate toward the center axis 17 of the pattern. The skew angle ".alpha." which is formed between the rollers in set 12 and a line which is perpendicular to the center axis 17 is different than the skew angle ".beta." of the rollers in set 14. The conveyor also comprises outside frame supports 19 and a center frame support 20 which provide attachment points for the rollers 15 in a manner which is known in the art. Because the conveyor does not rely on abutments positioned along the sides of the conveyor bed to unscramble packages, the outside frame supports do not have to project above the rollers 15 in order to support such abutments. As a result, lateral motion of packages on the conveyor is unrestricted and potential jam conditions are avoided by allowing a package overload to spill over the sides of the conveyor.

Each set of elongated rollers 12 and 14 is divided into zones of progressively increasing velocity. FIG. 1 shows three zones, however in practice the number of zones may be varied in order to achieve the desired result. The rollers in each zone on either side of the center axis 17 are driven by a belt positioned beneath the conveying surface in contact with the rollers in that zone. Belts 21a and 2lb are positioned beneath the rollers in zones 1a and 1b, respectively; belts 22a and 22b are positioned beneath the rollers in zones 2a and 2b, respectively; and belts 23a and 23b are positioned beneath the rollers in zones 3a and 3b, respectively. It will be appreciated that instead of belts, the rollers may be driven by chains or gears or may be self powered, as desired, without departing from the spirit of the invention.

The belts in successive zones may be driven at increasing speeds in order to create gaps between articles on the conveyor 10. Those skilled in the art will recognize that the differing skew angles of the roller sets 12 and 14 will cause unequal force vectors on opposite sides of the center axis 17. As shown in FIG. 2, the force vector V.sub.1a exerted by the rollers in zone 1a may be resolved into a lateral force Vl.sub.1a perpendicular to the center axis 17 and an axial force Va.sub.1a parallel to the center axis 17. In a similar manner, the force vector V.sub.1b exerted by the rollers in zone 1b may be resolved into a lateral force Vl.sub.1b and an axial force Va.sub.1b. By proper choice of the speeds of the drive belts 21a and 21b, the lateral forces Vl.sub.1a and Vl.sub.1b may be made equal in magnitude, and, since they are directed in opposite directions, their net effect will be to center an article on the center axis 17 of the conveyor. At the same time, the forces V.sub.1a and V.sub.1b produce axial forces Va.sub.1a and Vb.sub.1b which are unequal in magnitude but in the same direction. The net effect of these forces on an article which is straddling the center axis 17, will be to twist or rotate the article around its own axis. As a result, the overall effect of the forces V.sub.1a and V.sub.1b exerted by the rollers in zones 1a and 1b will be to force an article to the center axis 17 and to convey the article along the center axis while twisting the article about its own axis.

The rollers in successive zones are driven at increasing speeds in order to assist in the unscrambling of the articles into a single line as the articles travel down the conveyor axis. Referring to FIG. 3, the rollers in zone 2a are driven at speed V.sub.2a which produces forces Vl.sub.2 a and Va.sub.2a. The rollers in zones 2b are driven at speed V.sub.2b which produces forces Vl.sub.2b and Va.sub.2b. The speeds V.sub.2a and V.sub.2b are chosen so that forces Vl.sub.2a and Vl.sub.2b are equal in magnitude and opposite in direction. The forces Va.sub.2a and Va.sub.2b are equal in direction and unequal in magnitude. The overall effect of the forces V.sub.2a and V.sub.2b will be to convey an article along the central axis 17 at a speed which is greater than the speed the article is conveyed in zone 1 while twisting the article about its own axis.

In a similar manner, the rollers in zone 3 are driven at a speed which is greater than the speed of the rollers in zone 2. Referring to FIG. 4, the rollers in zone 3a are driven at speed V.sub.3a which produces forces Vl.sub.3a and Va.sub.3a. The rollers in zone 3b are driven at speed V.sub.3b which produces forces Vl.sub.3b and Va.sub.3b. The speeds V.sub.3a and V.sub.3b are chosen so that forces Vl.sub.3a and Vl.sub.3b are equal in magnitude and opposite in direction. The forces Va.sub.3a and Va.sub.3b are unequal in magnitude and in the same direction. The overall effect of the forces V.sub.3a and V.sub.3b will be to convey an article along the central axis 17 at a speed which is greater than the speed at which the article is conveyed in zones 1 and 2 while twisting the article about its own axis.

To fully appreciate the operation of the invention, an understanding of the properties of static friction and dynamic friction is necessary. Static friction or "friction at rest" is greater than Dynamic friction or "friction in motion". An article being conveyed on a surface such as a belt or roller will assume the direction and velocity of the surface on which it is being conveyed unless it is restricted from doing so. When an article is traveling at the same velocity and direction as its supporting conveying surface, the relationship between the article and the surface conveying it is that of Static friction. When an article is traveling in a different direction or at a different velocity than its supporting conveying surface, the relationship between that article and the surface conveying it is that of Dynamic friction. If an article is supported on a conveying surface which is moving in more than one direction or at more than one velocity, or both, it will be in a Static friction relationship with a portion or none of that surface depending upon the relationship of the center of mass of the article to the direction and velocity of the area supporting the majority of the mass of the article. Thus, it can be seen that an article traveling down the center axis 17 of the conveyor 10 in FIG. 1 is supported by conveying surfaces moving in more than one direction, or at more than one velocity, and is generally in a relationship of Dynamic friction with the conveying surface. A first article which is supported entirely by rollers in one zone will be driven at the same velocity as the rollers and is in a Static friction relationship with those rollers. If in traveling at that velocity and in that direction (towards the center axis 17) the first article comes into contact with a second article which is in a Dynamic friction relationship with its conveying surface, the dynamics of the first article in the static relationship will predominate and all other things being equal, the first article will squeeze its way into line displacing the second article until both articles are in an area of Dynamic friction straddling the center axis 17. As a rule, an article on the conveyor will be driven towards the center axis 17 and any article deviating from the center of the conveyor will be urged to return. This area of the conveyor is called the "neutral zone" and in the conveyor shown in FIG. 1, the neutral zone coincides with the center axis 17.

FIG. 5 shows a preferred embodiment of the invention in which the driving zones of the conveyor of FIG. 5 on opposite sides of the center axis 40 are offset or staggered with one another. Where a zone of driven rollers on one side of the conveyor overlaps two zones of driven rollers on the opposite side of the conveyor which are driven at different speeds, there is a shift in the relationship of the vectors controlling the motion of the article. As shown in FIG. 5, drive belt 4lb in zone lb is driven slower than drive belt 41a in zone 1a, drive belts 41a and 42b in zones 1a and 2b, respectively, are driven at the same speed, drive belts 42a and 43b in zones 2a and 3b, respectively, are driven at the same speed which is a higher speed than belts 41a and 42b, and drive belts 43a and 44b in zones 3a and 4b, respectively, are driven at the same speed which is a higher speed than belts 42a and 43b. Zone 2b overlaps zones 1a and 2a and zone 3b overlaps zones 2a and 3a.

The force vectors for the conveyor in FIG. 5 are shown in FIGS. 6-10. In the region of the conveyor where zones 1a and 1b overlap, the force vectors are shown in FIG. 6. As explained above in connection with FIG. 2, forces V.sub.1a and V.sub.1b produce lateral forces Vl.sub.1a and Vl.sub.1b respectively. By proper selection of the magnitude of the forces V.sub.1a and V.sub.1b, Vl.sub.1a and Vl.sub.1b are equal in magnitude but opposite in direction producing a zero net force in the lateral direction. This creates a neutral zone axis 46 in this region of the conveyor which coincides with the center axis 40. In that region of the conveyor where zone 1a overlaps zone 2b, as shown in FIG. 7, force V.sub.1a produces lateral force Vl.sub.1a and force V.sub.2b produces lateral force Vl.sub.2b. Force Vl.sub.2b is greater than force Vl.sub.1a producing a shift in the neutral zone axis 46 onto the rollers which are driven by belt 41a. At the same time, axial forces Va.sub.1a and Va.sub.2b are equal, eliminating the twisting forces. In that area of the conveyor where zone 2b overlaps zone 2a as shown in FIG. 8, force V.sub.2a produces lateral force Vl.sub.2a and force V.sub.2b produces lateral force Vl.sub.2b. Since the lateral forces are equal and in the opposite direction the net lateral force is zero resulting in a shift of the neutral zone axis 46 back to the center axis 40 of the conveyor.

In that region of the conveyor where zone 3b is opposite zone 2a as shown in FIG. 9, force V.sub.2a produces lateral force Vl.sub.2a and force V.sub.3b produces lateral force Vl.sub.3b. Since lateral force Vl.sub.3b is greater than force Vl.sub.2a, the neutral zone axis 46 shifts from the center axis 40 onto the rollers which are driven by belt 42a, and the twisting forces are eliminated since Va.sub.2a and Va.sub.3b are equal. In that region of the conveyor where zone 3b is opposite zone 3a as shown in FIG. 10, force V.sub.3a produces lateral force Vl.sub.3a and force V.sub.3b produces lateral force Vl.sub.3b. The two lateral forces are equal and in opposite directions producing a net lateral force equal to zero. This shifts the neutral zone axis 46 to the central axis 40 of the conveyor. The axial forces Va.sub.3a and Va.sub.3b are unequal producing a twisting force. Successive regions of the conveyor have similar operating characteristics.

Thus it can be seen that by selecting the proper drive speeds for the various staggered drive sections, a condition is created whereby articles traveling down the conveyor shift laterally in relationship to the center axis 40 to follow the neutral zone axis 46 and will be intermittently urged to rotate or twist while in a state of Dynamic friction with the conveying surface. This motion causes randomly arranged articles on the conveyor to undergo a mixing motion that enables them to squeeze into spaces between other articles created by the progressive velocity increase of the rollers in the downstream direction of the conveyor to form a single file line of articles.

It should be noted that the drive velocities selected do not have to be such that the opposed lateral forces are exactly equal in order to create the desired effect. The intensity of the mixing motion is dependent on the relationship of the skew angles ".alpha." and ".beta.", the relationship of the roller drive velocities, the co-efficients of friction, (static and dynamic) and the mass and velocity of the articles as they pass over the conveyor.

It should also be noted that an elongate narrow article will align itself with the longitudinal axis of the neutral zone as it is conveyed downstream. This is due to the fact that the forces of the rollers produce a force moment along the entire length of the article and in response the article will assume a neutral position at which these forces are in equilibrium. The neutral position for elongated articles is one in which the long axis of the article is aligned with the longitudinal axis of the neutral zone.

FIG. 11 shows an alternate embodiment of the invention in which two flow splitters are placed side by side in order to create a combined flow splitter-unscrambler conveyor 50. The conveyor comprises four sets of elongated rollers 52-55 which are positioned at alternate angles to form two side-by-side herringbone patterns. The roller sets 53 and 54 are coupled at their upstream end to an input conveyor 57, the roller sets 52 and 53 are coupled at their downstream end to an output conveyor 58, and the roller sets 54 and 55 are coupled at their downstream end to an output conveyor 59. Articles entering the conveyor 50 from the input conveyor 57 are driven by the diverging rollers of sets 53 and 54 to the two axes 61 and 62 at the intersection of roller sets 52 and 53, and 54 and 55, respectively. The roller sets 52 and 53 form a herringbone pattern of rollers similar to the conveyor of FIG. 5 and are driven by belts 52a and 53a to form successive zones of increasing speeds. The rollers of sets 52 and 53 are positioned at differing angles to the axis 61 and due to the staggered locations of the belts 52a and 53a, a neutral zone axis 64 will be formed which is similar to the neutral zone axis 46 in the conveyor of FIG. 5. In a similar manner, the rollers of sets 54 and 55 are positioned at differing angles to the axis 62 between them and are driven by belts 54a and 55a to form zones of increasing speeds. Due to the staggered locations of the belts 54a and 55a, a neutral zone axis 65 will be formed which is similar to the neutral zone axis 46 in the conveyor of FIG. 5. As a result, a mass of articles delivered to the conveyor 50 by the input conveyor 57 will tend to divide and each of the articles will be driven to one of the two axes 61 and 62. The increasing speeds of the zones along the two axes 61 and 62 and the staggered arrangements of the drive belts will cause the articles to form two single file lines of articles, to rotate or twist around their own axis, and to follow the neutral axes 64 and 65 as they proceed to the two output conveyors 58 and 59. In this way, the conveyor 50 will both singulate and split a mass of articles into two separate lines of articles.

Having thus described the invention various alterations and modifications will occur to those skilled in the art, which modifications and alterations are intended to be within the scope of the invention as defined in the appended claims.

Claims

1. An unscrambling conveyor for processing a randomly arranged mass of articles into a single file line of articles comprising;

a conveyor surface comprising an array of elongated conveyor rollers;

a first and second set of rollers comprising the array of rollers;

a plurality of drive means for driving the conveyor rollers, wherein the first and second sets of rollers are selectively driven at two different speeds;

an axis dividing the first and second sets of rollers, said first and second sets of rollers being disposed at two different angles relative to the axis; whereby the force vectors acting on an article on the conveyor from the first set of rollers are different than the force vectors acting on an article from the second set of rollers.

2. The unscrambling conveyor of claim 1 further comprising:

a plurality of successive zones comprising the first set of rollers and a plurality of successive zones comprising the second set of rollers; and

a drive belt means for each zone, wherein zones in the first and second sets of rollers are directly across the axis from one another.

3. The unscrambler conveyor of claim 1 further comprising:

a plurality of successive zones comprising the first set of rollers and a plurality of successive zones comprising the second set of rollers; and

a drive belt means for each zone, wherein zones in the first set of rollers are staggered across the axis from zones in the second set of rollers.

4. The unscrambler conveyor of claim 3 wherein the drive belt means in the successive zones on each side of the axis drive the rollers in the zones at increasingly higher speeds.

5. An unscrambling conveyor for processing a randomly arranged mass of articles into a single file line of articles comprising;

a conveyor surface comprising an array of elongated conveyor rollers;

a first and second set of rollers comprising the array of rollers;

a plurality of drive means for driving the conveyor rollers, wherein the first and second sets of rollers are selectively driven at two different speeds;

a plurality of successive zones comprising the first set of rollers and a plurality of successive zones comprising the second set of rollers;

a drive means for each zone, wherein zones in the first and second set of rollers are directly across the axis from one another; and

an axis dividing the first and second sets of rollers, said first and second sets of rollers being disposed at two different angles relative to the axis; whereby the force vectors acting on an article on the conveyor from the first set of rollers are different than the force vectors acting on an article from the second set of rollers.

6. The unscrambler conveyor of claim 5 wherein the drive belt means in the successive zones drive the rollers in the zones at increasingly higher speeds.

7. The unscrambler conveyor of claim 5 further comprising:

an absence of abutments along the sides of the conveyor surface, whereby lateral motion of articles on the conveyor is unrestricted.

8. An unscrambling conveyor for processing a randomly arranged mass of articles delivered to the conveyor into a single file line of articles, the conveyor comprising:

a conveyor surface comprising first and second sets of elongated conveyor rollers;

an axis extending in the longitudinal direction of the conveyor separating the first and second sets of conveyor rollers;

first drive means for the first set of rollers and second drive means for the second set of rollers;

means for generating a first force vector with the first set of rollers having a first lateral component and a first axial component; and

means for generating a second force vector with the second set of rollers having a second lateral component and a second axial component;

whereby the first and second lateral components have the combined effect of conveying an article to the axis and whereby the first and second axial components have the combined effect of conveying an article downstream of the conveyor while twisting the article around its own axis.

9. The unscrambling conveyor of claim 8 wherein the first set of rollers is positioned at a first angle to the axis and the second set of rollers is positioned at the second angle which is different than the first angle to the axis.

10. An unscrambling conveyor for processing a randomly arranged mass of articles delivered to the conveyor into a single file line of articles, the conveyor comprising;

a conveyor surface comprising first and second sets of elongated conveyor rollers;

an axis extending in the longitudinal direction of the conveyor separating the first and second sets of conveyor rollers wherein the first set of rollers is positioned at a first angle to the axis and the second set of rollers is positioned at the second angle which is different than the first angle to the axis;

first drive means for the first set of rollers and second drive means for the second set of rollers wherein the first set of rollers is driven at a speed which is different than the speed of the second set of rollers;

means for generating a first force vector with the first set of rollers having a first lateral component and a first axial component; and

means for generating a second force vector with the second set of rollers having a second lateral component and a second axial component;

whereby the first and second lateral components have the combined effect of conveying an article to the axis and whereby the first and second axial components have the combined effect of conveying an article downstream of the conveyor while twisting the article around its own axis.

11. The unscrambling conveyor of claim 10 further comprising:

a plurality of first zones of rollers comprising the first set of rollers and a plurality of second zones of rollers comprising the second set of rollers;

means for driving each of the first zones of rollers at progressively increasing speeds; and

means for driving each of the second zones of rollers at progressively increasingly speeds.

12. An unscrambling conveyor for processing a randomly arranged mass of articles delivered at the input of the conveyor into a single file line of articles, the unscrambling conveyor comprising:

an axis extending in the longitudinal direction of the conveyor and separating the conveyor into two halves;

means for generating first force vectors on a first side of the axis having first lateral components and first axial components; and

means for generating second force vectors on a second side of the axis having second lateral components and second axial components;

whereby in a first section of the conveyor the first and second lateral components have the combined effect of conveying an article to the axis and the first and second axial components have the combined effect of conveying an article in the downstream direction of the conveyor while twisting the article around its own axis, and in a second section of the conveyor the first and second lateral components have the combined effect of shifting an article off of the axis and the first and second axial components convey an article in the downstream direction of the conveyor.

13. The unscrambling conveyor of claim 12 further comprising:

means for generating a plurality of first force vectors each having a first lateral component and a first axial component; and

means for generating a plurality of second force vectors each having a second lateral component and a second axial component;

wherein said plurality of first and second first vectors each increase progressively in the downstream direction of the conveyor.

14. The unscrambling conveyor of claim 13 further comprising:

a first set of elongated rollers for generating the first force vectors and a second set of elongated rollers for generating the second force vectors.

15. The unscrambling conveyor of claim 14 further comprising:

first drive means for the first set of rollers and second drive means for the second set of rollers.

16. The unscrambling conveyor of claim 15 further comprising:

a plurality of first drive belts comprising the first drive means, each of the first drive belts driving a plurality of rollers in the first set of rollers, wherein the speeds of the first drive belts increase progressively in the downstream direction of the conveyor; and

a plurality of second drive belts comprising the second drive means, each of the second drive belts driving a plurality of rollers in the second set or rollers, wherein the speeds of the second drive belts increase progressively in the downstream direction of the conveyor.

17. The unscrambling conveyor of claim 16 further comprising:

means for positioning the first set of rollers at a first angle to the axis and means for positioning the second set or rollers at a second angle to the axis.

18. The unscrambling conveyor of claim 17 further comprising:

a plurality of first zones of rollers comprising the first set of rollers, each of the first zones of rollers being driven by one of the first drive belts; and

a plurality of second zones of rollers comprising the second set or rollers, each of the second zones of rollers being driven by one of the second drive belts; wherein the first and second zones of rollers are positioned directly across from one another on either side of the axis.

19. The unscrambling conveyor of claim 17 further comprising:

a plurality of first zones of rollers comprising the first set of rollers, each of the first zones of rollers being driven by one of the first drive belts; and

a plurality of second zones of rollers comprising the second set or rollers, each of the second zones of rollers being driven by one of the second drive belts; wherein the first and second zones of rollers are staggered with respect to one another on either side of the axis.