US3846021A

US3846021A - Reproduction apparatus using an endless image-bearing belt

Info

Publication number: US3846021A
Application number: US00433052A
Authority: US
Inventors: M Vola
Original assignee: Oce Van der Grinten NV
Current assignee: Canon Production Printing Holding BV
Priority date: 1971-10-26
Filing date: 1974-01-14
Publication date: 1974-11-05
Anticipated expiration: 1991-11-05

Abstract

Reproduction apparatus utilizing a moving endless belt to receive and transport an electrostatic charge image or a pigment image is provided with belt positioning means of simple form to hold the belt taut and in required alignment. A convexly curved smooth guiding surface, advantageously stationary, has the belt pressed against it and slid over it under tension as the belt enters a processing path in which it receives the image, and edge guides are disposed almost or actually in contact with the opposite side edges of a relaxed portion of the belt approaching the region where the belt is pressed against the slide surface. Belt driving means beyond the processing path keep the belt under tension sufficient to hold it flat and straight in an imagereceiving flight of its travel. In a return region of its path beyond the driving means a length of the belt is kept under substantially no applied tension.

Description

[ Nov. 5, 1974 REPRODUCTION APPARATUS USING AN ENDLESS IMAGE-BEARING BELT [75] Inventor: Mathias J. J. M. Vola, Sittard, Netherlands [73] Assignee: Oce-van der Grinten, N.V., Venlo,

Netherlands [22] Filed: Jan. 14, 1974 211 App]. N0; 433,052

Related US. Application Data [63] Continuation-in-part of Ser. No. 298,783, Oct. 18,

1972, abandoned.

[30] Foreign Application Priority Data 3,536,401 10/1970 Mason et al. 355/73 X 3,556,374 l/l97l Demke 226/119 3,646,866 3/1972 Baltazzi et al. 355/16 X Primary Examiner-Samuel S. Matthews Assistant Examinerl(enneth C. Hutchison Attorney, Agent, or FirmAlbert C. Johnston 57 ABSTRACT Reproduction apparatus utilizing a moving endless belt to receive and transport an electrostatic charge image or a pigment image is provided with belt positioning means of simple form to hold the belt taut and in required alignment. A convexly curved smooth guiding surface, advantageously stationary, has the belt pressed against it and slid over it under tension as the belt enters a processing path in which it receives the image, and edge guides are disposed almost or actually in contact with the opposite side edges of a relaxed portion of the belt approaching the region where the belt is pressed against the slide surface. Belt driving means beyond the processing path keep the belt under tension sufficient to hold it flat and straight in an image-receiving flight of its travel. In a return region of its path beyond the driving means a length of the belt is kept under substantially no applied tension.

19 Claims, 5 Drawing Figures REPRODUCTION APPARATUS USING AN ENDLESS IMAGE-BEARING BELT This is a continuation-in-part of copending application Ser. No. 298,783 filed Oct. 18, 1972 now abandoned.

This invention relates to a reproduction apparatus of the type making use of a moving endless belt with means for forming on this belt an image to be transferred to a receiving surface as an elecrostatic charge pattern or pigment pattern.

Many varieties of reproduction apparatus of that type are known. For instancez-German Pat. publication No. 1,925,684 (FIG. 2) and U.S. Pat. No. 3,646,866 show an apparatus having a photoconductive endless belt on which, by electrostatic charging, exposing and powdering steps, a powder image is formed which is then transferred to a sheet of receiving material. US. Pat. No. 2,990,278 (FIG. 1) shows an apparatus in which a powder image is formed on a photoconductive drum from which this image is transferred first to an endless belt and then from the belt to a receiving sheet. US. Pat. No. 3,051,568 (FIG. 5) shows an apparatus in which a charge pattern created by exposure on a photoconductive drum is transferred to an endless belt and developed on this belt by powdering it, after which the powder image thus formed is transferred to a sheet of print paper.

In order to produce sharp and blur-free images with this type of apparatus, it is necessary that theendless belt be kept flat and aligned very accurately.

It is difficult to keep endless belts in alignment, and

particularly so when the belts are relatively wide, e.g. wider than '20 cm. Such belts have a strong tendency to move sideways on the drive and/or guide rollers over which they are moved. This tendency may be caused by any of a variety of factors, such as unevenness in the belt material and/or variations in the parallelism or the friction of the belt positioning rollers.

Many expedients have been proposed for preventing or correcting lateral movement of endless belts.

A system frequently used for conveyor belts makes use of a steering roller the position of which can be changed with the aid of electrical, mechanical or prieumatic actuating means for correction of the path of the in the formation of an image, because it'may debase objectionably the sharpness of the image obtained. Furthermore, the steering roller system is quite expensive and complicated, and the required detection and switching means'are operated so frequently that they have a short service life.

Another system for aligning conveyor belts makes use of guide means in the form of arrests or rollers installed at each side edge of the belt, with which the belt makes contact when it moves sideways so as to cause a reactive force urging the belt back into the correct path. Under certain conditions a favorable result can be obtained with such a system. On the other hand, the forces exerted on the edges of the belt are relatively great, so that it is not practicable to use a system of this kind for positioning thin or delicate belts such as those desirably used in reproduction apparatus of the type to which the present invention relates. The edges of such belts would be damaged by frictional wear, creasing, or folding, at the locations where the correcting forces are exerted upon them. I

' Other means of various sorts are known for aligning narrow conveyor belts and driving belts, such as the use of special forms of the belts and of the belt positioning rollers. These may be made trough-shaped, V-shaped, conical shaped, etc. in order to assure the required alignment. Such means, however, offer no remedy for the problem of positioning the relatively wide belts required in the type of apparatus under consideration here.

The principal object of the present invention is to provide an effective yet technically simple way and means of keeping accurately aligned and transversely flat an endless image-carrying belt of reproduction apparatus of the type above mentioned.

A further object of the invention is to provide in such apparatus an arrangement for positioning such a belt by which a flight of the belt will constantly be kept straight and flat in a definite path, for the reception of an image projectedoptically onto its surface.

According to the present invention, these objects are attained by means of a belt positioning system in which the endless belt is driven at a location downstream from the processing path wherein the image forming and image transfer are effected, and at a location upstream therefrom the belt is kept pressed constantly in contact with and slid over a guiding surface that does not move at the linear speed of the belt, so that the length of the belt in said path is heldtaut, while a further length of it'in a belt return region between the driving means and said surface is kept relaxed, and in which, at the opposite, sides of the path of movement of a relaxed portion of the belt approaching the region where the belt is pressed against said surface, edge guides are provided with which the side edges of the belt are kept almost if not actually in contact.

According to a further feature of the invention, the belt driving means comprises a driven roller provided with an elastic sheathwhich engages the full width of the belt and is sufficiently soft to be depressible by tension in the belt. The belt is laid upon and held to the surface of the elastic roller sheath and by rotation of the roller is drawn from the processing path with the force required for sliding it over the guiding surface at the upstream side of that path. It has been found that the elastic sheath contributes importantly to the maintenance of the desired alignment of the belt. This is believed to be dueto the fact that when irregularities of tension occur in local regions of the width of the belt, corresponding incremental variations of the effective radius, hence of the peripheral belt driving speed, re-

sult in local regions of the elastic sheath so as to counteract the variations of tension.

Surprisingly it has been found that the invention enables the belt to be maintained in highly accurate alignment and that, at most, only very slight forces are exerted on the side edges of the belt when they make contact with the edge guides. This makes it possible to use very thin and/or delicate belts without risk of damage to them. It has been found, for instance, that belts of less than 1 mm in thickness and made of dielectric materials (e.g. rubber impregnated cloth, polyester, or polyolefme), with or without an electrically conductive layer (e.g. a vacuum deposited metal coating), as well as belts made of electrically conductive materials (e.g.

metal, paper, or conductive plastic), with or without a photoconductive or dielectric coating, can be controlled excellently in this way.

A suitable guiding or slide surface that does not move at the linear speed of the belt can be provided by use of a roller that rotates with movement of the belt but at a peripheral velocity lower than the speed of movement of the belt, or by use of a roller that rotates in a direction opposite to the direction of movement of the belt. It has been found especially advantageous, however, to utilize a belt guiding surface that is stationary. Remarkably good results are obtained by the use of a convexly curved stationary surface, such as a semicyclindrical surface.

The belt may be kept pressed in contact with the guiding surface by means of tension applied to the belt, such as by a tensioning roller, as the belt is driven and pulled overthe convex guiding surface. Instead of or in combination with such a tensioning of the belt, the belt may be kept pressed in contact with the guiding surface by means of a web or cloth, with or without soft hairs, flexibly stretched along and against the belt upon that surface.

The guiding surface with which the belt is kept in contact may be composed of any of a variety of materials. The material to be used is, of course, selected with a view to minimizing wear of the belt as a result of its sliding motion under pressure relative to the guiding surface. Accordingly, verysmooth surfaces, for instance chromium-plated metal surfaces or surfaces coated with Teflon (i.e., polytetrafluoroethylene), are eminently useful.

It has been further found, however, that it is advantageous to make the guiding'surface of a soft hairy or napped material, for instance, of felt or of a mohair or nylon plush. Where there is a risk of dust or other abrasive material (e.g. the remains of a powder image) entering between the guiding surface and the belt, the surface made of hairy material will reduce the risk of scratching of the sliding parts by accepting dust or other loose particles in the spaces between its hairs.

The invention will be further understood from the following detailed description and the accompanying drawings of an illustrative embodiment thereof. In the drawings:

FIG. 1 is a schematic longitudinal sectional view of an electrophotographic copying apparatus provided with an endless photoconductive belt in accordance with the invention;

FIG. 2 is a schematic front view, partly in schematic cross section, of the same apparatus;

FIG. 3 is a perspective view of the stationary belt aligning means of the apparatus of FIGS. 1 and 2;

FIG. 4 is a schematic sectional view of an electrophotographic apparatus using a wide, zigzag-folded endless photoconductive belt; and

conductive coating (e.g. metallized polyester film) to which a layer of a photoconductive composition (e.g. zinc oxide in an insulating binder) has been applied.

With the aid of flash lamps 3 and 4, mirrors 5 and 6 and objective 7, the image of an original lying on glass plate 2 is projected onto the surface of belt 1 in a vertically disposed flight la thereof, after this portion of the belt has been electrostatically charged by means of a corona device at 8. The latent image or charge pattern formed on the photoconductive belt by the flash exposure is developed into a powder image by means of a magnetic brush device at 9. The powder image is then transferred to a sheet 11 of copy paper, fed from a tray 10. Subsequently, the paper sheet carrying the powder image is fed between the rollers 12 and I3 which. being heated by heating elements 14 and 15, fuse the powder image and thus fix it to the paper. The copy thus obtained is finally deposited on the top side of the apparatus by means of

feed rollers

16 and 17. After transfer-v ring the powder image, the belt 1 is cleaned with the aid of a rotating polishing brush 18 which removes any residual powder from the photoconductive belt surface.

The belt 1 is arranged and held in working position and driven at the required speed by being trained along its upper reaches over the guide rollers 19 and 20 and over a driven roller 22 which drives the belt continuously and evenly. Roller 22 is provided with a resilient sheath 22a, for example of an elastic rubberhaving a hardness of 50 to 70 Shore and a thickness of 10 to 20 mm. Along its lower reaches the belt 1 is guided by a roller 21 and thence over a stationary aligning means 23 whichis more fully illustrated in FIG. 3. This aligning device consists of a convexly curved plate 24 of substantially semi-cylindrical. form, the surface of which is constituted by a layer 25 of a soft hairy or napped material, such as felt, and which is provided with end flanges 26 and 27 protruding radially beyond the napped surface. The distance between the flanges 26 and 27 is only slightly greater(e.g. 1 mm greater) than the width of belt 1.

The guide roller 21, in order to avoid damaging the photoconductive belt surface, may be coated with a soft or napped material such as mohair.

The belt 1 is so arranged that it is held over its entire width in close contact with the napped surface 25, so that when it is being driven by roller 22 it slides over surface 25 while being pressed evenly against this surface. Due to the pressure and resulting friction between the belt and surface 25 and the driving action of roller 22, the length of the belt, including flight la, which extends through the processing path between surface 25 and roller 22 is kept under a certain desired tension, while the further length of the belt which extends through the return path or region beyond roller 22 is relaxed. As a portion of the relaxed length of the belt approaches the region where the belt is pressed against the slide surface 25, it is guided to the desired position FIG. 5 is a perspective view of the slide surface and belt pressing means of the apparatus of FIG. 4.

Theapparatus represented schematically in FIGS. 1

and 2 of the drawings is provided with a thin (e.g.-

0.l0.5 mm thick) endless flexible belt 1. This belt is made, for example. of an electrically conductive material (eg. metal) or of a material having an electrically on that surface by the fixed side edge guides which in the embodiment shown are the flanges 26 and 27. It will be understood that little force needs be exerted on the side edges of therelaxed portion of the belt in order I to keep it directed toward and upon the same region of tension maintained in the length of belt 1 downstream from surface 25, the flight la of the belt onto which the image is projected is easily kept perfectly flat and straight as required in order to assure that a sharp image of each original exposed on plate 2 may be projected onto the belt surface.

In the region of its return path beyond the driving roller 22 the belt 1 need not be tensioned at all, or may be under considerably less tension than in the flight la where it receives the projected image-forming light. The provision of such a freely suspended" region of the belt path is particularly desirable when the belt is made mainly of nonelastic material such as metal or paper. Thisis also advantageous when a very long belt is used, in which case a very large part of the belt length, not under tension and substantially constituting a stock of photoconductive belt material, may be maintained in the return region between the belt driving roller and the guiding surface over which the belt is slid into the processing path.

The close contact between the belt and the guiding surface may of course be effected by pressing these members together in various ways. For example, a web 28 of a woven textile, or cloth, may be stretched over the belt 1 on surface 25 and held taut with the aid of springs or weights so as to press the belt against said surface. Such a cloth 28 working over the entire width of the belt is represented schematically by dotted lines in FIG. 1. Instead of such a wide web or cloth, a number of narrower webs or cloths lying side by side at intervals can of course also be used.

FIG. 4 and Fig. 5 are schematic views of an embodiment of the invention as it is adapted for use in a photocopying apparatus which also makes use of a photoconductive endless belt pre-folded in zigzag manner according to the invention disclosed in US. Pat. application Ser. No. 370,680 filed June 18, 1973 as a continuation of application Ser. No. 226,413 filed Feb. 15, 1972. As described more particularly in that application, such apparatus is provided with a long endless belt 30 of flexible photoconductive material which, as a result of its previous zigzag folding, is composed of a large number of equally sized panels or sections (e.g. 625 sections, each of approximately 40 cm in length) which are easily disposable one upon another for storage in a compact stack. The belt 30 may have, for example, a width of 25 30 cm and a total length of approximately 250 meters, and it may be composed, for example, of a paper weighing about 120 g/m coated with a layer weighing about g/m of zinc oxide in an insulating binder.

The major part of the belt length is disposed in a stack 31 of belt sections laid on top of each other in" a magazine 32 of the type disclosed in a copending US. Pat. application Ser. No. 226,870, now US. Pat. No. 3,765,488. A shorter part of the length of belt 30 is guided from the bottom of the magazine over and along a smooth convexly curved guide, or slide, surface 35 of a stationary belt guiding member 34, from which it passes through a processing path wherein the belt is po-.

When the driving roller 42 is rotated, the belt 40 is driven evenly, for example at a speed of 15 m/min., and the belt material is drawn correspondingly from the under side of stack 31, then along and over the aforesaid slide surface 35, thence through the processing path located between that surface and the nip of the rollers 42 and 43, and thence in a belt return region which in this embodiment comprises the stack 31 of belt sections laid one upon another in the magazine.

In order to maintain a desired tension in the belt length drawn through the processing path, and also to prevent the belt from flipping" over the surface 35 at the moments when a belt section between two foldsis drawn from the magazine 32, the belt is constantly pressed in contact with surface 35 by means of a holddown means 54. This means (see FIG. 5) comprises a fixed rod 55 having two bands 56 of a napped, or hairy, material (,e.g. bands or webs of nylon plush about 5 cm wide) fastened to it at one of their ends. The other ends of these bands are fastened to a loose rod 57 which is pulled under a substantially constant force by a tension spring 58. The bands 56 are stretched along and over a portion of the belt 30 engaged between them and the convex slide surface 35, so that the belt is constantly pressed against that surface with a force sufficient to keep the belt length being drawn into the processing path under a desired tension. The magnitude of the pressing force exerted by the bands 56 can, of course, be varied by adjustment of the tension applied by spring 58.

Due to the driving action of roller 42 and the friction which must be overcome in order to cause the pressed region of the belt to slide over the guide surface 35, the length of the belt extending between surface 35 and roller 42 is held taut, while the remaining length of the belt, being mainly disposed in stack 31 under no tension, is relaxed.

An electrophotographic image is formed on a section of the belt length extending through the processing path and is transferred from it to a receiving material by the action of suitable processing meanssuch as a charging unit 44, an exposure system optically exposing a panel of the photoconductive surface in zone 45 between

rollers

37 and 38, a developing unit 46, and a transfer unit at 47. Finally, before being deposited back into the stack, the belt is acted upon by a cleaning unit 48.

As a result of the tension maintained in the belt length extending between surface 35 and the driving roller 42, the folds between the successive belt sections are flattened in that length of the belt so that it is held in straight runs between the guide rollers in the processing path. Accordingly, sharp electrophotographic images can be applied to these sections of the traveling belt, without any objectionable influence being exerted thereon by the folds, and the images can be transferred effectively to a receiving material 49 brought into contact with the belt 30 at the transfer station 47 The portion of the belt 30 which has been used to form and transfer an image is laid section by section onto the top of the stack 31 with the aid of oscillating movements of roller 43 and of rocking movements of hold-down means 50, 51 and 52, as described more particularly in the aforesaid US. Pat. No. 3,756,488. Upon being thus redeposited in the magazine 32, the

stacked sections of the belt are brought laterally to a which lie transverse to the axis of its curvature and guide the edges of the belt passing to and over it in the manner of the flanges 26 and 27 of H68. 1 and 3. Consequently, the successive belt sections leaving the magazine will always slide to and over the same region of the surface 35, so that the belt is kept properly in alignment with the processing units as it is drawn into the processing path.

The edge guide strips 53 and/or those at 35a and 35b apply hardly anyforce or friction to the side edges of the belt; so the belt can be passed a great number of times (eg more than 200 times) through the apparatus without showing edge wear, even when it is composed of a quite thin or delicate material such as a coated paper.

The use of two hold-down strips pressing the belt against the curved slide surfaces is advantageous in that these strips can be arranged to press against marginal regions of the belt that ordinarily do not take part in the formation of images on the belt; so no damage to the image forming regions of the belt by undesirable friction is likely to occur.

What is claimed is:

1. ln reproduction apparatus including an endless belt, means positioning said belt for movement thereof through a processing path and thence through a region for return to said path and means respectively for forming an image on and transferring the image from a portion of the belt in said path, the combination which comprises means for driving the belt engaging said belt at a location downstream from said path between said path and said return region, a slide surface engaged by said belt at a location upstream from said path and over which the belt slides as it is moved by said driving means, means for pressing said belt against said slide surface with a force whereby tension is created in the length of the belt extending between said surface and said driving means so as to hold the belt taut in said path, the portion of the belt in said return region being longer than the shortest distance provided for belt travel therein and thus being substantially tensionless therein, and edge guides at the opposite side edges of a tensionless portion of said belt approaching said slide surface for guiding the belt constantly over a selected portion of said surface.

2. Apparatus according to claim 1, said driving means comprising a driven roller provided with an elastic sheath which engages the full width of said belt and is sufficiently soft to be depressible locally by local variations of tension in the belt.

3. Apparatus according to claim 1, said slide surface being a smooth convexly curved surface.

4. Apparatus according to claim 1, said slide surface being the surface of a roller having means to enable movement of it only at a peripheral speed different from the speed of the movement of said belt by said driving means.

5. Apparatus according to claiml, said slide surface being a smooth substantially semi-cylindrical outer surface portion of a stationary belt guiding member.

6. Apparatus according to claim 5, said surface portion being constituted by chromium plated metal.

7. Apparatus according to claim 5, said surface portion being constituted by a coating of polytetrafluoroethylene on metal.

8. Apparatus according to claim 5, said surface portion being constituted by a layer of a napped material.

9. Apparatus according to claim 1, said slide surface being a smooth outer surface of a substantially semicylindrical portion of a stationary belt guiding member, said member having thereon flanges which protrude substantially radially from opposite ends of said semicylindrical portion and constitute said edge guides.

10. Apparatus according to claim 1, said edge guides being mutually spaced apart by a distance not more than about 1 mm. greater than the width of said belt.

11. Apparatus according to claim 1., said path com prising a flat exposure zone having a flight of said belt extending therethrough and said belt being pressed against said slide surface with sufficient force that the tension on said belt in said path holds said belt flight straight and flat.

12. Apparatus according to claim 1, said pressing means comprising at least one flexible web of a material having little frictional resistance to movement of said belt and means holding said at least one flexible web stretched under tension against and along said belt over a region of said slide surface.

13. Apparatus according to claim 12, said at least one flexible web having a smooth napped surface on the side thereof bearing against said belt.

14. In electrophotographic reproduction apparatus including an endless belt having a photoconductive surface, means positioning a length of said belt in and for movement through a processing path comprising a straight path part, means for projecting an optical image onto said surface in said path part to form an electrostatic image thereon, and means positioning a further length of said belt in a belt return region away from said path, the combination which comprises belt driving means including a roller engaging said belt downstream from said path and rotatable to drive said belt from said path into said return region, said roller having an elastic sheath which engages the full width of said belt and is sufficiently soft to be depressible locally by local variations of tension in the belt, a stationary convexly curved belt guiding surface engaged by said belt at a location upstream from said path and over which the belt slides as it is driven by said roller, means for pressing said belt against said guiding surface with sufficient force that the belt length between said guiding surface and saidroller is kept taut and is held flat in said straight path part, said further length of said belt being longer than the shortest distance provided for belt travel through said return region and thus being substantially tensionless in said return region, said pressing means including at least one flexible web of a material having little frictional resistance to movement of said belt and means holding said at least one flexible web stretched under tension against and along said belt .over a region of said guiding surface, and belt edge guides at the opposite side edges of a tensionless portion of said belt approaching said pressing means for guiding the belt constantly over a selected portion of said guiding surface.

15. Apparatus according to claim 14, said at least one flexible web having a smooth napped surface on the side thereof bearing against said belt.

16. Apparatus according to claim 15, said belt guiding surface being a smooth substantially semicylindrical outer surface portion of a stationary belt guiding member, said at least one flexible web having a smooth napped surface on the side thereof bearing against said belt.

17. Apparatus according to claim 16, said surface portion being constituted by a layer of napped material.

Claims

1. In reproduction apparatus including an endless belt, means positioning said belt for movement thereof through a processing path and thence through a region for return to said path and means respectively for forming an image on and transferring the image from a portion of the belt in said path, the combination which comprises means for driving the belt engaging said belt at a location downstream from said path between said path and said return region, a slide surface engaged by said belt at a location upstream from said path and over which the belt slides as it is moved by said driving means, means for pressing said belt against said slide surface with a force whereby tension is created in the length of the belt extending between said surface and said driving means so as to hold the belt taut in said path, the portion of the belt in said return region being longer than the shortest distance provided for belt travel therein and thus being substantially tensionless therein, and edge guides at the opposite side edges of a tensionless portion of said belt approaching said slide surface for guiding the belt constantly over a selected portion of said surface.

5. Apparatus according to claim 1, said slide surface being a smooth substantially semi-cylindrical outer surface portion of a stationary belt guiding member.

9. Apparatus according to claim 1, said slide surface being a smooth outer surface of a substantially semi-cylindrical portion of a stationary belt guiding member, said member having thereon flanges which protrude substantially radially from opposite ends of said semi-cylindrical portion and constitute said edge guides.

11. Apparatus according to claim 1, said path comprising a flat exposure zone having a flight of said belt extending therethrough and said belt being pressed against said slide surface with sufficient force that the tension on said belt in said path holds said belt flight straight and flat.

14. In electrophotographic reproduction apparatus including an endless belt having a photoconductive surface, means positioning a length of said belt in and for movement through a processing path comprising a straight path part, means for projecting an optical image onto said surface in said path part to form an electrostatic image thereon, and means positioning a further length of said belt in a belt return region away from said path, the combination which comprises belt driving means including a roller engaging said belt downstream from said path and rotatable to drive said belt from said path into said return region, said roller having an elastic sheath which engages the full width of said belt and is sufficiently soft to be depressible locally by local variations of tension in the belt, a stationary convexly curved belt guiding surface engaged by said belt at a location upstream from said path and over which the belt slides as it is driven by said roller, means for pressing said belt against said guiding surface with sufficient force that the belt length between said guiding surface and said roller is kept taut and is held flat in said straight path part, said further length of said belt being longer than the shortest distance provided for belt travel through said return region and thus being substantially tensionless in said return region, said pressing means including at least one flexible web of a material having little frictional resistance to movement of said belt and means holding said at least one flexible web stretched under tension against and along said belt over a region of said guiding surface, and belt edge guides at the opposite side edges of a tensionless portion of said belt approaching said pressing means for guiding the belt constantly over a selected portion of said guiding surface.

16. Apparatus according to claim 15, said belt guiding surface being a smooth substantially semi-cylindrical outer surface portion of a stationary belt guiding member, said at least one flexible web having a smooth napped surface on the side thereof bearing against said belt.

18. Apparatus according to claim 14, said belt guiding surface being a smooth outer surface of a substantially semi-cylindrical portion of a stationary belt guiding member, said member having thereon flanges which protrude substantially radially from opposite ends of said semi-cylindrical portion and constitute said edge guides.

19. Apparatus according to claim 14, said sheath being a layer about 10 to 20 mm. thick of a rubber having a Shore hardness of 50* to 70*.