US3736056A

US3736056A - Apparatus for imparting intermittent rotation to a first member in rotating registry with a second member

Info

Publication number: US3736056A
Application number: US00221310A
Authority: US
Inventors: R C Burnet; W F Slack
Original assignee: DYK RESEARCH CORP VAN
Current assignee: DYK RESEARCH CORP VAN; VAN DYK RESEARCH CORP US
Priority date: 1972-01-27
Filing date: 1972-01-27
Publication date: 1973-05-29
Anticipated expiration: 1990-05-29
Also published as: CA977025A; GB1401359A; DE2302986A1; JPS4884639A

Abstract

An office copier in which a rotating photoconductive drum is exposed by an intermittently rotating scan mirror. A slip-free clutch intermittently couples the scan mirror to a shaft which rotates in synchronism with the photoconductive drum. When the clutch is engaged, the scan mirror is caused to rotate in a given direction in precise synchronism with rotation of the photoconductive drum, thus insuring that there is substantially no scan slip in the resulting latent image formed on the drum surface. When the scan mirror has completed its scan, the clutch is disengaged and a spring rapidly returns the scan mirror to an initial rest position in preparation for the next scan. Since the amount of rotation of the scan mirror may be varied by varying the time of engagement of the slip-free clutch, this arrangement is especially suitable for office copiers requiring variable scan length capability.

Description

United States Patent [191 Burnet et al.

[451 May 29,1973

[54] APPARATUS FOR IMPARTING INTERMITTENT ROTATION TO A FIRST MEMBER IN ROTATING REGISTRY WITH A SECOND MEMBER [75] Inventors: Robert C. Burnet, Hackettstown; William F. Slack, Andover, both of NJ.

[73] Assignee: Van Dyk Research Corporation,

Whippany, NJ.

[22] Filed: Jan. 27, 1972 [21] Appl. No.: 221,310

Primary Examiner-Robert P. Greiner Attorney-Arthur L. Lessler [5 7] ABSTRACT An office copier in which a rotating photoconductive drum is exposed by an intermittently rotating scan mirror. A slip-free clutch intermittently couples the scan mirror to a shaft which rotates in synchronism with the photoconductive drum. When the clutch is engaged, the scan mirror is caused to rotate in a given direction in precise synchronism with rotation of the photoconductive drum, thus insuring that there issubstantially no scan slip in the resulting latent image formed on the drum surface. When the scan mirror has completed its scan, the clutch is disengaged and a spring rapidly returns the scan mirror to an initial rest position in preparation for the next scan. Since the amount of rotation of the scan mirror may be varied by varying the time of engagement of the slip-free clutch, this arrangement is especially suitable for office copiers requiring variable scan length capability.

20 Claims, 3 Drawing Figures PATENTED W29 I975 3; 7 36 O56 SHEET 1 OF 2 PATENTEB MAY 2 9 I975 sum 2 or 2 APPARATUS FOR IMPARTING INTERMITTENT ROTATION TO A FIRST MEMBER IN ROTATING REGISTRY WITH A SECOND MEMBER BACKGROUND OF THE INVENTION This invention relates generally to the field of motion transmitting apparatus and specifically to apparatus for imparting intermittent rotation to a first member in rotating registry with a second member.

For purposes of this specification and the claims appended hereto, the terms rotating registry and registered rotation are utilized to define a relationship between two rotatable members connected through a selectively engagable coupling means whereby, from the instant of engagement to the instant of disengagement of the coupling means, the two rotatable members are rotating at a predetermined angular velocity ratio. Thus, the terms contemplate that upon the occurrence of engagement of the coupling means between the two rotatable members, any inertia effects are absorbed or compensated for externally of the coupling means between the two rotatable members.

Those concerned with the mechanical linkage and component interconnection arts have long concerned themselves with the problems attendant to rotatably linking two or more members for intermittent registered rotation in accordance with a predetermined angular velocity ratio. These problems have been particularly troublesome in the office copier and related arts wherein. the rotation of a scan mirror or other scanning device must be synchronized with the rotation of apparatus such as a photoconductive drum. In such apparatus, efficient operation dictates that during the scanning phase of the machine cycle, the rotation of the scanning device with respect to the transfer drum be strictly in accordance with the predetermined angular velocity ratio. Absent such control there may occur a phenomenon sometimes described as scan skid wherein improper control of the relative angular velocities of the scan mirror and the transfer drum causes a smearing of the image.

As should be recognized by those skilled in the art, the difficult time for maintaining registry between an intermittently rotating member and a constantly rotating member occurs at the commencement of the rotation of the intermittently rotating member. This occurs because at this stage in the operational cycle the inertia of the intermittently rotating member must be overcome. In prior art office copiers one approach to overcoming the inertia of the scanning device is to provide an early start for the scan mirror. This method of insuring registry is inefficient, however, because the time taken in starting the scan cycle early represents a wastage of machine time and an operating loss.

The principal prior art approach to the problem of maintaining rotating registry between two rotating members has been to connect the second rotating member to a cam and connect the first rotating member to a cam follower which is in operable engagement with the cam. With such a structural arrangement, rotation of the motive means causes rotation of the second rotatingmember and the cam which in turn displaces the cam follower thereby causing the first rotating member to be displaced in registry with the second rotating member.

The problems attendant to such an approach are legion. Initially, the manufacture of a cam to achieve the desired function is expensive. Secondly, the wearing of the cam and follower can result in the occurrence of non-registry and scan skid. Thirdly, the length of scan is fixed by the machined cam configuration which, in most prior art devices, causes three cam cycles to occur for each full rotation of the photoconductive drum. In addition to limiting the apparatus to a fixed length of scan, the repeated use of the same three sections of the photoconductive drum has been found to result in fatigue of the drum material as well as the occurrence of incomplete discharge of prior images and the reproduction of ghost images on subsequent copies. Finally, most known devices of the cam follower type involve the utilization of cable or metallic tape in connecting systems between the moving parts, which connecting systems are known in the art to be subject to fatigue, inaccuracy, slippage and difficult maintenance.

In addition to the,manufacturing and operating problems attendant to the cam/follower type apparatus, there are commercial disadvantages. One of the most significant of the commercial disadvantages is that the cam/follower devices are limited to a fixed length of possible scan. Thus, once a particular cam is installed in a machine, the scan length of the machine is fixed, e.g., such as to handle paper having a dimension in the direction of scan of 8% inches, and any change in scan length can be accomplished only by'a change in the cam.

A second commercial disadvantage of prior art machines is that their operation involves an inherent wastage of machine time. Specifically, in most prior art apparatus, the cams are continuously rotating. Because the cam configuration defines the points in the drum rotation when a cycle may be initiated, any attempt to initiate a cycle other than at these predetermined points will result in a time delay until the cam cycle initiation point is reached.

SUMMARY OF THE INVENTION It is an object of the present invention, therefore, to provide an apparatus for imparting intermittent rotation to a first member in rotating registry with a second member wherein the amount of rotation imparted to the first member may be selectively varied without structurally modifying the equipment.

A further object of the present invention is to provide an apparatus for imparting intermittent rotation to a first member in rotating registry with a second member wherein no cam and cam follower structures are utilized to generate the scanning motion.

A still further object of the invention is to provide an apparatus for imparting intermittent rotation to a first member in rotating registry with a second member wherein the intermittent rotation of the first member may be initiated at any point of angular rotation of the second member.

These objects and others, which although not enumerated, will be recognized by those skilled in these arts, are achieved by the apparatus of the present invention, one embodiment of which may include a first member mounted for rotation, a second member mounted for rotation, coupling means for selectively coupling the first member and the second member for intermittent registered rotation, and motive means for imparting rotation to the first and second members.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the present invention may be had from the following detailed description thereof, particularly when read in the light of the accompanying drawings wherein:

FIG. 1 is a schematic view of an electrostatic copying system with respect to which the present invention may be utilized;

FIG. 2 is a partial perspective view of apparatus according to the invention of the type which is utilized in the electrostatic copying apparatus of FIG. 1; and

FIG. 3 is a side view, partially in cross-section through the plane 33 of FIG. 2.

DETAILED DESCRIPTION The problems attendant to the development of apparatus for imparting intermittent rotation to a first member in rotating registry with a second member are particularly apparent when such apparatus is desired to be used in conjunction with electrostatic copying devices or similar devices. Accordingly, there is shown schematically in FIG. 1 an electrostatic copying system designated generally by the reference numeral 10, in which system is incorporated an apparatus for imparting intermittent rotation to a first member in rotating registry with a second member according to the invention.

Considering briefly the structure of system and its operation, a curved support plate 12 is provided for supporting a document (not shown) bearing an image to be copied. The image to be copied is illuminated by a suitable light source (not shown) and scanned by a rotatably mounted reflecting means viz. mirror 14, which reflects the light from the document to be copied through a suitable lens 15 and thereafter against a fixed reflecting means viz. mirror 16 from which the light is reflected to a photoconductive drum 18 to lay down 'a suitable image on the drum surface.

The effect of the image on the surface of the photoconductive drum 18 may be best understood from a consideration of the electrostatic copying process which is utilized with respect to the apparatus 10 of FIG. 1 and typical of those generally known in the art. Initially, a discharged surface of photoconductive drum 18, the material of which may be any of those known in the art, e.g., selenium or zinc oxide, is passed under a corona emitter 20 which establishes on the surface of the photoconductive drum a uniform electrostatic charge density. The charged surface of the photoconductive drum 18 is passed under a slit 21 through which the image scanned from the document positioned on document support plate 12 is projected. Exposure of the electrostatically charged photoconductive drum surface to the image results in an electrostatic field image on the drum surface. Thereafter, the now exposed drum surface is subjected to a cascade of developer in developer tank 22 thereby permitting electrostatically charged areas on the drum surface, which correspond to the image to be reproduced, to be coated with the toner from the bath. After exposure to the developer, the photoconductive drum continues to rotate so as to position the developed drum image in proximity with a moving paper 24. Thereafter, the paper is passed in registry with the drum over an image transfer corona emitter 25 to induce the transfer of toner from the drum to paper 24. The paper then is separated from the drum and passed under a heater (fuser) 27 to melt the toner and set the image on the paper.

After the paper is separated from the drum surface, the drum surface is exposed to a further corona emitting device 29 to neutralize any residual charge thereon. A mechanical brushing apparatus 30 is utilized to mechanically remove any excess toner from the surface of the drum. Finally, the drum is exposed to a light source 32 to insure adequate discharge of the activated surface of the drum. Thereafter, the drum surface is recharged at the beginning of the next cycle by being passed under corona emitter 20.

The present invention relates to that portion of the apparatus 10 which functions to scan the document to be copied and present the scanned image of the document to be copied to the photoconductive drum such that the rate of scan and the rate of rotation of the photoconductive drum are in registry thereby avoiding scan skid as discussed above. Referring, therefore, to FIG. 2, an apparatus for imparting intermittent rotation to a first member in rotating registry with .a second member, e.g., the scan portions and the transfer drum of apparatus 10 of FIG. 1, according to the invention, is shown perspectively and designated by the general reference numeral 40. For purposes of description of the invention, apparatus 40 will be described as being a portion of the electrostatic copying apparatus disclosed in FIG. 1 and corresponding elements will be designated by corresponding reference numerals where appropriate.

Accordingly, referring to FIGS. 2 and 3, apparatus 40 can be seen to comprise a scan mirror 14 which is mounted for rotation on a longitudinally axially extending shaft 42. As may best be seen in FIG. 3, scan mirror 14 is mounted in a cradle 43 such that the longitudinal axis of rotation of shaft 42 lieslongitudinally centrally along its upper or reflecting surface. Mirror 14 is a front or first surface mirror, the embodiment shown being a first surface mirror with a reflective coating of sufficient flatness as to not distort the image, such mirrors being generally available.

Spaced longitudinally along shaft 42 from the position of mirror 14 is a clutch 45 which, when engaged, transmits rotational movement along shaft 42 to mirror 14 from a belt driven sheave 48. As will be discussed below in greater detail, clutch 45 is a fast pick-up, short slip-time, no engagement-slip clutch with a low inertia driven member, the clutch being engageable at any position in the rotation of the driving member.

Sheave 48 is rigidly secured to shaft 42, e.g., by a taper lock bushing 49. The surface of sheave 48 is a machined cylindrical surfacesubstantially without runout.

As is shown in FIG. 3, shaft 42 is mounted for precise rotation about its longitudinal axis by a plurality of ball bearing assemblies 51 which are supported by suitable pedestals 52. It should be noted, however, that any low friction no-play mounting means may be utilized torotatably support shaft 42.

Referring to FIG. 2, photoconductive drum 18 can be seen to be supported by a longitudinally axially extending shaft 60 which is parallel to shaft 42 and which is rotatably mounted through bearing 62. Rigidly secured to shaft 60 are a first sheave 64 and a second sheave 68. First sheave 64 is adapted to receive belt 65 therearound which belt also passes around the sheave 48 on shaft 42. Second sheave 68 is adapted to receive a drive belt 69 therearound, which drive belt is driven by a sheave 70 mounted on the output shaft of a motive means 71. In this regard, motive means 71 can be any of many types known to those skilled in the art, e. g. an electric motor.

First sheave 64, in a similar manner to sheave 48, is also provided with a machined cylindrical surface substantially without run-out so as to define a proper surface for receiving belt 65. In the disclosed embodiment of the present invention, belt 65 is a metal belt of rectangular cross-section. The material for belt 65 may be stainless steel, beryllium copper or other material having a high Youngs modulus of elasticity. It should be noted, however, that the drive system between the photoconductive drum 18 and scan mirror 14 need not necessarily utilize cylindrical sheaves and a belt having a rectangular cross-section. Rather, other forms of drive may be utilized, e. g. a timing belt drive, so long as a high-friction, non-slip surface-to-surface contact is maintained so as to permit registered rotation between photoconductive drum l8 and scan mirror 14.

Disposed on shaft 42 between central bearing 51 and clutch 45 is a lever arm 72 which is angularly displaceable with shaft 42. Secured to the angularly displaceable end of lever arm 72 is a spring 74 which is secured to a rigid support member 75. Spring 74 is a tension spring for establishing a torque which opposes the rotation of shaft 42 during the engagement of clutch 45. Thus, during the engagement of clutch 4S, shaft 42 is rotated clockwise as shown in F IG. 2 thereby elongating and loading spring 74. Upon the disengagement of clutch 45, spring 74 acting through lever arm 72 rotates shaft 42 and therewith mirror 14 counter-clockwise until lever arm 72 comes in contact with a stop means 77 which is positioned to limit the counter-clockwise rotation of shaft 42. The positioning of stop means 77 is such as to define the initial position of shaft 42 for any scan cycle.

Considering now the operation of apparatus 40, motor 71 may be energized to cause rotation of this output shaft and therewith sheave 70. The rotation of sheave 70 causes displacement of belt 69 and therewith the rotation of sheave 68 which in turn causes rotation of shaft 60 and photoconductive drum 18. Additionally, the rotation of shaft 60 causes rotation of sheave 64, which rotation is transmitted to sheave 48 on shaft 42 through belt 65. Rotation of sheave 48 causes rotation of that portion of shaft 42 up to the driving side of clutch 45. Thus, whenever motor 71 is actuated, the entire system is in continuing rotation up to and including the driving side of clutch 45.

Upon the occurrence of the engagement of clutch 45 in response to a control signal through a control system (not shown), all portions of shaft 42 including scan mirror 14 are joined for rotation.

Clutch 45 can be engaged at any point in the rotation of photoconductive drum 18. Thus, a scan cycle can be initiated at any point in machine time and the cycle will commence immediately, without regard for the position of drum 18 or the elements of the connector system between drum 18 and mirror 14. With clutch 45 engaged, shaft 42 continues to rotate until a signal to disengage the clutch is received whereupon the energy stored in spring 74 acts through lever arm 72 to cause counter rotation of shaft 42 and therewith scan mirror 14 until lever arm 72 comes in contact with the upper surface of stop element 77. At this point the system is ready for a next scan cycle.

As noted above a principal feature of this invention is to provide an apparatus wherein rotating registry between a constantly rotating member, viz. photoconductive drum 18 in the disclosed embodiment, and an intermittently rotating member viz. scan mirror 14 in the disclosed embodiment, is achieved with consistency. The unique structural coupling arrangement disclo'sed above for achieving this result includes sheave 64 which is rigidly secured to shaft and which drives belt 65 whereby sheave 48 is driven. The sheave-belt surface relationship is such as to establish a sufficient frictional engagement between the belt and sheave to preclude slippage at these points during either the engagement or the engaged drive of clutch 45. In this regard, it will be recognized by those skilled in'the art that various means may be utilized to tension belt 65 if desired. Specifically, a tensioning sheave (not shown) may be utilized to bear against the surface of belt 65 thereby increasing the frictional engagement between the belt and sheaves 64 and 48 to insure no-slip relationship -for the particular torques established during engagement and disengagement of clutch 45.

Similarly, clutch 45 must be chosen such as to preclude any effective slippage while engaged and during engagement whereby to obviate the possibility of scan skid as discussed above. In this regard, it has been found that an electromagnetic disc clutch having the following characteristics is satisfactory for carrying office copier loads wherein the driven member inertia is in the range of .267 lb. inches Acceptable specifications for a satisfactory clutch for such loads are as follows:

a. rotor and armature diameters 2.62 inch b. bore diameter inch with key c. coil volts 24 D.C.

d. coil watts 6.8

e. elapsed time to generation of 15 inch lb. torque 3.0 milliseconds f. maximum transmitted torque 60 inch lbs.

It should also be noted that a clutch for use in the present invention must be capable of being engaged at any point in the rotation of the driving member.

The relationship between the belt drive system and the clutch is such as to preclude effective slippage between scan mirror 14 and photoconductive drum 18. The engagement and disengagement of clutch 45 requires that a certain amount of inertia be overcome and, as will be recognized by those skilled in this art, the compensation for such inertia is often a cause of slippage. The structure of the present invention, however, provides that the increased inertia resulting from such an engagement of clutch 45 will not disturb the rotating registry between photoconductive drum 18 and scan mirror 14 but rather will be reflected only in a change in speed of rotation of motor 71. Thus, although the speed of rotation of motor 71 may, for a short period, increase or decrease in response to a change in load condition, the angular velocity ratio between photoconductive drum 18 and scan mirror 14 will remain constant during all times when clutch 45 is engaged. Thus, the structure of the present invention permits generation of the scanning motion without the utilization of reciprocating mechanical devices such as cams and cam followers, and the maintenance of a rotating registry between the constantly rotating member and the intermittently rotating member. Accordingly, this structural approach to the problem presents a significant step forward in these arts.

It is to be recognized that although the invention has been disclosed only with respect to utilization in the office copier arts, it may be utilized at any time to lay down a scanned image on a moving surface without departing from its teaching. Thus, a moving photoconductive belt could be substituted for the photoconductive surface of the drum. Similarly, a scanned image may be laid down directly on the surface of a treated paper on which the image is to be reproduced.

We claim:

1. Scanning apparatus for forming an image of an object on a surface, comprising:

a rotatable drive shaft;

drive means for rotating said drive shaft and moving said surface in mutually synchronous slip-free relationship; an optical element mounted for rotation on a second shaft to scan said object and project an image thereof on said surface when said second shaft is rotated in synchronous relationship with the motion of said surface; means for biasing said second shaft at a given initial angular orientation with respect to said object;

means including a slip-free clutch for coupling said shafts for mutually synchronous slip-free rotation whenever said object is to be scanned by said optical element, any change in the angular velocity of said drive shaft resulting from the sudden increase in the load presented to said drive means when said clutch is engaged being transmitted to said moving surface, so that the synchronous relationship between said optical element and said moving surface is unaffected by said angular velocity change, said biasing means returning said second shaft to said initial angular orientation when said clutch is disengaged after said optical element has scanned said object. I

2. The apparatus according to claim 1, wherein said surface is photoconductive and said image is an electrostatic charge pattern.

3. The apparatus according to claim 2, wherein said biasing means comprises a spring.

4. The apparatus according to claim 3, wherein said surface is the exterior of a rotatable drum.

5. The apparatus according to claim 4, wherein said shafts are collinear.

6. The apparatus according to claim 5, wherein said drive shaft and said drum are interconnected by a belt drive.

7. The apparatus according to claim 6, wherein said belt drive includes a third shaft uponwhich said drum is rotatably mounted, a first sheave secured to the drive shaft, a second sheave secured to said third shaft, and a belt engaging said sheaves.

8. The apparatus according to claim 7, wherein said sheaves include generally cylindrical belt engaging surfaces, said belt being flat and in operative engagement with said belt engaging surfaces.

9. A scan system for a copying apparatus, comprising:

a first mirror;

first mounting means for mounting said first mirror for rotation about a first longitudinal axis; photoconductive drum means;

second mounting means for mounting said photoconductive drum means for rotation about a second longitudinal axis, said second longitudinal axis being spaced from and parallel to said first longitudinal axis;

coupling means for selectively coupling said first mirror and said photoconductive drum means for registered rotation, said coupling means including a clutch means;

motive means for imparting rotation to said first mirror and said photoconductive drum, said motive means causing rotation. of said first mirror in a first angular direction when said clutch means is engaged; and

means for rotating said first mirror in a second angular direction opposite said first angular direction, said means for rotating being operable in response to the disengagement of said clutch means.

10. A scan system according to claim 9, including:

longitudinally extending first shaft means secured to said first mirror, said longitudinally extending first shaft means being coaxially with the first longitudinal axis; longitudinally extending second shaft means secured to said photoconductive drum means, said longitudinally extending second shaft means being coaxial with said second longitudinal axis; and wherein said clutch means is in operable engagement with said first shaft means.

11. A scan system according to claim 9, wherein said first mounting means comprises first bearing means for rotatably supporting said first shaft means.

12. A scan system according to claim 9, wherein the response time for achieving 25 percent maximum torque upon engagement of said clutch is less than 3 milliseconds.

13. A scan system according to claim 9, wherein said coupling means includes a belt drive for rotatably connecting said first mirror and said photoconductive drum. v

14. A scan system according to claim 13, wherein said belt drive includes a first sheave operably secured to said first mirror, a second sheave operably secured to said photoconductive drum, and belt means engaging said first and second sheave means for imparting said registered rotation therebetween.

15. A scan system according to claim 14, wherein said motive means is operably connected to said transfer drum.

16. A scan system according to claim 10, wherein said coupling means includes a belt drive for rotatably connecting said first shaft means and said second shaft means.

17. A scan system according to claim 16, wherein said belt drive includes a first sheave secured to said first shaft means, a second sheave secured to said second shaft means, and belt means engaging said first and second sheave means for imparting said registered rotation therebetween.

18. A scan system according to claim 17, wherein said clutchmeans is in operable engagement with said first shaft and disposed between said first sheave means and said first mirror.

19. A scan system according to claim 9, wherein said means for rotating said first mirror in a second angular direction comprises a spring operably secured to said first shaft means, said spring being displaced and subject to load in response to rotation of said first mirror gaged positions without regard to the angular position in said first angular direction. of said first shaft means whereby to permit selective ro- 20. A scan system according to claim 10, wherein tation of said mirror to achieve any desired scan length. said clutch is operable between engaged and disen-

Claims

1. Scanning apparatus for forming an image of an object on a surface, comprising: a rotatable drive shaft; drive means for rotating said drive shaft and moving said surface in mutually synchronous slip-free relationship; an optical element mounted for rotation on a second shaft to scan said object and project an image thereof on said surface when said second shaft is rotated in synchronous relationship with the motion of said surface; means for biasing said second shaft at a given initial angular orientation with respect to said object; means including a slip-free clutch for coupling said shafts for mutually synchronous slip-free rotation whenever said object is to be scanned by said optical element, any change in the angular velocity of said drive shaft resulting from the sudden increase in the load presented to said drive means when said clutch is engaged being transmitted to said moving surface, so that the synchronous relationship between said optical element and said moving surface is unaffected by said angular velocity change, said biasing means returning said second shaft to said initial angular orientation when said clutch is disengaged after said optical element has scanned said object.

5. The apparatus according to claim 4, wherein said shafts are collinear.

7. The apparatus according to claim 6, wherein said belt drive includes a third shaft upon which said drum is rotatably mounted, a first sheave secured to the drive shaft, a second sheave secured to said third shaft, and a belt engaging said sheaves.

9. A scan system for a copying apparatus, comprising: a first mirror; first mounting means for mounting said first mirror for rotation about a first longitudinal axis; photoconductive drum means; second mounting means for mounting said photoconductive drum means for rotation about a second longitudinal axis, said second longitudinal axis being spaced from and parallel to said first longitudinal axis; coupling means for selectively coupling said first mirror and said photoconductive drum means for registered rotation, said coupling means including a clutch means; motive means for imparting rotation to said first mirror and said photoconductive drum, said motive means causing rotation of said first mirror in a first angular direction when said clutch means is engaged; and means for rotating said first mirror in a second angular direction opposite said first angular direction, said means for rotating being operable in response to the disengagement of said clutch means.

10. A scan system according to claim 9, including: longitudinally extending first shaft means secured to said first mirror, said longitudinally extending first shaft means being coaxially with the first longitudinal axis; longitudinally extending second shaft means secured to said photoconductive drum means, said longitudinally extending second shaft means being coaxial with said second longitudinal axis; and wherein said clutch means is in operable engagement with said first shaft means.

13. A scan system according to claim 9, wherein said coupling means includes a belt drive for rotatably connecting said first mirror and said photoconductive drum.

18. A scan system according to claim 17, wherein said clutch means is in operable engagement with said first shaft and disposed between said first sheave means and said first mirror.

19. A scan system according to claim 9, wherein said means for rotating said first mirror in a second angular direction comprises a spring operably secured to said first shaft means, said spring being displaced and subject to load in response to rotation of said first mirror in said first angular direction.

20. A scan system according to claim 10, wherein said clutch is operable between enGaged and disengaged positions without regard to the angular position of said first shaft means whereby to permit selective rotation of said mirror to achieve any desired scan length.