Dynamically stable sheet clamping system for high speed sheet handling drums

A sheet clamping system for high speed rotatable drums is provided in which light-weight clamping bars, formed of relatively thin metal sheet stock, are configured and/or oriented for maximum resistance to deflection radially of the axis of drum rotation as well as positively secured in a sheet medium clamping position at axially spaced intervals along the length of the drum. The system further includes centrifugally actuated, axially extending rods positioned to underlie the operative portion of each clamping bar in a manner such that leading and trailing edges of a retained sheet are sandwiched between the rods and the clamping bars under a clamping force generally proportional to the square of drum rotational speeds.

BACKGROUND OF THE INVENTION 
This invention relates to improvements in sheet clamping apparatus for high 
speed sheet handling drums and, more particularly, it concerns a 
light-weight arrangement of axially orientated, drum-carried clamping bars 
by which the leading and trailing edges of sheet media may be secured 
firmly along the full length of each such edge by any of several forms of 
actuating mechanisms supported independently of the drum. 
Laser printing technology has developed to a stage where high resolution 
continuous tone images of a quality commensurate with or superior to those 
now provided by chemical processing of photographic sheet materials may be 
produced. In general, laser printing requires modulated laser light to be 
focused to a point in a sheet medium contained focal plane which must be 
held to within a fraction of a micrometer to optimize the resolution 
attainable in the laser modulated light. The focused laser light must then 
traverse the entire area of the image to be formed in a series of 
contiguous tracks or lines. This latter operation is ideally formed by 
supporting the sheet medium about the cylindrical periphery of a rotating 
drum and traversing the axial dimension of the medium with the focus point 
of the modulated laser light. Because of the large number of tracks which 
must be traversed, the time required to record an image on the sheet 
medium in this manner is almost entirely a function of the speed at which 
the medium is moved by the drum. 
To be cost effective in relation to more conventional techniques for 
forming continuous images on sheet media, the laser printing operation is 
preferably carried out at drum speeds on the order of 1500-6000 rpm. At 
such speeds, centrifugal forces acting on the sheet medium tend to 
separate the medium radially from the peripheral surface of the drum to a 
degree giving rise to displacement of the medium as well as tangential 
forces acting upon the clamped margins of the medium. It is important, 
therefore, that the mechanism used to secure the medium to the drum be 
capable of resisting such forces. In addition, dynamic balance of the drum 
at such high speeds requires that the drum carried clamping mechanism be 
embodied in an assembly of components which is maintained balanced during 
drum rotation. At the same time, the clamping mechanism must be capable of 
resisting movement and/or deflection, or compensate therefor, under the 
centrifugal forces incurred. 
While the problems of retaining a sheet against the centrifugal forces 
developed by carrying the sheet on a high speed rotating drum have been 
addressed in such relatively low tolerance applications as sanding drums 
and the like, the clamping arrangements employed in the prior art are 
relatively complicated and incapable of maintaining the sheet medium on a 
drum in a predictable relationship required by the optics of a laser 
printing system. In a commonly assigned copending application Ser. No. 
034,665, filed Apr. 6, 1987, the problem associated with retaining sheet 
clamping bars against displacement under centrifugal force during drum 
rotation at speeds on the order of 1600 rpm are addressed. In the 
disclosure of this application, pivotal centrifugally actuated members are 
employed to draw the central portion of the sheet clamping bars inwardly 
in opposition to the centrifugal force. The approach represented by the 
disclosure of the afore-mentioned co-pending application is satisfactory 
for the drum revolution speeds contemplated by that disclosure. However, 
the relatively moveable organization of components supported by the drum 
gives rise to problems associated with balancing the drum at speeds in 
excess of 1.600 rpm, possibly up to 6000 rpm. 
In light of the current state of the art relative to drum clamping 
arrangements for retaining sheet media about the periphery of the drum and 
the commercial potential of laser technology applications in the 
production of continuous tone images, there is need for an improved 
apparatus for securing a sheet medium to a high speed drum without concern 
for displacement of the sheet medium or the clamps from a predictably 
precise radial as well as tangential position on the drum. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a sheet clamping system for high 
speed rotatable drums is provided in which light-weight clamping bars, 
formed of relatively thin metal sheet stock, are configured and/or 
oriented for maximum resistance to deflection radially of the axis of drum 
rotation as well as positively secured in a sheet medium clamping position 
at axially spaced intervals along the length of the drum. The system 
further includes centrifugally actuated, axially extending rods positioned 
to underlie the operative portion of each clamping bar in a manner such 
that leading and trailing edges of a retained sheet are sandwiched between 
the rods and the clamping bars under a clamping force generally 
proportional to drum rotational speeds squared. 
In a preferred embodiment, the clamping system includes a mounting channel 
having a pair of inwardly diverging flanges joined at a common web secured 
to the drum near the cylindrical periphery. The drum is formed with 
divergent slots along its length to receive the mounting channel flanges 
in this orientation. To each of the mounting channel flanges, an assembly 
of an axially slidable actuating bar and a radially adjustable clamping 
bar is secured by headed pins extending through axial and radial slots in 
the respective actuating and clamping bars to provide for such movement. 
Each actuating bar carries transverse pins which engage spaced axial slots 
in the mounting channel flange and correspondingly spaced axially inclined 
slots in the clamping bar. Thus, axial movement of the actuating bars 
operates to cam the clamping bars between operative retracted sheet 
clamping positions and radially extended positions for loading a sheet on 
to the drum. 
The centrifugal force responsive rods are supported by means permitting the 
rods to move radially of the drum, such as, for instance, resilient leaf 
springs attached to the mounting channel web and which extend through 
aperture windows in the clamping bars to support the rods under a clamping 
lip of each clamping bar. In a relaxed condition of the springs supporting 
the rods, the rod surfaces lie substantially in the cylindrical periphery 
of the drum so that in the clamping position of the clamping bars, the 
clamping lips thereof will lightly engage the rods. Thus when a sheet 
medium is loaded onto the drum, opposite ends of the same will be 
positioned over the rods so that when the clamping bars are moved to their 
operative clamping position, the ends of the sheets are sandwiched between 
the rods and the clamping bar lips. During rotation of the drum the 
centrifugal force acting upon the rods and, hence, against the clamped 
margins of the sheet medium, compensates for the tangential forces tending 
to remove the margins from their clamps. The rods prevent tangential 
forces acting upon the sheet margins, from dislodging the sheet from the 
clamp. The centrifugal force acting on the rods is proportional to the 
square of the rotational speed of the drum. 
In a preferred embodiment, a system is provided by means of which a sheet 
of medium may be wrapped around the cylindrical surface of the drum and 
clamped thereon in such a manner that the sheet is not separated from the 
drum surface during high speed rotation thereof. In its simplest form, 
such a system may comprise a roller mounted in parallel to the axis of the 
drum and extending the length thereof. Preferably, the roller is radially 
movable relative to the axis of the drum into and out of engagement with 
the sheet. Movement of the roller may be synchronized with the clamping of 
the leading edge of a sheet in a clamping slot of the drum, and the roller 
may then stay in engagement with the surface of the drum until the 
trailing edge of the sheet has been clamped in the other slot of the drum. 
Thereafter, the roller may be moved out of engagement with the sheet. 
Accordingly, a principal object of the present invention is to provide an 
improved sheet clamping system for high speed drums which is simply 
actuated, effective to limit radial displacement of the sheet under 
centrifugal forces incurred at high drum rotational speeds by providing 
means effective to retain the tangential position of the leading and 
trailing edges of the sheet, and which may be operated during sheet 
loading and unloading operations by means supported off the ends of the 
drum. 
Other objects and further scope of applicability of the present invention 
will become apparent from the detailed description to follow taken in 
conjunction with the accompanying drawings in which like parts are 
designated by like reference characters.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In. FIG. 1 of the drawings, an exemplary high speed sheet handling drum 
incorporating the sheet clamping system of the present invention is 
generally designated by the reference numeral 10 and rotates about a 
longitudinal axis A. The drum 10 has a pair of radial end faces 14 and 16 
and a precision formed cylindrical outer surface 18 interrupted by an 
axially extending opening 20 which accommodates the sheet medium clamping 
system 22 of the present invention. Each of the end faces 14 and 16 
includes a recess 24 and 26, respectively, for mounting counterbalancing 
weights (not shown) diametrically opposite the clamping system 22. 
The clamping system 22 includes an axially oriented mounting channel 28 
having a pair of inwardly diverging flanges 30 and 32 and a common web 34 
secured to the drum near the cylindrical periphery 18. At the opening 20, 
the drum 10 is formed with divergent slots 36 and 38 along its length to 
receive the diverging mounting channel flanges 30 and 32. The system 22 
further includes two clamping assemblies designated generally by the 
reference numerals 40 and 42, respectively. Each of the clamping 
assemblies, in turn, includes an axially slidable actuating bar 44 and a 
radially adjustable clamping bar 46 secured by headed pins 48 (FIGS. 2 and 
3) to each of the flanges 30 and 32. Each of the clamping bars 46 
cooperates with a respective centrifugal force responsive clamping rod 50 
to clamp the leading and trailing edges of a sheet (not shown) to the drum 
10 as is described in greater detail below. 
Since the actuating bar 44, clamping bar 46, and headed pins 48 of each of 
the clamping assemblies 40 and 42 are of identical construction and, as 
such, designated by like reference characters, only the clamping assembly 
40 will be described in detail below. It is understood that the clamping 
system 22 of the present invention incorporates both clamping assemblies 
40 and 42 which cooperate with respective clamping rods 50 and are 
attached to the mounting channel 28 with the clamping assemblies in 
reversed orientation with respect to one another and, thus, facing in 
opposite directions. The actuating bars 44, clamping bars 46, and the 
channel member 28 are made from light-weight, relatively thin metal sheet 
stock. Because each of the clamping assemblies 40 and 42 are formed of 
identical components, the weight of the clamping system 22 is evenly 
distributed about the midline of the channel member 28. As such, balancing 
of the drum 10 is facilitated. 
As shown most clearly in FIG. 3 of the drawings, the inclined flange 30 of 
the channel member 28 has a plurality of cylindrical, threaded openings 52 
and a pair of axial, elongate guide slots 54. The clamping bar 46 has a 
planar support web 56 and an inclined clamping lip 58 extending at an 
acute angle therefrom. The support web 56 includes a plurality of radial 
slots 60, inclined cam slots 62 and rectangular apertures 64. Each of the 
inclined cam slots 62 has an axially extending upper end 66 which provides 
a flat 68 at the top end of each of the cam slots. The lower surface of 
the clamping lip 58 is covered with a friction enhancing sheet media 
contacting material 70 (FIG. 2). The undersurface 70 of the clamping lip 
58 is selected from a variety of materials depending on the particular 
sheet medium being used. Any resiliency in the material forming the 
surface 70 serves to reduce damage to the edge of sheet media otherwise 
due to compression of the sheet media between the clamping rod 50 and the 
clamping lip 58. 
The actuating bar 44 carries a pair of transverse pins 72 which engage the 
axial guide slots 54 in the inclined flange 30 and the inclined cam slots 
62 in the clamping bar 46. The actuating bar 44 also has a plurality of 
axial slots 74 corresponding to the threaded openings 52 in the inclined 
flange 30 and the radial slots 60 in the clamping bar 46. The actuating 
bar 44 has one end bent to provide a right angle tab 76 and an opposite 
straight end 78. The clamping rod 50 is shown to be attached to the common 
web 34 of the rigid support member 28 by a plurality of resilient leaf 
spring elements 80 which pass through the apertures 64 (FIG. 3) in the 
clamping bar 46. Of course, other means, such as pivoted arms (not shown) 
allowing movement of the rod 50, may be used instead of the springs 80. 
In FIG. 1 of the drawings, the clamping bar 46 is shown in an extended 
sheet medium leading or trailing edge loading position. In this position, 
the clamping lip 58 extending from the planar web 56 of the clamping bar 
46 is spaced from the upper surface of the associated clamping rod 50 by 
an amount sufficient to allow the insertion of the edge of a sheet 
therebetween. When the clamping bar 46 is in this sheet loading position, 
the tab 76 of the actuating bar 44 is located beyond the lateral end 14 of 
the drum 10. 
Once inserted into the gap formed by the extended clamping bar 46 and the 
clamping rod 50 (see FIG. 2), which may be accomplished by the edge of the 
sheet 11 being pushed into the gap at a speed in excess of the rotation 
(arrow B) of the drum 10, the clamping bar 46 is lowered into clamping 
engagement with the edge of the sheet 11. 
At substantially the same time a roller 19 being of substantially the same 
length as the drum 10 and being preferably coated with an elastomeric 
material is moved into engagement with the sheet 11 to press it into 
intimate contact with the cylindrical outer surface 18 of the drum 10 
during the rotation thereof. While shown to be mounted for idling on a 
pair of arms 21 which are pivotable between a rest position shown in 
phantom (FIG. 2) and an operative position shown relative to the drum 10, 
the roller 19 may be suspended in any other manner permitting its movement 
toward and away from the drum 10. At the end of a rotation of the drum 10 
the trailing edge of the sheet 11 is pushed into the gap formed by the 
other clamping bar 46 and clamping rod 50 shown to the right of the 
clamping arrangement just described. The roller 19 may then be withdrawn 
from contact with the sheet 11. 
In FIG. 2 of the drawings, the clamping bar 46 of the clamping assembly 40 
is moved from the extended sheet medium receiving position shown in 
phantom lines to a fully retracted clamping position shown in solid lines. 
The clamping bar 46 is retracted in response to movement of the actuating 
bar 44 from an axially extended position as shown in FIG. 1 to an axially 
depressed position, to be described in more detail below, but in which the 
tab 76 of the actuating bar 44 is adjacent one end of the inclined flange 
30 of the channel member 28 and the opposite end 78 (FIG. 3) of the 
actuating bar 44 extends beyond the opposite end of the inclined flange 30 
of the channel member 28. 
Even though the particular mechanism for axially moving each of the 
actuating bars 44 is not part of the present invention, it will be 
appreciated that the tabs 76 provide for the use of push-pull mechanisms 
located adjacent, but not mounted on, the ends 14 and 16 of the drum 10. 
Since the drum 10 need not carry the mechanisms for axially moving the 
actuating bars 44, the dynamic stability of the drum is enhanced. Such 
push-pull mechanisms would be mounted adjacent the ends 14 and 16 of the 
drum 10 opposite the ends of the clamping system 22 when the drum 10 is in 
the sheet loading orientation shown in FIG. 1. The ends of the channel 
member 28, clamping bars 46, and clamping rods 50 are recessed with 
respect to the radial end faces 14 and 16 of the drum 10 to provide 
adequate access to the tabs 76 (FIG. 1). 
With reference to FIGS. 1 and 2 of the drawings, the clamping system 22 is 
attached to the drum 10 by a plurality of fastener elements 82, such as 
counter sunk screw fasteners or rivets. Each fastener 82 extends through 
one of the leaf spring elements 80, the common web 34 of the channel 
member 28, a spacer 84, and into the drum. The height of the spacer 84 is 
chosen to compensate for the thickness of the particular sheet medium to 
be used and for manufacturing tolerances. The spacer 84 ensures that when 
the clamping bars 46 are in the retracted clamping position, the clamping 
lip undersurfaces 70 are tangent to the substantially cylindrical 
periphery of the sheet medium wrapped around the drum. As such, the edges 
of clamped sheet medium are not bent. 
In FIG. 3 of the drawings, the inclined flange 30, actuating bar 44 and 
clamping bar 46 of the clamping assembly 40 are separated in order to show 
the arrangement of cam slots and pins which provide for axial movement of 
the actuating bar 44 and radial extension and retraction of the clamping 
bar 46. It being understood that in use the actuating bar 44 and clamping 
bar 46 are attached to the inclined flange 30 by the headed pins 48 as 
shown in FIG. 2. Each of the headed pins 48 has a threaded end 86, a 
cylindrical shaft 88 and a flat head 90. The diameter of the threaded end 
86 of each of the pins 48 is less than that of the shaft 88 so that a 
shoulder 92 is formed at the junction of the threaded end 86 with the 
shaft 88. Since the actuating bar 44 and clamping bar 46 of the clamping 
assembly 40 are of identical construction to the actuating bar 44 and 
clamping bar 46 of clamping assembly 42, FIG. 3 and the description to 
follow provide a full understanding of the assembly and actuation of both 
clamping assemblies 40 and 42. Each of the elongate axial guide slots 54 
of the inclined flange 30 is dimensioned to receive one end of one of the 
transverse pins 72 extending from the actuating bar 44. The other end of 
each of the pins 72 extends from the front surface of the actuating bar 44 
and is received within one of the inclined cam slots 62 in the clamping 
bar 46. Each of the three threaded openings 52 in the inclined flange 30 
is adapted to receive the threaded end 86 of one of the headed pins 48. 
Each of the cylindrical shafts 88 of the pins 48 is received within one of 
the axial slots 74 in the actuating bar 44 and one of the radial slots 60 
in the clamping bar 46. Each of the pin shafts 88 has a sufficient length 
to allow for radial sliding movement of the clamping bar 46 and axial 
movement of the actuating bar 44 between the inclined flange 30 and flat 
heads 90 of the pins 48. The length of each of the cylindrical shafts 88 
is substantially equal to the combined widths of the actuating bar 44 and 
clamping bar 46 so that the ends of the pins 72 remain in the guide slots 
54 and cam slots 62. Further, when the clamping assembly 40 is assembled, 
the threaded end 86 of each of the pins 48 is fully received in one of the 
openings 52 so that the shoulder 92 on each of the pins 48 abuts the 
inclined flange 30, and each of the pin heads 90 abut the support web 56 
of the clamping bar 46. 
The axial slots 54 and 74 and the corresponding pins 48 and 72 limit the 
actuating bar 44 to only axial movement. The radial slots 60 and pins 48 
limit the clamping bar 46 to move radially only. The radial travel of the 
clamping bar 46 is limited by the vertical reach of the radial slots 60 
and the inclined cam slots 62. 
The clamping bar 46 is retracted from the extended sheet loading position 
(FIGS. 1 and 3) to the retracted sheet clamping position (FIG. 2) in 
response to axial movement of the actuating bar 44 in the direction 
indicated by the arrow B in FIG. 3. Since the clamping bar 46 is prevented 
from moving axially by the radial slots 60 and pins 48, axial movement of 
the pins 72 within the inclined ca slots 62 forces the clamping bar 46 to 
move downward in the radial direction shown by the arrows C in FIG. 3. In 
this manner, each of the pins 72 moves from the right hand end to the left 
hand end of the slots 54 and at the same time from the bottom to the top 
of each of the inclined slots 62. When each of the pins 72 abut the left 
hand end of one of the axial slots 54, each of the shafts 88 of the headed 
pins 48 abut the right hand end of one of the axial slots 74. 
The axially extending upper end 66 of each of the inclined cam slots 62 
facilitates locking the clamping bar 46 in the retracted sheet clamping 
position shown in FIG. 2 in the following manner. When the clamping bar 46 
is in the retracted sheet clamping position shown in FIG. 2, each of the 
flats 68 is biased against the lower surface of one of the pins 72 by a 
radial force exerted by the clamping rod 50 to the undersurface of the 
clamping lip 58. As such, the clamping bar 46 is positively locked in the 
sheet clamping position. During rotation of the drum 10 at high speeds, 
such as 1500-6000 rpm, the positive locking force between each of the 
flats 68 and one of the pins 72 increases due to centrifugal forces 
tending to force the clamping bar 46 radially upward and the clamping rod 
50 up against the sheet medium and clamping lip 58. The leaf springs 80 
allow the clamping rod 50 to move radially outwardly under the influence 
of centrifugal force. 
The fact that the clamping bar 46 is positively locked in the sheet 
clamping position (FIG. 2) at more than one location along its length 
ensures that the clamping system 22 is dynamically stable. During high 
speed rotation of the drum 10, the clamping system 22 is virtually static 
since the channel member 28 is rigidly attached to the drum, the clamping 
bars 46 and actuating bars 44 are locked in position, and radial travel of 
the clamping rods 50 is limited by the clamping lips 58. As such, dynamic 
movement of the clamping system during high speed rotation is limited to 
the radial movement of the clamping rods 50 allowed by compression of the 
sheet contacting surfaces 70 and by any elastic deflection of the clamp 
assembly. This limited radial movement of the clamping rods 50 provides 
for a sheet medium clamping force which is proportional to the speed of 
drum rotation and resulting centrifugal force on the sheet medium. 
With reference again to FIGS. 1 and 2, the incline of the clamping 
assemblies 40 and 42 provides numerous advantages. First, the up and back, 
extended sheet loading position of the clamping bars 46 facilitates proper 
alignment of the leading edge of a sheet medium under one of the clamping 
lips 58. For example, when the leading edge of a sheet is fed up under the 
clamping lip 58 of the clamping bar 46 of the clamping assembly 40, it may 
or may not come into contact with the support web 56. During retraction of 
the clamping bar 46 to the sheet media clamping position shown in FIG. 2, 
the clamping lip 58 and upper end of the support web 56 not only move 
downward but also forward because the support web 56 lies in a chordal 
plane subtending an arc of less than 180.degree. in the cylindrical 
periphery of the drum. This forward movement helps to ensure that the 
support web 56 contacts the edge of the sheet medium and aligns the sheet 
medium with respect to the clamping assembly 40. 
Second, the incline of the clamping assemblies 40 and 42 allows the 
trailing edge of a sheet to fall radially against the upper surface of one 
of the clamping bars 50 without having to be bent in order to pass by one 
of the clamping lips 58. For example, after the leading edge of the sheet 
has been clamped in the clamping assembly 40, the drum 10 is rotated 
clockwise while the sheet is held against the peripheral surface 18 of the 
drum 10. The drum 10 is rotated until the trailing edge of the sheet falls 
radially down against the upper surface of the clamping bar 50 of the 
clamping assembly 42. Then, the actuating bar 44 of the clamping assembly 
42 is depressed axially so that the clamping bar 46 retracts and clamps 
the trailing edge of the sheet between the clamping lip 58 and clamping 
rod 50 of the clamping assembly 42. 
Third, the incline of the clamping assemblies 40 and 42 ensures that the 
clamping bars 46 and the actuating bars 44 and the inclined flanges 30 and 
32 extend in a generally radial direction. Thus, bowing due to centrifugal 
forces acting on these components of the clamping assemblies 40 and 42 may 
be minimized. Further, the incline of the clamping assemblies makes 
provision for the clamping lip 58 to extend at an acute angle with respect 
to the support web 56. In this configuration, the clamping lip 58 resists 
bending or bowing due to centrifugal forces. This allows the channel 
member 28, clamping bars 46 and actuating bars 44 to be made of relatively 
thin, lightweight, metal sheet stock. 
Thus it will be appreciated that as a result of the present invention, a 
dynamically stable and highly effective clamping system is provided. From 
the foregoing description and accompanying drawing illustrations it will 
be apparent to those skilled in the art that variations and/or 
modifications of the disclosed embodiment may be made without departure 
from the invention. Accordingly, it is expressly intended that the 
foregoing description and accompanying drawings are illustrative of a 
preferred embodiment only, not limiting, and that the true spirit and 
scope of the present invention be determined by reference to the appended 
claims.