Abstract:
A sheet restrainer is used to restrain movement of continuously traveling glass sheet and includes arms on either side of the glass sheet. A driving device coupled to the arms moves the arms from a retracted position in which the arms are withdrawn from the glass sheet to an engaged position in which the arms are near the glass sheet and in alignment with each other. Rollers are connected to the arms and contact the glass sheet in the engaged position. A damping device applies an adjustable damping force to at least one of the arms dampening movement of the arm in the engaged position thereby restraining movement of the sheet. In a method of operation, the damping devices restrain angular and/or lateral movement of the sheet by transmitting movement of the sheet against the rollers into a resistive damping force of the damping devices.

Description:
TECHNICAL FIELD 
       [0001]    The field is restraining movement of continuously traveling glass sheet during scoring and separation of the sheet. 
       TECHNICAL BACKGROUND 
       [0002]    A fusion process (e.g., downdraw process) forms high quality thin glass sheets that can be used in a variety of devices such as flat panel displays. Glass sheets produced in a fusion process have surfaces with superior flatness and smoothness when compared to glass sheets produced by other methods. The fusion process is described below with respect to  FIG. 1  (Prior Art) but for a more detailed description refer to co-assigned U.S. Pat. Nos. 3,338,696 and 3,682,609, which are incorporated herein by reference in their entireties. 
         [0003]      FIG. 1  shows a schematic view of an exemplary glass manufacturing system  10  which utilizes the fusion process to make a glass sheet  12 . As shown, the exemplary glass manufacturing system includes melting vessel  14 , fining vessel  16 , mixing vessel  18 , delivery vessel  20 , fusion draw machine (FDM)  22 , and traveling anvil machine (TAM)  24 . Typically, components  16 ,  18  and  20  are made from platinum or platinum-containing metals, but they may also comprise other temperature resistant metals. 
         [0004]    Melting vessel  14  is where the glass batch materials are introduced as shown by arrow  26  and melted to form molten glass  28 . Melting vessel  14  is connected to fining vessel  16  by melting to fining vessel connecting tube  30 . Fining vessel  16  has a high temperature processing area that receives molten glass  28  (not shown at this point) from melting vessel  14  and in which bubbles are removed from molten glass  28 . Fining vessel  16  is connected to mixing vessel  18  by a finer to stir chamber connecting tube  32 . Mixing vessel  18  is connected to delivery vessel  20  by a stir chamber to bowl connecting tube  34 . Delivery vessel  20  delivers molten glass  28  through a downcomer  36  into FDM  22  which includes inlet  38 , forming vessel  40  (e.g., isopipe), and pull roll assembly  42 . 
         [0005]    As shown, molten glass  28  flows from downcomer  36  into inlet  38  which leads to forming vessel  40  which is typically made from a ceramic or a glass-ceramic refractory material. Forming vessel  40  includes opening  44  that receives molten glass  28  which flows into trough  46  and then overflows and runs down two lengthwise sides  48  (only one side shown) before fusing together at what is known as root  50 . Root  50  is where the two lengthwise sides  48  come together and where the two overflow walls of molten glass  28  rejoin (e.g., refuse) to form glass sheet  12  that is then drawn downward by pull roll assembly  42 . The glass sheet cools as it is drawn, transitioning from a molten state at the root, to a visco-elastic state and finally to an elastic state. Pull roll assembly  42  delivers drawn glass sheet  12  which, at the bottom of the isopipe is substantially flat, but which later in the process may develop a slightly bowed or curved shape across the width and/or length of the glass sheet  12 . This bowed shape may remain in glass sheet  12  all the way to TAM  24 . Continuous beads are formed along the outer periphery of first and second sides  63 ,  64  of the glass due to the pull rollers contacting the glass. A quality region of the glass is the major surfaces of the glass between the beads, whereas non-quality regions are the regions from the beads to the outer edges at the first and second sides  63 ,  64  of the sheet. 
         [0006]    TAM  24  comprises a laser-mechanical scoring device (or a mechanical scoring device)  52  and nosing device  54  that are used to score the drawn glass sheet, while the TAM moves vertically at the same rate as the glass sheet, so it can then be separated into distinct pieces of glass sheets  56 . TAM  24  is located in the elastic region of the sheet in an area referred to herein as a bottom of the draw  58 . TAM  24  operates in cycles, the cycle beginning at the first side  63  of the glass at a location that is above the location where the glass will be bent and separated. An optical head and quenching nozzle assembly mounted to the TAM move along the score line from first side  63  toward second side  64  of the glass, while the glass and the TAM continue to travel vertically downward at the same rate. The TAM then reaches the end of its stroke at second side  64  once the laser scoring and quenching processes are completed. The glass bending is carried out along the score line and the robotic equipment located near but below the score line at this point of downward travel of the sheet, separates an individual glass sheet from the continuous sheet. The TAM moves upward, returning to the beginning of the stroke at first side  63  of the glass. 
         [0007]    Nosing, pressing, ironing caused by scoring and separating processes cause motion in the glass sheet which in turn contributes to the creation of stress variation within the glass sheet. Sheet motion at the bottom of the draw is mainly driven by sheet scoring and separation processes. Depending on the bottom of the draw setup, robot tooling can also introduce sheet motion. Post separation sheet dangling can be another source of sheet motion if nosing retraction is not controlled properly. On the other hand, over constraint of the sheet by fixed rollers can cause sheet breakage and sheet crackout during the scoring process. Sheet motion produced by any of the above mechanisms, or any other mechanism, can propagate upward into the visco-elastic region of the glass sheet, and becomes especially troublesome in the region where the sheet transitions from a visco-elastic state to an elastic state. Here, stresses caused by movement of the sheet can be frozen into the sheet, and manifest later as, for example, shape changes when the sheet is separated or otherwise further processed. 
       SUMMARY 
       [0008]    We have developed a pair of sheet restraining units on each of the first and second sides of the sheet, which can be installed above the score line. The sheet restraining unit functions in such a way that it restrains the sheet but complies with the sheet shape, i.e., having a clamping force sufficient to hold the sheet without forcing sheet shape change. It will not block sheet vertical motion and will allow the sheet to move in a horizontal plane. It will absorb the kinematic energy from the sheet while moving with the sheet and effectively dampen the sheet motion during separation and post separation. It will also help to simulate the off-line manual separation process by restraining the sheet above the score line to provide a much more symmetrical bending separation setup. It can either be engaged throughout the TAM cycle or during partial processes such as scoring and separation, and will contact the sheet in the non-quality region and close to the beads. The device can also help with the reliable separation of large size sheet (e.g., generation 8 and above) with an improved bending separation condition. In addition, it provides a tool for laser scoring bending separation process optimization for less sheet motion and better edge quality. 
         [0009]    The sheet restrainer will be used as a bottom of the draw sheet motion reduction tool, especially during separation and post separation periods. The sheet restraining device is installed above the score line and can engage the sheet throughout the TAM cycle, during partial processes such as scoring and separation, or it can constantly engage the sheet when it is mounted to a fixed frame. The device will contact the sheet in the non-quality areas close to the beads. It will allow for free sheet travelling in the vertical direction and free sheet motion in the horizontal plane with no change to sheet shape. It is designed to absorb the kinematic energy from the sheet while moving with the sheet in the horizontal plane and damping the sheet motion during engagement. The coefficient of damping is adjustable based on the process needs. 
         [0010]    A first embodiment is a sheet restrainer for restraining movement of a continuously traveling glass sheet, comprising arms on either side of the glass sheet. A driving device (such as a pneumatic or hydraulic cylinder) is coupled to the arms and moves them from a retracted position in which the arms are withdrawn from the glass sheet to an engaged position in which the arms are near the glass sheet and in alignment with each other. Rollers are connected to the arms and contact the glass sheet in the engaged position. Reference to alignment of the arms of a unit does not mean perfect alignment but means that the rollers are on either side of the glass sufficient to impart a clamping force to the glass between them. A damping device applies an adjustable damping force to at least one of the arms dampening movement of the arm in the engaged position thereby restraining movement of the sheet. 
         [0011]    Referring to details of the sheet restrainer, the damping device can include a piston movable within a pneumatic or hydraulic cylinder that provides the damping force. Alternatively, the damping device can include magnets that provide eddy currents as the damping force. 
         [0012]    A pivotable variation of the sheet restrainer includes a frame. The arms are pivotally mounted to the frame. The driving device includes a hydraulic or pneumatic cylinder connected to one of the arms and a piston rod connected to the other of the arms extending from a piston movable in the cylinder. Extension or retraction of the piston and piston rod causes the arms to pivot between the retracted and engaged positions. The arms each comprise a first arm portion that is pivotally mounted to the frame and a second arm portion that is pivotally mounted to the first arm portion. The rollers are carried on the second arm portion. The damping device includes a first dashpot connected to the frame and the first arm portion of one of the arms dampening movement of the arms due to lateral movement of the sheet, and a second dashpot connected to the first and second arm portions of one of the arms dampening pivotable movement of the second arm portion relative to the first arm portion due to angular movement of the sheet. 
         [0013]    A linearly movable variation of the sheet restrainer includes a frame. A carriage is mounted to the frame in which each of the arms can slide in a linear direction. The driving device includes a pneumatic or hydraulic cylinder mounted to the frame and connected to each of the arms causing the arms to move linearly between the retracted position and the engaged position in the carriage. The cylinder also functions as the dampening device, dampening linear movement of the arms due to lateral movement of the sheet. 
         [0014]    In the pivotable and linear sheet restrainers the frame may be part of a traveling anvil machine that travels at a rate of the glass sheet, or it may be fixed relative to movement of the glass sheet. 
         [0015]    A second embodiment of the invention features a method of restraining movement of the continuously traveling glass sheet. A continuously traveling vertically oriented glass sheet is provided. The sheet comprises a visco-elastic region and an elastic region. Also provided is the sheet restrainer. The method includes moving the arms in a seeking phase until the rollers contact the sheet in the engaged position and the arms are in alignment with each other. The glass can be clamped at this point from rollers on both sides of the sheet and at both ends of the sheet. Next, an adjustable damping force is applied to the arm in response to movement of the sheet restraining the sheet movement in a sheet restraining phase. 
         [0016]    Referring to details of the method, in the case of the pivotable sheet restrainer, applying a dampening force comprises providing a first dashpot connected to the frame and a first arm portion of one of arms, the first arm portion being pivotably connected to the frame, and providing a second dashpot connected to the first arm portion and the second arm portion of one of the arms, the second arm portion being pivotably connected to the first arm portion. Pivotable movement of the first arm portion is dampened when lateral movement of the sheet causes at least one of the arms to move against a force of the first dashpot. Pivotable movement of the second arm portion is dampened when angular movement of the sheet causes the second arm portion to move against a force of the second dashpot. In the case of the linearly movable sheet restrainer, moving the arms comprises driving a hydraulic or pneumatic cylinder causing the arms to move linearly between the retracted and engaged positions in the carriage. Applying a damping force comprises damping movement of the arms when lateral movement of the sheet causes the arms to move against the force of the cylinder. 
         [0017]    Regarding further details of the method, the sheet restrainer clamps the sheet in the seeking phase and throughout the sheet restraining phase without changing a shape of the sheet. The sheet restraining phase takes place at least during scoring and separation of the sheet. The sheet restrainer is ideally suited for processes that use laser scoring because this reduces sheet movement during separation. The sheet is clamped near where laser scoring of the sheet is conducted resulting in more symmetrical bending of the sheet. The clamping of the sheet results in less sheet motion transferred to the sheet at a fusion draw machine above the sheet restrainer which reduces stress in the sheet. The clamping of the sheet is conducted at a force that is not conducive to sheet breakage and sheet crackout. 
         [0018]    The following advantages are provided by the sheet restrainer. The sheet is restrained but complies with sheet shape which is a major difference from existing edge guides/rollers at the bottom of the draw and offers benefits to forming process stability. Sheet motion is dampened by absorbing the kinematic energy from the sheet while moving with the sheet. A better bending separation setup is established for both large size (generation 8 and above) and laser scoring processes. A generic tool is provided to control or restrain the sheet without introducing excessive stress (binding) or changing sheet shape. It helps to avoid crackout during mechanical scoring. A low cost approach is provided to minimize bottom of the draw sheet motion during separation and post separation. 
         [0019]    Many additional features, advantages and a fuller understanding of the invention will be had from the accompanying drawings and the detailed description that follows. It should be understood that the above Technical Summary provides a description in broad terms while the following Detailed Description provides a more narrow description and presents embodiments that should not be construed as necessary limitations of the broad invention as defined in the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  (Prior Art) is a schematic view of a prior art apparatus including a fusion draw machine; 
           [0021]      FIG. 2  is a side view showing a first embodiment of a pivotable sheet restraining device in position on a glass sheet mounted to a TAM; 
           [0022]      FIG. 3  is a perspective view identified by dotted lines in  FIG. 2 ; 
           [0023]      FIG. 4  is a bottom plan view of the device of  FIG. 3 ; 
           [0024]      FIG. 5  is a bottom plan view of the device showing lateral and angular movement of the glass sheet being restrained by lateral and angular control dashpots of the device; 
           [0025]      FIG. 6  is a bottom plan view of a device of the first embodiment using magnetic dampening devices; 
           [0026]      FIG. 7  is a perspective view showing a second embodiment of a linear sheet restraining device in position on a glass sheet mounted to a fixed frame; 
           [0027]      FIG. 8  is a perspective view identified by dotted lines in  FIG. 7 ; 
           [0028]      FIGS. 9 and 10  are top plan views showing the linear sheet restrainer device in a retracted position and engaged position away from and on the glass sheet; and 
           [0029]      FIGS. 11 and 12  are top plan views of the linear sheet restrainer device where the glass is perpendicular to the device and applying angular motion to the device, respectively. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    The sheet restrainers shown in the embodiment of  FIGS. 2-5  are attached to TAM carriage  24  located above score line  62 . The sheet restrainers are individual units, one on first side  63  of the glass sheet and the other on second side  64  of the sheet. Two sheet restrainer units  65 ,  66  are needed to restrain the sheet from both sides A, B (bead areas  67 ) once the sheet is clamped by each unit. Each unit includes two swing arms  68  movable about pivots  70  connected to frame  72  mounted to the TAM. Each swing arm  68  includes a main arm or first arm portion  74  and an angular control arm or second arm portion  76 . Main arm  74  moves about pivot  70  and has an end  78  proximal to the sheet and an end  80  distal to the sheet. Angular control arms  76  are pivotally mounted at pivots  77  to proximal end  78  of each main arm  74 . Each unit (e.g., unit  65 ,  FIG. 3 ) includes a set of four rollers (a pair of rollers  82 , and a pair of rollers  84 ) in contact with\glass sheet  12  on each side (A, B) of the sheet, which allow the sheet to move vertically with respect to the restrainer without constraint. Although four rollers are shown, a different number of rollers may be employed, for example, two rollers on each side of each unit. All rollers in this disclosure can have compliant surfaces such as heat resistant rubber or other polymer. The rollers are mounted on axles  86  that rotate in bearings  88  in an H-shaped frame  90  connected to angular control arm  76  so that the rollers can conform to the sheet surface. The angular control arm transmits angular motion of the glass sheet shown by the arrows  92  ( FIG. 4 ) to an angular control dashpot unit  94 , which controls and dampens the angular movement of the angular control arm. Dashpot unit  94  may be, for example, a pneumatic or hydraulic dashpot. Cylinder  96  of the angular control dashpot unit is fixed to main arm  74  while a free end of piston rod  98  of the dashpot is mounted at pivot point  100  on the angular control arm. A pivot point, such as pivot point  100 , may comprise, for example, a bolt, a cotter pin, a wrist pin, or any other suitable fastener that allows pivoting of piston rod  98  about the pivot point. Digital pressure regulator  102  is used to control the coefficient of damping of angular control dashpot  94 . Although only a single pressure regulator is shown controlling the cylinder and two dashpots for clarity, multiple pressure regulators could be used. For example, one pressure regulator can be devoted to each cylinder (e.g. three). 
         [0031]    Swing arms  68  are used in a seeking phase to contact and clamp the glass. Hydraulic or pneumatic clamping air cylinder unit  104  is used to open and close swing arms  68  for engagement (clamping) and disengagement (unclamping) with the sheet. Cylinder  106  is fastened to distal end  80  of one of the swing arms while a free end of piston rod  108  extending from the piston of the cylinder is pivotably fastened to the distal end of another of the swing arms using fastener  110 . Fastener  110  provides a pivot point for piston rod  108 . The extension and retraction of the piston and piston rod in the cylinder enables the swing arms to function like scissors, wherein each swing arm  68  pivots about pivot point  70 , shown by arrows  112 , causing rollers  82 ,  84  to alternately clamp onto and release the glass sheet. Air cylinder  104  also controls the pressure of the rollers on the glass (how tight the glass sheet is clamped). Digital pressure regulator  102  is used to control the clamping force of the rollers on the glass by regulating the pressure output from pressure regulator  102  and hence the force applied by cylinder  106 . 
         [0032]    Once the glass sheet is clamped, swing arms  68  accommodate the sheet lateral motion (in the horizontal plane shown by arrows  116 ) and transmit lateral movement of the glass sheet to lateral control dashpot unit  118  mounted to frame  72 . Hydraulic or pneumatic cylinder  120  of lateral control dashpot unit  118  is mounted to the frame while a free end of piston rod  122  extending from the piston of the lateral control dashpot unit is pivotably connected to fastener  124  attached to a distal end  80  of a main arm  74  of one of the swing arms. Fastener  124  provides a pivot point for piston rod  122 . The lateral control dashpot unit  118  controls and dampens movement of swing arms  68  resulting from lateral movement  116  of the glass sheet. Pivotable movement  112  of the swing arms while in a clamped position moves piston rod  122  into and from cylinder  120 . Digital pressure regulator  102  is used to control the lateral control dashpot&#39;s coefficient of damping so that lateral movement of the clamped sheet is damped. 
         [0033]    The pneumatic or hydraulic cylinders for the swing arms, lateral control dashpot and angular control dashpot function to apply a constant predetermined pressure against their respective pivot points; and their clamping pressure or damping coefficients are adjusted by digital pressure regulator  102 . Digital pressure regulator  102  may be, for example, a Series ITV digital pressure regulator by SMC Corp. An example of a suitable type of dashpot is Airpot Corp. Product No. 2KS160P2.0NY. The timing for applying the seeking pressure and clamping force and magnitude of the seeking pressure and clamping force applied by the swing arm cylinder, and the damping coefficients of the angular and lateral control dashpot units, are controlled by programmable logic controller (PLC)  125 . 
         [0034]    The process sequence for operating sheet restraining units  65 ,  66  is as follows (refer to  FIGS. 3 and 4 ), which will be described only with respect to unit  65 , it being understood that sheet restrainer unit  66  is operated the same way. Prior to engagement of the rollers with the glass sheet, the PLC instructs via pressure regulator  102  the clamping air cylinder  104  to retract to open swing arms  68  so that they are out of the way of the glass sheet. At this stage, the PLC sets the pressure regulators controlling the dashpot units such that the dashpots move freely (minimal damping). To engage the rollers of the sheet restrainer unit with the glass, the PLC instructs the piston and piston rod  108  of clamping air cylinder  104  to extend, causing swing arms  68  to pivot about their respective pivot points  70 , which closes the swing arms at ends  78  and moves rollers  82 ,  84  into contact with the glass sheet (in the engaged position,  FIG. 3 ). Pressure regulator  102  is adjusted via PLC  125  to control the pressure of the rollers on the glass sheet. A flow control regulator attached to the air cylinder is controlled by the PLC or manually set at one setting to control the closing speed of the arms. 
         [0035]    As swing arms  68  and rollers  82 ,  84  close on the glass sheet, they move to the position of the sheet and then stop because of the low driving force (pressure) and slow engagement speed. A position sensor (not shown) may also be used to signal to the PLC when the arms have contacted the glass via the rollers, which can provide information to the PLC instructing it to stop the extension of the clamping cylinder piston rod. After the rollers reach a final engagement position against the glass sheet, the PLC resets pressure regulator  102  controlling the dashpot units to higher pressure so that the dashpot units effectively dampen the lateral and angular movements of the glass sheet. Therefore, the sheet restrainer units act as flexible holding devices to restrict motion of the sheet resulting from the separating of the glass sheet below the score line, and as sheet motion dampeners to dampen high frequency sheet movements after separation by absorbing energy from sheet motion. 
         [0036]    The sheet restrainer units may be attached to TAM  24  where they either constantly engage the sheet even during the return upward stroke of the TAM, or they can retract during the upward stroke and then reengage the glass at the start of the next downward stroke. The sheet restrainer units will contact the glass sheet at least during scoring and separation processes. The sheet restrainer units can also be operated to constantly engage the glass while mounted to a fixed frame. 
         [0037]    Referring to  FIG. 6 , another embodiment uses magnetic damping “dashpots units”  126  instead of air or oil dashpot units  94  or  118 . Each dashpot includes magnets  127  supported near a movable metal arm  128 . The magnets generate eddy currents in the arm  128 , which generates electric field  130 . This dampens movement of arms  128  in either direction shown by the double-headed arrow on the arm. 
         [0038]    Referring to the embodiment of  FIGS. 7-12 , linear sheet restrainers are shown. The linear sheet restrainers are individual units, one unit  132  or pair of devices at first side  63  of the glass and another unit  134  or pair of devices at second side  64  of the glass. The operation of only one linear sheet restrainer unit  132  will be discussed for the sake of clarity, it being understood that the other unit  134  operates the same way. Fixed frame  136  is located a predetermined distance above the score line. Arms  138  are each adapted to be moved linearly in carriage  140  mounted to the frame. Pneumatic or hydraulic cylinder unit  142  is mounted to the frame, and performs both a clamping and a dampening function. Piston rod  144  extending from a piston in cylinder  143  is connected to arm  138  at its free end. Movement of piston rod  144  out of or into cylinder  143  moves arm  138  inside carriage  140  alternately away from or towards the cylinder. An example of a suitable low friction, damping air cylinder unit is an Airpot Corp. AIRPEL™ type of cylinder, Product No. E16D20U. The cylinder unit  142  is controlled by pressure regulator  146 , which receives electronic signals from PLC  148 . 
         [0039]    On the end of arm  138  is mounted a set of four rollers (one pair of rollers  150 , and another pair of rollers  152 ) carried on H-shaped frame  154 . These linear sheet restrainer units may have different numbers of rollers, for example, each unit may have two rollers on each side of the sheet. The rollers are mounted on axles  156  that rotate in bearings (not shown) carried by the H-frame. The H-frame is pivotally mounted to rail  138  using fastener  158  to form a pivot point. 
         [0040]    Sheet restrainer unit  132  includes two subunits  157 ,  159  mounted to the frame on either other side of the glass sheet that move toward the sheet during clamping so as to be in alignment with each other. When piston rod  144  is extended out of the cylinder of both subunits the set of rollers  150 ,  152  of each subunit move from a retracted position ( FIG. 9 ) to an engaged position ( FIG. 10 ) in contact with the glass sheet on both sides of the glass and at both the first and second sides  63 ,  64  of the sheet, clamping the sheet between them. The rollers contact a non-quality region  160  of the glass. A position sensor (not shown) may be used to determine when the rollers have contacted the glass sheet. 
         [0041]    Linear sheet restrainer  132  dampens lateral movement  162  of the glass sheet ( FIG. 11 ) as such movement causes the pistons to move into and from the cylinders on each side of the glass. The H-shaped frame on which the rollers are mounted is permitted to pivot at points  158  in direction  164  ( FIG. 12 ) to accommodate angular movement of the glass, but this angular movement is not dampened. Therefore, when the glass sheet moves, for example, toward the bottom left side of  FIG. 8 , piston rod  144  compresses the piston of lower subunit  157  in that figure while the piston rod of upper subunit  159  is extended and the piston of that cylinder is expanded. As both cylinders are maintained at a constant pressure via PLC  148  and pressure regulators  146 , the pneumatic or hydraulic cylinders adjust automatically to movements of the glass. The range of movements of the pistons in the cylinders are preset to accommodate a range of movements of the glass sheet, typically on the order of at least 20 millimeters total or at least 10 millimeters for each device on a side of the glass. The pneumatic or hydraulic cylinder unit is a low friction cylinder, which functions not only to clamp the rollers to the glass but also offers linear damping while the rail moves back and forth in the carriage. The linear sheet restrainer units of the present embodiment constantly engage the glass sheet when mounted to a fixed frame. However, the linear sheet restrainer units may also be attached to TAM  24  where they either constantly engage the sheet even during return upward stroke of the TAM, or can retract during the upward stroke and then reengage the glass at the start of the next downward stroke. 
         [0042]    In operation, in a seeking phase, PLC  148  instructs pressure regulator  146  to be set at a low pressure output. This controls the cylinder causing the piston rod to extend which extends arm  138 . This extension continues at a low pressure for a predetermined period of time or until the rollers contact the glass at both sides. Arms  138  stop upon the rollers contacting the glass at the low pressure. A position sensor can also be used to determine when the rollers contact the glass and send electronic signals to the PLC which in turn instructs the cylinders to stop extending. Once the rollers have engaged both sides of the glass in a clamping position, the PLC instructs the pressure regulators to increase the clamping pressure. During the sheet restraining phase, the cylinders act as dampening devices, restraining lateral movements of the glass, while maintaining a clamping force on the glass. 
         [0043]    Many modifications and variations of the invention will be apparent to those of ordinary skill in the art in light of the foregoing disclosure. Therefore, it is to be understood that, within the scope of the appended claims, the invention can be practiced otherwise than has been specifically shown and described.