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This application claims benefit of U.S. Provisional application Ser. No. 60/353,079, filed Jan. 30, 2002, the entirety of the disclosure of which is incorporated herein by reference thereto. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to a system for adjusting an auxiliary, or, extension, screed of a paving machine with respect to a main screed. 
   In conventional asphalt paving operations, a self-propelled vehicle, known as a “tractor” is used having a hopper on the front end thereof. This hopper receives asphalt paving material, typically from a dump truck, and the tractor generally engages and pushes the truck forwardly as the truck empties its contents into the hopper. 
   The asphalt material is transferred from the hopper to the roadbed or other surface being paved, and the asphalt empties onto the roadbed in front of transversely extending screw augers. These augers transport the asphalt material laterally in front of an elongated plate, or “screed”, which compresses and compacts the asphalt downwardly to form a “mat” of paving material, ideally of uniform thickness and surface finish. 
   The screed is typically pulled behind the tractor and may move upwardly or downwardly with respect to the tractor, such screed being connected to the tractor by tow arms, or bars. The tow bars are pivotally connected to the tractor and pivot about an axis, or “tow points.” This arrangement effectively allows the screed to “float” with respect to the tractor as the screed is towed behind the tractor. 
   In order to control the thickness of the asphalt mat being formed by the asphalt screed, the height of the screed is generally varied by positioning the tow bars at selected elevations. The angle of attack of the screed must also be controlled to achieve the desired asphalt mat thickness and surface finish, and this is typically done by means of a crank at each end of the screed, rotation of the cranks causing corresponding height adjustments on each end of the screed. 
   A conventional screed is of a set width. However, in certain paving applications, particularly in applications such as driveways, parking lots, and the like, where varying asphalt mat widths are required, an adjustable, or extendable, screed arrangement is desirable. Extendable screeds have become common in the asphalt paving industry to achieve varying widths of a paved surface without interruption of the paving process. Typically, extendable screeds consist of a main screed section of fixed width and hydraulically extendable auxiliary screed sections capable of extending from each end of the main screed unit, such auxiliary sections generally being referred to as “extensions” or “extension screeds.” 
   In the normal operation of an asphalt paver, an operator makes adjustments in the attack angle of the screed to affect the depth of the asphalt mat being laid. This is achieved by raising or lowering the tow point on the tractor with a hydraulic cylinder unit, or more commonly in smaller paving machines, with the rotatable cranks. The cranks cause the main screed to pivot relative to the tow arm attached to the tractor, to thereby change the angle of attack of the main screed with respect to the direction of travel. With each of these adjustments of the main screed, the corresponding position of the extension screed will change due to the fact that it is mounted to the main screed. In other words, the extension screed is slaved to the movement of the main screed. 
   When the asphalt mat thickness changes, and the main screed is adjusted to float at a new paving depth, the main screed and extension screed will move about an arc with respect to the axis of rotation of the tow bars and with respect to the tractor. Consequently, this rotation of the screed about the tow point axis induces a change in the extension screed elevation with respect to the main screed elevation, since the extension screed, which either leads or trails the main screed, rotates in an arc having a different radial distance from the axis of rotation, than does the main screed. For example, the extension screed, if it is positioned in front of the main screed, moves through an arc having a shorter radial distance with respect to the main screed, or, in the event the extension screed follows behind the main screed, the extension screed would rotate through an arc having a longer radial distance compared to the main screed. In either case, the screed extension rotates to an elevation which is different relative to the main screed, and this creates a discontinuity, or “step”, between the asphalt mat formed by the main screed and the asphalt mat formed by the extension screed. This step is undesirable in that it results in an overall asphalt mat having a height difference in the surface thereof. 
   The extension screeds are preferably adjusted such that the asphalt mat surface produced by such screeds is matched to the surface of the main screed, and there is no step or discontinuity between the mat heights. The physical characteristics of the screeds being used and the depth of the asphalt mat being formed ordinarily influence the frequency and the degree to which the extension screeds require adjustment. Further, other factors contribute to the extension screed adjustment, such as the weight of the screed mechanism towed by the tractor, the length of the main and extension screed plates itself from front to rear, and the relative location of the trailing edge-of the extension screed plate in relation to the trailing edge of the main screed plate. 
   In order to avoid a step or discontinuity in the mat, operators generally attempt to adjust the height of the extension screed relative to the main screed. This may result in a trial and error approach which can be time consuming and inefficient. 
   When smaller paving machines are used, such as the type ordinarily used to form residential driveways, parking areas, and the like, the length of the tow arms is shorter than on larger paving machines. The relatively short length of the tow arms of the small utility paver thus make the effect of depth differentials between the main screed mat and the extension screed mat more pronounced and dramatic, and the requisite skill in adjusting for such mat height differentials more acute. Given the general utility nature of such paving machines and the variety of applications which they encounter, frequent depth adjustment of the mat being formed is common. Thus, the operator of the paver must constantly closely monitor the paving operation, and often times the matching of the mat heights for the main and extension screeds is performed inadequately. 
   Devices for adjusting the extension screeds for paving machines have been patented. For example, U.S. Pat. No. 4,379,653, issued to Brown; U.S. Pat. No. 4,702,642, issued to Musil; U.S. Pat. No. 6,203,243B1, issued to Birtchet; and U.S. Pat. No. 5,222,829, issued to Mogler, et al., disclose screed adjustment arrangements. 
   Accordingly, there exists a need for a reliable, and easy to use screed adjustment system for use on paving machines. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide an adjustable screed system for a paving machine. 
   Another object of the present invention is to provide an adjustable screed system having means for allowing a main screed and an extension screed to be simultaneously adjusted with respect to one another through use of a single adjustment means. 
   Yet another object of the present invention is to provide an adjustable screed system having a mechanical link between a main screed and an extension screed for allowing simultaneous adjustment of both the main and extension screeds. 
   Another object of the present invention is to provide an adjustable screed system having a single crank on each end of a main screed for allowing each end of the main screed and the extension screed to be simultaneously adjusted through use of the cranks. 
   A further object of the present invention is to provide an adjustable screed system having automated means for adjusting an extension screed with respect to a main screed. 
   A still further object of the present invention is to provide a method for adjusting a main screed and an extension screed. 
   The present invention includes, in a preferred embodiment, a mechanical linkage having a geometry designed to substantially hold the plane of the main screed and the plane of an extension screed at a generally constant relative angle with respect to one another through a predetermined range of screed depth adjustments. The linkage is configured to allow the extension screed to be raised and lowered by an amount sufficient to compensate for attack angle and elevation changes in the main screed. In particular, the linkage compensates for the effect of the radial distance differences from the axis of rotation of the tow bars (connected to the tractor) of the main screed and extension screed as the main screed pivots around the tow point of the tow bars for different asphalt mat depths. 
   In another embodiment, the present invention includes powered actuators, such as hydraulic or pneumatic cylinders, motorized mechanisms, etc., together with sensors, which effectively sense the position of the extension screed with respect to the main screed and automatically compensate for changes in position of the main screed to properly orient the extension screed with respect to the main screed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing, as well as other objects of the present invention, will be further apparent from the following detailed description of the preferred embodiment of the invention, when taken together with the accompanying specification and the drawings, in which: 
       FIG. 1  is a perspective view of a paving machine having an adjustable screed system constructed in accordance with the present invention, showing a screed extension in an extended position; 
       FIG. 2  is a perspective view of the adjustable screed system illustrated in  FIG. 1 ; 
       FIG. 3  is a plan view of an adjustable screed system constructed in accordance with the present invention; 
       FIG. 4  is a side elevational view of an adjustable screed system constructed in accordance with the present invention; 
       FIG. 5  is a side elevational view of an adjustable screed system constructed in accordance with the present invention, in a paving position; 
       FIG. 6  is a side elevational view of the adjustable screed system shown in  FIG. 5  showing asphalt in engagement with a transport auger; 
       FIGS. 7 and 8  are dimensioned views of an adjustable screed system constructed in accordance with the present invention; 
       FIGS. 9A through 9C  are side elevational views of an adjustable screed system constructed in accordance with the present invention shown in use paving asphalt mats of varying thicknesses; 
       FIG. 10  is a perspective view of a tow bar arrangement constructed in accordance with the present invention; and 
       FIG. 11  is a schematic representation of an alternate embodiment adjustable screed system constructed in accordance with the present invention, which uses powered actuators for adjusting the respective heights and angles of attack of a main screed and an extension screed. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The accompanying drawings and the description which follows set forth this invention in its preferred embodiment. However, it is contemplated that persons generally familiar with paving equipment and techniques will be able to apply the novel characteristics of the structures illustrated and described herein in other contexts by modification of certain details. Accordingly, the drawings and description are not to be taken as restrictive on the scope of this invention, but are to be understood as broad and general teachings. 
   Referring now to the drawings in detail, wherein like reference characters represent like elements or features throughout the various views, the adjustable screed system of the present invention is indicated generally in the figures by reference character  10 . 
   Referring now in more detail to the embodiment chosen for the purposes of illustrating the present invention,  FIG. 1  illustrates a paving machine, generally P, having a tractor portion, generally T, and a trailing, screed portion  10  incorporating the present invention. Screed portion  10  includes a main screed, generally  12 , and extension screeds, generally  14  and  16 . Extension screeds  14 ,  16  are configured for moving inwardly and outwardly with respect to main screed  12 , and are shown in an extended position, in FIG.  1 . Hydraulic cylinders  18 , are connected both to main screed  12  and extension screeds  14 ,  16  and serve to extend and retract extension screeds  14 ,  16  with respect to main screed  12 . 
   Screed system  10  is towed by tractor T during use, and screed system  10  is connected to tractor T by tow bars, or, arms, generally  20 ,  22 . Tow arms  20 ,  22  are allowed to pivot with respect to tractor T about a pivot axis via receivers  23  on conventional pivot pins (not shown) on each side of tractor T, only one of which being illustrated in FIG.  1 . Tow arms  20 ,  22  are connected to screed  12  by pins  58 , and pivot with respect thereto. 
   Also connected to screed  12  are two actuator, or, crank arrangements, generally C, one crank  30  being positioned on one end of screed  12 , and the other crank  32  being positioned on the other end of screed  12 . Rotation of cranks  30 ,  32  control the paving depth of main screed  12 , and in the present invention, also control the paving depth of extension screeds  14 ,  16  in a manner to be described in more detailed below. 
     FIG. 2  illustrates screed system  10  as detached from tractor T. Cranks  30 ,  32  are carried for rotation in journals  34  which are carried on plates  36  attached to tow arms  20 ,  22  and to frame members  38 ,  40 . Tow arms  20 ,  22  include lower portions  42 ,  44  which are fixedly attached to screed  12 . Lower portions  46 ,  48  of frame members  38 ,  40  are likewise fixedly connected to screed  12 . 
   Cranks  30 ,  32  each include a threaded rod  50  which threadingly engages a cross member  52  which is pivotally attached between link members  54 ,  56 , which are connected to pin  58  ( FIG. 4 ) for pivotal movement with respect to screed  12 . 
   Accordingly, rotation of crank  30  and/or  32  causes corresponding movement of links  54 ,  56  with respect to main screed  12  and tow arms  20 ,  22 , and such turning of cranks  30 ,  32  causes a corresponding change in the angle of attack of main screed  12  and extension screeds  14 ,  16 , which are slaved to the movement of main screed  12 . 
   Connecting links  60 ,  62  are pivotally connected to a pin  63  ( FIG. 5 ) in links  54 ,  56 , respectively, at one end thereof. The other end of connecting links  60 ,  62  are connected to a shaft  64  on each of extension screeds  14 ,  16  and serve to support extension screeds  14 ,  16  with respect to screed  12 , and also to alter and maintain changes in the angle of attack of extension screeds  14 ,  16  with respect to main screed  12 , when cranks  30 ,  32  are selectively rotated. 
   Generally L-shaped link members- 68  also connect extension screeds  14 ,  16  to main screed  12 . Links  68  include openings  70  for receipt of a shaft  72  carried in each of extension screeds  14 ,  16 . Links  70  also include a bore  74  for receipt of a pivot pin  76  and also a bore  78  for receipt of shaft  64 . Each of bores  70 ,  74 , and  78  allow for pivoting of links  68  with respect to shaft  72 , pin  76  (FIG.  5 ), and shaft  64 , respectively, to allow adjustment of extension screeds  14 ,  16  with respect to main screed  12  in proper relationship, as necessary during paving. It is noted that shafts  64 ,  72  act as guide rods for extension of screeds  14 ,  16 , which themselves are carried in housings, generally  79 . 
     FIG. 3  illustrates transport augers, generally A, which travel with screed extensions  14 ,  16  to transport asphalt deposited by hopper H ( FIG. 1 ) of tractor T during operation. Strike off-plates, generally  80 , may be provided at the extreme ends of extensions  14 ,  16 , for containing asphalt from spilling outward during paving. Main screed  12  includes a walkway, generally  82 , on which an operator may stand during operation of paver P. 
     FIGS. 4 and 5  illustrate screed system  10  in an operative paving position from the right side, as shown in  FIG. 3 , although it is to be understood that the description of construction and operation of the right side of screed system  10  also applies to the left side as well. In particular,  FIG. 5  illustrates an asphalt mat, generally M, having been formed as screed system  10  is pulled forward, in a direction to the right, shown in FIG.  5 . Main screed  12  includes a screed plate, generally  84 , having a curved leading edge portion, generally  86 . Extension screed  14  also includes a screed plate, generally  88 , having a curved leading edge portion, generally  90 . 
   As shown in  FIG. 6 , asphalt  92  has been deposited by hopper H and has been transported by auger A to position along the leading edge  90  of extension screed  14 . As can be seen from  FIG. 6 , both screeds  12  and  14  are angled upwardly from trailing edges  94 ,  96 , respectively thereof, and this upward angle is known as the “angle of attack” of screeds  12 ,  14 . As tractor T moves forwardly, asphalt  90  is received beneath leading edges  90  and  86  of screeds  14 ,  12 , respectively, and due to the weight of screed system  10  and the downwardly inclined angle of screed plates  84  and  88 , asphalt  90  is compacted and compressed to ultimately form asphalt mat M, as trailing edge  94  of screed  12  passes over asphalt  90 . 
   EXAMPLE 
     FIG. 7  illustrates certain example dimensions of one preferred embodiment of the present invention, for a screed system  10  constructed of steel and weighing of approximately lbs, such dimensions shown in tabular form below: 
   
     
       
             
             
             
           
             
             
             
           
         
             
                 
                 
             
             
                 
               Reference 
               Dimension 
             
             
                 
               Character 
               In Inches 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               a 
               6.02 
             
             
                 
               b 
               12.00 
             
             
                 
               c 
               7.55 
             
             
                 
               d 
               7.57 
             
             
                 
               e 
               14.50 
             
             
                 
               f 
               1.00 
             
             
                 
               g 
               7.25 
             
             
                 
               h 
               12.00 
             
             
                 
               i 
               8.25 
             
             
                 
               j 
               8.20 
             
             
                 
               k 
               6.02 
             
             
                 
               l 
               8.25 
             
             
                 
                 
             
           
        
       
     
   
     FIG. 8  illustrates dimensions for the same embodiment shown in  FIG. 7 , except that in the  FIG. 8  illustration, the angle of attack of screeds  12 ,  14  is substantially zero. 
   
     
       
             
             
             
           
             
             
             
           
         
             
                 
                 
             
             
                 
               Reference 
               Dimension 
             
             
                 
               Character 
               In Inches 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               a′ 
               6.028 
             
             
                 
               b′ 
               12.000 
             
             
                 
               c′ 
               7.550 
             
             
                 
               d′ 
               5.750 
             
             
                 
               e′ 
               14.250 
             
             
                 
               f′ 
               1.000 
             
             
                 
               g′ 
               7.250 
             
             
                 
               h′ 
               12.000 
             
             
                 
               i′ 
               8.250 
             
             
                 
               j′ 
               8.22 
             
             
                 
               k′ 
               6.028 
             
             
                 
               l′ 
               8.250 
             
             
                 
                 
             
           
        
       
     
   
   While the foregoing examples set forth specific dimensions of components of the present invention, it is to be understood that the present invention is not to be limited to such embodiment and dimensions, and that the present invention could be in a variety of other configurations, in accordance with the teachings and disclosure of the invention herein. 
     FIGS. 9A ,  9 B, and  9 C illustrate screed system  10  in use paving asphalt mats M of varying thickness. For example, in  FIG. 9A , the asphalt mat M could be approximately three-quarters of an inch. Note the relatively slight angle of attack of screeds  12  and  14 . Note also that link  60  is generally horizontal and that the back edge  98  of link  54  is substantially vertical. Similarly, upstanding leg  100  of link  68  is generally vertical. 
   In  FIG. 9B , the operator has rotated crank  30  in order to increase the angle of attack of screed  12 . This single adjustment by the operator simultaneously causes a corresponding change in the angle of attack of screed  14  such that the angular relationship of extension screed  14  with respect to screed  12  is maintained. Note also from  FIG. 9B  that link  68  has moved upward slightly due to the increase in the angle of attack. An example mat thickness as shown in  FIG. 9B  could be two inches. 
   In  FIG. 9C , a mat thickness of approximately five inches is being formed. The operator, at this point, has further rotated crank  30  to further increase the angle of attack of screeds  12  and  14 . Note that the lower portion  42  of link  68  has risen further, and that the back edge  98  of link  54  is now inclined rearwardly. Also, leg  100  of link  68  is now rearwardly inclined, and the forward end  102  of link  60  is now downwardly inclined. Through the range of motion shown in  FIGS. 9A through 9C , the angle of attack relationship between screeds  12  and  14  remains generally constant such that the mat M formed by screed  14  is at essentially the same height as mat M formed by screed  12 , thereby eliminating any discontinuity, or step, between the mats, and also, the mats formed by screeds  12  and  14  have essentially the same surface finish. 
     FIG. 10  illustrates tow arm structure  20  and the connection of crank  30  via plate  104  thereto. 
   The distance between the two pivot pins  58 ,  76  of tow arms  20 ,  22  is preferably matched to the distance between the two pivot pins  63 ,  64  of upper link  60 . This creates parallel motion between main screed  12  and extension screed  14  when screed  12  is moved by crank  30 . The bottom of extension screed  14  is mounted to housing  79  at the desired angle of attack for optimum paving performance relative to the main screed  12  during initial set up. Thus, the parallel motion of the linkage  60  and the link formed by the lower portion of arm  20  between the two pivot pins  58 ,  76  keep the angular relationship between extension screed  14  and main screed  12  substantially the same through the normal range of paving depths. 
   The main screed  12  trailing edge  94  is preferably located a slight distance behind the center of the rear pivot pin  58  of tow arm  20 . The trailing edge  96  of screed  14  is a significantly further distance forward of the front pivot pin  76  of tow arm  20 , and this difference allows for relative motion of the trailing edge  94  of the main screed with respect to the trailing edge  96  of the extension screed  14 . The direction and amount pf pivotal motion of the screeds per degree of attack angle depends on the tow arm length, the weight of the screed, the physical dimensions of the main screed and extension screed plates, and the position of extension screed  14  with respect to the attack angle of main screed  12 . Matching the amount of motion per degree of angle change in the main screed  12  allows the screed extension  14  to be matched to the main screed  12  with minimal error through a reasonable range of paving depths, for example, from three-quarter inches to six inches in depth, without requiring the operator to make any adjustments to correct the extension screed  14  elevation. 
     FIG. 11  illustrates an alternate embodiment of the present invention. Adjustable screed system  110  does not require the linkages discussed above with respect to screed system  10 . Instead, sensors  120 ,  122  (which can be of conventional design) are provided on extension screed  14 ′ and main screed  12 ′. Sensors  120  and  122  are positioned near the trailing edge of each of screeds  14 ′,  12 ′ to determine the elevation of the mats M′ formed respectively by screeds  14 ′ and  12 ′. Sensors  120 ,  122  output to a conventional controller, generally  126 , such as a programmable logic controller (PLC), microprocessor, or the like, with the respected mat elevation data. The controller  126  includes a set point adjustment and feedback control loop which, in response to the data input from sensors  120 ,  122 , provides a signal to an actuator, generally  130 , such as a reversible motor, stepping motor, or the like, which drives an adjusting rod  132  to raise or lower screed extension  14 ′ with respect to main screed  12 ′. Alternately, instead of using an actuator  130 , controller  126  could output a correction signal, to an output, such as a meter, LED readout, an audible signal, etc. (none shown) to the operator, the operator could then adjust a crank (not shown) accordingly, to selectively adjust the height of extension screed  14 ′. 
   The present invention is not limited to the mechanical linkages illustrated or described herein, and linkages of a variety of other configurations could be designed without departing from Applicant&#39;s invention. The present invention provides the operator with one less operation to monitor and control while also improving the surface of the asphalt mat formed. Further, the present invention of automatically compensating for screed extension height can be accomplished on existing screed designs, without using a linkage system, but instead by incorporating controls which are added to the extension and a powered actuator for adjusting the extension depth. The powered actuator, depth sensors and controllers, could extend the present invention to application on conventional pavers, while achieving substantially the same end result of the present self-compensating screed extension disclosed herein. 
   While preferred embodiments of the invention have been described using specific terms, such description is for a present illustrative purposes only, and it is to be understood that changes and variations to such embodiments, including, but not limited to, the substitution of equivalent features or parts, and the reversal of various features thereof, may be practiced by those of ordinary skill in the art without departing from the spirit or scope of the present disclosure.

Summary:
A mechanical linkage having a geometry designed to substantially hold the plane of a main screed and the plane of an extension screed at a generally constant relative angle with respect to one another through a predetermined range of screed depth adjustments. The linkage is configured to allow the extension screed to be raised and lowered by an amount sufficient to compensate for angle of attack and elevation changes in the main screed. In particular, the linkage compensates for the effect of the radial distance differences from the axis of rotation of tow bars of the main screed and extension screed as the main screed pivots with respect to the tow point of the tow bars for different asphalt mat depths. An alternate embodiment includes actuators, together with sensors, for sensing the position of the extension screed with respect to the main screed and automatically compensating for changes in main screed position to properly orient the extension screed with respect to the main screed.