Abstract:
A paving machine employs an electrically heated screed assembly to uniformly heat a screed plate of the machine. Uniform heating is achieved by inserting a thermally conductive plate between electrical heating elements and the screed plate. An insulation layer may be provided above the heating elements to direct the heat downward into the thermally conductive plate. The heat spreads relatively uniformly throughout the thermally conductive plate, thereby uniformly heating the screed plate. A clamping mechanism is also provided that, when tightened, provides a compressive force, thereby holding the assembly in place. When released, the pressure is alleviated, thus permitting a heating element to be removed for repair or replacement without the need to remove the screed plate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to paving machines and, more particularly, relates to an improved method and apparatus for uniformly heating a screed plate of a paving machine by providing a conductive plate between an electrical heating element and the screed plate, and for providing a clamping mechanism that permits the electrical heating element to be easily replaced without the need to remove the screed plate. 
     2. Discussion of the Related Art 
     Paving machines are well known for working paving materials into a mat to produce roads and other paved structures. Specifically, the typical paving machine transports paving materials from a hopper along a conveyor system and ultimately to a distributing auger, where the paving materials are distributed onto a roadway or another surface, where a screed plate then paves the paving materials into a mat. While the paving materials could be any of various known materials, hot mix asphalt (HMA) is commonly used and, for the sake of convenience, the paving materials will hereinafter be referred to as HMA. 
     The screed plates of HMA paving machines are typically preheated to a temperature of about 200° F. to 300° F. before paving commences and are maintained at this temperature during paving to prevent the hot asphalt being leveled by the screed plate from congealing on the face of the screed plate. Screed plates have traditionally been heated by oil or gas burners mounted above the screed plate such that the flames from the burners impinge sheet metal plates on top of the screed plate. Such burners supply intense heat to localized portions of the screed plates which results in uneven heating and congealing of the HMA onto the screed plate. Additionally, if the process is not carefully controlled, the screed plate may warp and become ineffective. Furthermore, as the flames become progressively dirtier, noxious fumes are emitted for the operator to contend with. 
     Systems have been proposed which are designed to avoid or to at least alleviate some of the problems associated with traditional screed heaters. In one such system, a heater heats the screed plate of a paving machine via heat transfer from heating oil stored in a low pressure reservoir mounted directly on top of the screed plate. Oil in the reservoir is drawn from the reservoir, pressurized by a high pressure pump, and then fed through a pressure release valve or other suitable flow restrictor which creates a pressure drop in the range of about 700 to 800 psi, thereby heating the oil to a temperature of about 275° F. The thus-heated oil is then returned to the reservoir for heat transfer to the screed plate. 
     This heated oil system suffers from several drawbacks and disadvantages. Most notably, the large pressure drops needed to provide the necessary heating require that the heating oil be pressurized by a pump to a relatively high pressure in the range of 800 to 1000 psi before undergoing the pressure drop in the flow restrictor. This requires the use of high pressure hoses and connections throughout the system, thus increasing the cost and complexity of the system and also increasing the dangers of leaks which could render the system ineffective. Moreover, if for any reason the pump and relief valve are not capable of providing a sufficiently large pressure drop to adequately heat the oil, the system then becomes incapable of boosting the oil temperature to the required level. 
     It therefore became desirable to develop a screed plate heating system that involves no moving parts, runs clean, emits no noxious fumes, and is capable of uniformly heating the screed plate. 
     One known system that strives to meet at least some of these goals involves the installation of electrical heating elements that are in direct contact with the screed plate to heat the screed plate. Being electrically powered by a sufficiently sized generator, this system does not have the disadvantages associated with combustion, and also ensures that sufficient energy is supplied to the electrical heating elements so that the screed is adequately heated. Furthermore, the generators associated with the electrically heated systems allow the use of higher-wattage lights than the conventional twelve-volt lights used on traditional paving machines, thus facilitating night operation. However, the direct contact between the heating elements and the screed plate gives rise to heat distribution problems similar to those encountered by oil-heated screeds. Specifically, hot spots develop on the screed plate at the point where the heating element contacts the screed plate, and the screed plate cools progressively at points more distant from the contact. This uneven heat distribution can also lead to relatively high temperature gradients, and possible warping of the screed plate. Another disadvantage arises when the electrical heating elements require either repair or replacement. In order to remove a heating element from this system, the screed plate must first be removed before an operator is able to access the heating element. This removal requirement is very time consuming and labor intensive. 
     The need has therefore arisen to provide an electrically heated screed assembly, which is capable of uniformly heating the screed plate while allowing easy access to and replacement of the heating elements without having to remove the screed plate. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is therefore a first object of the invention to provide an electrically heated screed assembly for a paving machine that heats the screed plate uniformly, thus preventing both paving material congealing and screed warping. 
     A second object of the invention is to develop an electrically heated screed assembly that allows for easy removal of the heating elements of the assembly for repair or replacement without having to remove the screed plate or otherwise disassemble the screed assembly. 
     A third object of the invention is to develop an electrically heated screed assembly that has one or more of the aforementioned advantages and that is extendible to widen the screed assembly, thus permitting paving of a wider area. 
     In accordance with a first aspect of the invention, a subframe attaches to the frame of the screed assembly. A screed plate is mounted onto the bottom of the subframe and a thermally conductive plate, such as aluminum, is disposed adjacent to the screed plate in a manner so as to span the length of the screed plate to a point just short of the midpoint of the screed plate&#39;s length. An electrical heating element, which may comprise a metallic material having a resistive coil wound inside it, is placed onto the thermally conductive plate. The heating element is wired to a power generator which supplies energy to the coil, thus heating the heating element. The thermally conductive plate then becomes uniformly heated and supplies this heat to the screed plate. The thermally conductive plate therefore effectively acts as a heating element and, because it is in thermal contact with a substantial area of the screed plate, it operates to heat the screed plate uniformly. An insulation layer may be placed above the electrical heating element to maximize the percentage of generated heat that is directed downwards toward the conductive plate and screed plate. Several electrical heating elements may be placed in strategic locations throughout the screed plate. To permit the screed plate to crown during operation, the heating elements and conductive plate preferably do not span the entire length of the screed plate. They instead span to a point short of the midpoint of the screed plate&#39;s length, and a complimentary assembly is located on the other side of the screed so as to also span to a point just short of the midpoint of the screed plate. Several rows of heating elements may be installed so that the entire screed plate is sufficiently heated. 
     In accordance with a second aspect of the invention, a clamping mechanism is installed on the heating element that, when tightened, compresses the associated heating element against the screed plate. When the clamping mechanism is loosened, the compressive force is relieved from the heating element, thus permitting an electrical heating element to be removed by an operator simply by pulling it in a longitudinal direction away from the screed assembly without first having to remove the screed plate. A new or repaired heating element may then be inserted into the system before re-tightening the clamping mechanism. 
     In a preferred embodiment, the clamping mechanism comprises a tubular beam that is placed above the insulation or, alternatively, directly above the heating element. A bracket is mounted on top of the beam and a vertical hole is created in the bracket&#39;s upper horizontal surface. Likewise, a vertical hole is formed in the upper horizontal surface of the subframe. The subframe hole is aligned with the hole in the bracket so that a bolt or other suitable threaded fastener may be inserted into both holes. 
     In one embodiment of the invention, the hole through the subframe surface is tapped and threadedly engages the bolt threads. 
     In another embodiment, a first nut is mounted on the subframe&#39;s upper horizontal surface, and the bolt is inserted into both holes. A second nut is mounted onto the bolt at a point located between the hole in the bracket and the beam. Therefore, when the bolt is tightened, the beam is lowered and provides a compressive force on the screed assembly. Conversely, when the bolt is loosened, the second nut exerts an upward force on the bracket, thus raising the beam and relieving the compressive force. The tapped hole through the subframe surface, mentioned above and preferred, achieves the same effect. 
     Additionally, a series of pusher bolts may be added to the clamping mechanism, that extend through the upper horizontal surface and contact the beam to provide uniform pressure throughout the screed assembly. 
     In accordance with a third aspect of the invention, an extension is provided that can be attached to the pre-existing screed assembly. Specifically, the extension includes a subframe having a vertical wall that is bolted onto a vertical wall of the frame of the paving machine. The extension also includes an electrical heating element and a thermally conductive plate, as well as the aforementioned clamping mechanism. This is particularly useful when an operator needs to pave a wider surface than usual. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred exemplary embodiments of the invention are illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which: 
     FIG. 1 is a side elevation view of a paving machine that incorporates an electrically heated screed assembly constructed in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a sectional side elevation view of the screed assembly of the paving machine of FIG. 1, on an enlarged scale relative thereto; 
     FIG. 3 is a sectional rear elevation view of the screed assembly with the exterior frame removed; 
     FIG. 4 is a fragmentary sectional side elevation view of one of the clamping mechanisms of the screed assembly with a cutaway portion of the frame, taken along the plane  4 — 4  in FIG.  2  and on an enlarged scale relative thereto; 
     FIG. 5 is an exploded perspective assembly view of the screed assembly; 
     FIG. 6 is an exploded perspective view of one of the heating elements and the associated clamping mechanism of the screed assembly; 
     FIG. 7 is a sectional side view of a portion of the heated screed assembly, on an enlarged scale relative to FIG. 2; 
     FIG. 8 is a sectional end elevation view of a portion of the screed assembly, taken along the plane  8 — 8  in FIG.  7  and on a slightly reduced scale relative thereto; 
     FIG. 9 is a rear elevation view showing the two halves of the screed plate of the screed assembly, on a slightly reduced scale relative to FIG. 3; 
     FIG. 10 is a fragmentary sectional side elevation view showing an extension mounted onto the screed assembly, on an enlarged scale relative to FIG. 9; and 
     FIG. 11 is an exploded perspective view of the extension. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Pursuant to the invention, a paving machine is provided which employs an electrically heated screed assembly including a screed plate and electrical heating elements that are in contact with a thermally conductive plate which is in contact with the screed plate. In this manner, the screed plate is uniformly heated. Clamping mechanisms are also installed in the screed assembly which, when loosened, allow for easy removal and replacement of the electrical heating elements without removing the screed plate. When tightened, the clamping mechanisms supply a compressive force to the heating elements, thereby preventing removal of the heating elements in the tightened state. 
     Referring to the drawings and initially to FIG. 1 in particular, a paving machine  20  is illustrated that includes a self-propelled chassis  22  on which is mounted an engine  24 ; a hopper  26 ; and a paving apparatus including a distributing auger mechanism  28  and a screed assembly  30 . The chassis  22  is mounted on two front axles  32  and rear axle  34 , receiving front steering wheels  36  and rear driving wheels  38 , respectively. The front  32  and rear  34  axles are steered and powered hydrostatically by engine  24  in a known maimer. 
     The hopper  26  preferably has a total capacity of about twelve tons to conform with industry standards and is designed to receive the paving materials  40  and to temporarily store them pending their delivery to the paving apparatus. While the paving materials  40  may comprise any known material, HMA is typically used and, for the sake of convenience, the paving materials  40  will hereinafter be referred to as HMA. A conveyor assembly  42  transports the HMA from a rear discharge opening of the hopper  26  to the auger mechanism  28  of the paving apparatus. 
     The distributing auger mechanism  28  of the paving apparatus may be any conventional mechanism and, in the illustrated embodiment, is of the type employed by the paving machine manufactured by Roadtec of Chattanooga, Tenn. under the Model No. RP  180-10.  The distributing auger mechanism  28  thus includes a hydrostatically driven bolt-type distributing auger extending transversely across the chassis  22  and mounted on a slide (not shown) which is raiseable and lowerable with respect to a stationary frame. 
     The screed assembly  30  comprises a pair of transversely spaced apart tow arms  44  (only one of which is shown in FIG.  1 ), and a heated (and preferably vibratory) screed plate  46  pivotally suspended from the rear ends of the tow arms  44 . Each tow arm  44  is raiseable and lowerable with respect to the chassis  22  at its front end via a first hydraulic cylinder (not shown) and at its rear end via a second hydraulic cylinder  48 . The front of each of the tow arms  44  is also pivotally connected to the chassis  22  at a tow point, formed from a bracket assembly, so as to permit vertical adjustment of the screed assembly  30  using the hydraulic cylinders mentioned. 
     In operation, the paving machine  20  is positioned on the surface to be paved  50 , and the hopper  26  is filled with the preferred paving material  40 , HMA. The conveyor assembly  42  then is activated to transport the HMA to the paving apparatus. An operator (not shown), when seated at a station or console  52 , then controls the engine  24  to propel the paving machine forward, in the direction of the arrow shown in FIG.  1 . Paving is commenced by discharging HMA  40  from the hopper  26  to the distributing auger  28 , which then remixes and distributes the HMA  40 . The screed assembly  30  then works the HMA into a mat  54  on the paving surface  50 . 
     Referring now also to FIG. 2, the screed assembly  30  further includes a main frame  56  and a subframe  58  mounted on the bottom of the main frame  56 . A screed plate  46  is then mounted on the bottom of the subframe  58 , thereby providing the foundation for the installation of the heating elements  60 . The screed plate  46  is covered by, and is in direct contact and thermal communication with, a thermally conductive plate  62 . The thermally conductive plate  62  is in thermal communication with the screed plate  46 , preferably by direct contact. In the present embodiment, the thermally conductive plate  62  is formed from aluminum, but it should be noted that any suitable thermally conductive material would suffice. 
     Turning next to FIG. 5, it will be noted, when the thermally conductive plate  62  is placed onto the screed plate  46 , that studs  64  in the screed plate  46  are able to extend through corresponding holes  66  in the thermally conductive plate  62 , enabling the plates  62  and  46  to be fixed to the subframe  58 . This manner of assembly not only fixes the thermally conductive plate  62  to subframe  58  but also prevents relative movement with respect to the screed plate  46 . A plurality of heating elements  60  are disposed directly above the thermally conductive plate  62 , and an additional insulation layer  68  is disposed between the subframe  58  and the conductive plate  62 . Each heating element  60  is held in place by a dedicated clamping mechanism  74 , as is shown in FIG.  8 . 
     Referring back to FIG. 2, the several illustrated electrical heating elements  60  are shown as being arranged in four rows of laterally-disposed heating elements  60 , so spaced longitudinally relative to the front and back of the paving, machine  20  (FIG. 1) as to effectively span the width and a major portion of the length of the screed plate  46 . 
     As shown in FIGS. 3 and 8, each row of heating elements includes two electrical heating elements  60 , disposed on opposite lateral sides of the screed plate  46 , in a known maimer, to form a gap midway along the length of the screed plate  46 , thereby allowing the screed plate  46  to crown during operation. Of course, the number and location of heating elements  60  may vary depending on, for example, the size of the screed plate  46 . 
     In this embodiment, each heating element  60  comprises a rigid hollow bar of steel or another metallic material having a resistive coil wound inside it that heats when energized, as is well known in the art. The heating element is wired to an electric generator (not shown) by lead wires  70  (shown in FIGS. 3 and 8) in a known, conventional manner to supply energy to the coil. The generator also provides additional power for high voltage lighting, thus facilitating night operation. An insulation layer  72  (shown in FIGS. 7 and 8) is disposed directly above the electrical heating elements  60  to inhibit heat transfer to the associated clamping mechanisms  74  (detailed below), thereby maximizing the transfer of energy downwards toward the thermally conductive plate  62  and screed plate  46 , thus increasing the efficiency of the system. While the insulation layers  68  and  72  are not essential for the operation of the present invention, their nonuse will decrease the efficiency of the system. It must also be noted that the thermally conductive plate  62  is not necessary to comply with all aspects of the invention, but it is implemented in this embodiment to supply heat uniformly to the screed plate  46 . If the thermally conductive plate is not used, the electrical heating elements  60  will be in direct contact with the screed plate  46 , and a higher number of more closely spaced heating elements would likely be employed. 
     Turning next to FIGS. 7 and 8, each clamping mechanism  74  is seen to include a pair of clamps  76  located at both ends of a tubular beam  78 , which is mounted above the insulation layer  72 . Mounted directly to the underside surface of the beam  78  is a bent plate  95  that is designed to captively retain the insulating layer  72  and electrical heating element  60  relative to conductive plate  62  when the clamps  76  are tightened. 
     As shown in FIG. 7, one such clamp  76  can be used to pull the beam  78  upwardly (as depicted by the arrow) away from the heating elements  60 , when loosened, thereby allowing the heating elements  60  to be easily removed from the paving machine, as desired, without first removing the screed plate  46 . 
     As shown in FIGS. 2-4 and  7 , the clamps  76  are seen to exert a downward force on the beam  78  when tightened, thereby providing a compressive force to the associated heating element  60 . Each clamp  76  preferably includes a bracket  80  that is welded to or otherwise mounted on beam  78 . Both the subframe  58  and bracket  80  (FIGS. 2 and 4) comprise horizontal surfaces  92 ,  94 , respectively, shown in FIG. 4, in which aligned holes  82 ,  84  exist, respectively, as shown in FIGS. 5 and 7. 
     In the illustrated embodiment of FIG. 4, a nut  86  is shown as welded to or otherwise mounted on the underside of the hole  82  in the subframe  58 . To achieve the same effect, the illustrated hole  82  may be tapped in a known manner to form a threaded hole through the upper surface of subframe  58 . 
     As shown in FIG. 4, a bolt  88  is inserted through the hole  82  in the subframe  58  and accompanying nut  86  and is further inserted through the hole  84  in the bracket  80 . If holes  82  of the subframe  58  are tapped, as noted above, the nuts  86  will not be necessary if the threads of hole  82  mesh with the threads of bolt  88 . 
     Once the bolt  88  is inserted into the bracket  80 , a nut  90  is mounted onto the bolt  88  at a point between the bracket  80  and the beam  78 . As shown in FIGS. 4 and 7, the bolt  88  may then be tightened relative to subframe  58  until such bolt  88  buttresses up against beam  78 . After that, a nut  90 , threadedly engaging bolt  88  between surface  94  and tubular beam  78  (as shown in FIG.  4 ), may be raised along the bolt  88  until such nut  90  is closely adjacent the underside of the horizontal surface  94  of the bracket  80 , after which such may then be fixed to the bolt  88  using a spring pin (not shown) or any other known method of fastening. 
     In this manner, when clamp  76  is tightened, the downward movement of each beam  78  provides a compressive force to associated heating elements  60 . Conversely, when the clamp  76  is loosened, the nut  90  exerts an upwards force on the bracket  80 , thus raising the beam  78  away from the associated electrical heating element  60 . Once the beam  78  is raised, the electrical heating element  60  can be removed by sliding it in a longitudinal direction that is generally parallel to the beam  78  until it is free from the system, as shown in FIG. 8. A second electrical heating element may then be installed by sliding it into the system in a direction generally parallel to the beam  78 . Alternatively, the electrical heating element  60  may be repaired and reinstalled into the assembly  30 . Note that the screed plate  46  is not removed during this process. Referring to FIG. 7, the clamping mechanism  74  on the left is shown in the open position while the remaining clamping mechanisms  74  are shown to be tightened. 
     Referring to FIGS. 6 and 8 optional pusher bolts  96  are installed at spaced-apart locations longitudinally between the clamps  76  in accordance with the preferred embodiment of the invention. These pusher bolts  96  function to provide uniform compression to the beam  78  if the beam  78  is sufficiently long that the clamps  76  alone might not adequately compress the heating elements  60 . The number of necessary pusher bolts  96  is indicative of the length of the associated beam  78 . Thus, if the beam  78  is sufficiently short, no pusher bolts  96  will be necessary. If necessary, one such pusher bolt  96  can be installed by drilling a vertical hole  97  in the subframe  58 . Preferably, the hole  97  is tapped in a known manner so as to have threads that mesh with the threads of pusher bolt  96 . As still another an alternative embodiment, nut  86  may be welded to or otherwise mounted on the underside surface of subframe  58 . In the illustrated alternative embodiment, the pusher bolt  96  is shown to be inserted through the hole  97 , threaded through the nut  86  and, when tightened, buttressed up against the beam  78 . Further tightening of the pusher bolts  96  compresses the associated electrical heating element  60 , thereby holding the heating element  60 . Note that if the pusher bolts  96  are installed, they are first loosened before the clamps  76  are raised. 
     Lateral clamps  98 , best seen in FIGS. 2,  4  and  6 - 8 , are also integrated into each clamping mechanism  74  (FIGS. 6 and 7) to prevent the clamping mechanism  74  from collapsing while the bolts  88  and  96  are tightened against the beam  78 . Otherwise, the compressive force from clamp  76  and pusher bolts  96  could cause the base of beam  78  to slip out from underneath of the screed assembly  30 . Each lateral clamp mechanism  98  is seen to include: 1) a hole  104  (FIGS. 2 and 7) in a vertical surface  100  of subframe  58 ; 2) vertical slots  102  (FIG. 6) in the side walls of the beam  78  that are laterally aligned with the holes  104  in the subframe  58 ; and 3) a bolt  114  inserted into the slots  102  so as to extend into hole  104 . Nuts  105  and washers are installed as shown (FIG. 6) to secure the beam&#39;s lateral position with respect to the subframe  58 , as shown in FIGS. 7 and 8. The vertical slots  102  in the beam  78  (FIG. 4) permit beam  78  to be raised and lowered, as desired, during operation of the clamping mechanism  74 , as shown in FIGS. 7 and 8. In this manner, lateral movement of the clamping mechanism  74  is fixed with respect to the subframe  58 . 
     It must be further noted that while a clamping mechanism  74  in accordance with the preferred embodiment has been described, any clamping device that can be loosened to permit the easy removal of the electrical heating element without removal of the screed plate  46  may be used. 
     Turning now to FIGS. 9,  10 , and  11 , a lateral extension component  106  having an electrically heated screed assembly  130  (FIGS. 9 and 10) is shown connected to the above-described screed assembly  30  by bolts  108  (FIG. 9) extending though holes in a side wall  110  (FIG. 10) of main frame  56  and through mating holes in a corresponding side wall  112  of frame member  156  of extension  106 . The extension  106  is particularly useful when a wider surface area than normal is to be paved. Extension  106  comprises a screed plate  146  with studs  164  (FIG. 11) extending through holes  166  in a conductive plate  162  that are fixed to holes  192  in the frame  156 . Electrical heating elements  160  and insulation layers  172  are positioned above the thermally conductive plate  162 . The extension  106  is shown further to include two clamping mechanisms  174  (FIG.  9 ), one of which is provided for each heating element  160 . Each clamping mechanism  174  (FIG. 11) includes a beam  178  and two clamps  176 . An alternative embodiment may further include a bent plate (not shown), as previously described above in connection with FIGS. 2,  4  and  6 - 8 . Each illustrated clamp  176  (FIGS. 10 and 11) is seen to include bolts  188 , brackets  180 , and nuts  190 . Also as mentioned above, bolts  188  can threadedly engage threaded holes in the frame  156 , or may threadedly engage nuts  186 . As shown in FIG. 10, the clamping mechanism  174  on the left is loose while the clamping mechanism  174  on the right is tightened, as previously described. However, in the extension  106 , the clamping mechanism  174  and heating elements  160  extend laterally of the above-described screed assembly  30 . Note also that the heating elements  160  and clamping mechanisms  174  are sufficiently short that pusher bolts (none shown) are accordingly not needed to ensure that the heating elements  160  are sufficiently compressed, nor are lateral clamps necessary. 
     A method of assembling the screed assembly  30  will now be described. First, the clamping mechanism is assembled as follows. Such assembly includes positioning the subframe  58  and brackets  80  in a manner such that their respective holes  82  and  84  are aligned, and securing the brackets  80  to subframe  58  using bolts  88  and nuts  90 , as shown in FIG.  7 . Next, the conductive plate is placed on top of the screed plate  46 , as shown in FIG.  3 . Then, with the conductive plate  62  on screed plate  46  (as shown in FIG.  7 ), with the heating elements  60  placed on the top of conductive plate  62  in spaced-apart fashion (see, e.g., FIGS.  5  and  7 ), and with insulation layers  72  longitudinally placed on top of corresponding associated heating elements  60  (see, e.g., FIGS.  6  and  7 ), the clamping bar portion of the clamping mechanism  74  is sub-assembled by first aligning bolts  114  with opposite slots  102  through beam  78 , then passing the bolts  114  through the holes  102 , and using nuts  105  and washers (as shown in FIG. 6) in a known manner, to position the plural (or several) tubular beams  78  (shown in FIG. 5) onto subframe  58  relative to the insulation layers  72  mounted on the heating elements  60  that have been placed on plate  62 , as shown in FIG.  7 . Next, the heating elements  60  with insulation layers  72  on top are together laterally slid inwardly, as can be appreciated by referring to FIG.  8 . Finally, the several bolts  88  are separately rotated about their longitudinal axes in a known manner relative to subframe  58 , to cause the tubular beam  78  to move toward the screed plate  46 . The several heating elements  60  and corresponding supermounted insulation layers  72  are separately longitudinally aligned with the bent plate  95  of each respective tubular beam  78  (see, e.g., FIG. 6) before each tubular member  78  is brought into abutting engagement with a respective insulation layer  72 , for purposes of fixedly urging the insulation layer against conductive plate  62 , as shown in FIG.  7 . While the longitudinal axes of bolts  88  are preferably disposed perpendicular to upper horizontal surface of subframe  58 , those skilled in the art can appreciate that bolt orientation that is somewhat offset from the perpendicular may, on occasion, be desirable for certain design purposes. Such bolt orientation is within the scope of the present invention. 
     To remove an electrical heating element  60 , the associated pusher bolts  96  are first loosened, and the bolts  88  of the two clamps  76  are also then loosened to raise the beam  78 . The electrical heating element  60  to be replaced or repaired is then removed by sliding it longitudinally out of the screed assembly  30 . A second heating element may then be inserted into the assembly  30  by sliding it in longitudinally above the thermally conductive plate  62 . If necessary, the insulation layer  72  may be placed on top of the replacement heating element before insertion. Once the new heating element  60  is in place, the clamping mechanism  74  is tightened to lower the beam  78  onto the heating element  60 , thus rendering the system operational. Note that replacement of the heating element  60  takes place without removal of the screed plate  46 . 
     Many changes and modifications may be made to the invention without departing from the spirit thereof. The scope of these changes will become apparent from the appended claims.