Heat Transfer for Asphalt Screed Plates

TECHNICAL FIELD

[0001] The present disclosure relates to asphalt paving machines, and more particularly to an electrically heated screed arrangement.

BACKGROUND

[0002] The laying of asphalt paving material on road surfaces entails spreading paving material consisting of an aggregate filled bituminous mixture on a prepared roadbed. The paving material is spread while hot and is then compacted so that upon cooling a hardened pavement surface is formed. Conventional paving machines utilize a heavy assembly termed a “screed” that is drawn behind the paving machine. The screed includes a replaceable screed plate that is constructed of a suitable steel, to spread a smooth even layer of paving material on the prepared roadbed. The weight of the screed assembly aids to compress the paving material and perform initial compaction of the paving material layer. Screed assemblies can include vibratory mechanisms placed directly on the screed plate or separate vibratory tamper bars connected in tandem with the screed plate to aid in the initial compaction of the paving material.

[0003] To facilitate laying of the paving material, the screed is typically heated, to a temperature in the range of about 82° to 171° C. (180° to 340° F.). Heating the screed assists the paving material in flowing under the screed and reduces adhesion of the paving material to the screed. If the screed is not adequately heated, the bituminous mixture contacts the bottom of the screed and begins to harden, resulting in buildup of paving material and excessive drag. Furthermore, preheating of the screed results in a more uniform texture longitudinally along a paved mat.

[0004] US 6,981,820 describes the use of thermal grease on a screed plate to aid in conduction. Such an arrangement results in inefficient heat transfer to the screed plate.

SUMMARY

[0005] One aspect of the present disclosure is directed to a screed plate for a paving mat, the screed plate comprising: an elongated flat plate having a screed plate upper surface and a screed plate lower surface, the screed plate lower surface being configured to be in contact with the paving mat; a radiused corner comprising a forward leading edge of the screed plate; and a trailing edge of the screed plate, the trailing edge being disposed opposite the forward leading edge; and a thermally conductive pad having a thermally conductive pad upper surface and a thermally conductive pad lower surface, the thermally conductive pad lower surface being disposed on the screed plate upper surface so as to facilitate heat transfer from the thermally conductive pad to the elongated flat plate, wherein the elongated flat plate, radiused corner, forward leading edge, and trailing edge comprise a first material, wherein the thermally conductive pad comprises a second material different than the first material.

[0006] Another aspect of the present disclosure is directed to an apparatus for transferring heat to a paving mat, the apparatus comprising: at least one electric heating element configured to generate heat; a screed plate having a screed plate upper surface and a screed plate lower surface, the screed plate lower surface being configured to be in contact with the paving mat, the screed plate comprising a first material; a thermally conductive pad having a thermally conductive pad upper surface and a thermally conductive pad lower surface, the thermally conductive pad lower surface being disposed on the screed plate upper surface so as to transfer heat from the thermally conductive pad to the screed plate, the thermally conductive pad comprising a second material different than the first material; and an adapter plate having an adapter plate upper surface and an adapter plate lower surface, the adapter plate lower surface being disposed on the thermally conductive pad upper surface so as to transfer heat from the adapter plate to the thermally conductive pad, the adapter plate comprising a third material different than the first material and the second material, wherein the at least one electric heating element is in thermal contact with the adapter plate upper surface so as to transfer heat from the at least one electric heating element to the adapter plate.

[0007] A further aspect of the present disclosure is directed to a system for heating a paving mat, the system comprising: a screed heating arrangement, comprising: at least one electric heating element configured to generate heat, and at least one retaining mechanism; and a screed assembly, comprising: a screed plate having a screed plate upper surface and a screed plate lower surface, the screed plate lower surface being configured to be in contact with the paving mat, a thermally conductive pad having a thermally conductive pad upper surface and a thermally conductive pad lower surface, the thermally conductive pad lower surface being disposed on the screed plate upper surface, and an adapter plate having an adapter plate upper surface and an adapter plate lower surface, the adapter plate lower surface being disposed on the thermally conductive pad upper surface, wherein the screed heating arrangement is attachable to the screed assembly by attaching the at least one retaining mechanism to the adapter plate, and wherein heat generated by the at least one electric heating element is transferred to the paving mat through the adapter plate, thermally conductive pad, and screed plate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 shows an exemplary asphalt paving machine towing a screed assembly;

[0009]FIG. 2 shows a top view of the screed assembly of FIG. 1;

[0010]FIG. 3 shows an enlarged end view of a screed assembly and a screed heating arrangement;

[0011]FIG. 4 shows a top view of a screed plate;

[0012]FIG. 5 shows a bottom view of the screed plate of FIG. 4;

[0013]FIGS. 6 shows another view of a screed plate;

[0014]FIGS. 7A–7C show various views of a screed plate; and

[0015]FIG. 8 shows an electrical heater assembly for use with a screed plate according to the present disclosure.

DETAILED DESCRIPTION

[0016] Referring to FIG. 1, an asphalt paving machine 10 is shown with a screed assembly 12 attached to the back thereof. The asphalt paving machine 10 is supported by a propelling arrangement 14 that is driven by an engine 16 in a conventional manner, or by another known power source. The asphalt paving machine 10 may be used to pave paving mat P, which may be, for example, an asphalt surface.

[0017]The screed assembly 12 is pivotally connected behind the asphalt paving machine 10 by tow arms 18. The screed assembly 12 may be any of a number of configurations such as a fixed width screed or a multiple section screed that includes extensions. As shown in FIG. 2, the screed assembly 12 is provided with a main screed section 20 with a left and a right screed section 22, 24. The left and right screed sections 22, 24 are hingedly connected to one another along a longitudinal centerline 26 so that various operations, such as crowning of the paving mat P, can be performed. A screed extension 28 may also be provided behind and adjacent to both the left and right screed sections 22, 24. It should also be understood that screed extensions 28 may be positioned in front of the main screed section 20. Screed extensions 28 are slidably movable, such as by actuators, so that varying widths of paving material can be laid.

[0018]As shown in FIGS. 1-2, the screed assembly 12 may also include a tamper bar arrangement 29 positioned forward of the main screed section 20. Alternatively, some screed assemblies 12 include a vibratory mechanism positioned above the left and right screed sections 22, 24 and the screed extensions 28 to aid in the initial compaction of the paving material being laid down on paving mat P.

[0019]Referring now to FIG. 3, each of the screed sections 22, 24, 28 may include a screed plate 30 that is removably connected to and supported by a frame 32 that is reinforced by end plates 34, as shown in FIG. 2. Screed plate 30 includes an elongated flat plate 31. A forward leading edge 38 of the screed plate 30 defines a radiused corner 40, with the screed plate 30 also defining a rearward trailing edge 42. The radiused corner 40 blends into the forward portion of the frame 32. As used herein, “forward” refers to the portion of the screed assembly 12 that faces the asphalt paving machine 10, while “rearward” refers to the portion distal from the asphalt paving machine 10. In use, the screed assembly 12 is pulled in the forward direction behind the asphalt paving machine 10, so that the paving material is fed under the forward leading edge 38 and radiused corner 40. The screed plate 30 also defines a screed plate upper surface 46 and a screed plate lower surface 48 positioned between the forward leading edge 38 and the trailing edge 42.

[0020] Each screed plate 30 or screed assembly 12 may be heated by a screed heating arrangement 49, which includes an electric heating element 50 and a retaining mechanism 54. The electric heating element 50 is positioned on an adapter plate 35 associated with each screed plate 30. Each electric heating element 50 may be configured as a thin, elongated sheet and formed from a resistive conductor 52, as shown in FIG. 8. The resistive conductor 52 within each electric heating element 50 terminates with a set of leads or electrical conductors 60 that protrude from the electric heating element 50.

[0021]The electric heating element 50 may be fixedly secured to adapter plate upper surface 36 of adapter plate 35 with a retaining mechanism 54 so as to secure electric heating element 50 and facilitate conducting heat to the screed plate 30. Thus, at least one electric heating element 50 is attached to each screed plate 30. The screed plate 30 of the screed extensions 28 may only have one electric heating element 50 fixedly secured thereto. The shape and number of each electric heating element 50 may vary.

[0022]Each electric heating element 50 may be connected to an electric power supply 64, which is shown in FIG. 1. An electric power supply 64 may be, for example, an electrical generator 66, with the output connections of the electrical generator 66 being connected to the leads 60 of a corresponding electric heating element 50. The electrical generator 66 is operatively connected to the engine 16 of the asphalt paving machine 10, such as by direct connection or powered by a hydraulic motor, that is in turn connected to a hydraulic system of the asphalt paving machine 10. The generator 66 may be either an AC or DC generator such as a twelve or twenty-four volt DC or 120 or 240 AC generator.

[0023]FIGS. 4-7C show various details of a screed plate 30 for a paving mat P. Screed plate 30 may include an elongated flat plate 31 having a screed plate upper surface 46 and a screed plate lower surface 48. The screed plate lower surface 48 may be placed in contact with the paving mat P during a paving operation of asphalt paving machine 10, as shown in FIG. 1. Screed plate 30 may also include radiused corner 40 that comprises a forward leading edge 38 of the screed plate 30. Screed plate 30 may also include a trailing edge 42 disposed opposite the forward leading edge 38. “Opposite” in this sense means on opposing sides of elongated flat plate 31.

[0024] The screed plate 30 may also include a thermally conductive pad 56 having a thermally conductive pad upper surface 57 and a thermally conductive pad lower surface 58. The thermally conductive pad lower surface 58 may be disposed on the screed plate upper surface 46 so as to facilitate heat H transfer from the thermally conductive pad 56 to the elongated flat plate 31.

[0025]In an embodiment, the elongated flat plate 31, radiused corner 40, forward leading edge 38, and trailing edge 42 may comprise a first material _M1, while the thermally conductive pad 56 may comprise a second material _M2 different than the first material _M1. In an embodiment, the first material (M₁) may comprise at least one of cast steel and chromium carbide. In an embodiment, the second material (M₂) may comprise a thermally conductive filling.

[0026] In an embodiment, the thermally conductive pad lower surface 58 may be adhered to the screed plate upper surface 46. For example, the thermally conductive pad lower surface 58 may be adhered to the screed plate upper surface 46 using an adhesive 70. Other methods of adhering thermally conductive pad 56 to screed plate 30 (e.g., glue, etc.) are possible and within the scope of the present application. For example, thermally conductive pad 56 may be inherently tacky or sticky, negating the need to apply an adhesive 70 to adhere thermally conductive pad 56 to screed plate 30.

[0027]In an embodiment, the screed plate upper surface 46 may be machined, as shown in FIG. 6. The screed plate 30, which may be cast, is a surface that may have a rough surface (e.g., peaks and valleys). Machining that rough surface results in the surface being relative smooth, but some low spots in the casting, including their peaks and valleys, may remain. Due to the compliance (e.g., soft durometer) of the thermally conductive pad 56, the thermally conductive pad 56 may at least in part fill those peaks and valleys, all resulting in more efficient transfer of heat H to screed plate 30.

[0028]In an embodiment, the thermally conductive pad 56 may be a thermally conductive electrically isolating gap filler pad. The thermally conductive pad 56 may have a thermal conductivity T of at least 3.0 W/m-K. One exemplary thermally conductive pad 56 that provides these characteristics is the Therm-A-Gap^TM Pad 30 manufactured by Parker Chomerics, as discussed herein.

[0029]FIG. 3 shows an apparatus 68 for transferring heat H to a paving mat P. The apparatus 68 may include at least one electric heating element 50 configured to generate heat H. Screed plate 30 may have a screed plate upper surface 46 and a screed plate lower surface 48, the screed plate lower surface 48 being in contact with the paving mat P during a paving operation. In an embodiment, screed plate 30 may be made of a first material M₁.

[0030]Apparatus 68 may include a thermally conductive pad 56 having a thermally conductive pad upper surface 57 and a thermally conductive pad lower surface 58. The thermally conductive pad lower surface 58 may disposed on the screed plate upper surface 46 so as to transfer heat H from the thermally conductive pad 56 to the screed plate 30. In an embodiment, the thermally conductive pad 56 may be made of a second material M₂ different than the first material M₁.

[0031]Apparatus 68 may include an adapter plate 35 having an adapter plate upper surface 36 and an adapter plate lower surface 37. The adapter plate lower surface 37 may be disposed on the thermally conductive pad upper surface 57 so as to transfer heat H from the adapter plate 35 to the thermally conductive pad 56. In an embodiment, the adapter plate 35 may comprise a third material M₃ different than the first material M₁ and the second material M₂. For example, the first material M₁ may be at least one of cast steel and chromium carbide, the second material M₂ may be a thermally conductive filling, and/or the third material M₃ may be aluminum.

[0032] Apparatus 68 may also include at least one electric heating element 50. The at least one electric heating element may be in thermal contact with the adapter plate upper surface 36 so as to transfer heat H from the at least one electric heating element 50 to the adapter plate 35. In an embodiment, the at least one electric heating element 50 may comprise a plurality of electric heating elements 50.

[0033]A system 62 for heating a paving mat P may include a screed heating arrangement 49 and a screed assembly 12. The screed heating arrangement 49 may include at least one electric heating element 50 configured to generate heat H and at least one retaining mechanism 54. The screed assembly 12 may include a screed plate upper surface 46 and a screed plate lower surface 48, the screed plate lower surface 48 being in contact with the paving mat P during a paving operation.

[0034] The screed assembly 12 may also include a thermally conductive pad 56 having a thermally conductive pad upper surface 57 and a thermally conductive pad lower surface 58. The thermally conductive pad lower surface 58 may be disposed on the screed plate upper surface 46. The screed assembly 12 may also include an adapter plate 35 having an adapter plate upper surface 36 and an adapter plate lower surface 37. The adapter plate lower surface 37 may be disposed on the thermally conductive pad upper surface 57.

[0035] In an embodiment, the screed heating arrangement 49 may be attachable to the screed assembly 12 by attaching the at least one retaining mechanism 54 to the adapter plate 35. Such an arrangement may be referred to as a quick-change screed plate arrangement or a quick-change screed plate. In an embodiment, heat H generated by the at least one electric heating element 50 is transferred to the paving mat P through the adapter plate 35, thermally conductive pad 56, and screed plate 30.

[0036] In an embodiment, the at least one electric heating element 50 may be disposed between the at least one retaining mechanism 54 and the adapter plate 35. In an embodiment, the at least one electric heating element 50 may comprise a plurality of electric heating elements 50. In an embodiment, the at least one retaining mechanism 54 may comprise a plurality of retaining mechanisms 54.

[0037] Other variations of the preceding methods are also possible and within the scope of the present application. For example, various steps of the methods could be omitted and/or reordered without departing from the scope of the present application.

INDUSTRIAL APPLICABILITY

[0038] The screed plates, apparatuses for transferring heat to a paving mat, and systems for heating a paving mat of the present application facilitate more efficient heat transfer from a heating element to a paving mat, particularly in the case of a quick-change screed plate used on an asphalt paving machine. Preheating a screed plate in this manner results in less adhesion between the screed plate and the paving mat at a start of a paving operation, as well as more uniform texture longitudinally along a paved mat.

[0039] The screed plates, apparatuses for transferring heat to a paving mat, and systems for heating a paving mat of the present application employ a thermally conductive pad to help make heat transfer to the paving mat more efficient. The thermally conductive pad may be a thin (e.g., ~1mm thick), compliant (e.g., soft durometer) gap pad. One example of such a pad is a Therm-A-Gap^TM Pad 30 manufactured by Parker Chomerics. Such a pad is a thermally conductive very low compression force gap filler pad with a thermal conductivity T of approximately 3.2 W/m-K, although other types of thermally conductive pads and thermal conductivities T are possible and within the scope of the present application.

[0040] The thermally conductive pad may be disposed between a screed plate and an adapter plate, such as an adapter plate typically used in a quick-change screed plate arrangement, to fill a gap therebetween and increase a thermal conductivity of an interface between those components. For example, an adhesive may be applied to one side of the thermally conductive pad to adhere the thermally conductive pad to the screed plate, while another side of the thermally conductive pad could be attached to the adapter plate with another attachment mechanism. Alternatively, the thermally conductive pad may be inherently tacky or sticky such that it can adhere to the screed plate without the need for a separate adhesive. In either case, when pressure is applied to the screed plate by, for example, the attachment mechanism and/or the weight of the screed assembly sitting on the screed plate when the screed plate is in contact with the paving mat, the thermally conductive pad will fill gaps (e.g., peaks and valleys due to material variations, casting, machining, etc.) between the screed plate and the adapter plate. The presence of the thermally conductive pad between two components, such as the screed plate and the adapter plate, helps facilitate more efficient heat transfer between those components.