Screed assembly for an asphalt paving machine

A screed assembly has a screed plate that is formed of a material having a thermal conductivity of less than about 25 W/mK, and a thermal barrier material having a thermal conductivity of less than about 0.5 W/mK that substantially covers an upper surface of the screed plate. The low thermal conductance properties of the screed plate obviate the need for auxiliary heating systems to preheat or maintain the screed plate in thermal equilibrium with a paving material.

TECHNICAL FIELD 
This invention relates generally to an asphalt paving screed and more 
particularly to a screed plate having low thermal conductivity. 
BACKGROUND ART 
Asphalt paving screeds typically require auxiliary heaters, or burners, to 
apply heat to the material contacting plate of the screed to prevent 
sticking of asphalt paving material to the screed plate. U.S. Pat. No. 
3,557,672, issued Jan. 26, 1971 to Albert L. Shurtz, describes a burner 
system and baffle arrangement to direct a flow of heat from a plurality of 
burners to selected surfaces of the screed assembly. An alternative 
arrangement in which the screed of an asphalt paver is heated by oil 
maintained in a reservoir that is in heat-transferring contact with the 
screed plate is described in U.S. Pat. No. 5,096,331 issued Mar. 17, 1992 
to Larry Raymond. The burners or other heating arrangements of presently 
known screed systems typically require hydraulic or fuel systems, 
controls, and electrical systems to heat the material contacting plates. 
Such systems are costly to construct and troublesome to maintain. 
The present invention is directed to overcoming the problems set forth 
above. It is desirable to have a screed assembly that does not require any 
form of auxiliary heating to prevent freezing, or sticking, of the paving 
material to the screed plate. It is also desirable to have a screed plate 
that is economical to produce and maintain. 
The present invention overcomes the above described problems by providing a 
material contacting member, or screed plate, that has low heat transfer 
properties, i.e., low thermal conductance, and limited heat energy storage 
capacity. As a result of these characteristics, the screed plate embodying 
the present invention is capable of being heated at a desirable rate and 
maintained at a temperature sufficient to prevent sticking of the asphalt 
material to the plate, solely by heat transferred from the paving 
material. Furthermore, the present invention effectively eliminates the 
large thermal gradient that, heretofore, was inherently present between 
the plate and paving material. 
DISCLOSURE OF THE INVENTION 
In accordance with one aspect of the present invention, a screed assembly 
for an asphalt paving machine has a frame member and a screed plate 
attached to a frame member and a thermal barrier material in contact with 
the screed plate. The screed plate has a material contacting surface, a 
second surface spaced from the material contacting surface and a thermal 
conductivity of less than about 25 W/mK. The thermal barrier material 
substantially covers the second surface of the screed plate and has a 
thermal conductivity of less than about 0.5 W/mK. 
Other features of the screed assembly include a cover plate disposed in 
contacting relationship with the thermal barrier material, in spaced 
relationship with the screed plate.

BEST MODE FOR CARRYING OUT THE INVENTION 
In the preferred embodiment of the present invention, a screed assembly 10 
includes a frame member 12 that is attachable to an asphalt paver 14 by a 
pair of conventional tow arms 16. The frame member 12 includes a plurality 
of vertically disposed wall members 18, each having a horizontally 
disposed mounting plate 20 attached by a weld joint at a lower edge of the 
wall members. 
The screed assembly 10 also includes a screed plate 22 having a material 
contacting surface 24 on the bottom side of the plate, and a second 
surface 26 on the upper side of the plate. The screed plate 22 is 
removably attached to the frame member 12 by a plurality of nut members 28 
threadably engaging a respective stud bolt 30. The stud bolts 30 are 
welded to the second surface 26 of the screed plate 22. 
Heretofore, screed plates have typically been constructed of carbon steel 
plate having a thickness of from about 12.7 mm (1/2 in) to about 19.05 mm 
(3/4 in). Carbon steel has relatively high thermal conductivity (about 47 
W/mK) and a hardness of only about Rockwell C21. In carrying out the 
present invention, it is essential that the screed plate 22 be constructed 
of a wear-resistant material having low thermal conductivity. In order to 
be effective, the thermal conductivity of the screed plate should be no 
greater than about 25 W/mK and preferably less than about 18 W/mK. In the 
preferred embodiment of the present invention the screed plate 22 is 
constructed of a heat resistant, martensitic stainless steel having a 
thickness of only about 6.35 mm (1/4 in), a thermal conductivity of 15 
W/mK, and a hardness of Rockwell C45. Thus, the screed plate 22 embodying 
the present invention has significantly lower thermal conductivity, less 
mass, and much greater wear resistance, i.e., hardness, than presently 
known screed plates. These properties provide a screed plate that conducts 
heat away from a heated asphalt material 31 in contact with the plate at a 
much slower rate and has less heat energy storage capacity due to its 
lower mass. 
Alternate materials that may be suitable for construction of the screed 
plate 22 embodying the present invention, include other grades of wrought 
stainless steel, certain wrought iron-base alloys, and structural ceramic 
materials. Preferably, the material selected for construction of the 
screed plate 22 has a thermal conductivity of less than about 25 W/mK and 
a hardness of at least about Rockwell C40. 
Preferably, the stud bolts 30 are also constructed of a material having low 
thermal conductivity characteristics. In the preferred embodiment of the 
present invention, the stud bolts 30 are constructed of stainless steel 
having thermal conductance properties similar to that of the screed plate 
22. 
The screed assembly 10 also includes a thermal barrier material 32 that is 
in contact with, and substantially covers, the second surface 26 of the 
screed plate 22. Desirably, the thermal barrier material has a thermal 
conductivity of less than about 0.5 W/mK, and preferably less than about 
0.1 W/mK. As can be easily understood, for a given amount of heat 
transfer, a lower thermal conductivity permits the use of a 
correspondingly thinner layer of the thermal barrier material. For 
example, in the preferred embodiment of the present invention, the thermal 
barrier material is a moisture resistant, woven fiberglass cloth having a 
thermal conductivity of about 0.046 W/mK and a thickness of only about 3.2 
mm (1/8 in). Other materials suitable for the thermal barrier 32 include 
stagnant air (which has a thermal conductivity of about 0.026 W/mK), dense 
or porous plastics, mats of plastic or ceramic fibers, and either dense or 
porous ceramic or organic materials. The selected thermal insulating 
material should also have a service temperature of at least about 
170.degree. C. to avoid degradation during prolonged use with hot asphalt 
materials. 
Preferably, the screed assembly 10 also includes a cover plate 34 that is 
disposed in nominal contacting relationship with the thermal barrier 
material 32. The cover plate not only provides protection for the thermal 
barrier material but also adds stiffness to the assembly comprising the 
screed plate 22, the thermal barrier material 32 and the cover plate 34 to 
compensate for the reduced thickness of the screed plate 22. In the 
preferred embodiment of the present invention, the cover plate 34 is 
essentially a flat plate formed of carbon steel having a thermal 
conductivity of about 47 W/mK and a thickness of about 6.35 mm (1/4 in). 
The cover plate also has an upper planar surface 36 and a lower planar 
surface 38, with the lower surface 38 being positioned adjacent the 
thermal barrier material 32. 
To avoid undue compression of the thermal barrier material 32, the cover 
plate 34 is preferably maintained in a spaced relationship with respect to 
the screed plate 22 by a plurality of spacers 40 interposed the second 
surface 26 of the screed plate 22 and the lower planar surface 38 of the 
cover plate. In the preferred embodiment of the present invention, the 
spacers 40 are in the form of narrow rings disposed about each of the stud 
bolts 30, have a thickness corresponding to the nominal thickness of the 
thermal barrier material 32, i.e., about 3.2 mm (1/8 in), and are formed 
of stainless steel having low thermal conductivity to avoid excessive heat 
transfer through the spacer members 40. 
Although not essential, it is desirable to have a seal member 42 disposed 
around the periphery of the thermal barrier material 32 to prevent 
infiltration of moisture and foreign matter. The seal member 42 is 
preferably a compression-type seal formed of a metallic, elastomeric, or 
similar material. Preferably, the seal member 42 also has relatively low 
thermal conductivity. The seal member is typically positioned adjacent the 
peripheral edge of the thermal barrier material 32, and between the second 
surface 26 of the screed plate 22 and the lower planar surface 38 of the 
cover plate 34. If the stagnant air is selected as the thermal barrier 
material, it is necessary for the seal member 42 to extend completely 
around the area to be sealed between the screed plate 22 and the cover 
plate 34. However, in some screed arrangements, portions of the 
sandwich-like assembly comprising the screed plate 22, the thermal barrier 
material 32 and the cover plate 34 may be protected, or shielded, by 
components of the screed assembly 10. In such arrangements, if a thermal 
barrier material other than stagnant air is selected, it may not be 
necessary for the seal member 42 to extend completely around the periphery 
of the thermal barrier material 32. 
A test of the screed assembly 10 constructed as described above with 
respect to the preferred embodiment of the invention, i.e., with a 
stainless steel screed plate 22 having a thickness of about 6.35 mm (1/4 
in) and thermal conductivity of about 15 W/mK, and a woven fiberglass 
thermal barrier material 32 having a thickness of about 3.2 mm (1/8 in) 
and thermal conductivity of about 0.046 W/mK sandwiched between the screed 
and cover plates, was conducted using a hot mix asphalt material. The 
asphalt mix was deposited in the hopper of the paver 14, conveyed to the 
rear, and distributed laterally by the augers mounted at the rear of the 
paver, ahead of the screed assembly 10. The paver was then moved forward a 
few feet to move the screed assembly 10 over the distributed hot asphalt 
mix, and bring the material contacting surface 24 of the screed plate 22 
into contact with the asphalt mix. Forward movement of the paver was then 
stopped for about 30 seconds to allow thermal equilibration of the screed 
plate 22 with the asphalt mix. After this short time period, the paver 
resumed forward movement and normal operation, during which time a 
continuous mat of asphalt material was deposited behind the paver. 
During this test, there was no evidence of sticking, or freezing, of the 
asphalt material to the material contacting surface 24 of the screed plate 
22. The surface of the mat formed by the screed plate 24 was smooth, 
without any significant surface defects, or other evidence, indicating 
that material was being picked up or dragged by the screed plate 22. 
Furthermore, after the test, the screed assembly 10 was raised and the 
material contacting surface 24 of the screed plate 22 examined for 
material deposits. The surface 24 was found to be relatively clean, with 
no significant deposits of asphalt material attached to the surface. 
As evidenced by this test, heating of the screed plate 22 with burners or 
other type of auxiliary heating apparatus was not required prior to 
commencement of paving operations. This is a significant discovery that 
will now make possible the construction of asphalt paving screeds without 
the cumbersome and expensive auxiliary heating apparatus, and the 
attendant controls for such heaters, that have heretofore been required. 
INDUSTRIAL APPLICABILITY 
The screed assembly 10 embodying the present invention is particularly 
useful for distributing, smoothing, and at least partially compacting, 
both hot and cold asphalt mixtures. The low heat conductivity and heat 
capacity characteristics of the screed plate 22 make it now possible, when 
required, to quickly bring the screed plate 22 into thermal equilibrium 
with the asphalt mix and effectively diminish, i.e., for all practical 
purposes eliminate, the thermal gradient, or temperature difference, that 
has heretofore been present between the screed plate/paving material 
interface. 
The screed assembly 10 embodying the present invention does not require 
auxiliary heating apparatus to preheat, or maintain a flow of heat energy, 
to the screed plate, and therefore is less costly to build, maintain, and 
control. 
Other aspects, objects and advantages of this invention can be obtained 
from a study of the drawings, the disclosure, and the appended claims.