Field of the Invention
This invention relates to surfacing apparatus, and in particular
to laying boxes for use in road-surfacing vehicles, of the type in which a road-surfacing
material is applied, by trailing application, under a screed that defines the
Background of the Invention
The desire to spread asphalt as thinly as possible, with little or
no surface preparation, consistent with acceptable ridability, is well known for
Traditionally, asphalts have been applied at a thickness of between
30 and 50 mm. They can be made to give good texture depth, essential for most UK
roads, by rolling in chippings while the surface is hot. A flat, prepared substrate
is required, to ensure a flat, finished surface for the applied asphalt. Some reprofiling
will result from the surface preparation as well as the mat thickness tolerance.
Material thickness and level are dependent upon the forces applied to the floating
screed and the wheel/track base of the paver and towing arm length.
Recently, surfacing products have been commercially applied, again
using a rigid screed, at a thickness of between 10 and 25 mm. These materials contain
aggregate grading which will give acceptable texture depths for UK needs, without
the need to roll chippings into the surface. However, in some cases, the thickness
of the applied material approaches the maximum particle size of the material.
Surface deformation is then caused, by the screed dragging and scraping the surface.
A flat, prepared substrate and screed clearance control are most important. Some
reprofiling results from the surface preparation and some rut filling by the material.
These products are applied using the same techniques as described above.
The application of thin veneer slurries/microasphalt is also well
known. These coatings vary in thickness between 3 and 6 mm, and are generally applied
with a flexible screed or strike-off plate which uses the maximum particle size
of the material to control the thickness. This technique will not reprofile the
road surface. Moreover, the texture depth of such a system is not acceptable for
most UK roads.
An extension of the slurry/microasphalt system uses a technique whereby
the road surface is first reprofiled using the slurry mix, by using a rigid screed
in order to fill all the deformities and ruts. A top coat of slurry/microasphalt
can then be applied using a rigid or flexible screed. Slurry/microasphalt screeds
are supported by the substrate being coated via skids, whereas asphalt pavers
employ a system whereby the screed floats on the material being applied.
It is clear that most if not all processes which result in acceptable
texture depths require some form of surface preparation unless the substrate is
acceptably flat. The problem with a system which tracks the substrate is that
the skids are of finite width and do not take into account any deformity between
the skids. Clearly depressions are not important but, should the substrate rise
and reduce the material thickness, then the resultant surface may be scraped or
It is known that some asphalt pavers use active control systems which
adjust the forces on the screed, to maintain finished surface flatness. The control
system uses either a laser beam or string line as a datum. However, this will
not necessarily minimise the material usage as compromises have to be made in relation
to minimum allowable material thickness and substrate deformation over the datum
length. Generally, these systems are used on "prepared" substrates.
Summary of the Invention
A laying box assembly suitable for use with a road-surfacing vehicle,
by means of which surfacing material can be applied by trailing application, to
a road or other substrate, comprises a laying box having mounted at its leading
edge at least one flap for detecting the substrate profile; a screed; means for
controlling the thickness of the material applied to the substrate; and means for
transmitting movement of the at least one flap caused by unevenness in the substrate
to the thickness control means.
In use of apparatus according to the invention, a control system
is provided, whereby material is applied in such a way that the surface flatness
is maintained at an acceptable level. Further, when a plurality of independently-mounted
flaps are used, which is preferred, the finished profile takes into account substrate
deformities. Material usage is thus minimised, and the maximum material thickness
is always greater than the maximum aggregate particle size used.
Description of the Invention
For the purpose of clarity, in this Application the leading edge
of the screed or laying box is that edge which, in use, passes over the substrate
first, and the trailing edge is that edge parallel to the leading edge.
In a first embodiment of the present invention, the screed is movable
independently of the laying box, and is mounted on elongate members that run on
the substrate and which are positioned at least on either side of the screed.
When a plurality of flaps is used, movement of any flap caused by unevenness in
the substrate is transmitted to the screed mounting associated with that flap,
causing the screed to rise at that place.
A two-part screed can be used, hinged along its central axis, transverse
to its leading and trailing edges. In this case, an elongate member supports the
screed along its hinge also. Alternatively, a plurality of screeds may be used,
each mounted at its sides on elongate members, or a multi-hinged screed with each
hinge being supported on an elongate member.
In a second and a third embodiment of the present invention, the
screed is immovably fixed to the laying box, and the thickness of material applied
to the substrate is controlled by varying the height of the laying box above the
substrate. The laying box is mounted, at least on either of its sides, on elongate
members that run on the substrate. As in the first embodiment, when a plurality
of flaps is used, movement of any flap is then transmitted to the associated laying
box mounting, causing the laying box to rise at that place. Typically, and preferably,
the laying box is hinged along its central axis to allow for, or provide, road
camber,and is mounted on a further elongate member along that axis.
In the second embodiment, the laying box is pivoted at its leading
edge about an axis which is at a fixed height above the substrate. The thickness
of material applied is controlled by raising and lowering the trailing edge of
the laying box, according to substrate profile.
In the third embodiment, the laying box is not pivoted about a fixed
axis, but instead its sides can be raised and lowered to substantially the same
extent at their leading and trailing edges.
As in both the second and third embodiments the screed forms a structural
part of the laying box, the whole assembly is stronger than that of the first embodiment
and is therefore more readily suited to the design of a variable-width laying
box. Such a laying box will have a contracted and an expanded state, and it is
preferred that in the contracted state the flaps are in overlapping relation,
but that in the expanded state they have substantially no overlap.
To ensure that the heights above the substrate of a pair of screed
plates, or of two hinged-parts of a screed plate or laying box, are adjusted only
as necessary, i.e. independently, two sets of detector flaps are provided, eg
two pairs, between the outer elongate members. In this case, the outer detector
flaps control the sides of the screed plate or laying box, and the detector flaps
each side of the central elongate member control the movement at its centre. The
detector flaps may be of equal length, but this may be changed if, for instance,
a more pronounced crowning of the surface is required.
For example, when a hinged laying box is used, in the third embodiment
movement of an outer flap causes only the side of the laying box associated with
that flap to rise, whilst maintaining the centre of the box and the other side
of the box in their original positions. This means that increased material thickness
is provided only where it is needed, ie. where the substrate is raised, resulting
in an efficient usage of that material. This represents an advantage over the
second, pivoted, embodiment, where raising one side of the laying box may well
result in some raising of its centre, and possibly also its other side, resulting
in the use of more material than is necessary.
If more than a pair of screed plates are used, or a multi-hinged
screed plate or laying box, at least one detector flap, and preferably at least
two flaps, should be provided for each plate or supported part.
The invention will now be described by way of example only with reference
to the accompanying drawings.
Figure 1 is a side view of a mobile blending and mixing machine.
Figures 2A and 2B are top and side views, respectively, of a laying
box embodying the present invention.
Figure 3 is a schematic view of a mechanism for transmitting movement
of a flap to a screed plate in a laying box according to a first embodiment of
the present invention.
Figure 4 is a schematic view of the mechanism of Figure 3, in greater
Figure 5 is a side view of a laying box according to a second embodiment
of the present invention.
Figure 6 is a side view of the laying box of Figure 5, in raised
Figures 7A to 7D are schematic views representing the movement of
a laying box in the second and the third embodiments of the present invention.
Firstly, as shown in Figure 1, a laying box assembly (1) is towed
behind a mobile blending and mixing machine (2) which supplies surfacing material
thereto, by means which can be conventional.
In Figures 2A and 2B a screed plate, strike-off plate or roller screed
(subject of EP-A-0693591) is supported, independently of a laying box, on skids
or skis (3,4). The skids ride on the substrate to be spread with asphalt. The
length of the two outside skids (3) is such to ensure acceptable stability (e.g.
2.5 m). The centre skid (4) controls the position of a hinge provided between two
essentially coaxial rollers, whose respective orientation can be used to provide
By contrast to the prior art, the present invention provides flaps
or plates (5) that detect any deformity likely to reduce the material thickness
to below the minimum. A series of detector flaps (5) is attached to the leading
edge of the laying box, via an axle (6). These independently-pivoted detector flaps
will move should the road surface rise between the skids. The flap movement is
transmitted via a mechanical linkage to the stylus of a double-acting hydraulic
valve which is attached to the body of a double-acting hydraulic cylinder whose
rod is connected to the skid assembly and whose body is connected to the strike
plate roller assembly; see Figures 3 and 4.
Figure 3 shows from left to right, an hydraulic cylinder (7) whose
body is for attachment to the screed plate/roller assembly, and whose rod is for
attachment to the laying box structure via an adjusting screw through the hollow
cylinder rod. These attachments are shown in Figure 4.
Attached to the cylinder body, and ported to it, is a manifold block
(8) containing a flow control valve (not shown) which is capable of controlling
the cylinder speed.
Attached and ported to the manifold block is a stylus-operated two-position
valve (9,9a), the stylus (10) being spring biased downwards, as drawn. With the
stylus spring out, the cylinder rod will move up. With the stylus forced in, the
cylinder rod will move down, as represented by arrows (11), causing the screed
plate/roller assembly to rise.
Figure 4 shows the cylinder (7) which is connected to a strike plate
(not shown) with the manual adjusting screw (12) connecting the cylinder rod to
the laying box structure at (13). The manifold block (not shown) and stylus-operated
two-position valve (9) complete the organisation of the hydraulic elements.
A linkage consisting of a first radus arm (14), a large rod (15),
a second radus arm (16), a compressive strut (17), a third radus arm (18), and
a detector flap axle (19) with a detector flap (20), complete the mechanical feed-back
linkage. The compressive strut acts to prevent excessive loads being transmitted
to the valve stylus. It is designed to start compressing only when the stylus
full compression load is exceeded by an acceptable design factor, e.g. a factor
In operation the following will occur, assuming that valve (9) is
in its neutral position.
When a flap is raised, due to substrate unevenness, it will force
the stylus upwards via the linkage comprising (18), (17), (16), (15) and (14),
in that order, and thereby actuate the hydraulic cylinder rod downwards. This will,
in turn, lift the screed plate and roller assembly (not shown), as well as the
cylinder body (7), manifold block (8) and stylus valve (9). As the valve (9) moves
away from radus arm (14), so the stylus returns to its original position and the
upward motion of the screed plate and roller stops.
If the flap falls, the opposite to what is described above occurs,
until the stylus regains its equilibrium position. In this way the motion of the
flap is reproduced by the roller and screed plate assembly.
A valve dead band of 1 to 2 mm is sufficient to prevent hunting,
and approximately 4 mm movement either side of the dead band is sufficient to move
the cylinder up or down. A further hydraulic flow control valve contained in the
circuit (not shown) controls the rise time of this device. Acceptable ridability
can be achieved with undulations of about 2 mm per metre. The distance between
the screed plate and the flap is, for example, about 1 m. The forward speed may
be, for example, about S m/sec. A rise time of 1 mm/sec is acceptable.
In Figure 5, a screed plate (21) and a roller (22) are immovably
fixed to a side plate (23) of a laying box. The height (x) of the side plate (23)
above the substrate (24) controls the thickness of material screeded by the screed
plate and/or the roller. Lifting and lowering the side plate of the laying box
in a direction parallel to the surface of the substrate is achieved by parallel
links (25) and (26), which attach a skid (27) to the side plate, and which are
of equal centre pivot distance. The motion of those parallel links, and consequently
of the side plate, is controlled by a double acting hydraulic cylinder (28) and
a double acting hydraulic control valve (29). Detector flaps (30), together with
a return spring (31), monitor the road surface profile between the skids upon which
the laying box is mounted; details of the detector flaps and skids are given in
relation to Figures 2A and 2B above.
In this embodiment of the invention, the mechanical feedback linkage
consisting of items (14) to (18) of Figures 3 and 4 is not necessary, as the detector
flaps, pivot, screed plate and roller move together with the side plate to the
same distance above the substrate. Consequently, when the detector flaps are disengaged
from the raised substrate they return to the equilibrium state, taking the remainder
of the system with them. Instead, a simpler system can be utilised, in which the
flap bears directly on the valve stylus via a radus arm (32). The radus arm moves
away from the double acting valve stylus when the flap engages a raised portion
of substrate between the skids. This prevents the valve stylus becoming overloaded.
The return spring (31) together with a stop (33) will protect the valve if the
flap is allowed to fall beyond the accommodation of the valve, eg. when returning
to its equilibrium position.
In Figure 6, the side of the laying box is shown in a raised position,
for instance with the height (x) being approximately 40 mm.
Figures 7A to 7D show the different kinds of movement achieved in
the second and third embodiments of the present invention. In Figure 7A, the base
of a laying box hinged along the line X-X is shown in an unraised position. In
Figure 7B, unevenness of the substrate causes side Y of the laying box to rise
parallel to the surface of the substrate and without influencing the position of
the screed at its centre (along X-X) or its opposite side. In Figure 7C, a laying
box is again hinged along the line X-X, and pivoted from its leading edge along
Z-Z. The raising of one side of the laying box, for instance at Y, influences the
position of the laying box both at its centre and its opposite side, leading to
an inefficient use of surfacing material.
It is clear that sharp changes in substrate profile will not be accommodated
unless the detector flaps are positioned some way ahead of the front of the laying
box. To enable the accommodation of sharp leading substrate profile changes of
some 10 mm or more, and based on a preset material thickness of around 15 mm with
a maximum chip size of around 10 mm, the detector flaps are typically 1-5 m, preferably
some 2.5 m, ahead of the strike plate, if a minimum thickness of around 10 mm is
to be maintained over the 10 mm rise.