Screed Attached Grade Reference Mechanism

Abstract

A mobile grade reference device is connected to the paving machine for travel therewith, such grade reference device being disposed forwardly of the screed and inboard of the lateral extremities thereof. A longitudinally extending grade reference beam has the forward end thereof pivotally mounted to the mobile grade reference device at the longitudinal midpoint of the latter, such pivot point being located rearwardly of the pivot axes of the draft arms engaged with the screed. The rearward end of this beam is pivotally mounted to the screed. The control system for raising and lowering the pivot axes of the draft arms has the sensor thereof supported from the screed adjacent to and forward of the forward portion of the screed; this sensor is engaged by a stringline mounted on said beam.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the present invention relates to improvements in road finishing machines or pavers of the type which lay and finish bituminous or like paving materials along a roadway or subgrade. More particularly, the present invention relates to a grade reference mechanism forming part of the automatic screed control system for such a paving machine.

As is known to those skilled in the art, an automatic screed control system for a paver includes a feeler or sensor which engages an external reference wire or stringline. This external reference establishes a reference plane independently of the paving machine. The feeler or sensor activates a control system, usually electrically or hydraulically operated, to vary the screed angle of attack by raising or lowering the pivot points of the draft arms which mount the screed or by pivoting the screed with respect to the draft arms. Such a system permits the paving machine to lay a pavement with a desirable profile notwithstanding the fact that the subgrade, over which the paving machine must travel, is somewhat irregular in that it contains hollows or humps.

2. The Prior Art

In the use of automatic screed control systems of the type referred to herein, some form of external grade reference datum must be provided. This grade reference may be a line or wire which is erected in fixed position parallel to the desired profile grade of the pavement. However, this type of grade reference is not the most desirable as erection of the line or wire is expensive, time consuming and constitutes an inconvenience to the ready movement of workmen and vehicles at the job site.

When a curb or gutter extends along the roadway, the upper surface thereof may be used as the external grade reference. Of course, whether this grade reference may be used will depend on whether such a curb or gutter is available along the particular roadway to be paved. The use of this form of external reference is obviously quite limited.

A preferred way of establishing the necessary grade reference datum for an automatic screed control system includes the use of a mobile grade reference device which supports the stringline engaged by the sensor of the control system. This mobile grade reference device is towed by the paving machine. Such a mobile grade reference device is most often constituted by a beam, ski or straight-edge, of some substantial length, usually 30 feet or so, but in any case of a length at least equal to the span of the track or wheels of the paving machine. A mobile grade reference device of this type is preferably constituted by a series of articulated arms supported by plural subgrade engaging skids or wheels; reference is made to the pending application of Munyon and Beaty, Ser. No. 202,127, filed Nov. 26, 1971, now U.S. Pat. No. 3,771,892, and assigned to the assignee of the present application, for a complete disclosure and explanation of this type of grade reference device. Such a device spans hollows and sags and lengthens humps by providing a ramp action on either side thereby ensuring a minimum thickness of material over such hump. A mobile grade reference device of this form actuates the screed control sensing means such that the hollows and sags are filled with asphalt material and the humps covered with the asphalt material to provide an optimum pavement profile.

As explained in Munyon U.S. Pat. No. 3,285,148, assigned to the assignee of the present application, optimum results are achieved by locating the sensor of the automatic screed control system a short distance forwardly of the leading portion of the screed. As set forth in the aforementioned application of Munyon and Beaty, optimum results are obtained from the use of an elongated mobile grade reference device when the sensor engages the stringline at the longitudinal midpoint of such mobile grade reference device.

Accordingly, in constructing a system embodying the advantageous principles referred to in both the aforementioned Munyon patent and application of Munyon and Beaty, it necessarily follows that the mobile grade reference device must be located outboard of the lateral extremity of the screed in order that the latter is cleared by the trailing portion of the former. In many instances, the mobile grade reference device cannot be so positioned because the surface of the area adjacent the surface to be paved is unsuitable for movement of the mobile grade reference device, due to roughness or other undesirable conditions, such as obstructions.

When these conditions are encountered, the only practical alternative is to mount the mobile grade reference device forwardly of the screed and within the lateral extremities thereof. However, such a location of the grade reference device results in a substantial reduction in accuracy of the automatic control system with consequent unevenness in the pavement profile. This undesirable result obtains because the effective length of the mobile grade reference device will be substantially reduced as the sensor will engage the stringline on the mobile grade reference device adjacent the trailing end of the latter and not at its longitudinal midpoint. It will be recalled that according to the teachings of the above mentioned Munyon patent, the sensor should be mounted just forwardly of the leading portion of the screed. In an arrangement such as the one just described, the effective length of the mobile grade reference device will be approximately two times the distance from the trailing end of the grade reference device (which will be located as near the leading edge of the screed as practical) to the point on the stringline established by engagement of the sensor, the stringline being of course carried by the mobile grade reference device. Such an arrangement will level only very short undulations in the subgrade and will more or less reproduce longer deviations from the means grade.

In Long U.S. Pat. No. 3,618,484, a grade reference device consisting only of a pair of side-by-side wheels is mounted forwardly of the screed and within the approximate lateral extremities thereof. A longitudinally extending beam carrying a stringline is supported at its opposite ends by a pair of vertically disposed, fluid operated cylinders. The piston rod of the cylinder at the forward end of the beam is supported by the aforesaid pair of wheels; the piston rod of the cylinder at the rear end of the beam is supported by the screed. A sensor unit is provided to actuate the fluid cylinders in a manner to maintain the beam level or at a predetermined slope. Therefore, according to this device vertical movements of the grade reference device constituted by the pair of wheels are not transmitted to the stringline but rather are compensated for by the forward fluid cylinder. Further, the mechanism shown in this Long patent is disadvantageous in that it does not employ the preferred form of grade reference device having plural, longitudinally spaced, subgrade engaging skids or wheels.

SUMMARY AND OBJECTS OF THE INVENTION

The present invention may be summarized as relating to means for attaching a mobile grade reference device to the screed of a paving machine having an automatic screed control system, which means permit mounting of the mobile grade reference device forwardly of the screed and within the lateral extremities thereof and which means actuate the sensor of the automatic system with such a high degree of accuracy that the hollows and sags are filled with asphalt materials and the humps covered with an adequate layer of such material to provide a highly desirable pavement profile.

A primary object of the present invention is the provision of unique means for attaching a mobile grade reference device to the screed forwardly and within the lateral extremities thereof.

Another object of the present invention is the provision of a mobile grade reference mechanism of the type described, which mechanism includes a grade reference beam extending longitudinally of the direction of travel of the paving machine, such beam having the forward end thereof pivotally mounted to the mobile grade reference device at the longitudinal midpoint thereof and at a location rearwardly of the pivot axes of the draft arms for the screed, such beam having its trailing end pivotally attached to the screed.

Still another object of the present invention is the provision of a mobile grade reference mechanism as referred to in the preceding object and wherein the sensor engages the longitudinally extending beam at a location forwardly of the leading portion of the screed.

These and other objects and advantages of the invention will become apparent from the following specification disclosing a preferred embodiment shown in the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a paving machine embodying the present invention;

FIG. 2 is a side elevational view of the screed attached grade reference mechanism of the present invention;

FIG. 3 is an enlarged section taken along the line 3--3 of FIG. 2; and

FIGS. 4A through 4E are diagrammatic views illustrating the operation of the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring in particular to FIG. 1, an asphalt paving machine of well known type is generally designated 10. This paving machine includes a screed 12 towed behind the machine by a pair of draft arms, one arm being illustrated and designated 14. The draft arms are mounted to the paving machine for pivoting movement -- the pivot axis for the draft arm 14 is designated 15 (FIG. 4A). Power means, such as hydraulic cylinders, are attached to the forward ends of the draft arms for raising and lowering the pivot axes thereof for varying the angle of attack of the screed thereby to control the thickness of the pavement layer or lift, all in a manner well known to those skilled in the art. Reference should be had to the aforementioned Munyon patent for a complete description of the screed and the means for mounting and controlling the same.

The mobile grade reference device, generally designated 18, includes a first arm or beam 19 pivotally mounted at its respective opposite ends to the longitudinal midpoints of other arms or subbeams 20 and 21. The arm 20 has the opposite ends thereof respectively pivotally engaged with a pair of skids or shoes 23, 24. Similarly, the respective opposite ends of the arm 21 are pivotally engaged with another pair of skids or shoes 25, 26. It will be understood that the skids 23 trough 26 engage and slide along the subgrade. As used herein, the term "subgrade" has reference to the surface which is to receive the bituminous or asphalt material being laid by the paving machine, whether such surface is formed by previously laid bituminous material, graded earth or a layer of aggregate placed on the earth.

The paver 10 mounts a laterally disposed towing arm 28 which is braced by a diagonal arm 29, one end of the latter being connected to the frame of the paving machine. The distal end of the towing arm 28 is pivotally connected with a towing link 30, the latter being suitably pivotally engaged with a bracket 31 mounted on the arm or subbeam 20. It will be apparent that the mobile grade reference device 18 will be towed by the paving machine 10. It is desirable to provide lateral stability to the mobile grade reference device by employing a link 33, one end being pivotally engaged with a bracket 34 on the arm 21 and the other end of such link being pivotally secured to the paver.

A post or column 36 is rigidly secured to the arm 19 at the longitudinal midpoint of the latter and which point is also the longitudinal midpoint of the mobile grade reference device 18. This column is telescopingly received within a tube 37; preferably, frictional means adapted to be operated by a handle 38 are provided for adjusting the amount of insertion of the column 36 within the tube 37.

As best seen in FIG. 3, the upper end of the tube 37 is rigidly connected to an upwardly disposed channel member 40, which member receives one end of a grade reference beam 42 which may be an I-beam or wide flange rolled section. A latch spring 43 extends over the beam 42 and has opposite ends thereof suitably secured to the channel member 40. The spring 43 provides for connecting and disconnecting the beam 42 and tube 37 and also permits pivotal movement between the beam 42 and the tube 37, such pivoting movement being about a horizontal axis designated 44 (FIG. 4A). At this time it should be mentioned, for reasons which will be explained hereinbelow, that this horizonal pivot axis between the tube 37 and the beam 42 must be disposed rearwardly of the pivot axis 15 of the draft arm 14.

The rearward or trailing end of the beam 42 is connected to a bracket 45 for pivoting movement about a horizontal axis. The bracket 45 is rigidly connected to an upright 46, the lower end of the latter being rigidly connected to an angle member 47. It will be understood that the angle member 47 is rigidly secured to the screed 12.

The screed mounts a post 48 adjacent one side thereof, which post in turn mounts a bar 49 in horizontally disposed fashion. Preferably, means are provided for adjustably positioning the bar 49 in a horizontal plane. One end of the bar 49 is engaged by a diagonal brace 52, the latter being secured to the post 48. This same end of the bar 49 mounts a post 54, the lower end of which post mounts a sensing mechanism 55, which sensing mechanism may be of the type disclosed in the above mentioned Munyon patent. The sensing mechanism 55 includes a sensor arm 56, one end of which is engaged with a stringline 57, opposite ends of such stringline being secured to upstanding pins 58 secured to the beam 42 adjacent its respective ends. Preferably, the bar 49 mounts the post 54 for vertical adjustable movement. It will be understood that the sensor mechanism 55 is mounted, consistent with the teachings of the above mentioned Munyon patent, forwardly of the forward portion of the screed 12 and preferably adjacent to the forward portion of the screed. In a preferred form of this invention, the beam 42 has a length twice the distance between the rearward pivot point thereof and the point of engagement between the sensor arm 56 and the stringline 57; in other words, the sensor arm engages the stringline at the midpoint of the beam 42.

It will be understood that movement of the sensor arm 56, in response to movement of the stringline 57, will activate the control system for actuating the hydraulic rams to raise or lower the pivot axes of the draft arms thereby to vary the angle of attack of the screed, all in a manner well known to those skilled in the art. Again reference is made to the Munyon patent for a description of the control system for so raising and lowering the pivot axes of the draft arms.

The operation of the present invention can be best understood by reference to FIGS. 4A through 4E.

FIG. 4A illustrates the system in what may be termed a static condition. In such condition, the sensing arm 56 is in a "zero" position for establishing the predetermined angle of attack of the screed.

Disturbances, i.e., errors, may be introduced at the screed itself, as well as at the draft arm pivot axes, in either case causing the screed to deviate from the desired path. By way of example, these errors at the screed can be the result of changes in the forward speed of the paving machine, changes in mix temperature affecting mix viscosity and variations in the quantity of the mixture in the distribution auger chamber. FIGS. 4B and 4C illustrate the manner in which the screed attached grade reference mechanism of the present invention makes corrections for disturbances at the screed itself; FIGS. 4D and 4E illustrate the manner in which the present invention corrects for disturbances or movements at the pivot axes of the draft arms in response to vertical displacements of the paving machine as it travels over humps or hollows in the subgrade.

Referring to FIG. 4B, an error at the screed has caused the latter to become elevated. It is apparent that the consequent movement of the stringline is such as to move the sensing arm 56 to activate the control system for moving the pivot axes of the draft arms downwardly. FIG. 4C illustrates the opposite situation wherein an error at the screed causes the latter to move downwardly. In this case, the stringline 57 causes movement of the sensing arm 56 so as to move the draft arm pivot axis upwardly thereby to restore the screed to the proper angle of attack.

It will be apparent that if the horizontal pivot axis 44 between the tube 37 and the beam 42 were disposed forwardly of the pivot axis 15 for the draft arm, the system would then be inoperable as the resultant movement of the sensor arm 56 would be the opposite of that just described causing the automatic screed control system to run out of control. Accordingly, in the use of the present invention it is critical that the pivot axis 44 be located rearwardly of the pivot axes of the draft arms. The horizontal displacement of the pivot axis 44 rearwardly of the draft arm pivots may be small to allow for maximum length of the grade reference beam 42.

FIG. 4D illustrates the situation wherein the pivot axis of one or both of the draft arms is moved upwardly as the result of corresponding upward movement of the paving machine itself. It is apparent that the sensing arm 56 will be moved so as to actuate the associated control system to move the pivot axis of one or both of the draft arms downwardly. FIG. 4E illustrates the opposite situation wherein the pivot axis of the draft arm is moved downwardly as the result of corresponding movement of the paving machine, in which situation the sensing arm 56 is moved for raising the pivot axis of the draft arm thereby to restore the screed to its proper angle of attack disposition.

It is preferable that the automatic screed control system with which the present invention is associated include power means for raising and lowering the pivot axes of both draft arms. However, power means may be provided for only one draft arm with manually actuated means being provided for raising and lowering the pivot axis of the other draft arm. As is known to those skilled in the art, a screed has sufficient torsional deflection properties to accommodate such a power and manual actuating mechanism for the respective draft arms. When using the preferred automatic screed control system including power means for adjusting the pivot axes of both of the draft arms, the screed attached grade reference mechanism of the present invention may be provided at both sides of the paving machine or at one side of the machine and in combination with transverse slope control mechanism, the latter being shown in the aforementioned Munyon patent.

It is apparent that the present invention permits the sensor of the sutomatic screed control system to be located in its optimum position forwardly of the forward portion of the screed. Further, it is apparent that the present invention permits mounting of the mobile grade reference device forwardly of the screed within the lateral extremities thereof and with sensings from the mobile grade reference device being taken at the longitudinal midpoint thereof. It is also noted that the present invention permits the mobile grade reference device to be of rather substantial length which is, of course, a desired feature.

Claims

1. In a paving machine of the type having a screed which is pulled at the rear of the machine by a pair of draft arms, the forward ends of said draft arms being pivotally engaged with the machine, wherein powered actuating means are provided for varying the angle of attack of the screed, and wherein a control system operates said powered actuating means in response to movement of a grade reference sensor, the improvement comprising:

a. a mobile grade reference device elongated longitudinally in the direction of travel of the paving machine;

b. a grade reference beam extending longitudinally of the direction of travel of the paving machine;

c. rigid connecting means securing the forward end of said beam to said grade reference device intermediate the ends thereof such that vertical movements of the latter are directly transmitted to the former whereby said forward end of said beam and the portion of said grade reference device adjacent said connecting means move vertically in unison, said rigid connecting means including first horizontal pivot means pivotally connecting said forward end of said beam with said grade reference device and defining a horizontal pivot axis located rearwardly of the pivot axes of the draft arms;

d. second horizontal pivot means pivotally connecting the rearward end of said beam with the screed;

e. support means mounting said sensor for movement with the screed and at a location intermediate the ends of said beam; and

f. said beam including an element engaged by said sensor for moving the latter thereby to activate said control system in response to relative vertical displacements between said mobile grade reference device and said

2. The improvement according to claim 1 further defined by said connecting means mounting said mobile grade reference device forwardly of said screed and within the lateral extremities thereof, said first pivot means including a member connected to said mobile grade reference device adjacent the longitudinal midpoint thereof, and said sensor being located

3. The improvement according to claim 1 wherein said second pivot means pivotally connects said beam with the screed adjacent the rearward portion

4. The improvement according to claim 1 wherein said support means mounts said sensor forwardly of the forward portion of the screed and adjacent

5. The improvement according to claim 1 wherein said element is defined by

6. The improvement according to claim 2 further defined by, said second pivot means including a member rigidly connected to said screed and upstanding therefrom so as to cooperate with said rigid connecting means to mount said beam in substantial horizontal disposition and at an

7. The improvement according to claim 6 wherein said one member of said second pivot means is connected to said screed adjacent the rearward

8. The improvement according to claim 2 further defined by, said mobile grade reference device including a plurality of longitudinally extending

9. In a paving machine of the type having a screed which is pulled at the rear of the machine by a pair of draft arms, the forward ends of said draft arms being pivotally engaged with the machine, wherein powered actuating means are provided for varying the angle of attack of the screed, and wherein a control system operates said powered actuating means in response to movement of a grade reference sensor, the improvement comprising:

a. a mobile grade reference device including a plurality of articulated arms supported by plural subgrade engaging members, which arms extend longitudinally of the direction of travel of the paving machine;

b. means connecting said mobile grade reference device to the paving machine for movement therewith and in a position forward of the screed and within the lateral extremities thereof;

c. a grade reference beam extending longitudinally of the direction of travel of the paving machine;

d. rigid connecting means securing the forward end of said beam to said grade reference device intermediate the ends thereof such that vertical movements of the latter are directly transmitted to the former whereby said forward end of said beam and the portion of said grade reference device adjacent said connecting means move vertically in unison, said rigid connecting means including first horizontal pivot means pivotally connecting said forward end of said beam with said grade reference device and defining a horizontal pivot axis located rearwardly of the pivot axes of the draft arms;

e. second horizontal pivot means pivotally connecting the rearward end of said beam with said screed;

f. support means mounting said sensor for movement with the screed and at a location intermediate the ends of said beam; and

g. said beam including an element engaged by said sensor for moving the latter thereby to activate said control system in response to relative vertical displacements between said mobile grade reference device and said

10. The improvement according to claim 9 wherein said support means mounts said sensor forwardly of the forward portion of said screed and adjacent thereto and at a location adjacent the longitudinal midpoint of said beam.

11. The improvement according to claim 10 wherein said second pivot means connects said beam to said screed adjacent the rearward portion of the

12. The improvement according to claim 11 wherein said element on said beam is engaged by said sensor adjacent the longitudinal midpoint of said beam.