Apparatus for sensing twisting of a paving machine screed

[57]

ABSTRACT

Apparatus for sensing twisting of a screed of a paving machine, the paving machine having a control system for changing the angle of attack of of each end of the screed relative to a road surface. The twist-sensing apparatus includes a sensor adapted to be mounted on the screed for sensing twist in the screed, the sensor comprising a signal generator for generating a signal in response to the screed twisting from a predetermined twist angle, this signal being adapted to effect energization of the control system for changing the angle of attack of one end of the screed thereby to restore the screed to its predetermined twist angle. The signal generator is rigidly secured to and is movable with the screed at a first location on the screed. The sensor further comprises a twist reference arm rigidly secured to and movable with the screed at a second location on the screed angularly displaced from the first location. The twist reference arm extends obliquely relative to the screed between the first and second locations, and is operatively connected to the signal generator, whereby a change in screed twist causes relative movement between the signal generator and the twist reference arm and thus generates a signal upon the twist angle of the screed deviating from its predetermined twist angle.

Claims, 7 Drawing Figures

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PATENTED MAR25I975

3,872,601

SHEET 1 OF 3

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PATENTED MAR 2 51975

3.872,601

SHEET 2 OF 3

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PATENTED HAR 2 51975

SHEET 3 OF 3

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9/7

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3,872 1

APPARATUS FOR SENSING TWISTING OF A PAVING MACHINE SCREED

BACKGROUND OF THE INVENTION

This invention relates to apparatus for sensing twist- 5 ing of an elongate element of a road-making machine, and is particularly concerned with such a sensor for sensing a change in twist angle of a torsionally flexible screed of an asphalt paving machine and for generating a signal in response to the twist angle of the screed de- 10 viating from its predetermined twist angle, this signal being adapted to effect restoration of the screed to its predetermined twist angle.

More generally, the thickness of the asphalt mat laid by a paving machine, the level of the mat in the direc- 15 tion of travel of the paver (i.e., the grade of the mat), and the transverse slope of the mat is controlled by the angle of the ends of the screed (as viewed from the side) relative to a predetermined road surface (this angle being referred to as the angle of attack of the 20 ends of the screed). By increasing or decreasing the angle of attack of the torsionally flexible screed, the grade and slope of the mat may be maintained to form a desired road surface. The screed of most conventional pavers is towed behind the propulsion unit of the 25 paver by a pair of pull arms, one at each side of the paver, pivotally attached to the propulsion unit. The angle of attack of the ends of the screed may be varied by moving the forward ends of the pull arms relative to the paver or by pivoting the screed relative to the pull ³⁰arms.

Automatic controls for controlling both the grade and slope of the mat are well known in the art. In my U.S. Pat. No. 3,564,986 one such grade and slope control system is described. Briefly, prior art grade control ³³systems include a grade sensor which may be mounted at various locations on the paver (preferably on one of the pull arms). The grade sensor includes a grade follower (e.g., an arm, a ski, or a shoe) which rides on a pre-established grade reference (e.g., a grade line ⁴⁰string, a curb, an old road surface, or a newly laid mat). If the propulsion unit of the paver or the screed moves up or down so that the vertical distance between the grade reference and the grade sensor varies (thus indicating that the angle of attack of the screed has ⁴³changed and hence the mat being laid is deviating or soon will deviate from its desired grade), the grade sensor generates a signal which automatically effects energization of a control system which in turn changes the angle of attack of the end of the screed adjacent the ⁵⁰grade sensor (i.e., the grade end of the screed), thus restoring the mat being laid to its desired grade. Prior art slope control systems typically involve the use of a gravity referenced slope sensor operatively connected to the screed (e.g., the gravity-referenced slope sensor ⁵⁵may be mounted directly on the screed, carried by a transverse beam extending between the pull arms, or carried by control arms extending forwardly from the screed) for sensing the transverse slope of the screed . relative to a gravity-established reference (e.g., this reference may be a pendulum as disclosed in U.S. Pat. Nos. 2,491,275, 3,029,7 16 and 3,334,560 or a body of mercury as shown in my U.S. Pat. No. 3.564,531). If the slope of the screed varies from its predetermined reference, the gravity referenced slope sensor generates a signal which automatically effects energization of the control system to change the angle of attack of the ,601 end of the screed opposite the grade control and thereby restores the screed to its desired slope. Thus, the automatic grade and slope controls cooperate to hold the mat surface at a desired grade and slope independently of variations in the roadbed surface or variations in conditions of the mix (e.g., temperature, aggregate size, etc.). However, one problem which has been inherent in all gravity-referenced slope sensors has been that, as the paver begins to enter a superelevated curve, a screed operator must manually change the predetermined slope reference as the paver makes the transition from a generally flat roadbed to the constantbanked portion of the superelevated curve, this changing of the slope reference being generally referred to as “dialing in” to a curve. Similarly, as the paver exits the superelevated curve, the slope reference must be “dialed out.” If the slope reference of the gravityreferenced slope sensor is not changed upon entering and exiting a superelevated curve, the gravityreferenced slope sensor will attempt to maintain the mat at the established slope of the level or flat roadbed. Thus the mat surface will not follow the desired slope for the superelevated curve. This “dialing in” and “dialing out” of superelevated curves requires a skilled screed operator because, as the transition from the flat roadbed to the superelevated curve is made, the slope reference must uniformly be changed over a long distance. Thus, in actuality, gravity-referenced automatic slope control systems are manual control systems when the paver enters and exits a superelevated curve. In paving many rural roads, much of the roadway to be paved involves transitions into and out of superelevated curves, and thus for a large portion of the time, the slope of the paving mat must be manually controlled. Gravity referenced slope sensors, especially the pendulum type are of relatively complicated construction, are not sensitive to small changes in slope of the screed and thus require expensive and complex electronic amplifiers to generate a signal of sufficient strength to effect engergization of the screed control system, and are sometimes overly sensitive to transient movements of the paver and thus result in unwanted deviations in the mat.

The screed twist-referenced slope sensor of this invention is intended to eliminate many of the problems associated with gravity-referenced slope sensors: Reference may be made to U.S. Pat. No. 3,264,958 which discloses a slope control for a paving machine which is in generally the same field as applicant’s twist sensing apparatus.

SUMMARY OF THE INVENTION

Among the several objects of this invention may be noted the provision of apparatus for sensing the twist of a torsionally twistable element of a road making machine, such as the screed of a paving machine, and thus constitutes a nongravity-referenced slope sensor; the provision of such a nongravity-referenced slope sensor which will automatically maintain the mat being laid by a paving machine at its desired slope as the paver enters, moves through and exits a superelevated curve; the provision of such a nongravity-referenced slope sensor which senses only changes in twist of the screed from its predetermined twist angle regardless of whether the change in twist is caused by variations in the road-bed or by changes in conditions of the asphalt mix; the provision of such a nongravity-referenced

3,872,601 slope sensor which may be mounted on the screed substantially without modification to the screed and without interfering with the paving machine operators as they move about the paving machine; the provision of such a non-gravity-referenced slope sensor which is able to accommodate extraordinarily large screed twist angles without damage to the sensor; the provision of such a non-gravity-referenced slope sensor which may readily be adjusted to establish a new slope of the mat being laid while the paver is in operation; and the provision of such a nongravity-referenced slope sensor which is of rugged and economical construction. Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.

Briefly, apparatus of this invention senses twisting of an elongate element of a road-making machine, such as a screed of a paving machine. Generally, these paving machines have means for changing the angle of each end of the element or screed relative to a road surface. The apparatus of this invention comprises a sensor adapted to be mounted on the element for sensing twist in the element, the sensor having means for generating a signal in response to the element twisting from a predetermined twist angle, the signal being adapted to effect energization of the means for changing the angle of one end of the element thereby to restore the element to its predetermined twist angle. This signalgenerating means is rigidly secured to and is movable with the element at a first location on the element. The sensor further includes a twist reference arm rigidly secured to and movable with the element at a second location on the element angularly displaced from the first location. The twist reference arm extends obliquely relative to the element between the first and second locations, the twist reference arm being operatively connected to the signal-generating means, whereby a change in screed twist causes relative movement between the signal-generating means and the twist reference arm and thereby generates a signal upon the twist of the element deviating from its predetermined twist angle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective of a paving machine having a screed at its trailing end with apparatus of this invention for sensing twisting of the screed operatively installed thereon;

FIG. 2 is an enlarged plan view of the screed illustrating the apparatus of this invention in an operative position on the screed, with details of the screed omitted for clarity;

FIG. 3 is an enlarged elevational view of apparatus of this invention taken on line 3—3 of FIG. 2 illustrating a signal generator and a twist reference arm operatively connected to the signal generator;

FIG. 4 is an enlarged rear elevation of the signalgenerating means;

FIG, 5 is a plan view of FIG. 4;

FIG. 6 is a view similar to FIG. 4 with the front cover of the signal generator removed illustrating details within the signal generator; and

FIG. 7 is a view similar to FIG. 3 illustrating relative movement between the signal generator and the twist reference arm.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, a road-making machine, and more particularly a self-propelled asphalt paver, is indicated in its entirely at 1. This paver is shown to include a crawler type propulsion unit 3 having a hopper 5 at its front end into which asphalt mix may be dumped. A conveyor and auger system 7 in the paver evenly distributes the mix on the roadbed to be paved immediately behind the propulsion unit. An elongate member or screed unit 9 is towed behind the paver by means of pull arms 11 having their forward ends pivotally attached to the propulsion unit at each side of the paver and having their rearward ends secured to a respective end of the screed. The screed unit is a generally channel-shaped member which is torsionally twistable about its longitudinal axis, and has a leading edge 13, a trailing edge 15 and a scraper blade 17 at its leading edge for striking off excess mix deposited on the roadbed by the augers. The screed unit rides on top of the freshly laid mat smoothing and compacting the mat as it rides thereover. The screed may be equipped with heaters (not shown) to heat the screed and thus allow it to better smooth the asphalt mix, and may also be equipped with vibrators (not shown) to further compact the mix. The screed may be flexed longitudinally from one end thereof to the other thereby to adjust the crown of the road by crown adjustment means, generally indicated at 19. A platform 20 is provided at the rear of screed to permit workmen (i.e., screed men) to ride on the screed above freshly laid mix as the paver moves forwardly. The ends of the screed are pivotally attached to a respective pull arm, as indicated at 21, adjacent the leading edge of the screed. A manually operated power screw 23 is threadably carried by each of the pull arms and is socketed, as indicated at 25, at the trailing edge of the screed at each end thereof aft of the pivot connection 21. Rotational screws 23 cause the ends of the screed to pivot about pivot connections 21, and thus change the angle of the ends of the screeds relative to the pull arms.

In addition to smoothing and compacting the asphalt mix as it rides thereover, screed unit 9 also controls the thickness and surface conditions of the mat being laid. More particularly, by controlling the angle of each end of the screed relative to a desired road surface (these angles being referred to as the angles of attack of the ends of the screed), the grade (i.e., the level of the mat in the direction of travel of the paver) and the transverse slope of the mat may be accurately controlled. The angles of attack of the ends of the screed may be controlled by power-operated means, such as generally indicated at 27. As shown in FIG. 1, power-operated means 27 is shown to comprise a hydraulic cylinder unit 29 at each side of the power pivotally connected to the crawler unit and having a piston rod 31 pivotally connected to the forward end of a respective pull arm 11. The pull arm is pivotally connected to the crawler unit by a link 33. Upon actuation of the hydraulic cylinder unit, the forward end of the pull arm may be selectively moved up and down thereby to vary the angle of attack of the screed. It will be understood that other suitable power-operated means, such as the hydraulic linkage shown in my U.S. Pat. NO. 3,564,986 or such as the electric motors operating power screws disclosed

3,872,601 in the U.S. Pat. No. 3,054,716, may be utilized to vary the angle of attack of the ends of the screed.

As indicated generally at 35, a grade control system is included on paver 1 to sense relative vertical changes between the paver and a grade reference 37. This grade reference is shown to be a previously laid mat of paving, but it will be understood that it may be a curb, an old roadbed, or a grade line supported by stakes at a pre-established grade. The grade control system includes a grade sensor 39 supported from a pull arm 11 (this pull arm being referred to as the grade pull arm) adapted to control operation of the hydraulic cylinder unit 29 which is connected to the grade pull arm so as to vary the angle of attack of the adjacent or grade end of the screed and thereby to maintain the mat at a desired grade. The details of one such grade control system are more specifically set forth in my abovementioned U.S. Pat. No. 3,564,986.

In accordance with this invention, apparatus, generally indicated at 41, is provided for sensing twisting of the screed unit 9, this apparatus comprising a twist sensor 43 adapted to be mounted on the screed for sensing twisting thereof. Sensor 43 includes means 45 for generating a signal in response to the screed twisting from a predetermined twist angle. This signal (via appropriate wiring and other electrical components, such as shown in my U.S. Pat. No. 3,564,986) is adapted to effect energization of means 27 for changing the angle of attack of the end of the screed opposite the side of the screed controlled by the grade control system 35 (this end of the screed controlled by the twist sensing apparatus being referred to as the slope controlled end) thereby to restore the screed to its predetermined twist angle. As best shown in FIG. 1, the signal generating means is rigidly mounted on a bracket 47. This bracket is rigidly secured to and is movable with the screed at a first location on the screed. Twist sensor 43 further includes a twist reference arm 49 rigidly secured to and movable with the screed at a second location on the screed angularly displaced from the above-mentioned first location so that the twist arm extends obliquely relative to the screed between the first and second positions. In FIGS. 1 and 2, the preferred location of the signal generating means (i.e., the above-mentioned first location) is shown to be at the middle of the screed above its trailing edge 15 and the preferred location of the twist arm 49 (i.e., the second location) is shown to be at the rear end of one of the pull arms 11 at the end of the screed above the leading edge 13 thereof, the twist arm being movable with the end of the screed therebelow. More particularly, twist arm 49 is shown to be rigidly mounted on a bracket 50, the bracket being rigidly secured to the screed at the front corner thereof and extending above pull arm 11 over pivot point 21. Thus, the bracket accurately reflects movement of this front corner of the screed. It will be understood that, in some applications, the twist arm may be secured directly to the pull arm. The twist arm is operatively connected to the signal generating means 45, whereby a change in screed twist causes relative movement between the signal generating means and the twist reference arm and thereby generates the above-mentioned signal upon the twist angle of the screed deviating from the predetermined twist angle.

As shown in FIGS. 3-7 signal generating means 45 comprises a rotary actuator 51, a rotary switch or other transducer 53 (see FIG. 6) actuable in response to rota6 tion of the actuator to effect generation of an electrical signal. Means 55 is provided for operatively connecting the twist arm 49 to the actuator whereby twisting of the screed causes relative movement between the twist arm and the actuator so as to actuate rotary switch 53.

More particularly, signal generating means 45 includes a housing 57, and actuator 51 comprises a rotary shaft 59 journaled in the housing and extended outwardly therefrom and an actuating arm 61 extending radially from the outer end of the shaft. As shown in FIG. 5, arm 61 is a generally paddleshaped member having generally parallel faces 63α, 63b. Rotary switch 53 includes a rotary cam 65 (see FIG. 6) mounted on shaft 59 for rotation with the shaft and a pair of contact assemblies, as indicated generally at 67α, 61b operable in response to rotation of cam 65 thereby to generate a signal. Cam 65 has two cam surface contours 69α, 69b on its periphery. As shown in FIG. 6, each contact assembly 67α, 67b is mounted in housing 57 so as to be adjacent respective cam contour 69α or 69b. Each contact assembly comprises a first or movable, flexibly resilient cantilevered contact arm 71 in resilient engagement with the cam surface of cam 65 and a second resiliently flexible, cantilevered contact arm 73 adjacent to and normally spaced from the first contact arm. Each contact arm carries a contact 75 on its outer end for engagement by the contact carred by the other arm as the first contact arm moves in response to rotation of the cam. The first contact arm is movable in and out relative to the cam by a respective cam contour 69α, 69b in response to rotation of the cam. Each contact assembly is so positioned relative to its respective cam contour 69α, 69b and the cam contours are so developed and phased so that with the cam in a predetermined angular position corresponding to the twist angle of the screed 9 when the screed is in its predetermined twist angle, both of the contact assemblies are open, and upon twisting of the screed in one direction or the other from its predetermined twist angle, rotation of cam 65 is effected thereby to close the contacts of one of the contact assemblies while the contacts of the other contact assembly remain open. Thus, the closing of contacts of one of the contact assemblies generates an electrical signal indicating a change in twist of the screed. The opening and closing of these contact assemblies is visually indicated on the exterior of housing 57 by means of indicator lights 76 and by means of a pointer 77. A 3-position switch 78 is provided on the front cover. This switch controls which of the pull arms 11 is responsive to the signal generated by rotary switch 53. Cables 79 supply electrical power to the switch contacts 75 and interconnect the switch assemblies 67α, 67b into the control system 27.

As indicated generally at 80 in FIG. 6, means are provided for selectively adjusting the position of the contact arm 73 of each contact assembly 67α, 67b relative to its movable contact arm 71 thereby selectively to vary the angular position of cam 65 relative to contact assemblies 67α. 67b at which the respective sets or pairs of contacts 75 of each contact assembly open and close thus to vary the sensitivity of the apparatus of this invention to detecting changes in twist angle of the screed. More particularly, means 80 is shown to comprise an adjustment screw 81 at each side of housing 57, each screw being threaded in an insulative block 82 in the side wall of the housing. With a contact arm 73 in its unrestrained position, it angles out

3,872,601 away from contact 71 towards the adjacent side of the housing, and with the end of adjustment screw 79 in engagement with an adjacent contact arm 73, turning of the adjustment screw causes the contact arm 73 to flex and to move in and out relative to the cam. It will be noted that the construction of contact arms 71 and 73 prevents damage to the contact arms in the event the contact arm 73 is moved to its innermost position and rotation of cam 65 causes arm 71 to move out beyond the position of arm 73. Because of the resiliently flexible contact arms 71 and 73, they readily bend as the cam rotates and then snap back to their initial shape without permanent deformation.

As shown in FIGS. 3-5 and 7, housing 57 is mounted in a guide 83 adapted to be secured, as by bolting, to bracket 47 on screed unit 9. More particularly, housing 57 is shown to be a rectangular box having a back side wall 85, side walls 87α, 870, a top wall 89 and a bottom wall 91 integrally cast together, and a removable front cover 93. As is shown in FIG. 5, flanges 95α, 950 extend outwardly of the housing along the juncture of side walls and the back wall, these flanges extending the height of the housing and constituting guide rails. Guide 83 has vertical grooves 97α, 970 therein for slidably receiving a respective guide flanges 95α, 950 thereby to mount the housing for vertical movement relative to the guide. Guide 83 threadably carries an adjustment screw 99 at its top threaded into a boss 101 (see FIG. 6) in top wall 89 of the housing whereby turning of the adjustment screw raises or lowers the housing relative to the guide. As shown in FIG. 3, guide 83 has a cut-out 103 through which the outer end of shaft 59 extends. This cut-out is of sufficient size to permit vertical movement of the housing relative to the guide within a limited range. By varying the vertical position of housing 57 relative to guide 83, the vertical position of switch 53 and actuator 51 relative to twist arm 49 may be selectively adjusted thereby to vary the predetermined twist angle of the screed. It will be understood that the predetermined twist angle of the screed and hence the slope of the mate being laid may be adjusted while the paver is in operation. Set screws 105 are provided in guide 83 (see FIG. 4) and are engageable with flange 950 of housing 57 thereby to positively hold or lock the housing relative to the guide in any desired position. As best shown in FIG. 4, housing 57 carries a pointer 107 and guide 83 has a scale 109 thereby to indicate the predetermined twist angle of the screed.

As shown in FIG. 2, twist arm 49 is a rigid tubular member cantilevered from bracket 50 above the left pull arm 11 above leading edge 13 of the screed unit 9. It will, however, be understood that the twist arm may alternatively be cantilevered from the right pull arm. Means 55 for operatively connecting the twist arm to actuator 51 includes a clevis 109 carried on the free end of the twist arm. As best shown in FIG. 4, opposed adjustment screws Illa, 111b are threaded through the spaced apart sides of the clevis, these adjustment screws being substantially coaxial. Each adjustment screw carries a bearing member 113α, 1130, respectively, each member having a low friction pad (not shown) of nylon or Teflon (registered trademark of E. I. Du Pont de Nemours &amp; Co.) in sliding engagement with a respective face 63α, 630 of actuating member 61. Each bearing member is socketed on the rounded end of its respective adjustment screw so as to permit universal rotation of the bearing about any axis within a limited range. Adjustment screw 1110 carries an annular flange 115 secured thereto intermediate its free end in the inner side of the clevis. A compression coil spring 117 is interposed between the flange and the bearing member 113 0 thereby to push bearing members 1130 into resilient engagement with face 630 of actuating member 61 and to hold the actuating member in resilient engagement with bearing member 113α. Thus the actuating member is resiliently gripped between, but movable relative to, the bearing members. With the above-described connection between the twist arm and the actuating arm, movement of the twist arm relative to the actuating member (as shown in FIG. 7) may readily be translated into rotation of the actuating member. Because the bearing members are resiliently held in engagement with the actuating member, the bearing members are able to slide on the actuating member as the twist arm moves relative to the actuating member so as to accommodate changes in the distance between the attachment point of the twist arm on its pull arm 11 and the location of the signal generating means 45. This connection also accommodates extraordinary large twist angles of the screed, as may be occasioned by lifting of the screed for transport, without damage to the apparatus of this invention.

Operation of the apparatus 1 of this invention is as follows:

With grade control system 35 controlling the angle of attack of one end of the screed (i.e., the grade end of the screed) and with the screed twist sensing apparatus 41 of this invention controlling the angle of attack of the other end of the screed (i.e., the slope end of the screed), both the grade and slope of the mat being laid may accurately be controlled. More particularly, if the angles of attack of the ends of the screed relative to a desired planar road surface are the same, the predetermined twist angle of the screed is zero (the screed is not twisted), and if the angles of attack of the ends of the screed are different, the predetermined twist angle of the screed will be some value other than zero (the screed is twisted).

As the paver 1 moves along the road bed to be paved, the grade control system 35 moves the grade controlled end of the screed to various angles of attack so as to maintain a desired grade of the mat being laid regardless of variations in the road bed or in the asphalt mix. For example, if only the grade controlled side of the propulsion unit 3 of the paver encounters a depression in the road so that it moves down relative to the grade reference 37, the grade sensor 39 immediately senses this deviation and causes the hydraulic cylinder unit 29 on the grade controlled side of the paver to raise the front end of the grade pull arm 11 so as to maintain the angle of attack of the grade end of the screed at its desired angle of attack and thereby to maintain the grade of the mat constant.

If the conditions of the asphalt mix change (e.g., the mix is too cool) so that with the screed at a given angle of attack the grade of the mat will deviate from its preestablished grade, the grade sensor will sense the grade deviation and move the grade pull arm in the appropriate direction thereby to change the angle of attack of the screed so as to restore the mat to its desired grade. This change in angle of attack of the grade end of the screed induces a change in screed twist. This change in twist of the screed effects relative vertical movement between the clevis end of twist arm 49 and shaft 59 of

3,872,601 sensor 43 thereby effecting rotation of the shaft and cam 65 which in turn closes the contacts 75 of one of the contact assemblies 67α, 67ft thereby to generate a signal indicating a twist deviation of the screed from its desired twist angle, this in turn indicating an incipient change in the slope of the mat. This signal is adapted to effect energization of hydraulic cylinder unit 29 connected to the slope pull arm 11 thereby to restore the twist angle of the screed to its predetermined twist angle and thus eliminate errors in the slope of the mat substantially before they have caused any significant change in the slope of the mat.

If both sides of propulsion unit 3 encounter a depression, grade control system 35 will maintain the grade end of screed 9 at its desired angle of attack, as heretofore described. However, as the slope controlled side of the propulsion unit drops into a depression, twist will be induced into the screed (because the grade controlled end of the screed is maintained at its desired angle of attack). This change in screed twist actuates 20 signal-generating means 45, as above described, and thus the screed is maintained at its predetermined twist angle and the mat is maintained at its desired slope.

It will further be noted that as paver 1 moves into or exits from a superelevated curve, grade control system 25 35 and screed twist sensor 41 together automatically maintain both the grade and slope of the mat laid on the superelevation curve at a desired grade, as established by the grade reference, and at a desired slope, as established by the predetermined twist angle of the ³⁰screed. Because the apparatus of this invention does not rely on gravity for a slope reference, the slope reference need not be changed (i.e., “dialed in” and “dialed out” ) as the paver enters into and exits from superelevated curves.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.