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Figure 16-44.-Mat thickness and crown controls.
Figure 16-46.-Screed extension accessories.

Equipment Operator Basic - Beginning construction equipment operators manual
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the mat and too little results in an open texture in the center. The trailing edge is what actually sets the crown. Crown adjustments may be made independently or simultaneously  during  the  paving  operation. AUTOMATIC  SCREED  CONTROLS.—  The screed  controls  must  be  adjusted  by  the  screed  operator as  paving  progresses.    Automatic  screed  controls  are designed to adjust automatically to place a uniform mat of the desired thickness, grade, and shape (fig. 16-45). Types and Operating Principles.— Automatic screed  controls  can  be  used  in  several  different  ways, but all automatic screed control operations require a reference system for the automatic system to follow. This reference system can be the base on which the asphalt hot mix is being placed, the lane next to the material  being  placed,  or  a  string  line.  When  a  string line is used as a reference, the automatic control will follow the height of the string line exactly, so the mat conforms to it; therefore, placement of the string line (or other reference system) must be precise. Automatic screed controls can also follow traveling reference  systems.  A  traveling  reference  system,  such as a ski attached to a control arm, notes changes in base contours  and  adjusts  the  screed  automatically  to A  string  line  or  traveling  reference  system  allows the  automatic  control  to  adjust  screed  height  as necessary  to  maintain  proper  longitudinal  (lengthwise) grade  of  the  pavement.  Automatic  screed  controls  use  a system, attached to a beam, running between the two screed   pull-arms   to   maintain   proper   transverse (widthwise)   grade. A pendulum in the slope control housing moves side to side with changes in the transverse grade of the roadway,  triggering  necessary  adjustments  in  the  slope control mechanism. Automatic control systems have several advantages over  manually  controlled  screed  systems.  Some  of  the advantages  are  as  follows: l Automatic controls compensate for changes in grade and slope more quickly than a screed operator could. l  Automatic  controls  help  disassociate  the  screed from the erratic vertical movement of the tractor unit. .  Automatic  controls  adjust  the  screed  tow  points to enable the screed to follow a path parallel to the grade and  slope  of  the  reference  system,  which  may  be compensate. different from the path plane of the tractor unit. Figure  16-45.—Automatic  screed  reference  system. 16-40







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