the mat and too little results in an open texture in thecenter. The trailing edge is what actually sets the crown.Crown adjustments may be made independently orsimultaneously during the paving operation.AUTOMATIC SCREED CONTROLS.— Thescreed controls must be adjusted by the screed operatoras paving progresses. Automatic screed controls aredesigned to adjust automatically to place a uniform matof the desired thickness, grade, and shape (fig. 16-45).Types and Operating Principles.— Automaticscreed controls can be used in several different ways,but all automatic screed control operations require areference system for the automatic system to follow.This reference system can be the base on which theasphalt hot mix is being placed, the lane next to thematerial being placed, or a string line. When a stringline is used as a reference, the automatic control willfollow the height of the string line exactly, so the matconforms to it; therefore, placement of the string line (orother reference system) must be precise.Automatic screed controls can also follow travelingreference systems. A traveling reference system, suchas a ski attached to a control arm, notes changes in basecontours and adjusts the screed automatically toA string line or traveling reference system allowsthe automatic control to adjust screed height asnecessary to maintain proper longitudinal (lengthwise)grade of the pavement. Automatic screed controls use asystem, attached to a beam, running between the twoscreed pull-arms to maintain proper transverse(widthwise) grade.A pendulum in the slope control housing moves sideto side with changes in the transverse grade of theroadway, triggering necessary adjustments in the slopecontrol mechanism.Automatic control systems have several advantagesover manually controlled screed systems. Some of theadvantages are as follows:l Automatic controls compensate for changes ingrade and slope more quickly than a screed operatorcould.l Automatic controls help disassociate the screedfrom the erratic vertical movement of the tractor unit.. Automatic controls adjust the screed tow pointsto enable the screed to follow a path parallel to the gradeand slope of the reference system, which may becompensate.different from the path plane of the tractor unit.Figure 16-45.—Automatic screed reference system.16-40
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