The free floating screed has become standard because of the smoothing or averaging effect it can have on the existing base course. Since the only connection between the asphalt paver and the screed is the tow arm, the screed can "float" vertically relative to the paver. This allows the paver to traverse uneven ground while the screed floats over the material placed in front of it.
The free floating screed has a number of forces acting on it that when in equilibrium allow the depth behind the screed to be constant.
If each of these forces is constant, altering the angle of the screed to the horizontal (angle of attack) will control the amount of material extruded behind the screed. Increasing the angle angle of attack will cause the screed to climb higher through the pile of material, and therefore raise its trailing edge, increasing the amount of material extruded behind the screed. Similarly, reducing the angle of attack will reduce the amount of material extruded.
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