US Patent 6890125 Adjustable screed system

A mechanical linkage having a geometry designed to substantially hold the plane of a main screed and the plane of an extension screed at a generally constant relative angle with respect to one another through a predetermined range of screed depth adjustments. The linkage is configured to allow the extension screed to be raised and lowered by an amount sufficient to compensate for angle of attack and elevation changes in the main screed. In particular, the linkage compensates for the effect of the radial distance differences from the axis of rotation of tow bars of the main screed and extension screed as the main screed pivots with respect to the tow point of the tow bars for different asphalt mat depths. An alternate embodiment includes actuators, together with sensors, for sensing the position of the extension screed with respect to the main screed and automatically compensating for changes in main screed position to properly orient the extension screed with respect to the main screed.