Bucknam and Archer
Blackboard eraser
 An improved blackboard eraser used for automatically erasing the chalk dust comprises a housing, in which several sets of brushes and rollers in number corresponding to the brushes are installed. The rollers are made of suitable chalk-erasing material and the bottom contacts the brushes, so that the brushes can erase the chalk dust on the rollers, then they let the chalk drop down into the trough, when the rollers are rotating. This method avoids the chalk dust flying in the air. The gear means are installed on each side of the rollers. Furthermore, the gear means are connected with the idle gears on two sides of the housing to form a gear system. The RPM between the rollers are different due to the different number of the gear teeth between the gear means and the idle gears; then a friction effect is generated because of the different speeds between them, so that the erasing effect for the blackboard is improved. 
B
B43L
inventional
Jin-Sheng
Chen