Movement No. 84 is a direction-selecting mechanism controlled by a vertically positioned rod. A double rack-frame (B) is suspended from rod (A) and sits around a continuously rotating cam (D). When the cam shaft is centered between the two racks, it engages neither, and the rack-frame remains still. Raising or lowering rod (A) brings either the upper or lower rack into contact with the rotating cam, causing the rack-frame to be driven left or right accordingly. This mechanism was notably used in steam engine governors, where rod (A) was linked to the governor and the rack-frame was connected to the throttle or regulating valve — allowing engine speed to automatically control the valve position.
84. The double rack-frame, B, is suspended from the rod, A. Continuous rotary motion is given to the cam, D. When the shaft of the cam is midway between the two racks, the cam acts upon neither of them; but by raising or lowering the rod, A, either the lower or upper rack is brought within range of the cam, and the rack-frame moved to the left or right. This movement has been used in connection with the governor of an engine, the rod, A, being connected with the governor, and the rack-frame with the throttle or regulating valve.