The purpose of a summing mechanism is combine two input motions into one output motion. A common application for summing link is on all mechanically actuated hydraulic power control units (see Figure 2). A summing link is shown in Figure 1. This a linear summing link. Summing can also be done rotationally.
Figure 1 Summing Mechanism
For the summing linkage shown in Figure 4, the two inputs are xA and xB. The ouput is xC.
As a side note, the linkage in Figure 4 becomes a subtracting mechanism if point A is the output. Letting d1=d2, then, xB and xC are combined via subtraction according to
Two applications of a summing mechanism are shown in Figure 2. First, the output of the dual automatic flight control actuators are summed together to drive the linkage controlling the power control units. The second example is the summing link that drives the servovalve of each rudder power control unit (PCU). In both examples of a summing link, the inputs are at the two ends of the link and the output is in the middle.
Figure 2 Dual AFCS Actuator Summing Link Mechanism
An example of a rotational summing link design is shown in Figure 3. The mechanism shown in Figure 3 has two inputs, which are the speedbrake input and roll input. The speedbrake input provides a symmetric spoiler panel deployment command and the roll input provides an asymmetric spoiler panel command. There are two outputs, one to each spoiler panel. The two inputs are summed together to provide a combined output command to each spoiler panel.
Figure 3 Rotational Summing Mechanism