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Mechanisms – Pogo (Load Limiter or Bungee)

A pogo are also referred to as a load limiter or a bungee. The function of a pogo is to allow operation of mechanism when a jam occurs downstream of the pogo component. A pogo is shown in Figure 1. When a pogo is assembled, the spring compression is adjusted to a desired force level. When a force less than the spring force is applied to either end of the pogo, the pogo acts like a rigid link. When a force greater than the spring force is applied to the pogo, the spring will compress. An applied greater than the spring force implies a jam has occurred on the output side of the pogo.

Figure 1 Pogo

Under normal operation, the spring force is large enough (relative to normal operating loads) to keep the pogo stick rigid. If one side jams, then it is possible to move the other side at an operating force large enough to compress the spring. To visual jam operation, refer to Figure 1 and let the outer rod be fixed and push on the inner rod so that the pogo is in compression. The inner rod lip (right hand side in Figure 1) will push on the slider, which will compress the spring. The spring force is reacted into the lip on the outer rod (left hand side in Figure 1). If the inner rod is pulled, so that the pogo is in tension, the behavior is similar except the applied force and spring force reaction point switch sides. Force transmitted through device will not exceed the spring force as long as no internal hard stop within the pogo is contacted.

The function of a pogo in a flight control system is to allow other portions of a mechanism to continue operation should a jam occur in a mechanism downstream of a pogo. However, because the spring must be compressed, the operating force in the mechanism will be higher. The increase in force is normally high enough to be quite obvious to a pilot.

An application of a pogo in a flight control system is shown in Figure 2. Figure 2 shows a pogo on the input link to the servo on each rudder PCU. If a rudder PCU servo should jam, the rudder pedals could still drive the other rudder PCU by compressing the spring in the pogo of the jammed servo. Clearly, the force level would higher but the function of one rudder PCU would be retained. Without the two pogos in the system, a jammed servo would jam the complete rudder control system.

Figure 2 Simplified Powered Flight Control System with Pogo