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Mechanisms – Lost Motion

Lost motion provides sequencing in a mechanism. There are many means to achieve lost motion in a mechanism. Lost motion is similar to dwell in that both methods allow sequencing in mechanical mechanisms or linkage (see also Mechanisms – Dwell Linkage). The difference between lost motion and dwell is that with lost motion there is no motion during sequencing down the lost motion path while for dwell there is always some motion of the mechanism (no motion on any path is exactly zero).

Figure 1 shows one method to achieve lost motion in a mechanism.



Figure 1 Lost Motion Through Slotted Pulley Arrangement


In Figure 1, a cable passes over a pulley, which is held in one end of a slot via a spring. Assuming a reaction force at point B on the cable, when a force is applied to cable at point A (in the direction of the arrow), the pulley will be pulled to the left. When the pulley travels the length of the slot and hits the left side of the slot the cable at point B will start to move. Thus the pulley axis can be connected to a component, which will be moved first, and the cable can be connected to a second component which will be activated second.

An example of the Figure 1 mechanism would be a landing gear freefall cable. In this case, the pulley will be connected to a hydraulic valve. When the pulley moves in the slot, the spool in the hydraulic valve is moved such that the actuator ports are opened to return. Once the spool is moved and hits the end of travel in the spool housing, the cable (at point B) moves releasing the landing gear uplocks. The gear will fall out of the wheel well through gravity and the hydraulic fluid in the landing gear actuator flows to the return as the gear extends.

When analyzing a mechanism with lost motion, there will be two (or more) operating modes for each output. The mechanism would need to be analyzed for each operating mode. Note there would be two operating modes for the lost motion mechanism in Figure 1. In general, there could more than two operating modes depending on the number of lost motion devices in a mechanism.

Another method to achieve lost motion in a mechanism is shown in Figure 2. Figure 2 illustrates lost motion through a tab in a slot for rotational motion. As shown, the input link is splined to a shaft so that it rotates with the shaft. When the tab contacts the outer link, the outer link will start to rotate. The lost motion occurs when the input tab is not contacting the output link. Note that lost motion could also be achieved by switching the input and output link, so that the output would occur at the splined shaft.



Figure 2 Rotational Example to Achieve Lost Motion


Alternative methods to achieve lost motion can be found in the many textbooks on mechanisms available in engineering libraries. The textbook examples are too numerous and too varied to list here. The general idea is for lost motion is to have a connection path disengaged (non-moving) while other parts of the mechanism are moving. Any means to achieve lost motion in a mechanism is considered a lost motion device or lost motion mechanism.