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Mechanisms – Overcenter Linkage

An overcenter linkage or device is used to provide a mechanical stop in a linkage and prevent any backdriving of the mechanism. Figure 1 shows an overcenter mechanism. The depiction in Figure 1 is simple in nature, but is adequate for explaining the principles of an overcenter mechanism. Overcenter implies a rotational mechanism.

Figure 1 Overcenter Mechanism

In position A, as the crank is rotated clockwise, the slider will move to the right. When the slider gets to its maximum stroke position (where the link is horizontal) the mechanism is considered to be in the “center” position. As the crank continues to rotate clockwise, the link moves past the center position, i.e., overcenter. In position B, the crank has rotated to the point where the moving stop has hit the fixed stop. In this position, any force that pushes on the slider cannot backdrive the mechanism. The function of the spring is to hold the crank against the stop so that vibration or any freeplay in the mechanism will not cause the mechanism to rotate back out of the overcenter position. The spring does impact the driving force for the mechanism, as the spring force must be overcome to move the mechanism. Setting the spring forces is a tradeoff between the impact on the driving force and holding the moving stop tightly against the fixed stop. When the crank goes overcenter, the slider is effectively in a dwell. Thus overcenter and dwell can be incorporated into the same mechanism motion.

Overcenter mechanisms are used to hold a mechanism at one end of the mechanism travel. Two overcenter devices can be put into a single mechanism to hold the mechanism in either end of the mechanism travel. The spring(s) will add to the normal operating load in the mechanism.