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Mechanisms – Dwell Linkage

Dwell is used in a mechanism when sequencing is required. From a theoretical standpoint, dwell occurs when the output velocity is zero, which implies very little movement of the output. In practice, it is not possible to achieve zero velocity at a dwell position so there will always be some movement of the output. Figure 1 shows a simple and common means to achieve dwell.



Figure 1 Dwell Mechanism


In Figure 1, as the crank is rotated from point A to point B, the slider will move to the right. However, as the crank is rotated from point B to point C, there is very little movement of the slider. The total travel of the slider as the crank goes from A to B is given by

    (1)

The velocity is given by

    (2)

As long as θ is small xdot will be small. Dwell occurs or nearly occurs during the rotation from A to C. In practice, the crank/slider mechanism shown in Figure 6 will be one part of a mechanism. Thus as the crank goes from A to C, other portions of a mechanism will be moving.

A practical example of a dwell linkage is shown in Figure 2. Figure 2 shows a latch pin and a rotating latch. The mechanism in Figure 2 is similar to the general mechanism of Figure 1 in that the latch pin will move very little during a portion of the input bellcrank rotation. During the initial input link movement, the latch rotates approximately 90 to encapsulate the latch pin. As the input link continues to move there will be very little movement of the latch. The portion of the motion where dwell occurs is indicated in the figure.



Figure 2 Mechanism with Dwell Component


Figure 3 shows another means to achieve dwell in a mechanism. The mechanism in Figure 3 is called a hypocycloidal mechanism. In this mechanism, the planet gear will roll along the inside of the ring gear as the input crank is rotated. Point A will then follow the dashed lines shown in the figure as the input crank is rotated. When Point A is in the position shown, there will be very little motion of the output slider. This is where dwell occurs in the mechanism. In practice, a mechanism of this nature will have a input crank motion around 60 degrees maximum where a portion of the stroke will cause slider travel and another portion will cause the slider to be in dwell.



Figure 3 Hypocycloidal Dwell Mechanism


Mechanism textbooks provide many other examples of dwell mechanisms. Many involve four bar linkages. Dwell in four bar linkages can be designed into linkage motion using the methods discussed in Four Bar Linkage – Design Methods.