Dwell Mechanisms

'A dwell-mechanism (either a linkage or cam-follower type) is an intermittent motion mechanism that alternates forward and return motion between holding position(s)'.[1]

Another definition might be: A 'dwell-mechanism' is one in which one part reciprocates or oscillates, but also remains 'reasonably' stationary for a 'reasonable' period of the machine cycle, while the input-crank continues to rotate.

Here, 'dwell-mechanisms' do not include 'cam-mechanisms' as it is obvious that a dwell segment can easily be inserted into the motion of a cam-follower. Thus, dwell-mechanisms only have parts and joints with simple kinematic-chains. Usually, they are 6-bar mechanisms, and sometimes more.

Mostly, a dwell-mechanism uses the properties of some section of a coupler curve. A coupler-curve is traced onto the mechanism plane by a point that is fixed to the coupler of a four-bar mechanism. A dyad is added to the four-bar to give a six-bar mechanism, and one of the Parts in the extra dyad remains stationary while the crank continues to rotate.

There are two properties of a coupler-curve that can be used to give a 'dwell mechanism'.

The coupler-curve has a section that...

...is straight

or

...has a constant radius

...then you can use the coupler-curve to design a dwell mechanism.

Reference: [1] Uicker, J. Pennock, G. & Shigley, J. (2010). Theory of Machines and Mechanisms (4th ed.). Oxford University Press, p. 201.

Dwell Mechanism -  from a Coupler-Curve with a Constant Radius

The coupler-curve has a section with a constant radius. An RRR Dyad is added to the four-bar such that one part has a length with the same radius as the coupler-curve. The other Part is joined to the frame, and its other end is joined to the first part such that the joint remains at the centre of the curve during the constant-radius period.

Video of 6-bar Dwell Mechanism that uses a Coupler Curve that has a 'Constant Radius'.

 

Dwell Mechanism - from a Coupler-Curve with a Straight Line

This mechanism has two Parts that have a dwell period. There are two coupler-curves given by the the 'connecting-rod' of a Slider-Crank Mechanism. Each coupler-curve has within it a straight-line. RPR Dyads are added to the four-bar, such that one Part of each new dyad is joined to the frame directly in-line with the axis of each straight-line. The other Part is joined with Pin-Joint to the Coupler at the point that coincides with the straight-line. The two Parts of each Dyad are joined together with Slide-Joints such that the other Parts of the Dyads moves along the axis of the straight-line.

Video of 6-bar Dwell Mechanism that uses a Coupler Curve with a 'Straight-Line'.

 

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