Dwell mechanism

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

Single vs. double dwell

Single dwell cam mechanisms

In a single dwell cam mechanism, there are three motion functions that follow each other and then repeat: a dwell, a rise, and a return function. It is sometimes desired to use a rise function such that its acceleration does not become zero at its end. To maintain continuity of acceleration, the return function should begin with a non-zero acceleration that matches the end of the rise function. There are a number of functions that can provide these motion requirements. A double-harmonic function is a common example used for single dwell.^[2]

Double dwell cam mechanisms

A typical arrangement of a double dwell Cam mechanism will contain the sections rise-dwell-fall-dwell and will be a piecewise function. The position of the follower remains stationary during the dwells causing velocity, acceleration, and jerk to equal zero.^[3]

Dwell with linkage mechanisms

Stephenson III dwell mechanism with prismatic joint. Simulation done with MeKin2D.

Linkage mechanisms can only provide approximate single dwell, and require a minimum of six links. A four-bar linkage mechanism must be picked first to provide the appropriate coupler curve with a circular region (or approximate circular region). A two-link dyad is then added to provide the output link with the desired dwell motion.^[4]

The concept for linkage dwell mechanisms is that a node located at the center of the circular arc segment of a coupler curve will remain relatively stationary. Therefore, actual dwell time will depend on the length of the approximate circular arc in the coupler curve. Initial designs may need optimization to improve the dwell characteristics.^[5]

Applications

Cam-follower dwell mechanisms are used in pairs in sewing machines to operate the four motion feed dogs, with one cam moving the dog up and down, and the other cam moving the dog forwards and backwards. The cams in this application are usually phased 90 degrees apart allowing a pause in the up/down movement of the dog while it is being moved forwards/backwards. A separate adjustable sliding block or link is used to control the amount of forwards/backwards movement of the dog.

Industrial applications include loading and unloading parts, or transporting a part to a machine and holding it in place for a manufacturing process.^[6]

Other applications include assembly lines, packaging machinery, machine tools, etc.

References

^ Uicker, J. Pennock, G. & Shigley, J. (2010). Theory of Machines and Mechanisms (4th ed.). Oxford University Press, p. 201.
^ Norton, R. (2008) Design of Machinery (4th ed.), McGraw Hill, p.427
^ Doane, J. (2015) Machine Analysis with Computer Applications for Mechanical Engineers (1st ed.), Wiley, p299
^ Norton, R. (2008) Design of Machinery (4th ed.), McGraw Hill, p.147
^ Doane, J. (2015) Machine Analysis with Computer Applications for Mechanical Engineers (1st ed.), Wiley, p167
^ Doane, J. (2015) Machine Analysis with Computer Applications for Mechanical Engineers (1st ed.), Wiley, p167

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

[2] Norton, R. (2008) Design of Machinery (4th ed.), McGraw Hill, p.427

[3] Doane, J. (2015) Machine Analysis with Computer Applications for Mechanical Engineers (1st ed.), Wiley, p299

[4] Norton, R. (2008) Design of Machinery (4th ed.), McGraw Hill, p.147

[5] Doane, J. (2015) Machine Analysis with Computer Applications for Mechanical Engineers (1st ed.), Wiley, p167

[6] Doane, J. (2015) Machine Analysis with Computer Applications for Mechanical Engineers (1st ed.), Wiley, p167

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