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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).
Contents
Single Dwell Cam Mechanisms
In a single dwell Cam mechanism it is desired to use a function for the rise that does not return its acceleration to zero at the end of the function. The fall function should begin with a non-zero acceleration matching the end of the rise. A double-harmonic function is a common example used for single dwell.
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.
Dwell with Linkage Mechanisms
Linkage (mechanical) 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.
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.
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 or unloading parts, or transporting a part to a machine and holding it in place for a manufacturing process.
Other applications include assembly lines, package machinery, machine tools etc.