6.1 SELECTION OF MECHANISM TYPES

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When trying to select a mechanism type one way is, (NOTE: there is no exact way to this, and you need to exercise some creativity)

1. Examine the motion profile required.

2. Determine the distinct phases of motion, and estimate the d.o.f. needs for each/all.

3. Look for crosses in path, dwell spots, motion reversal, straight line segments, etc.

4. Consider the various configurations you know of and how they satisfy parts or all of the motion. The dof can be used to suggest the number of links and types of joints required (i.e. Grashof's criteria).

5. If no satisfactory solutions are available, the look to a handbook of available mechanisms for other ideas.

6. Use design techniques for the specific mechanisms to determine dimensions

7. Test the design on Working Model, or by building a prototype.

Shigley and Uicker identify three types of common linkage problems to look for,

Function generation - in this case we want to drive an output shaft (angular displacement) using an input shaft (angular displacement). This function can also be performed using cams with oscillating followers.

Prescribed Path Shape - Here we follow a point on the mechanism as it follows a path through space.

Body Guidance - A combination of translation and rotation may be used to move an object.

Erdman and Sandor also introduce the idea of choosing mechanisms into stages,

1. Type synthesis

2. Dimensional synthesis