Parallel Crank Coupling 3d model
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Parallel Crank Coupling

Parallel Crank Coupling

by GrabCAD
Last crawled date: 7 months, 2 weeks ago
Neil Sclater, Mechanisms and Mechanical Devices Sourcebook, page 315 says:

A parallel-crank coupling drives the overhead camshaft on engines. Each shaft has at least two cranks connected by links. Each must have full symmetry for constant velocity action and to avoid dead points.

IMHO, the full symmetry yields collision between connecting bars (blue color) at certain angular position. The only option for both bars to vault over each other is full asymmetry as shown in the figure.
In addition, ways and means of dynamic balance must be considered.
Just drive the angular constraint in the model browser. IDE: Autodesk Inventor v.2017