Mechanical Design Handbook

In the previous post [ 3-Position Motion Generation Four-Bar Linkage Synthesis ], the locations of the fixed ground pivots (O 2 and O 4 ) were mathematically determined by the positions of points A and B. The Problem: Sometimes, these calculated fixed pivots land in impossible locations—inside another machine part, off the machine base, or too far away. The Solution: We use Alternate Moving Pivots . Instead of using the endpoints of the line AB, we create new points (C and D) that are rigidly attached to the moving body. By adjusting the location of C and D, we can steer the fixed pivots (O 2 and O 4 ) to desirable locations. Step 1: Define the Desired Motion Draw the coupler link AB in its three design positions: A 1 B 1 , A 2 B 2 , and A 3 B 3 . Step 2: Define Alternate Moving Pivots (C and D) This is the critical step. We attach a "virtual" rigid shape to line AB to define new points C and D. Procedure: 1....

In real-world engineering, a mechanism often needs to guide a part through more than just a start and end point. It usually requires passing through 3 specified positions to clear obstacles or perform complex tasks. This technique is known as 3-Position Motion Generation . We can extend the logic from our previous post [ Four-bar linkage Synthesis using Unigraphics NX4 Sketch ] to solve this problem geometrically within the CAD environment. The Design Challenge Assume we must design a mechanism to move Link AB through three specific positions (A 1 B 1 , A 2 B 2 , A 3 B 3 ) while avoiding an obstacle (represented by the rectangle below). Step-by-Step Synthesis 1. Define the Positions: Draw Link AB in its three design positions: A 1 B 1 , A 2 B 2 , and A 3 B 3 . 2. Geometric Synthesis for Pivot O 2 : To find the fixed pivot that allows point A to move through all three locations, we must find the center of the circle that passes through A 1...

In advanced Mechanism Design , we often face the challenge of moving a rigid body from one specific position to another. This process is known as Motion Generation Synthesis . While sophisticated solver software exists, you can perform this synthesis geometrically using the Constraint-Based Sketcher found in any modern CAD package like Unigraphics NX (Siemens NX), SolidWorks, or CATIA. The Goal: Moving a Line in a Plane Assume we need to design a 4-bar linkage that moves a coupler link from position AB to position A'B' . Step-by-Step Geometric Synthesis 1. Define the Positions: Draw the link AB (Start Position) and A'B' (End Position) in the NX Sketcher. 2. Locate the First Pivot (O 2 ): Draw a construction line connecting point A to A'. Create a Perpendicular Bisector of line AA'. Theory: Any point located on this bisector is equidistant from A and A', meaning it can serve as a fixed pivot point. ...