Skip to main content

Featured Post

Why I Wrote The Sheet Mechanic (And Why Calculations Aren’t Enough)

For engineers who already know the math—but still lose projects. For the last few years, I’ve been sharing technical guides here on Mechanical Design Handbook —how to size a motor, how to calculate fits, and (as you recently read) how to choose between timing belts and ball screws. But after 25 years in industrial automation, I realized something uncomfortable: Projects rarely fail because the math was wrong. They fail because: The client changed the scope three times in one week. A critical vendor lied about a shipping date (and no one verified it). The installation technician couldn’t fit a wrench into the gap we designed. University taught us the physics. It didn’t teach us the reality. That gap is why I wrote my new book, The Sheet Mechanic . This is not a textbook. It is a field manual for the messy, political, and chaotic space between the CAD model and the factory floor. It captures the systems I’ve used to survive industrial projec...
Post a Comment
NEW RELEASE: Stop trying to be a Hero. Start being a Mechanic. Get "The Sheet Mechanic" on Amazon »
Disclosure: As an Amazon Associate, I earn from qualifying purchases.

3-Position Linkage Synthesis: Motion Generation in CAD

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 CAD Sketcher] to solve this problem geometrically within a modern CAD environment like Siemens NX, SolidWorks, or CATIA.

Advertisement

The Design Challenge

Assume we must design a mechanism to move Link AB through three specific positions (A1B1, A2B2, A3B3) while avoiding an obstacle (represented by the rectangle below).

Diagram showing three required positions for linkage synthesis
Figure 1: Defining the three target positions (A1B1, A2B2, A3B3) relative to the obstacle.

Step-by-Step Synthesis

1. Define the Positions:
Draw Link AB in its three design positions: A1B1, A2B2, and A3B3.

2. Geometric Synthesis for Pivot O2:
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 A1, A2, and A3.

  • Draw construction lines connecting A1 to A2 and A2 to A3.
  • Create Perpendicular Bisectors for both lines.
  • The intersection of these bisectors is the fixed pivot O2.
  • Draw line O2A1. This is your Input Link (Link 2).
Geometric construction using perpendicular bisectors to find pivot O2
Figure 2: Intersecting the bisectors of A1-A2 and A2-A3 locates the first ground pivot O2.

3. Geometric Synthesis for Pivot O4:
Repeat the exact same logic for point B to find the Follower Link pivot.

  • Draw construction lines connecting B1 to B2 and B2 to B3.
  • Create Perpendicular Bisectors for both lines.
  • The intersection is the fixed pivot O4.
  • Draw line O4B1. This is your Follower Link (Link 4).
Repeating the bisector process for point B to find pivot O4
Figure 3: Finding the second ground pivot O4 using the positions of Point B.
Advertisement

Building and Validating the Kinematic Chain

Now we construct the linkage geometry to verify the motion path.

Constraining the 4-bar linkage geometry in NX Sketcher
Figure 4: Building the constraint network. Links O2M, O4N, and MN form the mobile mechanism.
  1. Draw line O2M equal in length to O2A1.
  2. Draw line O4N equal in length to O4B1.
  3. Draw coupler line MN equal in length to A1B1.
  4. Set an angular driving dimension (e.g., 15 degrees) between O4B1 and O4N.

Simulation via "Animate Dimension":
In the CAD Sketcher, select the angular dimension and set the limits based on the synthesized range.
Example: Lower Limit: 0 | Upper Limit: 56.355 | Steps: 150

Using the Animate Dimension tool to simulate motion
Figure 5: Configuring the animation parameters to drive the linkage through the synthesized path.

Video Result: 3-Position Synthesis

Watch how the synthesized mechanism perfectly guides the link through all three positions while avoiding the obstacle.

Recommended Reading on Kinematics

Labels:

Comments

Post a Comment

Popular posts from this blog

Hoeken's Linkage: Kinematics and Walking Robot Design

Figure 1: Animated simulation of the Hoeken’s Linkage showing the characteristic "tear-drop" coupler curve. 🚀 New Design Guide Available Don't just read about it—build it. Check out our new tutorial: How to Design a Hoeken’s Linkage in Excel (with Free VBA Simulator) » Introduction to the Hoekens Linkage The Hoekens linkage is a specialized four-bar mechanism designed to convert rotational motion into an approximate straight-line motion. While it serves a similar purpose to other straight-line generators, its unique coupler curve—a "tear-drop" shape—makes it exceptionally useful for intermittent motion and walking machines. One of the most fascinating aspects of kinematic theory is the concept of "Cognates." The Hoekens linkage is actually a cognate linkage of the Chebyshev Straight-line Mechanism . This means that while the physical structure and link lengths differ, they can generate...
Post a Comment
Read more about this Mechanical Design Article »

Dowel Pins & Locating Pins: The Basics of Fixture Design

Dowel pins are precision cylindrical pins used for accurate part alignment in assemblies. They control position, not clamping force. This guide explains tolerances, fits, sizing rules, and design best practices. Figure 1: A typical fixture setup. Notice how dowel pins (silver) provide precise location, while bolts (not shown here) provide the clamping force. In the world of Precision Engineering , the difference between a high-quality product and a scrap part often comes down to microns. While bolts hold parts together, they are terrible at positioning them. This is where Dowel Pins and Locating Pins become essential components in industrial tooling . Advertisement What is a Dowel Pin? Dowel pins are precision-ground fasteners used to secure the relative position of two parts. They are typically machined to extremely tight tolerances (often within 0.0001 inches) and are available in materials like: Hardened Steel: For high-wea...
Post a Comment
Read more about this Mechanical Design Article »

Ball Detent Torque Limiter – Working Principle & Selection

Figure 1: The ball detent mechanism provides precise overload protection by disengaging instantly when the torque limit is exceeded. The First Line of Defense: Overload Clutches In high-speed automation and heavy industrial machinery, a "jam" is not a matter of if , but when . Whether it is a cardboard box getting stuck in a packaging machine or a tool crashing in a CNC lathe, the resulting torque spike can destroy gearboxes, twist shafts, and burn out expensive servo motors in milliseconds. A torque limiter (or overload clutch) is the mechanical fuse of the drive system. While electronic monitoring (current limiting) is common, it is often too slow to prevent physical damage from the massive kinetic energy stored in the system inertia. A mechanical torque limiter provides a physical disconnect that operates in a fraction of a second. Search for Torque Limiters & Safety Couplings Advertisement Why Choose ...
Post a Comment
Read more about this Mechanical Design Article »