Every linear motion design starts with the same choice: How do you convert rotary motor motion into linear travel? The two most common answers are the Lead Screw (simple, cheap, friction-based) and the Ball Screw (complex, expensive, rolling-based). Making the wrong choice here is costly. Use a lead screw where you need precision, and you get backlash. Use a ball screw in a vertical lift without a brake, and your load crashes to the floor. In this guide, we compare them side-by-side. Table of Contents 1. The Physics: Sliding vs. Rolling 2. Efficiency & The "Back-Driving" Danger 3. Accuracy and Backlash 4. Selection Table Advertisement 1. The Physics: Sliding vs. Rolling The fundamental difference is friction. Lead Screws rely on Sliding Friction . The nut (often bronze or plastic) slides directly against the steel screw threads. This generates heat and wear. Ball Screws re...
In [3-Position Synthesis with Inversion Method - Part 2], we successfully determined the locations of the moving pivots (G and H) relative to our fixed ground pivots (O2 and O4).
However, finding the points is only half the battle. Before we commit to manufacturing or detailed 3D modeling, we must verify that the mechanism actually moves smoothly between all three positions without locking up (toggle positions) or deviating from the path.
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Constructing the Kinematic Chain
Now that we have our four critical points (O2, O4, G, H), we need to "build" the mechanism links within the CAD Sketcher environment:
- Input Link (Link 2): Draw a solid line connecting the fixed ground O2 to the moving pivot G.
- Output Link (Link 4): Draw a solid line connecting the fixed ground O4 to the moving pivot H.
- Coupler Link (Link 3): This is the most important part. You must draw a rigid triangle connecting G, H, and the original coupler line A1B1.
(Critical: Apply "Rigid" constraints or dimension the triangle sides to ensure G-H-A-B moves as one solid body).
Setting up the "Animate Dimension" Simulation
To simulate the motion without leaving the sketcher, we use the Animate Dimension command (standard in NX, SolidWorks, and Inventor). This tool drives a specific dimension through a range of values, acting as a virtual motor.
Step-by-Step Configuration:
- 1. Add a Driving Dimension: Place an angular dimension between the Input Link (O2-G) and the horizontal ground plane.
- 2. Launch Animation: Right-click the dimension or select Tools > Sketch Constraints > Animate Dimension.
- 3. Set Limits:
- Lower Limit: The angle corresponding to Position 1.
- Upper Limit: The angle corresponding to Position 3.
- 4. Resolution: Set "Steps per Cycle" to at least 100 to ensure smooth motion visibility.
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Visual Verification: What to Watch For
Watch the simulation video below. As the mechanism moves, observe the Coupler Line (AB).
Figure 1: Simulation verifying the coupler passes through all 3 target positions relative to the fixed ground.
Success Criteria Checklist
- Accuracy: The line AB must perfectly overlap with the "ghost" positions A2B2 and A3B3 as it passes through them.
- Stability (Branch Defect): The mechanism must not "jump" or flip configurations between positions. If the lines cross unexpectedly, the solution is invalid.
- Force Transmission: The transmission angles should remain reasonable (ideally between 40° and 140°) to ensure the mechanism doesn't bind or lock up.
Conclusion
By using the Inversion Method combined with Sketch Animation, we have solved a complex constraint problem—fitting a mechanism to pre-existing mounting holes—completely within the 2D CAD environment. This Virtual Prototyping technique saves hours of trial-and-error in the detailed design phase.
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