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 advanced Mechanism Design, engineers 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 Siemens NX, SolidWorks, or CATIA.
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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 (Start) to position A'B' (Target).
Figure 1: Defining the Start Position (AB) and the Target Position (A'B').
Step-by-Step Geometric Synthesis
The logic relies on finding the center of rotation for the moving points.
1. Locate the First Pivot (O2):
Draw a construction line connecting point A to A'. Then, 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.
Figure 2: Creating the perpendicular bisector for vector AA' to locate ground pivot O2.
2. Select Pivot O2:
Select any convenient point on the bisector to be the fixed ground pivot O2.
(Example: Length O2A = 500 mm). Draw line O2A to represent Link 1.
3. Locate the Second Pivot (O4):
Repeat the process for points B and B'. Draw a line from B to B' and create its perpendicular bisector.
Figure 3: Locating the second ground pivot O4 using the bisector of BB'.
4. Select Pivot O4:
Choose a point on this bisector to be fixed pivot O4.
(Example: We selected a 60° angle). Draw line O4B to represent Link 2.
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Constructing the Kinematic Chain
Now we build the full linkage to test the motion:
- Draw line O2M equal in length to O2A.
- Draw line O4N equal in length to O4B.
- Draw the coupler line MN equal in length to AB.
- Apply geometric constraints to ensure the chain stays connected.
Figure 4: The fully constrained kinematic chain ready for simulation.
Validation via "Animate Dimension"
To verify the synthesis, we use the Animate Dimension tool (available in NX, SolidWorks, and Inventor).
Set an angular dimension between the ground and the input link. Configure the animation range (e.g., 0 to 60 degrees) and step count (150 steps for smoothness).
Figure 5: Configuring the animation parameters to drive the input crank.
Watch the Kinematic Synthesis Result:
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