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 the field of High-Speed Industrial Automation, simple geometric curves often fail. To achieve the smooth, vibration-free motion required by modern CNC machines and textile equipment, engineers must turn to advanced mathematics: Polynomial Cam Functions.
Figure 1: High-speed automation requires mathematical precision that simple geometric curves cannot provide.
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The Fundamental Law of Cam Design
According to the "Bible" of mechanism design (Fundamentals of Machine Design, Robert L. Norton), any high-speed cam must obey two critical rules to avoid catastrophic machine failure:
CRITICAL DESIGN RULES:
1. Continuity: The cam function must be continuous through the first (Velocity) and second (Acceleration) derivatives across the entire 360-degree interval.
2. Finite Jerk: The jerk function (the derivative of acceleration) must remain finite across the entire interval.
Why "Jerk" Matters in Automation
Normally, cam motion isn't defined by a single equation. It is a sequence of segments:
(Rise → Dwell → Return → Dwell)
We cannot simply stitch these segments together without math. If there is a discontinuity in the acceleration graph (e.g., jumping from 0 to 10 m/s2 instantly), the derivative becomes infinite.
Figure 2: An "Infinite Jerk" (right) acts like a hammer blow to your machine, causing rapid failure.
This derivative is called Jerk. An infinite jerk theoretically requires infinite force, which in reality manifests as:
- Extreme Vibration and noise.
- Rapid wear of cam followers and linkages.
- Immediate need for expensive predictive maintenance.
Cycloid vs. Polynomial: The Flexibility Problem
The Cycloidal Motion we discussed in previous posts is excellent because it guarantees finite jerk. However, it has a major limitation: It typically must start and end at zero velocity.
Enter the Polynomial Cam Function.
Polynomials give the designer total freedom. With a polynomial, you can define specific non-zero velocities at the start or end of a segment.
Figure 3: Polynomials allow for "Flying Transfers" where the cam accelerates to match a conveyor's speed perfectly.
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The Fifth-Degree Polynomial
For most industrial applications, the Fifth-Degree Polynomial is the gold standard. It provides exactly enough coefficients to control 6 boundary conditions:
- Start Displacement
- End Displacement
- Start Velocity
- End Velocity
- Start Acceleration
- End Acceleration
The General Equation:
s = C0 + C1x + C2x2 + C3x3 + C4x4 + C5x5
Where:
s = Displacement
x = The ratio of current angle to total angle (θ / β)
C0...C5 = Constants to be solved based on boundary conditions
In the next post, we will apply the boundary conditions to solve for these constants and generate the final design equation.
Continue to Part 2:
Polynomial Cam Function (Derivation of Fifth-degree function) - Part 2
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