Natural frequency is the inherent vibration frequency at which a vibratory screener system will oscillate when disturbed and released, determined by the machine's total mass and the stiffness of its isolation springs. Measured in Hertz (Hz) or cycles per minute (CPM), natural frequency is the foundational engineering parameter that dictates where resonance occurs and how far above resonance the machine operates during normal production.
Every vibratory screener — from a compact 18-inch lab unit to a 60-inch production separator like the ScreenerKing SiftPro 60 — has a natural frequency defined by two variables: the vibrating mass (machine frame, motor, screens, and product load) and the spring rate (combined stiffness of all isolation springs). The standard formula is:
fn = (1 / 2π) × √(k / m)
Where k is the total spring stiffness in N/m and m is the vibrating mass in kg. Industrial round vibratory separators typically have a natural frequency of 3–6 Hz (180–360 CPM), while their motors operate at 1,200–1,800 RPM — well above the resonance zone.
Natural Frequency of Common Screener Configurations
| Screener Size | Typical Mass (kg) | Typical Spring Rate (N/mm total) | Approx. Natural Frequency | Operating Speed |
|---|---|---|---|---|
| 18" (lab/pilot) | 30 – 50 | 8 – 15 | 3 – 5 Hz | 1,200 – 1,800 RPM |
| 24" single-deck | 60 – 100 | 15 – 25 | 3 – 4.5 Hz | 1,200 – 1,750 RPM |
| 30" multi-deck | 100 – 180 | 20 – 40 | 2.5 – 4 Hz | 1,200 – 1,750 RPM |
| 48" production (SiftPro 48) | 200 – 350 | 40 – 80 | 2.5 – 3.5 Hz | 1,200 – 1,750 RPM |
| 60" production (SiftPro 60) | 300 – 500 | 60 – 120 | 2 – 3.5 Hz | 1,200 – 1,750 RPM |
Why This Matters
Natural frequency is the invisible engineering boundary that separates safe, productive operation from destructive resonance. It matters for three practical reasons:
- Spring selection — The isolation springs are the primary component that sets natural frequency. Installing springs with the wrong stiffness shifts the natural frequency and can move it dangerously close to the operating speed. Always replace springs with the correct specification.
- VFD programming — When using a variable frequency drive to adjust motor speed, the VFD must be programmed to skip past the speed range corresponding to natural frequency. Operating at or near natural frequency, even briefly, causes resonance that can damage the machine within seconds.
- Machine loading — Adding mass to the system (heavier product load, additional decks, auxiliary equipment) lowers the natural frequency. While this moves natural frequency further from operating speed (generally desirable), it also changes the G-force output for a given motor setting.
Related Glossary Terms
- Resonance — The destructive condition that occurs when operating frequency equals natural frequency
- Critical Speed — The motor RPM that corresponds to the natural frequency
- Vibration Isolation — The spring system that defines the natural frequency
- Spring — The component whose stiffness directly determines natural frequency
- G-Force — The force output that changes when mass or spring rate shifts
Natural Frequency FAQs
What is natural frequency in vibratory screening?
Natural frequency is the frequency at which a vibratory screener naturally oscillates based on its mass and spring stiffness. It is measured in Hertz (Hz) or cycles per minute. A screener's operating speed must be well above its natural frequency — typically 3 to 5 times higher — to avoid resonance. Natural frequency is calculated as fn = (1/2π) × √(k/m), where k is spring stiffness and m is mass.
Why does natural frequency matter for vibratory screener springs?
The isolation springs are the primary component that sets the natural frequency. Stiffer springs raise the natural frequency; softer springs lower it. If springs are replaced with incorrect stiffness, the natural frequency shifts and may move closer to the operating speed, increasing the risk of resonance. Always replace springs with the correct OEM specification or verified equivalent from ScreenerKing.
Correct Springs Maintain Safe Natural Frequency
ScreenerKing manufactures replacement springs engineered to match OEM natural frequency specifications for Sweco, Kason, Midwestern, Cleveland Vibratory, Russell Finex, and Rotex vibratory separators. Our engineering team can verify spring rate compatibility before you order.
