What Is Critical Speed?

Critical speed is the motor RPM at which a vibratory screener's operating frequency matches its natural frequency, triggering resonance and producing dangerous, uncontrolled vibration amplitudes. It is expressed as a specific RPM value (or narrow RPM band) that the screener must pass through quickly during startup and shutdown, never dwelling at this speed during operation. For most industrial round vibratory separators, critical speed falls in the range of 180 to 400 RPM.

During a normal startup, the vibrating motor accelerates from 0 RPM through the critical speed zone to its operating speed of 1,200–1,800 RPM in approximately 1–3 seconds. This brief pass-through produces a noticeable shudder or bounce that operators may observe, but the exposure time is too short to cause damage. The danger arises when the motor dwells at or near critical speed — a scenario most commonly caused by improperly programmed variable frequency drives (VFDs), mechanical issues preventing full-speed operation, or power supply problems.

Critical Speed vs. Operating Speed

Why This Matters

Critical speed is the single most important speed to avoid on a vibratory screener. Understanding it protects equipment and operators in three ways:

• VFD programming — Variable frequency drives must be programmed with a "skip band" or "critical frequency avoidance" zone around the critical speed. The VFD should accelerate through this band in under 2 seconds and never allow the motor to settle at any speed within the zone. Without this programming, even routine speed adjustments can trigger catastrophic resonance.
• Startup and shutdown protection — Across-the-line starters on vibratory screeners are preferred in many installations precisely because they bring the motor to full speed quickly, minimizing time at critical speed. Soft starters must be configured with aggressive ramp rates for the same reason.
• Maintenance awareness — Worn springs lose stiffness over time, which lowers the natural frequency and shifts the critical speed. While this typically moves critical speed further from operating speed, it changes the machine's dynamic behavior and should be monitored during routine maintenance.

Related Glossary Terms

• Resonance — The destructive condition triggered at critical speed
• Natural Frequency — The inherent system frequency that defines the critical speed point
• Vibration Isolation — Springs and mounts that set the natural frequency and critical speed
• G-Force — The controlled force output at proper operating speed above critical
• Spring — Component whose stiffness determines the critical speed

Critical Speed FAQs

What is critical speed on a vibratory screener?

Critical speed is the specific motor RPM at which the screener's operating frequency equals its natural frequency, triggering resonance. For most round vibratory separators, critical speed falls between 180 and 400 RPM, well below the normal operating range of 1,200 to 1,800 RPM. The machine passes through critical speed briefly during startup and shutdown. Extended operation at critical speed causes severe equipment damage.

Can a VFD cause a screener to operate at critical speed?

Yes. A variable frequency drive with a slow acceleration ramp can hold the motor at or near critical speed long enough to trigger damaging resonance. VFDs used on vibratory screeners must be programmed with fast ramp rates (typically under 3 seconds from 0 to operating speed) and must have the critical speed frequency range programmed as a skip zone that the drive accelerates through without stopping.