What Is Carry-Over in Vibratory Screening?

Carry-over in vibratory screening is the undesired passage of undersize particles into the oversize discharge stream, representing material that should have passed through the screen but was instead swept over the top with the rejected fraction. It is the most common symptom of a screener that is not operating at full efficiency, and reducing carry-over is the primary goal of most screening optimization efforts.

In an ideal screening operation, every particle smaller than the screen aperture would pass through to the undersize fraction and every particle larger would discharge as oversize. In practice, some undersize particles never reach the screen surface, never encounter an open aperture, or are held to larger particles by moisture or static charge. These particles "carry over" with the oversize discharge. Carry-over is quantified by sampling the oversize stream and measuring the percentage of particles that are smaller than the nominal screen opening. Industry-acceptable carry-over rates vary: safety screening may tolerate 10–20% carry-over, while precision classification may require less than 2%.

Common Causes and Solutions for Carry-Over

Why This Matters

Carry-over is the visible indicator that a screening operation is losing money or producing off-spec material:

• Yield loss — In operations where the undersize is the desired product (e.g., flour milling, powder classification), carry-over means good product is being discarded with the oversize waste. A 5% carry-over rate on a 5,000 lbs/hr operation means 250 lbs/hr of saleable product is lost.
• Oversize contamination — In operations where the oversize is recycled (e.g., grinding circuits), carry-over means already-sized material is being unnecessarily reprocessed, wasting energy and reducing circuit capacity.
• Quality indicator — Carry-over percentage is a direct measure of screening efficiency. Monitoring it over time reveals gradual degradation from screen wear, increasing blinding, or process changes upstream that alter the feed characteristics.
• Troubleshooting starting point — When product quality issues arise, carry-over measurement is the first diagnostic test. It immediately tells you whether the screener is the source of the problem and quantifies the magnitude.

Related Glossary Terms

• Screening Efficiency — The metric calculated from carry-over measurement
• Separation Curve — Shows the probability of carry-over across all particle sizes
• Feed Rate — The most common cause of excessive carry-over
• Residence Time — Insufficient time on screen causes carry-over
• Blinding — Clogged screen that forces carry-over
• Material Bed Depth — Deep bed prevents fines from reaching screen

Carry-Over FAQs

What is carry-over in vibratory screening?

Carry-over is undersize particles in the oversize discharge — material that should have passed through the screen but was swept over the top instead. It is measured as the percentage of undersize material in the oversize stream. Zero carry-over is theoretical; acceptable levels range from less than 2% for precision classification to 10–20% for safety screening, depending on application requirements.

What causes carry-over on a vibratory screener?

Common causes include excessive feed rate (deep bed traps fines), insufficient residence time, blinded screen cloth, low G-force, moisture causing agglomeration, and static charge. Systematic troubleshooting checks each factor to identify and correct the root cause.