What Is Oversize (Overs / Tails)? Definition & Role in Vibratory Screening

Oversize (also called overs or tails) is the fraction of material that is too large to pass through the screen aperture on a vibratory screener, remaining on the screen surface and discharging through the oversize outlet. Depending on the application, oversize may be the desired product, a reject stream, or an intermediate fraction destined for further processing. It is the complement of undersize (fines) — together, oversize and undersize account for 100% of the material fed to the screener.

In a multi-deck vibratory separator, each deck produces its own oversize stream. The top deck's oversize is the coarsest fraction; material that passes through the top deck becomes the feed for the second deck, which produces its own oversize, and so on. This cascading design allows a single piece of equipment to produce multiple graded oversize fractions in one pass.

Oversize in Different Applications

Oversize Quality and Carryover

A perfectly efficient screen would produce an oversize stream containing zero particles smaller than the aperture. In practice, some undersize particles are always carried over with the oversize — this is called carryover and is the inverse measure of screening efficiency. A screener operating at 90% efficiency has 10% carryover — meaning 10% of the undersize material in the feed ended up in the oversize discharge instead of passing through.

Carryover is caused by:

• Excessive feed rate — Material travels across the screen too quickly for all undersize particles to find an opening.
• Blinding — Blocked openings reduce the effective open area, leaving fewer paths for undersize to pass through.
• Deep material bed — Undersize particles buried beneath oversize material never contact the screen surface.
• Insufficient vibration — Without adequate amplitude, material does not stratify properly (fines do not settle to the screen surface).
• Moisture or static — Causes fine particles to cling to coarser particles instead of separating.

Reducing Carryover in the Oversize Stream

1. Reduce feed rate to increase residence time on the screen surface.
2. Install de-blinding aids to maintain effective open area.
3. Optimize counterweight settings to improve material stratification.
4. Increase screen area by upgrading to a larger diameter screener.
5. Re-screen the oversize through a second pass to recover trapped undersize.

Related Glossary Terms

• Undersize / Fines — The complementary fraction that passes through the screen
• Aperture — The opening size that determines the oversize/undersize split
• Screening Efficiency — Measures how cleanly oversize and undersize are separated
• Deck — Each deck produces its own oversize stream
• Mesh Size — Defines the cut point that determines what is oversize
• Blinding — Increases carryover in the oversize stream

Oversize / Overs / Tails FAQs

What is oversize material in vibratory screening?

Oversize material (also called overs or tails) is any material that is too large to pass through the screen openings on a vibratory screener. These particles remain on the screen surface and are discharged through a separate outlet. Depending on the application, oversize may be the desired product (in de-dusting) or waste (in safety screening where oversize represents contaminants or agglomerates).

Why is there undersize material in my oversize discharge?

Undersize material in the oversize discharge is called carryover and indicates imperfect screening efficiency. Common causes include excessive feed rate, screen blinding, insufficient vibration, and high bed depth. Reducing feed rate, adding de-blinding devices, and optimizing vibration settings will reduce carryover.

What is the difference between oversize, overs, and tails?

Oversize, overs, and tails all refer to the same thing — the material retained on the screen that does not pass through the apertures. "Oversize" and "overs" are the most common terms in vibratory screening. "Tails" is used more frequently in mining, mineral processing, and some chemical processing contexts. All three terms are interchangeable.