The angle of repose is the steepest angle at which a pile of granular material remains stable without sliding, measured from the horizontal base to the slope surface, indicating the material's flowability. It is expressed in degrees and provides a quick, practical assessment of how freely a material flows. A low angle of repose (under 30 degrees) indicates a free-flowing material like dry sand or plastic pellets. A high angle (over 45 degrees) indicates a cohesive or sticky material like wet flour or fine pigments that resists flow.
In vibratory screening, the angle of repose influences how material moves across the screen surface. Free-flowing materials with low angles of repose spread quickly and may cross the screen too fast, reducing residence time and screening efficiency. Cohesive materials with high angles of repose tend to pile up at the feed inlet and require higher amplitude or adjusted lead angle settings to maintain adequate material transport. ScreenerKing, Sweco, Kason, and Midwestern separator operators use angle of repose as one indicator when tuning vibration parameters for optimal screening performance.
Angle of Repose Classification
| Angle of Repose | Flow Characteristic | Example Materials | Screening Adjustment |
|---|---|---|---|
| 25-30 degrees | Very free-flowing | Dry sand, glass beads, plastic pellets | Reduce lead angle to slow transport; lower amplitude |
| 30-38 degrees | Free-flowing | Granulated sugar, dry salt, fertilizer | Standard settings; minimal adjustment needed |
| 38-45 degrees | Fair flowability | Flour, cement, fine powders | Increase amplitude; consider anti-blinding aids |
| 45-55 degrees | Cohesive/poor flow | Wet powders, fine pigments, cocoa | Maximum amplitude; ball deck or ultrasonic deblinding |
| Over 55 degrees | Very cohesive | Damp clay, sticky chemicals | Pre-condition material; special screen media may be needed |
Why This Matters in Vibratory Screening
- Material transport rate — The angle of repose predicts how quickly material moves from the feed inlet to the discharge spout. Free-flowing materials move fast; cohesive materials move slowly. Lead angle adjustments compensate for both extremes.
- Screen blinding risk — Cohesive materials with high angles of repose are more likely to cause plugging and blinding because particles stick to wire surfaces and to each other rather than flowing freely through openings.
- Feed distribution — High-angle-of-repose materials tend to pile at the feed point rather than spreading across the full screen area, reducing effective screening surface utilization.
- Vibration tuning — Materials with different flow characteristics require different amplitude and frequency settings. Angle of repose provides a starting point for vibration parameter selection.
Related Glossary Terms
- Bulk Density — Another key material property for screener sizing
- Lead Angle — Adjusted based on material flow characteristics
- Amplitude — Increased for cohesive, high-angle materials
- Hygroscopic Material — Moisture absorption increases angle of repose
- Stratification — Cohesive materials resist stratification
Angle of Repose FAQs
What is the angle of repose?
The angle of repose is the steepest angle at which a pile of granular material remains stable without sliding, measured in degrees from horizontal. A low angle (under 30 degrees) indicates free-flowing material; a high angle (over 45 degrees) indicates cohesive, poorly flowing material.
How does angle of repose affect screening?
Materials with high angles of repose are cohesive and resist flow across the screen, requiring higher amplitude and potentially ball deck or ultrasonic deblinding aids. Free-flowing materials with low angles move quickly and may need slower transport rates (lower lead angle) for adequate screening time.
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