Crusher reduction ratio is the feed particle size divided by the product particle size on the same measurement basis. For plant surveys, the clearest form is usually R80 = F80 / P80, where F80 and P80 are the sizes through which 80% of the feed and product pass by mass. A ratio of 5 means that the selected characteristic feed size is five times the corresponding product size; it does not mean every particle became five times smaller.
Use one basis and one unit:R80 = F80 / P80If F80 is 300 mm and P80 is 60 mm, R80 = 300 / 60 = 5.0. The result is dimensionless.
Start With Comparable Particle-Size Data
Reduction ratio is only meaningful when the numerator and denominator describe matching streams and use the same percentile. F80 comes from a representative size distribution of the material entering the defined crusher or circuit. P80 comes from the corresponding product stream after conditions have stabilized.
Do not divide the largest observed feed lump by P80, or F80 by the crusher’s closed-side setting, and label the result R80. Those quantities describe different aspects of the duty. Top size can help check receiving limits, while CSS is a machine setting; neither is automatically equal to an 80% passing size.
| Quantity | Meaning | Correct Use | Common Error |
|---|---|---|---|
| F80 | Feed size at 80% cumulative passing by mass. | Numerator in an R80 calculation for a defined feed stream. | Replacing it with maximum lump size or crusher gape. |
| P80 | Product size at 80% cumulative passing by mass. | Denominator measured from the corresponding product stream. | Assuming it equals CSS or a screen aperture. |
| Top size | Largest defined or observed particle size, depending on the stated method. | Receiving, safety, scalping and maximum-size checks when the method is explicit. | Mixing a top-size numerator with a P80 denominator. |
| CSS | Minimum discharge opening during the crusher cycle for the specified measuring convention. | Operating and chamber-setting control for the relevant crusher. | Treating the setting as a measured product size. |
Worked Example: Calculate One Crushing Stage
Assume a stable survey of a primary crushing stage gives an F80 of 360 mm and a P80 of 72 mm. Both values come from representative feed and product size distributions and use millimetres.
- Write the measured values: F80 = 360 mm; P80 = 72 mm.
- Divide on the same basis: 360 / 72 = 5.
- Report the result as R80 = 5.0, together with the survey date, throughput, crusher setting, feed condition and sampling locations.
This hypothetical result describes the measured size change during that survey. It is not a guaranteed ratio for every jaw crusher. Rock breakage characteristics, feed gradation, fines, moisture, chamber condition, setting, stroke, speed and feeding method can change the product distribution and available capacity.
How Crusher Reduction Ratio Changes Across a Circuit
For an idealized two-stage flow with no classification, bypass or blending between measurements, the stage ratios can be multiplied:
- Primary stage: 600 mm F80 to 120 mm P80 gives 5:1.
- Secondary stage: 120 mm F80 to 30 mm P80 gives 4:1.
- Overall circuit: 600 mm to 30 mm gives 20:1, which also equals 5 x 4.
Real circuits need more care. A vibrating screen can remove finished material and return oversize, while stockpiles, bypass streams and circulating load can change the distribution entering the next crusher. A screen classifies particles; it does not break them. Calculate each crusher ratio from its actual combined feed and product, and calculate the overall ratio from the defined circuit feed and final product. Do not multiply stage values unless the intermediate streams align. Chalmers University research on crushing-plant optimization examines how crusher settings, recirculating material and downstream screens interact at plant level.
This distinction matters when reviewing the complete primary crushing stage or comparing a single crusher with a multi-stage plant. The same ratio can describe very different process arrangements.
CSS Influences Product Size but Does Not Define It Alone
A smaller setting often shifts the product distribution finer, but the relationship is not one-to-one. Chamber profile, wear, feed distribution, material strength, crusher type and operating condition also matter. The guide to crusher closed-side setting explains how the setting interacts with throughput, recirculating load and product size.
Industrial survey data should therefore pair R80 with the measured CSS and the full feed and product distributions. Interpret changes against the installed crusher, chamber, material and operating condition rather than transferring a ratio from another duty.
What Crusher Reduction Ratio Cannot Tell You
R80 compresses two particle-size distributions into one number. It is useful for describing size reduction, but it cannot show whether the plant produced the required saleable fractions or achieved them efficiently.
- Capacity: identical ratios can occur at different tonnes per hour.
- Product shape: R80 does not describe flakiness, elongation or cubical shape.
- Fines: two products with the same P80 can contain different fine fractions.
- Energy: the ratio does not state power draw or specific energy.
- Wear: it does not identify liner profile, blow-bar condition or uneven chamber wear.
- Yield: it does not show how much material meets each final product specification.
Review ratio together with throughput, power, circulating load, screen efficiency, complete size distributions, product shape and wear observations. For compression crushing, compare the measured duty with the relevant cone crusher chamber and application rather than using a generic ratio as the only selection criterion.
Make Each Survey Reproducible
A reliable ratio needs a traceable measurement record. Identify the exact feed and product streams, allow the circuit to reach a representative condition, collect suitable samples under the site’s approved procedure, complete a consistent sieve analysis and retain the full cumulative passing curves.
Record throughput, crusher speed and setting, screen apertures, recirculating load where available, liner or wear-part condition, feed source, moisture and any abnormal events. Repeating the same method creates a trend that can support process diagnosis. Changing the sampling point or percentile without identifying the change makes comparison unreliable.
Use Reduction Ratio as One Circuit Constraint
Reduction ratio is most useful as a clearly defined size-change measure. Use R80 for like-for-like surveys, retain top-size and CSS checks for their own purposes, and evaluate the ratio with capacity, gradation, shape, power, wear and screening performance. The Vanore stationary crusher range provides equipment context for primary, secondary, tertiary and shaping duties; actual stage selection must still follow the material, feed distribution, target products and complete circuit arrangement.