Crusher reduction ratio explained: definition, calculation and limits by crusher type

Reduction ratio is the ratio between the size of what goes into a crusher and the size of what comes out. It is the number everyone quotes to compare machines, and the one that misleads the most when nobody says how it was calculated: two people talking about "a reduction of so much" are often not talking about the same thing, because one is comparing maximum sizes and the other is comparing characteristic points on size distribution curves.

This article fixes the useful definition, works through the calculation on an explicitly fictitious example, explains why reduction is spread over several stages rather than demanded from one machine, and describes what each crusher type typically does — without catalogue figures, because the real range depends on the model, the chamber and the rock, and only the manufacturer, or a test on your material, can state it.

Definition: F80, P80 and what the ratio measures

The oldest way to define reduction is to divide the largest admissible feed lump by the discharge opening. It has one merit, simplicity, and one flaw, imprecision: the largest lump is a single outlier, and a crusher's discharge is not sized to its setting — part of the product passes at the maximum opening.

The definition used today by design offices and manufacturers relies on size distribution curves. F80 is the screen size through which eighty percent of the feed mass passes, and P80 is the screen size through which eighty percent of the product mass passes. The reduction ratio is then:

R = F80 / P80

The choice of "eighty percent" is a convention: it characterizes a curve by a single point in its coarse region, where the fragmentation energy is decided, while ignoring the few extreme lumps that would distort a calculation based on maximum size. It is also the basis of the crushing energy calculation methods, which lets you go from ratio to power without changing definitions.

Two precautions. First, state which of the two definitions you are using: a ratio calculated on maximum sizes and one calculated on F80/P80 are not comparable. Second, a crusher's P80 is not its setting: it depends on the setting, but also on throw, chamber and rock, and it is the manufacturer who publishes that relationship for its model.

How to calculate it: a fictitious example

The values below are arbitrary, chosen to illustrate the mechanics of the calculation, and represent no machine and no site.

Suppose a feed with an F80 of 600 units and a primary product with a P80 of 150 units. The primary's reduction is 600 / 150 = 4. That product feeds a secondary that delivers a P80 of 30 units: secondary reduction 150 / 30 = 5. A tertiary then brings the P80 down to 10 units: reduction 30 / 10 = 3.

The total circuit reduction, 600 / 10 = 60, is the product of the stage reductions (4 × 5 × 3), not their sum. That property is why three modest machines do what no single machine can, and why adding a stage changes the order of magnitude while pushing an existing stage harder gains only a few units.

On a real site, the F80 comes from a size analysis of the feed — truck-bed image analysis, sieving of samples or blast data — and the P80 from sampling the product or from the manufacturer's curves for the chosen setting. A ratio built on assumed values is worth exactly what those assumptions are worth.

Why one stage is not asked to do it all

A crusher breaks rock by compression or by impact, and each mechanism fragments efficiently only within a limited size range. In a compression chamber, a lump has to be gripped between the surfaces — the nip angle — then compressed to failure, then the fragments have to drop and be caught again. The more reduction you ask of that chamber, the longer rock stays in it and the more times it is compressed, and three things degrade at once:

• Capacity falls, because the chamber fills with material that has not yet reached discharge size.
• Power per ton rises, and with it the load on bearings, eccentric and structure.
• Wear accelerates, because every extra compression rubs rock against the wear parts; the bottom of the chamber opens faster and the setting drifts.

There is also a mechanical limit: the chamber geometry — the relationship between the feed opening and the available setting range — bounds the reduction a machine can deliver without blocking at the top or generating excess fines. Tightening the setting below what the chamber allows produces a crusher that grinds rock on rock instead of letting it pass: that is the packing regime, and it is expensive.

Spreading reduction over successive stages, each in its comfortable range, gives stable capacity, acceptable product shape and predictable wear instead. The number of stages and the split between them is a circuit design decision, not a catalogue choice.

What each type typically does

The table below describes each type's role and the factors that bound its reduction. Numerical ranges are deliberately absent: they vary from model to model, chamber to chamber and rock to rock, and the manufacturer states them for its model and chamber profile.

The general rule: compression crushers — jaw, gyratory, cone — each give a moderate unit reduction and are combined in stages; the impact crusher gives a higher unit reduction, at the cost of wear that confines it to low-abrasion rock. The choice between these families is covered in which crusher for your rock and your throughput, and the mechanics of the secondary stage in how a cone crusher works.

Reduction, capacity, shape and wear: four linked variables

On a given machine, the lever for reduction is the setting — the closed side setting. Tightening it raises reduction and lowers capacity; opening it does the opposite. But the setting also acts on product shape and on wear, and those four variables cannot be adjusted independently.

A choke-fed cone crusher produces more cubical particles because rock is partly compressed against rock; that same regime raises the effective reduction without touching the setting. Conversely, an under-fed chamber lets flat and elongated particles through, reduces less, and wears the bottom of the chamber unevenly. The reduction obtained is therefore as much a matter of feed and fill level as of setting.

Finally, the reduction demanded determines the fragmentation energy, and therefore the installed power. Energy calculation methods use precisely F80 and P80 as inputs, which is one more reason to calculate the ratio on that basis rather than on maximum sizes.

The classic mistakes

The first is sizing the primary on the largest lump in the blast without looking at the curve: the opening is chosen for the extreme lump, and the setting needed to reach the target P80 falls outside the chamber's range. The circuit then runs with an overloaded secondary or an over-tightened primary. The correct method is described in sizing a primary crusher.

The second is comparing ratios calculated on different bases from one manufacturer to another, or taking the "maximum" reduction on a data sheet for the continuous operating reduction.

The third is ignoring that the feed F80 changes with the blast, the zone being mined and the season: a circuit designed for a study F80 will behave differently when blast fragmentation deteriorates — and a primary that receives larger lumps than planned blocks at the top of the chamber. Where those blockages become frequent, a fixed unblocking boom such as the IC Boom System keeps people out of the chamber, but it treats the symptom; the cause is in the feed curve.

Further reading

Reduction ratio is a circuit design tool before it is a machine characteristic: start from the real F80 and the P80 required downstream, split the reduction into stages, then choose machines whose setting range covers each stage with margin. That approach is part of our feasibility studies; for a specific circuit — new plant, added stage, change of ore — the request for proposal gets you an answer built on your curves, not on averages.

Frequently asked questions

Can the reduction ratio be calculated from the crusher setting? Only approximately: the P80 depends on the setting, but also on throw, chamber and rock. The manufacturer publishes product curves per setting for its model; sampling the actual product remains the only reliable measurement.

Why is total reduction the product of stage reductions and not their sum? Because each stage divides the characteristic size by its own ratio: the primary's P80 becomes the secondary's F80, and so on. Dividing successively by 4, by 5 and by 3 amounts to dividing by 60, with the fictitious values of the example.

Is a high reduction ratio always desirable? No. On a given stage, demanding more reduction than the chamber allows cuts capacity, raises power per ton and accelerates wear. The better answer is to spread reduction over an extra stage rather than push a machine outside its range.