Crusher Reduction Ratio and Circuit Stage Check
Reduction ratios MULTIPLY through a crushing circuit, which is why one machine cannot do what three can.
Example
You enter
- Feed size, 80% passing (in) 24
- Product size, 80% passing (in) 0.75
- Crushing stages in the circuit 2
- Machine ratio range, low 3
- Machine ratio range, high 6
- Measured size out of the first stage (in) 6
You get
- Circuit reduction ratio 32.00 to 1
- Per stage ratio 5.65685
- First intermediate (in) 4.24264
- Stages required 2
- Actual downstream ratio 8
Checked against published worked examples from: Metso.
Details, formula, and sources
ScopeA pit feeding 24 in run-of-quarry and making a 0.75 in product needs a total ratio of 32, and no crusher covers that in one pass. Split across two stages it is 5.66 to 1 each with a 4.24 in intermediate: past Metso's jaw (3-5) and cone (3-4) ranges, so both machines would be overdriven, which means wear, heat, and no margin when the feed gets blocky. Three stages puts it at 3.17 to 1 each, every machine in the comfortable middle of its range, which is why most aggregate plants are three-stage and why the third crusher usually pays for itself in liner life and uptime rather than in tonnage. THE FIELD VALUE IS DIAGNOSTIC RATHER THAN DESIGN. When a plant is not making spec or a machine is running hot and passing oversize, computing the ratio each machine is ACTUALLY being asked to perform identifies the offender in one line. If the jaw is producing 6.0 in rather than the 4.24 in the two-stage plan assumed, the cone is being asked for 8.0 to 1 -- outside its range -- and the cone is not the problem. No adjustment at the cone fixes something upstream of it, and that is the argument this puts on paper. Reduction ratio arithmetic on 80 percent passing sizes. It does not size a crusher, predict capacity or power draw, or generate a product gradation: those come from the manufacturer's capacity tables and closed-side-setting curves for the specific machine and material, and gradation depends strongly on rock friability and on whether the circuit is open or closed. Closed-circuit operation with recirculating load changes both the effective ratio and the tonnage through the machine substantially and is not modeled. It does not evaluate feed gradation, moisture, clay content, or the surge and screening capacity between stages, which is usually what actually limits a plant. The crusher manufacturer's selection data and the plant designer govern.
Formula or tablereduction ratio = feed size / product size on 80% passing sizes; circuit ratios MULTIPLY, so an even split is the total raised to one over the stage count; stages needed = ceil(log of the total over log of the machine's high ratio).
Source and editionThe reduction ratio relation and the multiplicative circuit rule by name, with typical machine ranges (Metso Crushing and Screening Handbook, 7th ed.: jaw 3 to 5, primary gyratory 6 to 8, secondary cone 3 to 4, tertiary cone 2 to 3.5; impactors run higher) entered rather than shipped. The crusher manufacturer's selection data and the plant designer govern.
Free accessDivision and a logarithm on sizes the user measures; no manufacturer capacity table or closed-side-setting curve is reproduced.
What governsEstimate. AHJ and licensed professional govern.
API field namesfeed_size_in, product_size_in, stages, machine_ratio_low, machine_ratio_high, actual_intermediate_in, total_ratio, per_stage_ratio, first_intermediate_in, stages_required, actual_downstream_ratio
- Ratios multiply, they do not add which is why one machine cannot do a circuit's workcrushing practice
- Three stages buy liner life, not tonnage every machine in the middle of its range rather than at the topcrushing practice
- The diagnostic beats the design computing what each machine is really being asked for names the offendercrushing practice