A Comprehensive Analysis of Jaw Crusher Designs and the Criticality of Oversize Calculations

The jaw crusher stands as one of the most fundamental and robust pieces of equipment in the comminution circuit of mineral processing, construction aggregates, and demolition recycling. Its principle is elegantly simple, yet its design evolution has been a story of continuous refinement aimed at enhancing efficiency, throughput, and reliability. Central to the effective operation of any jaw crusher is the accurate prediction and control of product size distribution, with a particular focus on preventing and calculating oversize material. This article provides a detailed exploration of modern jaw crusher designs and delves into the critical engineering calculations used to manage oversize, ensuring optimal plant performance.

Part 1: The Evolution and Mechanics of Jaw Crusher Designs

At its core, a jaw crusher operates on the principle of compressive force. Two vertical jaws—one stationary (fixed jaw) and one movable (swing jaw)—form a V-shaped chamber. The movable jaw exerts immense cyclic force onto the feed material trapped between them, fracturing it along its natural cleavage lines until it is small enough to escape through the opening at the bottom, known as the closed-side setting (CSS).

1.1 Key Design Classifications:

Jaw crushers are primarily classified by the motion of the swing jaw.

• Blake (Double Toggle) Jaw Crusher: This is the original design. In a Blake crusher, the swing jaw is pivoted at the top. A double-toggle mechanism causes the bottom of the swing jaw to move in a predominantly elliptical path. This design imparts a significant amount of rubbing action in addition to compression, which is beneficial for abrasive materials.

  • Advantages: Lower wear on the toggle plate and seats compared to single-toggle designs; generates less wear dust; historically known for handling harder and more abrasive rocks.
  • Disadvantages: More complex mechanism with more parts; generally heavier and more expensive; lower capacity-to-weight ratio compared to single-toggle crushers.

• Overhead Eccentric (Single Toggle) Jaw Crusher: This is the most common design in modern quarries and mines. The swing jaw is pivoted at the top, but it is actuated by an eccentric shaft located at the bottom. This creates a more complex rocking-and-compressing motion with a significant vertical component at the discharge point.

  • Advantages: Simpler design with fewer parts; lighter weight for a given capacity; higher capacity due to a more aggressive crushing stroke; better particle shape due to the rock-on-rock action in the lower part of the chamber.
  • Disadvantages: Higher wear rates on the jaw plates due to greater rubbing; generates more fines; higher maintenance costs associated with jaw plate replacement.

• Dodge Jaw Crusher: A less common design where the pivot point is located at the bottom of the swing jaw. This results in a minimal rubbing action and a uniform product with minimal fines. However, its low throughput and susceptibility to choking make it unsuitable for high-tonnage primary crushing applications.

1.2 Modern Design Innovations:

Contemporary jaw crushers are not merely about choosing between toggle types. They incorporate sophisticated engineering features:

• Chamber Design: Modern chambers are designed using advanced computer modeling (e.g., Finite Element Analysis for stress points and Discrete Element Modeling for material flow). A deep, symmetrical chamber prevents choking and ensures consistent throughput. “Wedge” systems allow for quick and easy adjustment of the CSS.
• Kinematics: The motion path of the swing jaw is optimized to maximize production while minimizing wear. The goal is to create an aggressive stroke at the top of the chamber to grip large rocks quickly (“nip angle”) and sufficient stroke at discharge to ensure material flows freely.
• Materials and Wear Parts: The use of high-strength, quenched-and-tempered steel for frames provides durability under extreme cyclic loads. Jaw plates are manufactured from manganese steel (typically 14-18%), which work-hardens under impact, extending service life significantly.
• Automation and Control: Modern crushers are integrated with programmable logic controllers (PLCs) that monitor parameters like power draw, hydraulic pressure (for clearing blockages), and CSS. These systems can automatically adjust feed rates or trigger alarms to prevent damage from tramp metal or uncrushable objects.

Part 2: The Imperative of Oversize Calculation

In crushing circuits, “oversize” refers to material in the crusher product that exceeds a target maximum dimension. This can be defined as material larger than either:
a) The desired top-size product specification for downstream processes.
b) The return opening in a closed-circuit system where undersize is screened out and oversize is recirculated back to the crusher (“closed-circuit crushing”).

Uncontrolled oversize poses significant operational hazards:

• Downstream Blockages: It can plug chutes, blind screens, or damage conveyor belts.
• Secondary/Tertiary Crusher Damage: Feeding large rocks into cone crushers or impactors not designed for such feed can cause severe mechanical damage.
• Reduced Overall Plant Efficiency: It forces recirculation loads higher than designed for (“circulating load”), consuming extra power without producing saleable product.

Therefore, predicting potential oversize during plant design phase through calculation allows engineers to select appropriate equipment sizes—especially screens—and configure circuits correctly.

2.1 Calculating Closed-Side Setting (CSS) vs. Product Size

A fundamental misconception is that all particles in a crusher’s product will be smaller than its CSS—the narrowest gap between the jaws at their closest point during a cycle.

The reality is that product size distribution follows what’s known as a “crusher curve.” While no particle can pass through when it’s oriented perpendicularly across an opening smaller than itself during minimum gap conditions , many particles will be presented at angles or fractured into slabs that pass through easily even if their longest dimension exceeds CSS . Therefore:

Oversize Ratio = Particle Size / CSS

An industry rule-of-thumb suggests that approximately 15-20% by weight (“percent passing”)of crushed product will have one dimension larger than CSS . For instance:
If you set your CSS = 150mm , you can expect around 80-85% passing 150mm screen .

However , this ratio varies significantly based on :

• Material characteristics : Harder , more brittle rocks tendto produce more slabby particles .
• Feed size distribution : Well-graded feed produces better inter-particle crushing .
• Chamber profile &crusher type .

2 .2 Using Manufacturer’s Capacity &Gradation Tables

The most reliable methodfor predictingproductgradationis tousemanufacturer-suppliedtechnicaldata . These tablesaregeneratedfromextensiveempiricaltestingandprovideestimatedoutputcurvesforgivenfeedconditionsandCSSsettings .

Example Extract fromaTypicalGradationTable(CSS=125mm):

Fromthistable , wecanseethatwithaCSSof125mm :
-The”oversize”(material>125mm )isapproximately15%(100%-85%) .
-Material>100mmscreenwouldbe30%(100%-70%) .

Thisdataiscriticalfordesigninga screen downstream . Ifyour targettopsizeproductis100mm , youknowthat30%ofthecrusherproductwillneedtoberecirculated ; thisdefinesyourcirculatingloadandscreenrequirement .

2 .3 MathematicalModelsforPredictingProductSizeDistribution

Foradvanceddesignwork , engineersemploymathematicalmodels . ThemostwidelyrecognizedistheWhitenCrusherModel(alsoknownasthePerfectMixingModel)whichdescribesthecrusherastransferfunctionthatbreaksparticlesbasedonaprobabilityfunctioncalledthe”BreakageFunction”andaclassificationmatrixrepresentingthedischargeopening .

Whilecomplex , themodelessentiallystatesthattheproductsize Pcanbecalculatedfromthefeedsize Fusing :

P = (I - C) * (I - B * C)^(-1) * F

Where:

• P = Productsizevector
• F = Feedsizevector
• I = Identitymatrix
• C = Classificationmatrix(definesprobabilityofaparticleleavingthecrushereachpass)
• B = Breakagefunctionmatrix(defineshowparticlesbreak)

Thismodelrequirescalibrationwithspecificcrusherandmaterialdata butprovidesahighlyaccuratepredictionoftheentireproductsizedistributionincludingoversizefractionsunderdifferentoperatingconditions .

Part3 : PracticalManagementofOversizematerial

Calculationisvitalforplantdesignbutoperationalcontrolisequallyimportant :

–FeedControl(SCS) : Usingapre-screen(“scalper”)beforethejawcrusheremovesfinesalreadyatsizethatdonotrequirecrushingthisreducesthecrusherloadandincreasescapacitywhileimprovingparticleshapebypromotingrock-on-rockcrushing(“inter-particlecommutation”)inlowerpartchamber .

–ClosedCircuitOperation : ForstringentproductsizespecificationsoperatingjawcrushersinclosedcircuitwithscreenisstandardpracticeThescreenreturnstheoversizefractionensuringitgetsrecrusheduntilitpassesthroughscreenopeningThisallowsoperatorstoruna tighterCSSproducingmorecontrolledproductsizedistributionathigheroverallefficiencythoughwithincreasedrecirculatingloadsandpowerconsumptiontrade-offsmustbeevaluatedeconomically .

–RegularMeasurement&Adjustment : WearonjawplatesgraduallyincreasesCSSevenifadjustmenthasnotbeenchangedleadingtogradualincreaseinproducttopsizeregularmeasurementofCSSusingleadslugsortraditionalgaugescombinedwithsieveanalysisoffinalproductiscrucialformaintainingqualitycontrolpreventingunexpectedoversizedownstreamissues

Conclusion

JawcrushershavetranscendedtheirsimplemechanicalloopstobecomehighlysophisticatedmachineswhoseperformancecanbepreciselymodeledUnderstandingthedesignnuancesbetweensingledoubletogglemodelsprovidesbasisforcorrectapplicationselectionMoreimportantlygraspingrelationshipbetweenclosed-sidesettingactualproductsizedistributionthroughrigorouscalculationmanufacturerdatamathematicalmodelingisnotmerelyanacademicexercisebutacornerstoneofefficientreliablesafemineralprocessingoperationByaccuratelypredictingmanagingoversizematerialengineerscanoptimizecrushingcircuitsmaximizethroughputminimizedowntimeensurefinalproductmeetsspecificationseverytime