A Comparative Analysis: The Blake-Type vs. The Dodge-Type Jaw Crusher

In the realm of comminution—the process of reducing the size of solid materials—jaw crushers stand as one of the most fundamental and historically significant machines. Among the various designs, the Blake Jaw Crusher and the Dodge Jaw Crusher represent two pivotal, yet distinctly different, approaches to achieving particle size reduction through compressive force. While both are considered “overhead eccentric” jaw crushers in a broad sense, their specific kinematic principles, mechanical designs, and operational characteristics set them worlds apart. Understanding the difference between these two types is not merely an academic exercise in engineering history; it provides crucial insight into the selection of appropriate technology for specific crushing applications.

This article provides a detailed, objective comparison of the Blake and Dodge-type jaw crushers, delving into their design philosophies, working mechanisms, advantages, disadvantages, and their respective places in modern mineral processing.

I. Foundational Principles and Historical Context

The Blake Jaw Crusher: The Archetype

Patented by Eli Whitney Blake in 1858, the Blake Jaw Crusher is widely credited as the first practical mechanical rock crusher. Its design was so effective and robust that it became the standard for primary crushing for over a century, and its basic principle is still employed in many modern jaw crushers. The defining characteristic of the Blake-type crusher is the location of the pivot point of the movable jaw.

• Pivot Point: In a Blake crusher, the movable jaw is pivoted at the top. This means the swinging jaw’s arc-like motion is greatest at the discharge point (the bottom) and minimal at the feed opening (the top).

The Dodge Jaw Crusher: The Alternative Approach

Developed later as an alternative, the Dodge Jaw Crusher features a fundamentally different pivot arrangement. Patented in 1881, its design sought to address some perceived limitations of the Blake design but introduced new ones.

• Pivot Point: In a Dodge crusher, the movable jaw is pivoted at the bottom. This results in a virtually uniform, parallel motion throughout most of its stroke near the discharge opening.

This single difference in pivot location dictates nearly every other aspect of their performance, from capacity and product gradation to susceptibility to choking and maintenance requirements.

II. Detailed Working Mechanism and Kinematics

Blake-Type Kinematics: The “Choking” Design

The motion of the swinging jaw in a Blake crusher can be described as a compound pendulum. As the eccentric shaft at the top of the crusher rotates, it imparts an elliptical motion to the top of the movable jaw. However, because it is hinged at the top:

1. The motion at the feed opening is primarily a slight back-and-forth “rubbing” or “scrubbing” action with very little vertical travel.
2. The motion at the discharge opening is a much larger elliptical path with significant vertical movement.

This kinematic profile has critical implications:

• Progressive Crushing: Rock entering from the top receives its initial compression but is not discharged immediately. As it moves down,the crushing strokes become more pronounced.
• Self-Cleaning Action: The significant vertical movement at the discharge end helps to push crushed material downward and out ofthe chamber,facilitating dischargeand reducingthe likelihoodof packing.
• Variable Wear: The greatest wear occurs in th e lower portion ofthe fixedjaw plate near th edischarge pointbecause thisis where th emost significantcrushing action occurs.This leads t oa more predictableand manageablewear pattern.

Dodge-Type Kinematics: The “Uniform Output” Design

The kinematics ofthe Dodgecrusheraresimplerbutmore restrictive.Thepivotatthebottommeansthatalmosteverypointonthemovablejawtravelsinasmallarc,movinginandoutina nearlyparallelorstraight-linemotionrelativetothefixedjaw.Thisresultsin:

1.Auniformcrushingactionalongtheentirelengthofthecrushingchamber.
2.Verylittleverticalmovementatthebottomdischargeend,makingitrelyongravityforproductdischarge.
3.Thegreatestmovement,andthusinitialbreakingforceisappliedatthetopofthechambernearthefeedopening.

Theimplicationsofthisdesignareprofound:

• Uniform Product Size: Because themotionisnearlyuniformthroughout,thetendencyistoproduceamoreuniformly-sizedproductcomparedtotheBlakecrusher.
• High RiskofChoking: Therelianceongravityfordischargeandthelackofapositiveejectingmotionmeansthatifthematerialdoesnotfallfreely,thecrusherisextremelypronetochoking(blockage),especiallywithclayeyorwetfeed.
• ConcentratedWear: Wearonthejawplatesismoreevenlydistributed,butbecause themajorcrushingforceisappliednearthetop,wearcanbeconcentratedinthatarea.

III. Comparative Analysis: Advantages and Disadvantages

*Note on Reduction Ratio: While often cited as an advantage for Dodge crushers due to their uniform nip angle allowing for better grip on particles,the practical limitationsofchokingoftenpreventtherealizationofthistheoreticalbenefitinscaleoperations.

IV. Modern Relevance and Conclusion

In today’s industrial landscape,the distinction between these two types has become somewhat blurred,yet their legacies endure.The overwhelming majorityofmodern,job-provenjawcrushersusedforprimarycrushingarebasedontheBlakeprinciple.Modernvariantslike singletogglejawcrushers(whichareakin tot heB lake designbutwithasimplermechanism)dominatethe marketfortheirreliabilityhighcapacity,andabilitytohandleawiderangeoffeedmaterialsincludingthose thataredamporsticky.TheBlakedesign’sresistancetochokingisits singlemostimportantoperationaladvantageinalarge-scaleprocessingenvironmentwheredowntimeiscostly.

TheDodge-typecrusher,havingfailedtogainwidespreadindustrialadoptionduetoitsfatalflawofbeingpronetochoking,hasseenitsusefadeintoobscurity.Itisseldomifeverusedinnewfull-scaleprocessingplants.Itsprimarylegacylivesoninlaboratorysettingswherecontrolledconditions(smallbatches,dryfeed)andtheneedforaspecificproductsizemakeitausefulanalyticaltool.Some small-scale or specialized applications might still employ it,but it is not considered aviableoptionformodernhigh-tonnageprimarycrushing.In essence,theindustryvotedwithitsfeet:thereliabilityandcapacityoftheB lake-typedesignprovedfarmorevaluablethanthe theoreticalproductsiz euniformityoftheDodge type.In conclusion,thedifferencebetweenBlack(acommonmisspellingofBlake)andDodge-typecrushersboilsdowntoafundamentaldifferenceinengineeringphilosophy.Oneprioritizedrobustnessandreliableoperationunderarduousconditions(theBlake);theotherprioritizedatheoreticalidealinproductsiz econtrol(theDodge).Historyandpracticalexperiencehavedemonstratedthatforthevastmajorityofindustrialcrushingapplications,t hedesignphilosophyembodiedbyEliWhitneyBlake’soriginalinventionremainsthecorrectchoice