Jaw Crushers

Design and Operation These crushers may be divided into two main groups (Fig. 20-21), the Blake, with a movable jaw pivoted at the top, giving greatest movement to the smallest lumps; and the overhead eccentric, which is also hinged at the top, but through an eccentric-driven shaft which imparts an elliptical motion to the jaw. Both types have a removable crushing plate, usually corrugated, fixed in a vertical position at the front end of a hollow rectangular frame. A similar plate is attached to the swinging movable jaw. The Blake jaw (Fig. 20-22) is moved through a knuckle action by the rising and falling of a second lever (pitman) carried by an eccentric shaft. The vertical movement is communicated horizontally to the jaw by double toggle plates. Because the jaw is pivoted at the top, the throw is greatest at the discharge, preventing choking.

Crushing angles in standard Allis-Chalmers-Svedala Blake-type machines generally are near 0.47 rad (27°) (see Fig. 20-21). The reduction ratios at minimum recommended settings and with straight jaw plates average about 8:1. Curved (or concave) jaw plates are designed to minimize choking.

The overhead eccentric jaw crusher (Nordberg, Telsmith Inc., and Cedarapids) falls into the second type. These are single-toggle machines. The lower end of the jaw is pulled back against the toggle by a tension rod and spring.

The choice between the two types of jaw crushers is generally dictated by the feed characteristics, tonnage, and product requirements (Pryon, Mineral Processing, Mining Publications, London, 1960; Wills, Mineral Processing Technology, Pergamon, Oxford, 1979.) Greater wear caused by the elliptical motion of the overhead eccentric and direct transmittal of shocks to the bearing limit use of this type to readily breakable material, although Cedarapids has reduced the elliptical wear effect by moving the bearing over the crushing chamber. Overhead eccentric crushers are generally preferred for crushing rocks with a hardness equal to or lower than that of limestone. Operating costs of the overhead eccentric are higher for the crushing of hard rocks, but its large reduction ratio is useful for simplified low-tonnage circuits with fewer grinding steps. Double-toggle type crushers cost about 50 percent more than similar overhead-eccentric-type crushers.

Comparison of Crushers The jaw crusher can accommodate the same size rocks as a gyratory, with lower capacity and also lower capital and maintenance costs, but similar installation costs. Therefore they are preferred when the crusher gape is more important than the throughput. If required throughput in tons/h is less than the square of the gape in inches, a jaw crusher is more economical (Taggart, Handbook of Mineral Dressing, Wiley, New York, 1945). Relining the gyratory requires more effort than for the jaw, and also more space above an d bel ow the crusher. Improved alloys have reduced the need for relining, however. Gyratories are preferred for larger throughputs. In metal mines continuity of operation favors gyratories over jaws because of their low maintenance. Quarries on the other hand can use hammer or other impact crushers, while maintenance is done on second shift.

Blake

Blake Jaw Crusher Characteristics
FIG. 20-22 Blake jaw crusher. (Allis Mineral Systems Grinding Div., Svedala Industries, Inc.)

Performance Jaw crushers are applied to the primary crushing of hard materials and are usually followed by other types of crushers. In smaller sizes they are used as single-stage machines. Jaw crushers are usually rated by the dimensions of their feed area. This depends on the width of the crushing jaws and the gape, which is the maximum distance between the fixed and movable jaws at the feed opening. The setting of a jaw crusher is the closed (or close) or the wide opening between the moving jaws at the outlet end, usually measured between tips of the corrugations. The reciprocating motion of the jaws causes the opening to vary between closed and wide, and the difference is the throw. Specifications are usually based on the closed settings. The setting is adjustable.

The throw of Blake jaw crushers is determined by the hardness of the ore as well as the size of the machine [Mollick et al., Engineering & Mining J., 181(6), 69-171 (1980)]. It may vary from d in for hard but friable ores to 3 in for resilient material. The Big Bite jaw crusher (Kue-Ken Div, Process Technology Inc.) is a Blake double-toggle type in which the fulcrum pinion is positioned well over the center ofthe grinding chamber [Anon., Quarry Management, 18(1), 25-7 (1991)]. This increases the throw at the top of the jaw opening, allowing it to better crush large rocks. It can be shipped in sections. Capacities of Kue-Ken jaw crushers (Blake type) are given in Table 20-8, including both standard and Big Bite types. Performance data of overhead eccentric crushers with straight jaw plates are given in Table 20-9.

Dragon single-toggle primary jaw crushers are supplied (by Fives-Cail Babcock) in eight sizes. The smallest is the MR53 with feed opening 550 X 350 mm and the largest is the MR200 with feed opening 2000 X 1600 mm, the corresponding capacities being from 15 to 800 ton/h. These units can be supplied fully mobile or on skid frames. The BCS series are extra heavy-duty crushers. High flywheel inertia enables them to accept large lumps of limestone and achieve throughputs up to 6000 ton/h. Pegson Telsmith in UK has a new single-toggle primary jaw crusher with roller bearings sized at 1000 X 800 mm. It is expected to be the first of a new line that incorporates the latest manufacturing techniques. Universal Engineering Corp. offers 27 sizes of overhead eccentric crusher, with long jaws and small nip angle to reduce rebound.

Smaller jaw crushers are available from Sturtevant Inc, with capacities of 1 to 10 hp (0.7 to 7 kW). Jaw settings range from f to 2.5 in (3 to 65 mm).

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Responses

  • portia
    How to measure gap between fixed and moving jaw plates in double toggle jaw crusher?
    3 years ago
  • ashlyn
    What is the throw of 14x24 jaw crushers?
    2 years ago

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