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aggregate particles cause a limited number of fracture planes
when the aggregate particle is trapped between two crushing
surfaces, between other aggregate particles, or between a com-
bination thereof. As shown in Figure 18, cleavage produces
a few large particles. Impact-type crushers cause the particle
to shatter as it is propelled at high speeds against an anvil or
against other aggregate particles. Impact-type crushers pro-
duce the widest distribution of particle sizes. Barksdale (25)
noted that when producing the same size coarse aggregate
product, impact crushers tend to produce a greater percent-
age of particles passing the 4.75-mm sieve, but compres-
sion type crushers produce a greater percentage of material
passing the 0.075-mm sieve (dust) within the fine aggregate
fraction.
Figure 19. Schematic of a jaw-type crusher (156).
2.10.1 Types of Crushers
There are four major types of crushers used to produce
aggregate for HMA: jaw, gyratory, cone, and impact. Jaw, produce excessive fines by overcrushing. Overcrushing occurs
gyratory, and cone crushers are all forms of compression when rocks of the desired size are recrushed before they can
crushers. Compression-type crushers apply a compressive pass out of the crusher and be removed from the crushing
force to the aggregate that is trapped between crushing sur- stream by screening.
faces. A common characteristic of these machines is that the There are two types of impact crushers, horizontal shaft
aggregate must pass through a fixed opening. The fixed open- and vertical shaft. Horizontal shaft impact crushers use one
ing is adjustable and is referred to as the "close-side" setting or more rotors, hammers, or rotating pins mounted on a cage.
(25). Jaw and gyratory crushers apply the crushing force The rotors or hammers directly impact the rock as well as
slowly, producing cleavage and abrasion. Cone crushers, a propel the rock against aprons, anvils, or other aggregate par-
subclass of gyratory crushers, apply their energy approxi- ticles where further impact occurs. Horizontal shaft impact
mately twice as fast, producing fracture by shatter as well as crushers can produce a high reduction ratio, from 15 through
by cleavage (25). Examples of jaw and cone crushers are 20 to 1. Horizontal shaft impact crushers are only suitable
shown in Figures 19 and 20. A complete description of for low-abrasion aggregate feeds. In a vertical shaft impact
crusher types is provided in Barksdale (25). crusher, the aggregate feed is introduced into a shoe or pump
Typical reduction ratios for jaw-type crushers are 71. spinning on a vertical axis. The aggregate feed is thrown cen-
Gyratory or cone crushers can produce reduction ratios from trifugally against a series of anvils, pockets of aggregate par-
21 through 101. The use of high-reduction ratios tends to ticles (i.e., autogenous), or a combination thereof (25). Ver-
tical shaft impact crushers produce a small reduction ratio
Figure 18. Size distributions resulting from various
fracture mechanisms (Figure from Kelley [155] published Figure 20. Cutaway view of Symons cone crusher (156,
in Barksdale [25]). originally from Nordberg, Inc., Milwaukee, WI).
