A jaw crusher breaks rock by squeezing it between two heavy steel plates, one fixed and one moving. The moving plate swings back and forth in a reciprocating motion, compressing material against the stationary plate until it fractures. Crushed pieces fall through the narrowing gap at the bottom, emerging at a controlled size. It’s one of the simplest and most widely used machines in mining, quarrying, and construction.
The Basic Crushing Cycle
Material is fed into the top of the crusher, dropping into a V-shaped space between the two jaw plates. As the movable jaw swings toward the fixed jaw, it applies enormous compressive force to the rock caught between them. The rock fractures along its natural weak points and breaks into smaller fragments.
When the moving jaw pulls back, the gap widens and the broken pieces drop further down. Because the two plates are angled toward each other (wider at the top, narrower at the bottom), pieces keep getting caught and re-crushed as they fall. Each swing of the jaw breaks the material a little smaller. Once fragments are small enough to pass through the gap at the very bottom, they discharge from the machine. The width of that bottom gap determines the final product size.
Key Components and What They Do
The crushing action starts with an eccentric shaft, sometimes called the pitman shaft. This is a shaft where one section is offset from the rest, so as it rotates, that offset portion traces a wider arc. A bearing assembly mounted on the offset section converts the shaft’s rotation into a primarily up-and-down motion.
That motion transfers to the swing jaw through a set of toggle plates. These plates sit at an angle between an arm extending from the pitman and the swing jaw itself. As the pitman rotates and pulls the arm upward, the toggle plates push toward horizontal, which forces the swing jaw outward against the fixed jaw. When the pitman drops, the toggle plates relax and the swing jaw pulls back. This is the back-and-forth cycle that does the actual crushing.
A heavy flywheel mounted on the shaft stores rotational energy, carrying the machine through each crushing stroke so the motor doesn’t have to supply peak force on every cycle. The motor only needs to provide enough energy to keep the flywheel spinning. Most jaw crushers have a flywheel on each side of the frame for balance.
Single Toggle vs. Double Toggle Designs
The two main jaw crusher designs differ in how they move the swing jaw. In a single toggle crusher, one eccentric shaft sits at the top and drives both the jaw’s movement and the toggle plate in a single mechanism. This creates an elliptical motion at the jaw face, meaning the swing jaw applies both compressive pressure and a downward rubbing action to the material. That friction helps pull material through the crusher faster, making single toggle machines higher in throughput and popular for most commercial crushing operations.
A double toggle crusher uses two shafts and two toggle plates. One shaft pivots at the top while a separate eccentric shaft drives both toggles. This arrangement produces a purely compressive force with almost no rubbing. Double toggle machines are better suited for very hard, abrasive rock because the simpler motion reduces wear on the jaw plates, but they tend to be heavier, slower, and more expensive.
Blake and Dodge Variations
You’ll sometimes see jaw crushers described by where the moving plate pivots. In a Blake-type crusher, the swing jaw pivots at the top, so the widest movement happens at the discharge end. This gives it high capacity and makes it the standard design for industrial applications. A Dodge-type crusher pivots at the bottom instead, so the greatest motion is at the feed opening. Dodge crushers produce a more uniform product size, but their capacity is lower, and they’re used mainly in laboratory settings.
How Output Size Is Controlled
The size of material leaving a jaw crusher depends on two measurements at the discharge gap. The closed side setting (CSS) is the narrowest distance between the two jaw plates at the bottom, measured when the swing jaw is at its closest point to the fixed jaw. The open side setting (OSS) is the widest distance, measured when the swing jaw has pulled fully back.
CSS is the more important number for product sizing. It directly controls the maximum size of most particles leaving the crusher. Adjusting the CSS is usually done by changing shims or hydraulic spacers behind the toggle plate, which shifts the swing jaw’s resting position closer to or farther from the fixed jaw. Keeping the CSS consistent as jaw plates wear down is one of the most important maintenance tasks for producing a uniform product. As the plates erode, the effective gap widens, and the crusher starts putting out coarser material unless the setting is corrected.
What the Jaw Plates Are Made Of
The jaw plates are the parts that actually contact the rock, and they take tremendous abuse. Most are cast from high manganese steel, typically containing 12% to 22% manganese. This material has a remarkable property: it work-hardens under impact. Every time rock slams against the plate, the surface layer gets harder. At any given moment, only about 2 to 3 millimeters of the plate’s face is in this hardened state, but that thin shell is dramatically more wear-resistant than the softer steel beneath it. As the surface wears away, a new layer hardens to take its place.
The manganese percentage creates a trade-off. Higher manganese content (20-22%) hardens faster, which is useful in high-impact applications. Lower manganese content (12-14%) hardens more slowly but reaches a greater final hardness once it does, making it more wear-resistant over time. Some manufacturers add small amounts of molybdenum or boron to the steel, which can extend plate life by 10% to 30%. Choosing the right plate composition depends on the type of rock being crushed and how abrasive it is.
What Affects Crushing Efficiency
Several factors determine how well a jaw crusher performs. Feed size matters: material should be small enough to fit into the top opening without bridging across the gap. A common guideline is that the largest feed pieces should be no more than about 80% of the gape (the top opening width). Feeding oversized material causes blockages and uneven wear on the plates.
Rock hardness and shape also play a role. Flat, slab-shaped pieces tend to align vertically between the jaws and slip through without being fully crushed. Rounder, more equidimensional feed produces better results. Moisture and clay content can cause material to stick to the plates or pack in the discharge area, reducing throughput.
The crusher’s speed, measured in revolutions per minute of the eccentric shaft, determines how many crushing strokes happen per minute. Faster speeds increase capacity but reduce the time material has to fall between strokes, which can actually produce a coarser product. Slower speeds give more complete crushing per cycle. Most operators find a balance based on the specific material and desired output.