How a gyratory crusher works: parts, motion and settings

A gyratory crusher does the same job as a jaw crusher — bringing run-of-mine or blasted rock down to a size the next stage can accept — but it does it continuously, all the way around its circumference, with a chamber that closes and opens as it turns. It is the primary crusher of high-tonnage operations: open-pit mines, large quarries and some cement plants choose it because it can be fed directly by trucks, tolerates large lumps and delivers a high throughput for its footprint.

That capacity comes at a price: a tall machine, a deep pit, a massive foundation, and a setting you cannot see from the outside. Understanding what sits below the feed ring — and how the bottom of the chamber is adjusted — avoids the two most common mistakes: a gyratory fed as if it were a jaw, and a gyratory whose setting drifts with nobody measuring it.

The parts, top to bottom

The spider. A cast crossbeam bolted to the top of the top shell, with a central hub that carries the upper bearing of the main shaft. Its arms span the feed opening and are protected by wear caps, because they take the rock dumped by the trucks head-on. Together with the rim of the shell, it defines the feed ring through which lumps enter.

The shells. The top shell, in cast steel, is lined inside with rows of concaves — the fixed manganese-steel wear segments that form the outer wall of the chamber. The bottom shell carries the eccentric bushing, the countershaft with its pinion and the ring gear, and the discharge openings; it is the part that passes the crushing loads to the foundation.

The main shaft and the mantle. A long, heavy forging, suspended at the top in the spider bearing and guided at the bottom by the eccentric. It carries the head, on which the mantle — the moving conical wear part — is mounted. The chamber is the annular space between mantle and concaves: wide at the top where rock enters, narrow at the bottom where it leaves.

The eccentric. A bushing whose bore is offset from its axis of rotation. As it turns, the bottom of the shaft describes a circle while the top stays fixed in the spider: the shaft sweeps a cone. That gyration, not a rotation, is what does the work.

The hydraulic shaft-positioning system. Under the main shaft, a hydraulic piston supports the entire weight of the shaft, head and mantle. Raising the piston lifts the shaft and closes the chamber; lowering it opens the chamber. This is how the machine is adjusted, how wear is compensated and how tramp iron is released. A pressurized oil circuit, monitored for temperature and flow, lubricates the eccentric, the gears and the spider bearing.

The motion: gyration, not rotation

With every turn of the eccentric, every point around the chamber sees the mantle approach the concaves and then move away. The closing zone travels continuously around the machine. Rock descending under gravity is compressed on each pass, fractures, drops one step during the opening phase, and is caught again lower down — until it passes through the discharge opening.

The mantle also turns slowly on its own axis, driven by friction with the rock, which spreads wear over its whole surface. Unlike a swing jaw, which only crushes during half of each cycle, the gyratory is working somewhere in its chamber at every instant: that is where its high throughput and its steadier power draw come from. The distance between the most closed and the most open position at any one point is the throw, fixed by the eccentricity.

The setting: OSS, CSS and shaft position

Two dimensions describe the discharge opening. The CSS (closed side setting) is the minimum gap between mantle and concaves at the bottom of the chamber, on the closed side. The OSS (open side setting) is the maximum gap, on the opposite side, equal to the CSS plus the throw. By convention, primary gyratories are often specified and adjusted on the OSS, because that is the dimension that fixes the largest piece that can leave the machine — and therefore what the conveyor, the surge pile and the secondary crusher downstream must accept. The manufacturer states the setting range and the corresponding product curves for each model.

Adjustment is made through shaft position: raising the shaft tightens the chamber, lowering it opens it. On a machine with hydraulic positioning, the operator adjusts on the run from the control room, and a position sensor reports a computed setting. That value has to be recalibrated periodically against a physical measurement, because mantle and concave wear opens the chamber at a constant shaft position. The measurement method and the traps of setting adjustment are covered in our article on closed side setting: how to measure and adjust it.

The same system protects the machine: if tramp iron — a bucket tooth, a drill rod — enters the chamber and pressure rises past a threshold, the control system lets the shaft drop to open the chamber, then raises it again.

Direct truck feed and a deep foundation

The gyratory is designed to be fed directly, without a feeder or scalping grizzly, by trucks dumping into a hopper above the feed ring. It then works choke-fed, with rock partly compressing against rock, which helps fragmentation and evens out wear; the surging feed is absorbed by the height of the chamber.

The flip side of that simplicity is underneath. Product falls through the bottom-shell openings into a surge pocket, then onto an apron feeder that carries it to the conveyor. Machine, pocket and feeder stack vertically: a primary gyratory needs a deep pit, a reinforced-concrete foundation able to take large dynamic loads, and hopper steel sized for loaded trucks dumping into it. A gyratory is therefore chosen at the site study stage, never as an improvised replacement for a jaw: see our guide to sizing a primary crusher.

For each machine size the manufacturer states the tabulated capacity per setting, the largest admissible lump and the installed power. For the other machine in the table, see how a jaw crusher works.

Feed-ring blockages

A gyratory almost always blocks at the top. An oversize lump bridges between the spider and the rim of the shell, or two lumps arch against each other in the feed ring; sometimes a flat slab lands on a spider arm and never enters. The machine runs empty below, trucks queue above, and the shutdown, lockout and clearing sequence begins.

Clearing by hand, with a crane hook or with an excavator puts people above a chamber where the lump can tip without warning, and costs hours once the feed stoppage, restart and downstream catch-up are counted. That is why high-rate primary gyratories build in a fixed unblocking boom, mounted above the top shell, which reaches down into the feed ring and breaks or pushes the lump from a protected station. What to look at when choosing one — reach, hammer energy, interface with the steelwork — is explained in how to size a crusher unblocking boom; the available configurations are on the IC Boom System product page.

Maintenance: what really matters

• Lubrication: oil temperature, pressure and flow are watched every shift; a drift is the early warning of an expensive failure.
• The spider bearing and its wear caps: the bearing is greased separately and its clearance is checked; the caps wear quickly under truck dumping.
• Mantle and concaves: track the setting and the profile to plan the changeout, which needs a crane and a shutdown window.
• Foundation and anchor bolts: a gyratory that moves on its base cracks the bottom shell.

Concave and mantle replacement, main-shaft lifting and return to service are among the interventions our team plans under its installation and commissioning service.

Further reading

The gyratory is the primary crusher of high tonnages because it works continuously and takes trucks directly. Its constraints — pit, foundation, heavy maintenance, feed-ring blockages — are manageable when they are planned from the design stage. The related articles below cover the jaw, the cone and the cost of blockages; for a specific project — selection, unblocking boom, installation, parts — use the request for proposal.

Frequently asked questions

What is the difference between a gyratory crusher and a cone crusher? The principle is the same — a mantle gyrating inside concaves — but the gyratory is built for primary duty: a long, steep chamber, a shaft suspended in a spider, direct feed of large lumps. The cone has a shorter head, a chamber with a parallel zone, and receives pre-crushed material.

Why is a primary gyratory often set on the OSS rather than the CSS? Because in primary duty what matters is the largest piece leaving the machine, not the fineness of the product: the OSS fixes that top size, and therefore what the conveyor and the secondary must accept.

Can a jaw crusher be replaced by a gyratory on an existing plant? Rarely without heavy civil work: the gyratory needs a deeper pit, a more massive foundation and a truck-dump hopper. It is handled as a project in its own right, with a site and structural study before any decision.