Closed side setting (CSS) explained: how to measure and adjust it

The closed side setting — CSS — is the minimum gap between the two crushing surfaces at the bottom of the chamber, at the instant in the cycle when they are closest. It is the parameter that fixes product size, and through it capacity, power draw and wear rate. On a jaw, cone or gyratory crusher it is the only setting the operator changes routinely; everything else — throw, speed, chamber — is fixed by the manufacturer or changed during a major shutdown.

And yet, on many sites, the CSS is a number written on a board rather than a value that has been measured. Liners wear, the chamber opens, the product coarsens, the downstream stage overloads, and nobody connects those symptoms to the setting until a sieve analysis exposes it. This article covers what the CSS governs, how it is actually measured, why it drifts, and how it is adjusted on each machine type.

CSS and OSS: two dimensions, one throw

Every cycle, the moving surface — swing jaw, mantle — approaches the fixed surface and then moves away. The gap at the closest point is the CSS; the gap at the farthest point is the OSS (open side setting). The difference between the two is the throw, fixed by the shaft eccentricity. The three quantities are tied together: knowing the CSS and the throw means knowing the OSS.

What changes from one machine to another is the reference dimension. On a jaw or cone crusher the setting is quoted as CSS, because that is what fixes the fine end and the consistency of the product. On a primary gyratory the reference is often the OSS, because the concern is the largest piece leaving and what the secondary will have to accept. Before comparing two data sheets or two readings, check which dimension is meant — and the manufacturer states which one it uses to tabulate capacity and product curves for its model.

Another difference lies in where the measurement is taken. On a jaw, the CSS is measured at the bottom of the dies, peak to peak or valley to valley depending on the profile, and the manufacturer states its convention. On a cone or gyratory it is measured at the bottom of the chamber, in the parallel zone, on the closed side — where the mantle is closest to the concaves at that instant.

What the CSS governs

The CSS does not act alone. Product shape also depends on chamber fill, and capacity depends on the feed as much as on the setting. But at constant feed and chamber, it is the lever. It fixes the reduction obtained on the stage — see our article on reduction ratio — and therefore what the next stage receives. A CSS that opens without anyone correcting it shifts load onto the secondary or tertiary until that machine saturates.

Measuring the CSS: lead ball, aluminium block, sensors

Direct measurement remains the reference, and it is done with the machine stopped, locked out, chamber empty.

The lead ball. A lead sphere, slightly larger in diameter than the expected CSS, is lowered on a wire into the chamber on the side to be measured. The shaft is turned by hand or inched — following the manufacturer's procedure — so that the moving surface squeezes it at the most closed point. The ball comes out flattened; its thickness, measured with calipers, is the CSS at that location. On a cone or gyratory the measurement is repeated at several points around the circumference to check that the chamber is concentric.

The aluminium block. Same principle with an aluminium block or a lead slug, more stable in a jaw chamber, where it is placed at the bottom of the fixed jaw before the swing jaw is brought across. Some sites use a clay slab or a wax ball when the manufacturer accepts it; the principle is still measuring an imprint.

Sensors. On hydraulically adjusted machines, a shaft or bowl position sensor gives a computed CSS, displayed in the control room. That value is convenient, but it is a position, not a gap: as liners wear, the same position gives a larger CSS. The sensor therefore has to be recalibrated against a physical measurement at regular intervals — after every liner change at a minimum, and more often on abrasive rock. Chamber scanning systems, by laser or profile survey, complement the point measurement by giving wear over the full chamber height.

Whatever the method, the measurement is logged with the date, the tonnage since the last liner change and the measuring point. That series is what makes drift visible and planning possible.

Why the CSS drifts with wear

Wear parts — jaw dies, mantle, concaves — lose metal with every ton. The bottom of the chamber, where rock is most compressed and finest, wears fastest. At constant setting position the gap grows, the product coarsens, capacity rises and power per ton falls: the indicators appear to improve while the product goes out of specification.

Wear is not uniform. On a jaw, the profile hollows out at the bottom and changes the nip angle; on a cone, off-centre feed wears one side of the chamber more than the other and the CSS varies with where it is measured. That is why CSS tracking is the first indicator for deciding when to replace wear parts: measured drift, related to tonnage, gives the wear rate and the date beyond which the setting can no longer be recovered.

Adjusting the CSS on each machine

Jaw crusher. On conventional machines the adjustment is made at standstill: shims added or removed behind the toggle seat, or wedges moved by screw or by cylinder. On recent models a hydraulic cylinder allows adjustment without dismantling, sometimes with the machine running empty. The place of that setting in the machine's operation is described in our article on how a jaw crusher works.

Cone crusher. Depending on the design, the bowl is screwed in or out of the frame — which lowers or raises the concaves relative to the mantle — or the main shaft is moved by a hydraulic cylinder under the head. Modern machines do either on the run, from the control system, and can hold the CSS or track a target power as liners wear. That is what gives the cone crusher its product consistency over the life of a liner set.

Gyratory crusher. The setting is the main shaft position, supported by a hydraulic piston: raising the shaft tightens the chamber, lowering it opens it. Adjustment is made on the run, and the same circuit releases tramp iron. The details are in how a gyratory crusher works.

In every case: adjust in small steps, let the machine settle, and verify on the product — not only on the display.

Common mistakes

• Confusing position with setting: reading the CSS on the display without ever recalibrating it against a physical measurement. The drift goes unnoticed until the downstream stage complains.
• Measuring at a single point on a cone or gyratory, when the chamber wears off-centre.
• Tightening to compensate for a weak secondary: the primary leaves its range, its capacity falls and its liners wear out far faster for a marginal gain.
• Tightening below the chamber's admissible minimum: rock is ground on rock without passing, power climbs, and the machine eventually packs at the bottom.
• Changing the CSS without telling downstream: screens, conveyors and the next crushers are sized for a given product.
• Skipping the measurement after a liner change: new liners of a different profile give a different CSS at the same position.

Further reading

The CSS is a value you measure, log and correct — not a number you assume. Regular tracking, related to tonnage, is what allows wear parts to be planned rather than suffered; our after-sales and spare parts service relies on those records to anticipate mantle, concave and jaw die orders. Feed-opening blockages are a separate matter — they come from the feed, not the setting — and are handled by a fixed unblocking boom such as the IC Boom System. For a specific machine — measurement, adjustment procedure, parts supply — use the request for proposal.

Frequently asked questions

How often should the CSS be measured? At a minimum after every wear-part change and at regular intervals afterwards, tied to tonnage rather than the calendar; more often on abrasive rock. The manufacturer recommends a frequency for its model; your measurement series will tell you whether it is enough.

Is the CSS shown by the control system reliable? It is a shaft or bowl position converted into a gap for a given liner condition. It is reliable as long as it is recalibrated against a physical measurement; between recalibrations it underestimates the real opening as liners wear.

Can the CSS be adjusted while the crusher is running? On hydraulically adjusted cones and gyratories, yes, from the control room. On most jaw crushers, shim or wedge adjustment is done at standstill; some cylinder-equipped models allow it with the machine running empty. In every case, proceed in small steps and verify on the product.