Impact Crusher Curtains: New Blow Bars Do Not Change the Shape
An impact crusher reduces material twice. The blow bar throws it; the curtain catches it and throws it back. Producers replace the blow bar, because it is the part that visibly disappears, and then wonder why the product grading has barely moved. On most machines the shape is not made by the first blow at all — it is made by the second impact against the curtain, and the gap that governs it is measured from a circle that shrinks every hour the machine runs. New bars restore the first blow. They do not restore the second, and the second is what the buyer is rejecting.
This article covers the two blows, why the setting drifts without anyone touching it, how to read curtain wear, and what an impact crusher will not fix no matter how the curtains are set.
I. Two Blows, One Grading
The rotor does the breaking everyone talks about. The curtain does the work that decides whether the product is cubic or flat.
• The first blow splits the particle. A blow bar travelling fast enough shatters stone along its natural weaknesses. That produces fragments, not a size distribution.
• The second impact shapes it. Fragments thrown against the curtain break again, and this time the break is across the piece rather than along it. That is where angular, single-face fracture becomes a rounded, denser particle.
• The gap decides how many get a second chance. A particle that clears the gap untouched goes out as it was, flat sides intact. The gap is the only control the operator has over how much of the product received that second blow.
• Which is why the bar and the curtain are not interchangeable maintenance items. A worn bar reduces the first blow's intensity. A worn or widened curtain removes the second blow entirely, and no amount of new bar metal brings it back.
II. The Setting Is Measured From a Circle That Shrinks
A curtain gap is not measured from the rotor shaft. It is measured from the tip circle that the blow bars sweep. That circle is a wear surface, and it gets smaller from the first day of work.
• Set it with new bars and it is correct for one shift. As the bar wears, the tip circle contracts and the gap opens by exactly the amount of wear — with the setting mechanism untouched.
• So the machine produces coarser product while every adjustment stays where it was. The operator's notes say nothing changed. The drawing the gap is measured against had already moved.
• And the curtain wears in the same direction. Its face is not flat after a few months; it becomes concave, deepest where the material stream concentrates. The nominal gap then exists only at the edges.
• Two wear surfaces, one number on the adjustment sheet. That is the whole mechanism behind "we set it right and it still makes oversize".
III. Why Replacing the Bars Changes Nothing
| What the producer observes | What they usually replace | What was actually moving |
| Product got coarser with no setting change | Blow bars | The tip circle contracted; the gap opened untouched |
| Flakier, more elongated particles | Blow bars | The second impact is being skipped, not weakened |
| Oversize in the finished pile | Blow bars | Curtain face worn concave; gap only correct at the edges |
| Output dropped without a power change | Blow bars | The gap was tightened to compensate, and throughput paid for it |
All four rows are the same machine. In each case the bar is replaced because it is the only part with an obvious wear indicator, while the component that controls the grading has no indicator at all — just a shape that slowly stops being the shape it was set to.
• Measure the gap from the bar tip, not from the rotor body. A measurement taken from the shaft looks consistent and means nothing, because the surface the material actually meets is the tip circle.
• Measure at several points across the width. A single reading at the centre hides a concave face and a tapered gap. The edges are where oversize escapes.
• Record the bar tip diameter at every change. Three numbers over a year turn a drift into a trend you can plan around, instead of a rejection you discover.
IV. Reading Curtain Wear
The curtain tells you what the rotor has been doing, if you look at where the metal went rather than how much.
• Wear concentrated in a narrow band means the feed is not spread across the rotor width. The material stream is a stripe, and the rest of the curtain has never done any work.
• Polished, rounded wear suggests rubbing more than impacting — often a gap too tight for the feed, so particles are dragged through rather than struck.
• Deep local gouges or a broken section usually mean uncrushable material reached the curtain, or a plate that had already cracked was left in service.
• Even wear across the full width but heavy overall is a good sign for distribution and a sign that the curtain is doing the work the rotor should have done — check whether the bars are being run past their useful life.
• And a curtain that is worn on one side only is a frame or alignment question, not a wear-part question. Replacing it repeats the pattern.
V. What Curtain Setting Will Not Fix
• Too much reduction asked of one machine. An impact crusher is not a substitute for a primary stage. Feed that needs several reductions will be run at a tight gap, which raises wear, dust and power for a product still not where it should be.
• Feed that is already fine. Below a certain size a particle has nothing to break against; it passes without a second impact whatever the gap. Screening the fine fraction out before the crusher often improves the product more than any adjustment.
• Wet or sticky material. Coating on the curtains and build-up on the rotor change the effective gap continuously, and no setting survives a feed that is pasting itself to the chamber.
• An overloaded rotor. Feeding faster than the rotor can clear the chamber turns impact into a milling action against a bed of material, which changes both the grading and the wear pattern at once.
• A machine that is simply the wrong size. Curtains adjust a distribution; they do not add capacity. If the shape is acceptable only at a throughput the plant cannot live with, the problem is upstream of the adjustment.
VI. What to Ask Before You Re-Set or Re-Order
• What the gap is measured from — tip circle or rotor body — and at how many points across the width.
• What the minimum gap is for the feed size I run, and what happens to wear and throughput below it.
• How the curtains are adjusted — mechanically, hydraulically, from outside the frame — and whether it can be done while the machine is running.
• What the curtain wear limit is, in a dimension I can check with a straight edge rather than a judgement.
• Whether the curtains are reversible or have a second working face, and how many faces before replacement.
• Which curtain is the grading curtain, since many machines have more than one and only one of them is doing the shaping.
Frequently Asked Questions About Curtain Gaps
Q: I fitted new blow bars and the grading is the same. Why?
A: Because the bars only restore the first blow. The second blow happens at the curtain, and its gap is measured from the bar tip circle — which had already shrunk before you changed the bars, and which is why the gap was wider than the sheet said.
Q: Should I set the gap tighter to get a finer product?
A: Only within what the feed allows. Below that point particles are dragged rather than struck, throughput falls, wear rises, and the product gets flatter rather than better shaped.
Q: How often should curtain plates be replaced?
A: On condition, not on a date. The trigger is the wear profile: once the face is concave enough that the gap at the edges no longer matches the gap at the centre, the plate has stopped controlling anything.
Q: Does one curtain do all the work?
A: On a two or three-curtain machine, the first curtain sets the top size and the later ones refine shape. Adjusting the wrong one moves the grading in a direction you did not intend, which is a common reason adjustments appear to do nothing.
For impact crushers and the wear parts matched to your feed, see the impact crushing equipment range.
