PC Hammer Crusher: The Grate Sets the Product Size
A PC hammer crusher is judged by its hammers and specified by its grate — and most buyers have that the wrong way round. The hammers only break material; the grate slot decides what is allowed to leave. Get that relationship right and the machine is simple, cheap and consistent. Get it wrong and every problem looks like a hammer problem that new hammers never fix.
This article explains what the grate actually controls, the three settings that shape the product curve, and the one material property that rules this machine out before any of that matters.
I. What the Grate Really Does
Material falls into a chamber where hammers on a rotating rotor strike it and drive it against the breaker plates and the grate below. Anything small enough to pass the grate slots leaves; everything else stays in the chamber and is hit again, and again, until it does.
• The grate is a classifier, not a floor. Its slots are the maximum particle size the machine can produce, enforced mechanically rather than by hoping the rotor gets there.
• It narrows the curve. Because nothing oversized can leave, the product is more uniform than an open-discharge machine can manage — the reason PC crushers are popular where a top size is contractual.
• It buffers the effect of wear. A worn hammer produces coarser particles, but the grate still holds them back, so quality drifts more slowly than on a grate-less design.
• It also sets the machine's weakness. An aperture that controls size also blocks flow — and that is where every wet-day problem begins.
II. Three Settings That Shape the Curve
Once the grate is chosen, the product is set by three things, and they trade against each other rather than adding up.
1.Grate slot width
This is the hard ceiling on particle size, and it is the first number to specify. It is also the first thing to inspect: a worn or broken bar lets oversize through while everything else looks normal.
2.Hammer-to-grate clearance
Too large and material rides on the grate without being struck again — output falls while power stays the same. Too small and the hammer grinds the grate, so both wear faster. This clearance is a maintenance dimension, not a purchase dimension, which is why it is so often neglected.
3.Rotor speed
Higher speed means more impacts per tonne and a finer curve, at the cost of hammer life and power. Lower speed raises output and coarsens the product up to the limit the grate imposes.
The order matters. Choose the grate for the specification, set the clearance for throughput, then adjust speed for the curve. Buyers who start with speed end up with a machine that meets one grade and misses another.
III. Moisture Comes Before Everything
Every advantage of the PC design depends on material being able to fall through the slots. Wet or sticky feed does not: it packs between the bars, the open area closes, and the chamber fills faster than it can discharge.
• The symptom looks electrical. Current climbs, the drive trips, and the operator assumes the motor is undersized. The cause is a blinded grate.
• Clay is worse than water. Damp material dries; clay smears across the slots and stays there, and no amount of running clears it.
• Cleaning is manual. A blinded grate is opened, not washed — which means the cost is a shift, not a spare part.
• So the honest first question is about the feed, not the machine. If the material arrives wet or carries sticky fines, an open-discharge design is the right answer and this one is not.
IV. Where It Sits Among the Hammer Machines
| Type | Discharge | Product | Choose when |
| PC hammer crusher | Grate bars | Uniform, top size enforced mechanically | Dry, brittle feed with a contractual top size |
| Heavy hammer crusher | Usually open | Coarser, broader curve, high throughput | Large lumps, soft to medium rock, single stage |
| Reversible hammer crusher | Usually open | Stable curve, longer hammer life between changes | Coal, limestone, steady duty with scheduled reversal |
| Vertical compound crusher | Open, two zones | Fine but broad, oversize returns from bypass | Damp-tolerant fine crushing where a screen is accepted |
The line that matters is the second column. A grate buys size control and charges for it in moisture tolerance; an open discharge buys throughput and charges for it in top size. Every other difference between these machines follows from that one choice.
V. The Wear Loop
Hammers and grate bars wear against each other, and the loop runs in one direction until someone breaks it.
• Hammers wear short, clearance grows. Material sits on the grate instead of being re-struck, so the chamber holds more for longer.
• More material on the grate means more abrasion on the bars. The slots round over and widen, and top size creeps up without anything appearing broken.
• Output falls before quality does. The plant notices the tonnage first and the specification later, which is why a rejection often arrives as a surprise.
• Change them as a set. New hammers on a worn grate restore neither clearance nor slot width, and the mill will be opened again within weeks.
VI. Running It Properly
• Start empty, stop empty. A chamber full of material on shutdown sets around the grate; the next start is a hard one.
• Feed evenly, not in surges. A steady curtain keeps the chamber working as a mill rather than as a storage bin.
• Keep metal out. Unbreakable tramp is the one thing a hammer crusher cannot process, and the damage goes beyond the hammers.
• Inspect the grate at every hammer change. Slot width and bar condition are a two-minute check that prevents a specification failure.
• Do not add a screen to fix a worn grate. A screen downstream will hide the symptom while the chamber keeps overworking, and you will have paid twice.
VII. What to Ask the Supplier For
• The grate slot width, stated as a dimension, and the product curve it produces on material similar to yours.
• Hammer-to-grate clearance when new, and the wear limit at which bars must be replaced.
• Hammer and grate life in tonnes, priced per tonne of product rather than per set.
• The maximum feed moisture the design assumes, and what the supplier recommends if yours is higher.
• How the grate is changed — from below, through a side door, or by lifting the casing — and how long it takes.
• Rotor speed and whether it is fixed or adjustable, so the curve can be moved without new hardware.
Frequently Asked Questions About PC Hammer Crushers
Q: What does the "PC" designate?
A: It is the series designation for the conventional hammer crusher with a bottom grate. The distinguishing feature is not the letter but the grate — it is what separates this machine from the heavy and reversible types.
Q: Why does it block when the feed is damp?
A: The grate slots blind. Once the open area closes, the chamber fills and the drive current rises until it trips. Drying the feed or choosing an open-discharge machine are the two real answers.
Q: Do I still need a screen after it?
A: Usually not for top size, because the grate already enforces it — provided the bars are in good condition. A screen is still useful if you must separate several product sizes.
Q: Can it handle abrasive rock?
A: It can run, but hammers and bars wear together, so the cost per tonne climbs quickly. Ask for wear life in tonnes on your material before comparing it with a compression machine.
Q: Why is my output lower than before without any breakdown?
A: Almost always clearance. Worn hammers leave a gap above the grate, material circulates instead of passing, and the machine does more work for less product.
For hammer crushers matched to your feed and product, see the hammer crusher range.
