Horizontal shaft impact (HSI) crushers put blow bars through punishing cycles of high-speed impact. Wear is inevitable, but premature wear doesn't have to be.
According to Jarrod Adcock, crushing product manager at Superior, producers experiencing short blow bar life should look beyond the wear part itself. Feed size and gradation, rotor speed, apron settings, feed distribution, and blow bar metallurgy all influence how quickly a bar wears.
When those variables are balanced, producers can extend wear life while maintaining consistent production. When they're not, even a premium blow bar can wear quickly.
Oversized feed is one of the most common causes of accelerated blow bar wear. Since operators can't see inside an HSI while it's running, the first clue is often sound.
“If the feed size is too big, you'll start to hear a lot of isolated noises going on inside, with not a lot of discharge coming out of the machine,” says Jarrod Adcock, Superior's crushing product manager.
Sporadic banging combined with reduced throughput can indicate oversized material blocking the inlet or disrupting material flow.
Once the crusher is opened for inspection, oversized feed often leaves clear witness marks. Look for large gouges in the blow bars, side sheets, and apron liners. These marks occur when oversized rock sits above the blow bar and is repeatedly clipped instead of being struck cleanly.
Even though they may not require additional reduction, excess fines still take up crusher capacity.
"They also cushion the impact between the blow bar and larger feed material, reducing the power of the blow," says Adcock. "This can cause a reduced reduction ratio."
Instead of deep gouging, excess fines may polish the blow bar surface, reduce throughput, and lower production.
Wet fines introduce another problem. Material can pack into the aprons and create what Adcock calls a “dirt wall,” further reducing crushing performance.
Feed distribution matters, too. Concentrating feed in one area leads to uneven wear in that same area and can shorten usable blow bar life. Material should be spread evenly across the rotor.
Rotor speed is one of the primary variables operators can control, and small changes can have a significant effect on wear.
Increasing speed generally increases impact energy, which can improve reduction. But rotor speed must work with feed size, apron settings, and the number of blow bars on the rotor.
The objective is a clean strike on the front face of the blow bar.
If the rotor is running too fast for the application, rock can “foul tip,” skidding across the top edge of the blow bar instead of striking its face.
“That introduces a lot of premature wear because you're not striking the main face, but just knocking the edge,” Adcock says.
Material may also remain in the crushing chamber longer, creating additional wear.
Running too slowly creates a different problem. Material can build up inside the chamber, shifting wear from the blow bar face toward the rotor body. High amperage and chattering curtains can indicate that material isn't moving through the crusher efficiently.
Rotor speed shouldn't be adjusted in isolation. Feed size, entry velocity, apron settings, and blow bar configuration all influence the correct operating speed.
Most HSI crushers use primary and secondary aprons, or curtains, which help control material reduction as rock moves through the crushing chamber. The distance between each apron and the rotor determines the size of material that can pass through that stage.
As a general starting point, Superior recommends approximately a 2:1 ratio between the primary and secondary settings. A 4-inch primary setting, for example, might be paired with a 2-inch secondary setting.
The exact ratio depends on the application, but the objective is to distribute reduction between the two crushing stages.
Adcock has encountered crushers operating with a 9-inch primary setting and a ¾-inch secondary setting. That extreme difference forces the secondary stage to perform most of the reduction.
“The load is not balanced between curtains, which causes structural damage to the crusher,” he says.
Instead of efficiently impact-crushing the material, the machine can begin grinding material through the apron gap, which increases wear on the blow bars and other crusher components.
Chattering curtain springs are one warning sign that the crushing load may need to be rebalanced.
Once operating conditions are under control, blow bar metallurgy can be matched more closely to the application.
A common starting point for primary crushing is manganese, while secondary and recycling applications may start with medium chrome. Selection should then account for:
Available options can include manganese, low-, medium-, and high-chrome alloys, ceramic-enhanced bars, and different heat treatments.
Each involves a trade-off between impact resistance and wear life.
Manganese is relatively soft but highly impact-resistant. It can absorb significant impacts without cracking, making it useful where oversized feed or uncrushables are a concern. The trade-off is faster wear in abrasive applications.
High-chrome ceramic bars sit at the other end of the spectrum. They can deliver substantially longer wear life in the right application but are less tolerant of oversized material and tramp metal.
When blow bars are wearing faster than expected, don't immediately assume the metallurgy is wrong. Inspect the crusher and the entire feed circuit.
One of the best diagnostic tools is the blow bar itself.
“The profile of a worn blow bar tells a lot about how the material or how the crusher is being operated,” Adcock says.
Under good operating conditions, the leading edge of a blow bar generally develops a smooth, rounded wear profile. A sharp edge, deep gouges, polishing, or uneven wear can point operators toward problems with feed conditions, rotor speed, apron settings, or material distribution.
“Ensuring you're optimized and balanced properly for speed, apron settings, and feed are all massive points that lead to very efficient production,” Adcock says. "Before changing metallurgy, look at the complete crushing setup."