Impact Crusher Blow Bars: Selection, Maintenance and Failure

Understanding Blow-Bar Function and Failure

Blow bars are the main wear parts of horizontal-shaft impact crushers (HSI). Rotating bars strike the feed and throw it against impact aprons and curtain liners for size reduction. Circuit efficiency depends on bar condition, material and geometry.

Unlike cone liners, which mainly face compression and sliding wear, blow bars experience high-energy impact, abrasive erosion and thermal fatigue in combination. This demanding environment makes material selection and disciplined maintenance particularly important. For background on wear mechanisms, see our Wear-Part Failure Mode Analysis.

Comparing Blow-Bar Material Grades

Material selection largely determines performance. Three main material families serve different duties; the wrong grade can shorten life by more than 50%.

High-Manganese Steel (Mn13, Mn18, Mn22)

An important characteristic of manganese steel is work-hardening : repeated impact progressively hardens the surface, but the extent depends on the grade, heat treatment, impact energy and feed conditions. Suitability for a particular impact crusher must therefore be checked against its manual, existing part records and operating conditions.

• Mn13Cr2 — A standard choice for general primary crushing of medium-hard rock.
• Mn18Cr2 — Offers 25-35% longer wear life than Mn13 under high abrasion and impact, making it a preferred grade for abrasive ore.
• Mn22Cr2 — Maximum work-hardening capacity for extreme impact with very hard, highly abrasive feed.

Manganese steel needs sufficient impact energy to work-harden. In secondary or low-energy duty, the surface may not fully harden and wear can rise sharply. For more on its metallurgy, see our High-Manganese Steel Guide.

High-Chromium Cast Iron (Cr26, Cr28)

High-chromium iron relies on a network of hard chromium carbides (HV 1400-1800) in a martensitic or austenitic matrix rather than work-hardening. Its as-cast hardness of 58-65 HRC exceeds fully work-hardened manganese steel and provides strong wear resistance under low to moderate impact.

High-chromium bars perform well in secondary crushing of abrasive river gravel, basalt and recycled concrete. Their brittleness, however, makes them vulnerable to severe fracture under high impact or tramp metal. Cr28 is slightly harder than Cr26 but less tough.

Martensitic Alloy Steel

Martensitic bars sit between manganese steel and high-chromium iron. Heat treatment gives uniform 50-58 HRC hardness through the section, not only at the surface, for consistent wear. They are preferred formixed-feed conditions where both impact and abrasion matter, such as limestone with small silica inclusions.

Material Selection Matrix

As a rule of thumb, use manganese steel for hard primary feed and high-chromium iron for abrasive secondary feed. Martensitic alloy steel offers a balanced choice when conditions are uncertain.

Blow-Bar Maintenance Practices

Installation and Tightening

Installation condition can substantially affect blow bar life. Follow the equipment manufacturer's torque, sequence and reinspection intervals for wedge locking systems. Retightening intervals differ by model; a universal number of hours cannot replace the machine manual.

1. Clean All Mating Surfaces — Remove dirt, rust and old packing from rotor seats and bar backs.
2. Apply Threadlocker — Apply threadlocker to all clamping bolts to resist vibration loosening.
3. Follow the Tightening Sequence — Typically work outward from the centre, alternating sides for even clamping.
4. Retighten after 2-4 Hours — Initial vibration can reduce bolt preload by 10-20%.
5. Inspect the Wedges — Replace worn or damaged wedges that reduce clamping force.

Turning and Replacement Schedule

Most bars have two or more wear faces. Turning to a fresh face can double or triple usable life. Timing depends on the wear profile:

• Manganese Steel Bars — Turn when leading-edge wear reaches 30-35% of the original profile depth.
• High-Chromium Bars — Turn at 25-30% wear; lower toughness makes delay more consequential.
• Turn All Bars Together — Mixing worn and new bars causes imbalance, excessive vibration and bearing damage.
• Replace Complete Sets — Never mix new and used bars on the same rotor, even if they look similar.

Routine Inspection

A structured inspection plan identifies problems before they become major failures:

Failure Analysis: Common Blow-Bar Problems

The failure pattern provides clues to premature failure. Reading it correctly helps prevent recurrence.

Failure Mode One: Severe Fracture

Fracture is a costly failure: a bar breaks through its section, often damaging the rotor, aprons and housing. Causes include:

• Tramp Metal — The leading cause: bucket teeth, drill rods or scrap produce impacts beyond the bar's fracture resistance.
• Insufficient Feed — Individual rocks strike at full rotor speed without the cushioning of a full crushing chamber.
• Wrong Material Grade — Using high-chromium bars in high-impact primary duty.
• Incorrect Rotation Direction — Reverse rotation loads the trailing edge instead of the designed leading edge.

Failure Mode Two: Premature Wear

When bars remain intact but wear too quickly, material and duty are usually mismatched:

• Manganese Steel under Low Impact — Insufficient work-hardening leaves a soft surface vulnerable to abrasion.
• High-Chromium Iron under High Impact — Repeated microfracture of the carbide-bearing structure accelerates material loss.
• High Silica in Feed — Silica (quartz) has hardness HV 1000-1200, near that of work-hardened manganese steel. Even a small increase can sharply accelerate wear.
• Excessive Feed Moisture — Wet material compacts, reducing impact efficiency and promoting sliding wear.

Failure Mode Three: Uneven Wear

Uneven wear across rotor width causes imbalance, vibration and lower output. Greater wear at one end forms a tapered profile and reduces product quality. Common causes:

• Uneven Feed Distribution — Off-centre entry concentrates wear on one side.
• Worn Feed Chute or Distributor — A worn or misaligned chute deflects material to one side.
• Unequal Apron Settings — A closer setting on one side creates asymmetric crushing forces.

Check the feed system first when wear is uneven. Chute adjustment or distributor replacement can extend bar life by 20-30%.

Failure Mode Four: Edge Chipping and Spalling

Edge chipping indicates impact concentrated at the edge rather than spread over the face. Typical causes:

• Oversize Feed — Rocks above the recommended maximum strike the edges at high speed.
• Worn Curtain Liners — Recessed apron wear redirects rebounding rock onto bar edges.
• Excessive Rotor Speed — Higher tip speed produces impact energy beyond the edge's capacity.

Cost Optimization

Match Material to Duty

Material grade affects purchase price, replacement frequency and downtime. The table below only illustrates how to calculate cost per tonne. Its operating conditions and prices are hypothetical and do not predict returns at any site. An actual comparison must use the mine's throughput, service-life records, downtime costs and supplier quotations.

Under these assumptions, a higher unit price can still yield a lower cost per tonne. This result cannot be directly extrapolated to other materials or machines. Before changing grades, check tramp-metal risk, impact energy, feed size distribution and the materials permitted by the machine manufacturer. For the costing method, see our Crusher Operating Cost Guide.

Aftermarket versus OEM Blow Bars

Prices, lead times and documentation completeness for OEM and aftermarket parts vary by brand, model and supplier. Compare material certificates, dimensional reports, traceability, site service life and downtime risk in one assessment, without assuming a fixed price difference or equivalent performance:

Verification is essential when sourcing aftermarket parts. Request a heat certificate showing actual chemistry, and check hardness on site before installation. For the detailed procedure, see our Wear-Part Quality Inspection Guide.

Operating Factors That Affect Cost

Beyond material selection, these operating factors affect cost per tonne:

• Full-Chamber Feeding — A full chamber cushions direct high-speed impact and can extend life by 15-25%.
• Feed-Size Control — Remove oversize before the crusher to prevent edge chipping and uneven wear.
• Moisture Control — Keep feed moisture below 5% to prevent compaction and maintain impact efficiency.
• Apron Gap Optimization — Too tight increases back pressure and wear; too wide reduces the reduction ratio.
• Match Rotor Speed — Higher tip speed increases output and wear; find the best balance.

Diagnostic Illustration: Premature Blow-Bar Wear in Aggregate Crushing

Diagnostic illustration, not a verified customer case. In this illustration, a secondary impact crusher operates after a jaw crusher, and its manganese steel blow bars have not developed the expected work hardening. An engineering assessment requires more than the machine model: check feed size, impact energy, quartz content, tramp-metal records and actual wear morphology.

Check the following hypotheses against wear morphology and feed records; these are not verified customer findings:

1. Hypothesis to verify — impact energy in the secondary stage may be insufficient to work-harden the current manganese steel adequately. Verify this using hardness profiles, wear photographs and trial data.
2. High Silica Content — assess hard minerals such as quartz using ore analysis and wear morphology, without assuming content or wear-rate increases.
3. Uneven Feed — measure feed distribution and wear on both rotor sides to confirm any imbalance, without assuming a split percentage.

Assess material and operating changes, with implementation subject to manufacturer requirements and trial verification:

• Assess Cr26 high-chromium iron blow bars for suitability, first checking impact and tramp-metal risks against the material's limits.
• Repair the feed chute and assess flow-guiding measures from the measured distribution.
• Check the apron gap against the specific model, feed and product targets; do not use a universal gap value.

Trial Records and Procurement Assessment:

• Record usable hours and processed tonnage per set under equivalent feed and machine conditions.
• Calculate replacement demand from the same annual tonnage and measured life, without presenting assumed annual purchases as achieved results.
• Record unit price, usable life, tonnage processed and replacement labor before and during the trial.
• Record planned and unplanned downtime separately so that production fluctuations are not mistaken for material benefits.
• Calculate the outcome from site data: A procurement conclusion requires a comparison under equivalent feed and machine conditions.

This illustration explains verification, not achieved life or cost gains. Record the effects of material, feed and gap in controlled trials before reaching a procurement conclusion.

Frequently Asked Questions

Putting the Selection Together

Improve blow-bar performance through four coordinated actions:

1. Choose the Right Material — Manganese for high impact, chromium for high abrasion, martensitic steel for mixed duty.
2. Maintain Consistently — Correct torque, timely turning and complete-set replacement.
3. Diagnose the Failure Pattern — Fracture, premature wear, uneven wear and edge chipping have different causes.
4. Optimize Cost per Tonne — The cheapest bar is rarely the most economical over its life.

For further reading, see our Cone Crusher Wear-Part Selection Guide and Wear-Part Failure Mode Analysis.

Contact our engineering team for a free blow-bar review: we assess current duty, recommend material and compare cost per tonne.