Blow bars (also known on site as crusher bars, impact crusher hammers or impact bars) are the main wear parts fixed to the rotor of an impact crusher that strike the rock directly. Blow bars are one of the largest items in an impact crusher’s operating cost; with the right metallurgy, timely turning and correct operating conditions, bar life can be multiplied, while a wrong choice and neglect lead to early damage to both the bars and the rotor.
In this guide we cover in detail the function of blow bars, the material options and which one to choose for which application, what wear patterns tell you, turning and replacement steps, common mistakes, stock planning and spare part selection. For the general working principle of the impact crusher, see our impact crusher guide, and for the rotor, our rotor guide.
Function of the Blow Bar
- Breaks rock by impact: The bar rotating with the rotor strikes falling rock at high speed; the rock is broken both by this impact and by hitting the aprons it is thrown against.
- Protects the rotor: It takes the wear instead of the rotor body.
- Affects product size and shape: The bar profile and its wear condition determine crushing efficiency and particle shape.
Material Options and Comparison
| Material | Impact resistance (toughness) | Wear resistance | Typical application | Note |
|---|---|---|---|---|
| Manganese steel (Mn13–Mn18) | Very high | Low–medium | Primary crushing, coarse feed, metal risk | Needs real impact to work-harden |
| Martensitic steel | High | Medium | Recycling, mixed feed, secondary crushing | Balanced choice; good tolerance of tramp iron |
| Low/medium chrome cast iron | Medium | High | Secondary crushing, limestone, asphalt | Metal and coarse feed can cause breakage |
| High chrome cast iron | Low | Very high | Controlled feed, medium/fine sizes | Metal ingress causes breakage |
| Ceramic composite (MMC) | Low–medium | Highest | Abrasive material, well-disciplined plant | Feed control and metal separation are essential |
The basic rule: as wear resistance increases, impact tolerance decreases. Metallurgy should therefore be chosen not only by the abrasiveness of the material but by considering feed size, the risk of uncrushable material, rotor speed and crushing stage together. For details on material quality, see our manganese steel guide.
Which Bar for Which Application?
| Application | Recommended starting point |
|---|---|
| Primary impact crusher, quarry rock (limestone) | Manganese or martensitic |
| Secondary crushing, clean limestone | Medium/high chrome |
| Concrete and demolition waste (with rebar) | Martensitic |
| Asphalt recycling (RAP) | Medium chrome or martensitic |
| Tertiary crushing, fine product | High chrome or ceramic composite |
| Abrasive rock, strict feed control | Ceramic composite |
What Do Wear Patterns Tell You?
- Evenly rounded leading edge: Correct speed and feed; normal wear.
- Sharp edge and deep gouges: Oversized feed or metal.
- Polished face, little wear on the edge: Too much fine material; the impact is being cushioned.
- Heavy wear in the middle or at one end of the bar: Uneven feed distribution.
- Wear on the top edge, little wear on the face: The rotor is faster than necessary; rock hits the edge.
- Cracking or breakage: Metallurgy too hard for the application, metal ingress or the bar moving in its slot.
Turning and Replacement
Symmetrical blow bars are designed with two edges; when the leading edge reaches the wear limit specified by the manufacturer, the bar is turned round and the second edge is used. If the turning point is missed, the clamping area of the second edge is damaged and the bar is discarded after using only one edge, which means a large share of the wear material is wasted.
Replacement steps
- Preparation: The new bar set is weighed and the weights are recorded; wedges and clamping elements are prepared.
- Safe shutdown: The crusher and feeder are locked out; confirm that the rotor has stopped completely.
- Opening the housing: It is opened with the hydraulic housing opening system and the mechanical safety locks are fitted.
- Securing the rotor: The rotor is prevented from turning with the rotor locking device.
- Removing the bar: The wedge/clamping element is released; the bar is pulled out of its slot with the manufacturer’s extraction tool.
- Slot check: The slot is cleaned and checked for wear and deformation.
- Fitting: The new or turned bar is seated in the slot; opposite positions are given bars of equal weight.
- Clamping and checking: The wedges are tightened; the rotor is turned by hand (with the turning device) to confirm there is no contact with the aprons.
- Commissioning: Idle run; vibration and noise check. The wedges are checked again at the end of the first shift.
Effect of Operating Conditions on Bar Life
| Condition | Effect | Recommendation |
|---|---|---|
| High rotor speed | Wear increases rapidly, the top edge wears | The lowest speed that achieves the product |
| Incorrect apron ratio | The load becomes unbalanced, bars and aprons are strained | Front/rear apron about 2:1 |
| Too many fines in the feed | Impact is cushioned, efficiency drops | Pre-screening |
| Metal and uncrushable material | Breakage, slot damage | Magnetic separator, metal detector |
| Uneven feed distribution | Local wear, early replacement | Feed chute and deflector |
Common Mistakes
- Changing metallurgy to solve operating problems: Most “the bars don’t last” complaints are rooted in speed, apron or feed settings.
- Fitting bars without weighing them: Balance is upset and bearings are damaged.
- Missing the turning point.
- Not checking the slots: A loose bar breaks and hammers the rotor.
- Attempting weld build-up on unsuitable materials: Chrome and ceramic bars cannot be rebuilt by welding.
Troubleshooting Table
| Symptom | Possible cause | Solution |
|---|---|---|
| Bars breaking | Metal, oversized feed, metallurgy too hard, loose wedge | Metal separator, feed control, tougher metallurgy, wedge check |
| Bars wearing very fast | High speed, abrasive material, soft metallurgy | Reduce speed, more wear-resistant metallurgy |
| Gouged wear in the middle | Feed arriving through a narrow zone | Widen the feed chute, add a distributor |
| Vibration | Weight difference, broken bar | Weigh and match the set, replace the broken bar |
| Coarse product | Worn bars, increased apron gap | Turning/replacement, apron adjustment |
An Example from the Field
At a limestone plant, high chrome blow bars were breaking once or twice a month. Changing suppliers and trying harder bars had not helped. The investigation found that the CSS of the primary jaw crusher had grown because of wear, sending larger rocks than permitted to the impact crusher, and that the magnetic separator was faulty. The primary crusher was adjusted, the magnetic separator was repaired and the rotor speed was reduced to match product requirements. With the same metallurgy, the breakages stopped and bar life increased considerably.
Cost and Life Analysis
The right measure when choosing blow bars is not the “bar price” but the bar cost per tonne. A more expensive ceramic composite bar can process several times more tonnage in an abrasive application and reduce changeout downtime; but in a plant with metal ingress it can lose its whole advantage with a single breakage. For every metallurgy trial, therefore, the tonnage processed, number of changes, downtime and breakages should be recorded, and the decision made on total cost per tonne.
OEM or Aftermarket?
| Criterion | Check |
|---|---|
| Profile and dimensions | Full seating in the slot, wedge surfaces, length |
| Weight | Weight tolerance within the set |
| Metallurgy | Chemical analysis, hardness, heat treatment report |
| Casting quality | Porosity, cracks, flash |
| Support | Metallurgy recommendation for the application and stock |
Stock and Supply Planning
- At least one full set of bars and wedges/clamping elements should always be kept in stock.
- During metallurgy trials, a spare set of the previous metallurgy should also be kept.
- Order timing should be planned by tracking bar life by tonnage.
Pre-Purchase Checklist
- Have the crusher make/model and rotor type been identified?
- Are the material processed, feed size and metal risk known?
- Are the current bar metallurgy and its life recorded?
- Are wedges and clamping elements included in the order?
- Has a set weight tolerance been requested?
Safety
- The housing must not be opened until the rotor has stopped completely and been locked.
- Blow bars are very heavy; use the manufacturer’s extraction and lifting tools.
- Mechanical safety locks must be fitted while the housing is open.
Frequently Asked Questions
How long do blow bars last?
It varies greatly with the abrasiveness of the material, rotor speed and metallurgy. The best way to estimate life is to keep records by tonnage.
Should all bars be replaced at the same time?
For balance, they should be replaced in opposite pairs or as a full set.
Is a ceramic composite bar always better?
No. It gives very long life with abrasive material and controlled feed; in plants with metal risk or coarse feed, the risk of breakage is high.
Can blow bars be rebuilt by welding?
Manganese bars can in some cases be built up with suitable electrodes; chrome and ceramic composite bars cannot be rebuilt by welding.
Does a broken bar damage the rotor?
Yes; broken pieces can hammer the rotor, aprons and housing and upset the balance. After a breakage, the rotor and slots must always be checked.
Are hammer mill hammers and blow bars the same?
No. The hammers in a hammer crusher are attached to the rotor with pins and swing freely; blow bars in an impact crusher are fixed rigidly to the rotor. For hammer crushers, see our hammer crusher guide.
Spare Parts and Service for Blow Bars
As a manufacturer of impact crushers and tertiary impact crushers, CSP Mühendislik supplies manganese, martensitic, chrome and ceramic composite blow bars for different makes and models as part of its impact crusher spare parts range, and provides service support for choosing the right metallurgy for your application, bar replacement and rotor checks. Share your crusher model and the material you process on WhatsApp and we will recommend the right bar.
