An impact crusher (horizontal shaft impactor, HSI) breaks material not by compression but by striking it with blow bars fixed to a rotor turning at high speed and throwing it against aprons. It is one of the most economical crushers for medium-hard, low-abrasion rocks such as limestone, dolomite and gypsum, in concrete and asphalt recycling, and in aggregate production where a cubical particle shape is wanted.
The impact crusher’s greatest strengths are a high reduction ratio and excellent particle shape: reduction ratios of 10:1 to 15:1 can be reached in a single stage. Its weakness is the wear cost with abrasive rocks with a high silica content. This guide brings together the practical knowledge needed to set an impact crusher correctly and get the full life out of the blow bars.
How Does an Impact Crusher Work?
Material entering through the feed opening falls in front of the rotating rotor. The blow bars on the rotor strike the rock at high speed; the rock breaks both from this first impact and from hitting the impact aprons (breaker plates) it is thrown against. Pieces rebounding from the aprons meet the rotor again. The material leaves the crusher once it is small enough to pass through the gap between the apron and the circle described by the blow bars.
In this mechanism, breakage takes place along the rock’s own natural cracks and planes of weakness. The result is sharp-edged but cubical particles and a high proportion of fines. Impact crusher product therefore easily meets the particle shape specifications required for concrete and asphalt aggregate.
Primary, secondary and tertiary impact crushers
- Primary impact crusher: Heavy-rotor machines that can crush large blocks from the quarry (up to 1 m) directly. In limestone quarries, one can be used as a single machine in place of a jaw + cone crusher pair.
- Secondary impact crusher: Processes 0–200/300 mm material from the jaw crusher; the most common application in aggregate plants.
- Tertiary impact crusher: Optimised for fine aggregate and sand production with a smaller feed size, higher rotor speed and usually a three-apron (or apron + grinding path) design. It is an economical solution for producing chippings and 0–5 mm manufactured sand.
Main Components
| Component | Function | Maintenance focus |
|---|---|---|
| Rotor | Carries the blow bars and provides the kinetic energy | Rotor body wear, balance, bearing temperature |
| Blow bars | The main wear part that strikes the material | Turning/replacing, replacement in opposite pairs |
| Front and rear (primary/secondary) aprons | Surfaces the rock strikes, which determine product size | Apron liners and adjustment mechanism |
| Side liners and housing liners | Protect the housing against wear | Thickness checks, bolts |
| Apron adjustment rods / springs or hydraulics | Set the apron gap and open under overload | Spring preload, hydraulic seals |
| Main shaft and bearings | Carry the rotor | Grease quantity and interval, temperature |
| Hydraulic housing opening system | Opens the housing for maintenance | Safety locks |
For details on blow bars and other wear parts, see our blow bar replacement guide and our article on impact crusher spare parts.
Setting an Impact Crusher Correctly
The product size and wear behaviour of an impact crusher are determined by the combined effect of three settings: rotor speed, apron gaps and feed. When one is changed, the others should be reassessed.
Rotor speed
- As the rotor’s peripheral speed increases, the product becomes finer and the reduction ratio rises; but wear also increases rapidly.
- If the rotor turns too fast, the rock hits the top edge of the blow bar rather than its face (“skidding”); the top edge of the bar wears quickly and efficiency falls.
- If the rotor turns too slowly, material builds up in the chamber and wear shifts to the rotor body; motor current rises and apron chatter appears.
- Speed is usually changed by changing pulleys or with a frequency inverter, and even small changes noticeably affect wear; after a speed change, monitor the product curve and wear for a few days.
Apron gaps
Keeping the ratio between the front (primary) and rear (secondary) apron gaps at around 2:1 is a good starting point; for example, 100 mm at the front apron and 50 mm at the rear. Upsetting the ratio loads most of the crushing onto the rear apron, causing excessive wear, chatter and structural damage there. As the aprons wear, the gap increases, so the gaps should be measured and corrected periodically.
Feed
- Feed size: The maximum feed size specified by the manufacturer must not be exceeded. Large rocks cause gouging wear and breakage of the bars; single knocking sounds from the crusher and reduced output are warning signs.
- Fines: Too many fines in the feed cushion the impact and reduce efficiency; if wet, they form a “mud wall” on the aprons and housing. Fines should be removed before the crusher with a grizzly feeder or scalping screen.
- Distribution: Material should be spread evenly across the full width of the rotor. In a crusher fed in one zone, the bars gouge in the middle or at one edge; the bar is replaced early, but most of its weight has not been used.
- Metal: Uncrushable metal pieces break bars, especially ceramic-reinforced and high-chrome bars. A magnetic separator on the feed belt is essential; in recycling applications, additional measures should be taken for reinforcing steel from concrete.
Blow Bar Selection: Which Material When?
| Material | Impact resistance | Wear resistance | Suitable use |
|---|---|---|---|
| Manganese steel | Very high | Low–medium (depends on work hardening) | Primary crushing, coarse feed, risk of uncrushable material |
| Martensitic steel | High | Medium | Recycling, mixed feed, medium-size rock |
| Low/medium chrome cast iron | Medium | High | Secondary crushing, limestone, asphalt |
| High chrome cast iron | Low | Very high | Controlled feed, fine/medium size, metal-free material |
| Ceramic reinforced (MMC) | Low–medium | Highest | Abrasive material, plants with strict feed control |
The rule is simple: the harder (more wear-resistant) the material, the less impact tolerance. Before changing blow bar metallurgy, rotor speed, apron settings and feed quality should therefore be reviewed; very often the root cause of the complaint “the bars don’t last” is not the metallurgy but the operating conditions.
What Do Blow Bar Wear Patterns Tell You?
- Rounded, even leading edge: Correct speed and feed; normal wear.
- Sharp edge, deep gouges: Oversized feed or metal.
- Polished face, little wear on the leading edge: Too many fines, cushioning.
- Heavy wear at one end or in the middle of the bar: Uneven feed distribution.
- Wear on the top edge, little wear on the face: Rotor faster than necessary.
Turning and Replacement
- Symmetrical blow bars are turned over to use the second edge when the leading edge has worn to the limit specified by the manufacturer. If turning is late, the clamping area of the second edge is damaged and the bar can no longer be turned.
- For rotor balance, bars are always replaced/turned in opposite pairs or as a full set; bars of different weights cause vibration and bearing damage.
- At a bar change, check the retaining wedges, rotor arms and rotor caps; a worn rotor seat causes the new bar to move.
- Apron liners and housing liners should also be measured during a bar change and, if at their limit, replaced in the same shutdown.
Maintenance Schedule
| Interval | Check |
|---|---|
| Every shift | Bearing temperature, vibration, motor current, unusual noise, feed and discharge chutes |
| Daily | Blow bar and apron wear check (through the inspection door, under LOTO), bolts, belt tension |
| According to the manufacturer’s instructions (usually 8–40 hours) | Greasing the rotor bearings; the amount of grease should not be excessive |
| Weekly | Measuring and adjusting apron gaps, side liners, apron adjustment springs/hydraulics |
| Monthly | Rotor body and arms, housing welds, hydraulic opening system, safety locks |
| Yearly | Bearing clearances, shaft, rotor balance, comprehensive housing inspection |
Troubleshooting Table
| Symptom | Possible cause | Solution |
|---|---|---|
| Coarse product, low capacity | Apron gaps increased, bars worn, low rotor speed | Adjust the gaps, turn the bars, check the speed |
| Excessively fine product | High rotor speed, narrow gaps | Reduce speed, open the gaps |
| Vibration | Unbalanced bar set, broken bar, bearing damage | Stop immediately; check the bars and bearings |
| Apron chatter/knocking | Incorrect apron ratio, coarse feed, low spring preload | Re-establish the 2:1 ratio, check the feed |
| Bar breakage | Metal, coarse feed, metallurgy too hard for the application | Metal separator, feed control, suitable metallurgy |
| Material sticking in the housing | Wet, clayey fines | Remove fines beforehand; heating/lining solutions if needed |
| High bearing temperature | Too much/too little grease, contamination, belts too tight | Correct the greasing programme, check the seals |
Safety
- An impact crusher’s rotor keeps turning for a long time after the motor stops. Before opening the housing, make sure the rotor has stopped completely and is locked.
- When the housing is opened hydraulically, do not go underneath without the mechanical safety locks fitted.
- Use the manufacturer’s lifting devices for bar changes; a single blow bar can weigh hundreds of kilograms.
Impact Crusher or Cone Crusher?
The abrasiveness of the material is the deciding factor. For low-abrasion rocks such as limestone, an impact crusher has advantages in both investment and operating cost and gives a more cubical product. For abrasive rocks such as granite, basalt and quartzite, a cone crusher reduces wear costs many times over. For sand production and final shaping, a vertical shaft impactor (VSI) is preferred.
Frequently Asked Questions
Which materials is an impact crusher not suitable for?
Very abrasive rocks with a high silica (SiO₂) content and very sticky, clayey materials are challenging for an impact crusher. With such materials, wear costs and the risk of blockage should be calculated in advance.
What is the difference between a tertiary impact crusher and a VSI?
Both produce fine aggregate; a tertiary impact crusher is a horizontal-rotor design with aprons and gives economical production of chippings and sand from medium-hard rocks. A VSI, because it can work on the rock-on-rock principle, can also produce sand from abrasive material and gives very good particle shape.
Why don’t blow bars reach their full life?
The most common reasons are uneven feed distribution, excessively coarse or fine feed, unsuitable rotor speed and an incorrect apron ratio. These operating conditions should be corrected before changing metallurgy.
How Is Rotor Peripheral Speed Calculated?
The “speed” discussed in impact crusher settings is not the motor speed but the peripheral speed of the blow bar tip. The calculation is simple:
Peripheral speed (m/s) = π × rotor diameter (m) × rotor speed (rpm) / 60
For example, if a 1.2 m diameter rotor turns at 600 rpm, the peripheral speed is about 3.14 × 1.2 × 600 / 60 ≈ 37.7 m/s. Lower peripheral speeds are used in primary impact crushers and higher ones in secondary and tertiary crushers. When speed is increased by 10% with a pulley change, the peripheral speed also increases by 10%; but because impact energy increases with the square of the speed, the crushing effect and wear change more markedly. Speed changes should therefore be made in small steps, with the product curve and wear monitored together.
Apron Setting Step by Step
- Stop the crusher, wait for the rotor to stop completely and lock out the energy sources.
- Through the inspection door or by opening the housing, measure the gap between the blow bar tip circle and the apron liners. Turn the rotor by hand (or with the manufacturer’s turning device) and measure at the position of the longest bar.
- Bring the front apron to the target value; then set the rear apron to keep a ratio of about 2:1 with the front apron.
- On spring systems, check the spring preload; on hydraulic systems, check the setting pressure against the manufacturer’s value.
- After adjustment, run the crusher empty, then load it and take a product sample; confirm the target product with a sieve analysis.
If the apron gap is set too small, grinding occurs between the bar and the apron; both energy and wear increase and too many fines are produced.
Impact Crushers in Recycling
Impact crushers are widely used for crushing concrete, asphalt and demolition waste because they separate mortar from aggregate well and give a cubical product. Points to watch in recycling applications:
- Bars with high impact resistance (martensitic or manganese) against steel and hard foreign objects,
- A strong magnetic separator at the crusher discharge,
- A low rotor speed and water spraying to prevent sticking when crushing asphalt in hot weather,
- Reducing large blocks with a hydraulic breaker before feeding.
For details, see our article on recycling construction and demolition waste.
Spare Parts Stock
- Always in stock: At least one set of blow bars and wedge/retainer parts, one set of each apron liner, side liners, a belt set, bearing grease.
- Critical spares: Rotor bearings, main shaft, apron adjustment rods and springs or hydraulic cylinder seals.
- Planned: Rotor caps, housing liners, feed chute wear plates.
An Example from the Field: The Real Cause of Bar Breakages
At a limestone quarry, high-chrome blow bars in a secondary impact crusher were breaking frequently. Various trials, including changing supplier, had not worked. The investigation found three causes: the primary crusher’s CSS had grown because of wear and rocks larger than permitted were reaching the impact crusher; the magnetic separator was faulty; and the rotor speed had been raised for an earlier product demand and never reduced. The primary crusher’s CSS was corrected, the magnetic separator was repaired and the rotor speed was brought to a suitable value. The breakages stopped, and bar life with the same metallurgy increased considerably. This example shows that with blow bar problems, the whole circuit must be examined together.
Pre-Purchase Checklist
- Have the material’s abrasiveness index and silica content been determined? (The economics of an impact crusher depend on this.)
- Are the maximum feed size and feed grading known?
- What products and particle shape specifications are required?
- Are moisture and clay content high? Has scalping been planned?
- Is there a way to adjust rotor speed (pulley options or a frequency inverter)?
- Have maintenance space and lifting equipment been planned for opening the housing and changing bars?
- Are local spare parts and metallurgy options available?
More Questions
Can a primary impact crusher replace a jaw crusher?
For low-abrasion rocks such as limestone, yes; a primary impact crusher can reduce plant investment by providing a high reduction ratio in a single stage. For hard and abrasive rocks, a jaw crusher is more suitable economically.
Does it make sense to rebuild blow bars by welding?
It depends on the bar type. Manganese bars can in some cases be built up with suitable electrodes; high-chrome and ceramic-reinforced bars cannot be rebuilt by welding, as the heat causes cracks. Follow the manufacturer’s recommendation.
The product is too fine; what should I do?
The first steps are to reduce rotor speed, open the apron gaps and remove fines from the feed beforehand. Also check whether the recirculating load in a closed circuit is unnecessarily high.
Impact Crusher Solutions from CSP Mühendislik
CSP Mühendislik manufactures impact crushers and tertiary impact crushers, and supplies impact crusher spare parts such as blow bars, apron liners and rotor parts in metallurgy suited to your material. Contact us to assess your crusher settings and wear costs together.
