A hammer crusher (also called a hammer mill) is a type of impact crusher that breaks material by striking it with hammers attached so they swing freely on a rotor turning at high speed, and limits product size with the grate (screen) bars underneath. It gives a high reduction ratio in a single stage for medium and low-hardness, low-abrasion materials such as limestone, gypsum, coal, glass, salt and clay.
Cement raw material preparation, coal preparation plants, glass recycling and the lime industry are the areas where hammer crushers are most widely used. This guide summarises the hammer crusher’s working principle, glass and coal crusher applications and the maintenance practices needed for long-life operation.
How Does a Hammer Crusher Work?
Material enters the crushing chamber from above and is broken by the impact of the hammers rotating with the rotor. The hammers are attached to the rotor with pins and swing outwards under centrifugal force; when they meet an uncrushable object, they swing back and absorb the impact. The broken material strikes the breaker plates on the walls of the crushing chamber and is broken a second time. Particles smaller than the openings of the grate at the bottom of the chamber leave the chamber; coarse particles meet the hammers again until they are small enough.
This mechanism has two important consequences. First, the top size of the product is tightly controlled by the grate opening. Second, because material stays in the chamber until it passes through the grate, the proportion of very fine product is high; hammer crushers are therefore preferred in applications where fine product is an advantage, such as preparation before grinding, rather than in aggregate production where fines are unwanted.
Types of hammer crusher
- Single rotor, single direction: The most common type; can be built with a grate or without a grate (open discharge).
- Reversible hammer crusher: The rotor can turn in both directions; by changing the direction of rotation, both faces of the hammers and breaker plates are used.
- Double rotor hammer crusher: Used in cement and lime applications where a high reduction ratio is wanted in a single machine.
Glass Crushers
Glass crushers are hammer or impact crushers optimised for recycling bottle and flat glass waste. In producing cullet, the aim is to reduce the glass to the required size, make labels, caps and organic residues separable, and round off sharp edges. In glass crushers:
- Because glass is extremely abrasive, wear-resistant alloys are used for hammers and liners,
- Magnetic separation is carried out before the crusher for metal caps and foreign objects,
- Fine glass powder (usually below 1 mm) is screened off as a separate fraction; this powder can be used as a sand alternative or as fill,
- A dust extraction system and safety measures (cuts, silica dust) are essential.
Coal Crushers
Coal crushers reduce coal to a size suitable for the mill or boiler on the feed lines of thermal power plants, cement plants and coal preparation plants. Because coal is relatively soft and friable, hammer crushers are very efficient here. Key points in coal crusher applications:
- Moisture: Wet, clayey coal blinds the grate. Grate opening and rotor speed should be chosen according to the moisture content; if necessary, an open (grateless) design or pre-screening should be used.
- Dust and explosion risk: Coal dust is combustible. Dust build-up around the crusher must be prevented, and equipment and earthing complying with ATEX requirements must be used.
- Foreign material: Metal pieces and hard rocks arriving with the coal damage the hammers; a magnetic separator should be used on the belt and, if necessary, rock separation.
Parameters That Determine Performance
| Parameter | Effect |
|---|---|
| Grate opening | Determines the top size of the product; a narrow grate means a finer product but lower capacity |
| Hammer tip speed | Higher speed makes the product finer, so a wider grate can be chosen; wear and fines increase |
| Hammer-to-grate clearance | The clearance grows with wear; as it grows, capacity falls and oversize escapes |
| Hammer weight and number | Heavy hammers for coarse, hard feed; more, lighter hammers for fine feed |
| Feed size and rate | Overfeeding fills the chamber and raises the current; steady feeding is essential |
| Moisture | The primary cause of grate blinding |
Hammer Maintenance: Ways to Double Hammer Life
- Turning hammers: Most hammers are designed symmetrically; when one face wears, the hammer is turned over to use the second face. In some designs the hammer can be used on all four corners.
- Reversing rotation: On reversible crushers, changing the direction of rotation at regular intervals spreads hammer and grate wear over both sides.
- Moving hammers out: When the hammers wear and their distance from the grate increases, in some designs they can be moved to pin holes nearer the outside of the rotor; the grate can also be adjusted towards the centre.
- Balance: When hammers are replaced, the hammer weights on each pin row must be equalised, and opposite rows must have the same weight. A weight difference means vibration and bearing damage.
- Pins and rotor discs: Check the pins and pin holes at every hammer change; an ovalised pin hole can cause a hammer to break off.
Maintenance Schedule
| Interval | Check |
|---|---|
| Every shift | Vibration, bearing temperature, motor current, unusual noise, signs of grate blinding |
| Daily–weekly | Hammer wear, hammer-to-grate clearance, breaker plates, bolts |
| According to the manufacturer’s instructions | Bearing greasing (too much grease is also harmful) |
| Monthly | Pins, rotor discs, grate bars, belts and coupling |
| Yearly | Rotor balance, bearing and shaft check, housing liners |
Troubleshooting Table
| Symptom | Possible cause | Solution |
|---|---|---|
| Capacity has dropped | Grate blinded, hammers worn, clearance increased | Clean the grate, turn/move the hammers, adjust the grate |
| Coarse product | Grate bars worn or broken | Replace the grate |
| Vibration | Unbalanced hammer weights, broken hammer, bearing damage | Stop; weigh and rearrange the hammer set |
| High motor current | Overfeeding, wet material, grate blinding | Regulate the feed, check moisture |
| Excessively fine product | Rotor speed too high, grate too narrow | Reduce speed or widen the grate |
When Is a Hammer Crusher Not the Right Choice?
With hard, abrasive rocks such as granite, basalt or quartzite, hammer wear is not economical; for these materials a combination of jaw crusher and cone crusher should be preferred. If fines are not wanted in aggregate production, an impact crusher is a more suitable option.
Frequently Asked Questions
What is the difference between a hammer crusher and an impact crusher?
In an impact crusher the blow bars are fixed to the rotor and product size is set by the aprons; in a hammer crusher the hammers swing freely and product size is limited by the grate. A hammer crusher gives a finer product with a limited top size.
Where is the cullet produced by a glass crusher used?
Cleaned cullet can be used in new glass production (saving energy), in glass wool production, as a sand alternative for fill, and as blasting and filtration media.
Why does the grate blind in a coal crusher?
The most common reason is high moisture and clay content. Increasing the grate opening, removing fine coal by pre-screening or switching to a grateless design are possible solutions.
Tip Speed and Grate Selection: A Simple Calculation
Hammer tip speed (m/s) = π × hammer circle diameter (m) × rotor speed (rpm) / 60. For example, if a rotor with a 1.0 m hammer circle turns at 1,000 rpm, the tip speed is about 52 m/s. Tip speed and grate opening should be assessed together:
- A high tip speed reduces material more; a wider grate can be used for the same product size. A wider grate blinds less and increases capacity.
- A low tip speed means fewer fines and less wear; however, the grate must be chosen narrower to reach the target size.
- The grate opening is usually chosen slightly above the required product top size, because particles can pass through the grate at an angle. The exact value is determined by trials and sieve analyses.
Hammer Materials
| Material | Characteristics | Use |
|---|---|---|
| Manganese steel | High toughness, work hardening | Coarse, impact-heavy feed, limestone |
| High chrome cast iron | High wear resistance, low toughness | Small feed, abrasive but low-impact applications |
| Bimetallic (manganese body + chrome tip) | Toughness and wear resistance together | Coal, lime, cement raw material |
| Hardfaced hammer | Tip area hardened by welding | Medium duty, can be rebuilt |
Hammers are chosen according to the size and abrasiveness of the material crushed and the risk of uncrushable foreign material.
Hammer Replacement and Balancing Procedure
- Stop the crusher, lock the rotor and tag the energy sources.
- Open the housing cover; mark the positions of the old hammers.
- Weigh the new hammers one by one and record their weights.
- Group the hammers so that the total weight on opposite pin rows is equal; do not exceed the weight difference allowed by the manufacturer.
- Check the pins and pin holes; replace worn pins.
- Fit the hammers and check the pin retainers; turn the rotor by hand to confirm there is no contact with the grate.
- Run the crusher empty and measure vibration; then increase the load gradually.
Safety and Dust Explosions in Coal Crushers
Coal dust can explode when it reaches a certain concentration in air and meets an ignition source. In coal crushing plants:
- Dust build-up around the crusher and on structures should be cleaned regularly; cleaning with compressed air should be avoided,
- Electrical equipment should be chosen according to the zone classification,
- Bearing temperatures should be monitored; an overheating bearing can be an ignition source,
- Explosion vent panels and spark detection systems should be considered for dust collection systems.
Typical Flow of a Glass Recycling Line
- Reception and pre-sorting (large foreign objects, separation of ceramics and porcelain),
- Magnetic separation (metal caps),
- Size reduction in the glass crusher,
- Screening and label/paper removal (by air),
- Eddy current separator for aluminium,
- Colour sorting and optical classification (for high-quality cullet),
- Splitting into fractions and stockpiling.
The glass crusher is not a solution on its own but one link in this chain. The line’s capacity and product quality depend on the effectiveness of the separation steps.
Spare Parts Stock
- Always in stock: A full set of hammers (weight-matched), a pin set, grate bars, breaker plates, a belt set.
- Critical spares: Rotor bearings, coupling elements, shaft.
- Planned: Housing liners, rotor discs.
An Example from the Field: Grate Blinding
At a lime plant, capacity almost halved in winter. The cause was grate blinding as moisture and clay content increased in the rainy season. As a solution, a simple pre-screen was added in front of the crusher so that fine, wet material bypassed the crusher; the grate opening was increased by one step and rotor speed raised slightly. Winter capacity came close to summer levels. In hammer crushers, the solution to grate blinding often lies not in the crusher itself but in preparing the feed.
More Questions
Why does a hammer crusher produce such a fine product?
Material stays in the chamber and is struck again and again until it passes through the grate. A narrow grate and high tip speed increase this effect. To reduce fines, the grate can be widened and the speed reduced.
What is a grateless hammer crusher?
Designs with no bottom grate or a partly open one; they prevent blinding with wet and sticky material. Product size control is looser than with a grate design.
Which uses less energy, a hammer crusher or an impact crusher?
It depends on the application. Where a fine product is wanted, a hammer crusher is efficient in a single stage; in coarse aggregate production, an impact crusher usually works with fewer fines and less energy.
Hammer Crusher Solutions from CSP Mühendislik
CSP Mühendislik manufactures hammer crushers, glass crushers and coal crushers, and supplies crusher spare parts such as hammers, grates, breaker plates and pins. Contact us to choose the right crusher for your material.
