Component 1: the housing
Here LuT works with a heavily ribbed steel construction, in contrast to many other manufacturers. The rear part of the housing can be opened hydraulically for service work. Service work includes, for example, easy replacement of the blow bars and simple, problem-free removal of the complete rotor with mounted bearings.
The hinged-section mounting is designed as a box shape so that cracking of the webs is avoided. During operation the hinged housing section is secured by simple but very robust locks.
In order to reduce wear as far as possible, new housings – and older ones on request – are lined with Hardox 400 or Hardox 500 plate. We guarantee the use of original SSAB material (Hardox).
The wear parts are for the most part interchangeable, which considerably reduces the spare parts stock the operators of such plants have to keep.
With a rotor diameter of 1300 mm, the inlet – that is, the material intake – is designed at a height of one metre thanks to the inclination we use. The material opening remains the same size up to the rotor or becomes larger. This ensures that jamming grain does not normally occur. Other machine types with other diameters are correspondingly larger or smaller (mill 1600/1800 B: inlet = 1260 mm high).
In all our types, the inlet impact plate in the upper part of the housing forms a slide without a protruding edge together with the impact apron behind it, in every gap setting. This achieves an undisturbed material flow. Should material unexpectedly jam in the crushing chamber, the front impact apron can be raised hydraulically from the machine control station during operation. The jammed material can then pass through the crushing chamber unhindered.
Due to the constructions and material thicknesses we have chosen, the operating weight of our machines is very high in comparison with competitors.
Component 2: the impact aprons
In accordance with the demands placed on them, both impact aprons in LuT mills have an extremely elaborate and solid construction and weigh, for example, approx. 2,500 kg and 2,200 kg each in a 1300/1300 mill.
They are freely movable in the housing and, due to the type of suspension and their high weight, press towards the rotor. Adjusting spindles set the desired gap between the rotor and the respective impact apron and thus determine the grain size of the material. The rear impact apron is additionally held stable in the position described by compression springs.
For the necessary gap adjustment of the impact apron we supply a hydraulic assembly aid on request. Hydraulic gap adjustment is also possible on request.
All mounted impact plates are interchangeable and, like the blow bars on the rotor, can be used on both sides. A high degree of utilisation is thereby guaranteed.
Thanks to the movable suspension and the clearance of both impact aprons in the mill housing, there is always the possibility of yielding far enough in the event of foreign bodies or oversized feed material. The machine is therefore largely protected against mechanical damage.
Component 3: the rotor
The rotor is probably the most important component of the impact crusher, the heart of every machine, on which the desired grinding result depends.
Among other things we manufacture the so-called construction waste rotor. This rotor type is the most common in construction waste recycling. The LuT rotor, an open welded construction, was deliberately built around a blow bar model available on the market. These models can be repositioned several times.
The rear contact surface of the blow bar is a heavy, solidly forged beam. Locking bar and lock retainer prevent lateral displacement of the blow bar and lie so favourably in the wear shadow that these parts guarantee a very long service life. A machined retaining lug on the rear holding piece prevents the blow bar from coming loose from the rotor body during operation. Moreover, the blow bar model used can be used on both sides, which achieves a high degree of utilisation and, with it, a favourable price-performance ratio.
The holding shoes located in the front area, an exchangeable wear part, prevent or considerably reduce wear on the rotor discs. A design variant developed by us with clamping wedge fastening ensures a firm seat of the blow bars and is used at the customer's request.
The rotor body is mounted via the rotor shaft in heavy steel bearing housings manufactured especially by us. The rotor shaft is connected to the rotor body by a so-called clamping set connection and can be replaced easily if necessary.
In addition to the open design of rotors, we also supply the closed design depending on requirements. With this design, recycling is possible if the plant is charged accordingly. In the closed construction the rotor body acts as a drum in which 6 blow bars are mounted offset by 60 degrees to each other.
In contrast to the model described above, the blow bars used in such rotors (closed design) have a far smaller cross-section, which results in a lower weight of the bars. The advantage here is that a crane is no longer needed as an assembly aid for turning or changing the blow bars. This simplifies service work and results in shorter set-up times.
Special accessories
The construction of all LuT impact crushers is designed so that all machines can be fitted with an adjustable infeed beam and a crushing track. Retrofitting existing machines is possible with the corresponding conversion kit.
The adjustable infeed beam
A component that is mounted beneath the inlet slide plates and adjusted via a hand lever according to the degree of wear of the blow bars. This part prevents feed material from reaching the material discharge uncrushed in front of the rotor.
The crushing track
A construction that wraps around the rotor in a sickle shape behind the second, i. e. lower, impact apron. Due to the stepped design of the crushing track, three different crushing actions take place: impact, friction and squeezing.
The repeated impact guarantees almost complete crushing along the natural cracks and the breaking of flat material. Friction and squeezing serve to knock off the sharp edges. This achieves a very good impact crushing value and fine fractions. Optimum grain limitation is also ensured.
Like the impact aprons, the crushing track is set via spindle adjustment or hydraulically. As the component is fixed in place and cannot yield in the event of foreign parts, only material without iron content can be processed.
Mode of operation of an impact crusher
All impact crushers work on the principle of overcoming impact strength. Since impact strength is always lower than compressive strength, the energy required relative to the degree of crushing is always lower than with all other types of crushing or machines.
A formula states that the factor causing the crushing becomes greater the higher the speed of the rotor and the heavier the specific weight of the feed material. It can also be seen that with crushing machines working on this principle, flat final grain is practically ruled out, whereas that is not always the case with other crushers (e. g. jaw crushers).
Technically this is achieved because the feed material is caught by the fast rotating, horizontally mounted rotor and hurled against the impact aprons. Slow-motion studies have shown that the material is crushed mainly by the blow of the rotor bars and only secondarily by the impact on the impact plates.
As a considerable part of the energy is already converted into heat and crushing work when the bars strike, only a small amount of energy remains in the fragments with which they are hurled against the impact plates.
Another reason for the better crushing effect of the blow bars most probably lies in the edge effect of these bars, in contrast to the flat impact on the impact plates. The greater wear of the blow bars compared with the impact plates, which during the crushing process is roughly proportional to the increase in surface area achieved, confirms this observation.
As the feed material primarily breaks freely on the bars, the impact surfaces mainly have a guiding function. They are intended to ensure the constant repetition of the impact process and to delimit the impact chamber. It follows that the crushing chamber must not be too large, in order to avoid unnecessary flight distances, but must be matched to the maximum feed size.
In addition to impact and friction crushing, a kind of vibration crushing also occurs in the impact crusher. The lumps, often more than a cubic metre in size, “dance” on the blow bars of the rotor, with individual pieces being knocked off; the actual shattering, however, takes place through the enormous shock and vibration which break up the structure of the lump and cause it to fracture.
Feeding and material guidance
As a rule the material to be processed is fed into the crusher by a grizzly feeder or a screen. The inclination of the feeding equipment should not be chosen too steep, however, since on the one hand the dwell time of the material for selection is too short with screen feeding and on the other hand large round or roundish lumps place excessive load on the rotor. In the latter case the service life of the rotor is reduced. In order to reduce wear overall, the fine fractions of the material are screened off. Water spraying, however, should only be used at the material outlet.
The material is pre-crushed at the front impact apron and broken to the desired final grain size at the rear one. Of course fine fractions and oversize grain cannot be prevented entirely.
Gap setting and rotor speed
The overall speed of the rotor and the gap between the blow bar circle and the impact aprons have a significant influence on the degree of crushing and on the end product. With LuT mills the gap can be set during operation in order to avoid downtime. Care must be taken, however, that the gap does not become too narrow, which causes shearing and friction effects and leads to excessively high fine fractions.
A narrow gap can reduce oversize grain, but as is well known the impact crusher does not have a permanently set gap like the jaw crusher. Once a blow bar has passed the lower edge of the impact plate, this gap increases again by the height of the blow bar, and pieces larger than this gap width may occasionally pass through.
With the different materials that can be crushed, different circumferential speeds and numbers of blow bars naturally have to be used. A material that is easy to crush and therefore tends to produce a higher proportion of fines should be run at a lower circumferential speed and possibly with 2 normal blow bars and 2 so-called blind bars. The gap setting should likewise be adapted to the material.
In contrast to easily crushed materials, the machine should run at a higher circumferential speed and with 4 blow bars for hard and tough material. Unfortunately we cannot give you any general guidelines for these settings; they always depend on the materials to be crushed. You, the operators, will however be able to judge within a certain time which setting is the most favourable for you.
With proven construction waste that is recycled, there should be enough space at the crusher outlet to be able to remove the iron parts released during the crushing process without problems.
Noise and plant planning
Finally we would like to address the noise that occurs during the crushing process. This level is certainly high; nevertheless we have found that the noise is below the values that occur at screening plants and material chutes.
When planning plants in future, please make sure that the material is conveyed in a circuit wherever possible, i. e. feeding and discharge points should be located above or below one another so that no obstructive “edges” arise during transport and processing which counteract a steady material flow.
With timely and sensible service and replacement of the wear parts, the LuT impact crusher has a long life expectancy. Essentially, the mechanical work is performed by precisely these replaceable spare parts.
