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Analysis of stripping mechanism of roller press

Date:2019-07-04 15:51     writer:cement editor     Views:

Roller press is a widely used pre-grinding system in cement factory, which is composed of two rollers with the same speed and relative motion. The material is extruded between the two rollers from the upper part into the gap between the rollers (extrusion pressure is generally considered to be up to 300MPa). The roller sleeve of the roller press is mainly extruded and broken, which plays the role of pre-crushing and pre-wear. Its wear belongs to high stress three-body abrasive wear, and its working principle is shown in Fig. 1. The wear resistance of roller press is mainly guaranteed by stripe layer.

For vertical mill, roller press and other wear-resistant surfacing workpiece, especially roller press, the surface peeling off block often occurs in the process of use, the depth and area are uncertain, which belongs to the category of fatigue failure. Fatigue failure is due to the formation of microcracks in the local high stress region of parts and components under the action of variable stress, and then develops into macrocracks. The crack continues to propagate and eventually leads to fatigue failure. Therefore, in addition to ensuring wear resistance, the anti-fatigue peeling ability should also be considered, so the material of roll surface of roller press must have good plastic toughness.

Fatigue failure Analysis of 1 Roller sleeve

An empirical formula is obtained from the known fatigue limit of materials.

σ-1 ≤ 0.5 σ b-(1 ≤ 1)

Fatigue limit of σ-1 ≤-material

σ b ≤-tensile strength limit

in that case of normal use of the roll press, the pressure range of the roll surface of the roll press is 50-300 MPa, in general, the working pressure should be about 200 MPa, obviously, the working pressure is much lower than the fatigue limit of the main material 42CrMo steel (b = 1080 MPa,1-1-540 MPa) commonly used by the roller press, And the fatigue limit of the surface-facing material of the roller press is much higher than that of the numerical value. And the actual situation tends to occur at a position of 20-30 mm from the surface of the roller. this is on the one hand a stripe layer, The crack extends downward and reaches the depth of the maximum shear stress, while fatigue, on the other hand, causes the roll surface pressure to exceed the strength limit of the material at a certain time or the fatigue limit at a certain time, so that the material produces microcracks at the maximum shear depth.

The fatigue spalling of the roller press is caused by cyclic varying stress, tensile stress, and plastic strain. The cyclic variation stress causes the crack to form, the tensile stress expands the crack, and the plastic strain affects the whole fatigue process. If one of that three is absent, the fatigue is not possible to form and expand.

The flow of materials in the gap between two rollers forms two systems with different wear mechanisms, such as area I (full compaction stage) and regional II (lamination crushing stage and agglomeration discharge stage) in the figure. The roll surface in area I is worn by the low stress abrasive wear of the material, while the roll surface in area II is extruded and worn by the material at the same time, which belongs to the high stress abrasive wear. In region II, the roller surface fails due to wear failure or extrusion spalling. Which of the two is in the dominant position, mainly depends on the wear ability of the roll surface and the ability of the roll body to bear the periodically changing extrusion pressure, that is, the fatigue resistance.

Due to the periodic change of extrusion pressure, elastic deformation and even plastic deformation occur on the roll surface. The maximum shear stress is produced at a certain depth under the roll surface. Under the action of the shear stress, if the fatigue strength of the roll body is lower than the maximum value of the fatigue shear stress, fatigue cracks will occur at the position where the maximum fatigue shear stress is located. Under the continuous action of the fatigue shear stress, the new cracks or the original cracks produced in the process of surfacing the roll surface continue to expand and increase, resulting in the local spalling of the roll surface or even a large range of spalling. Finally, due to extrusion, wear and tear, serious damage to the roll surface, can not be used normally.

Analysis of maximum Shear stress depth of 2 Roller Press

It is necessary to discuss the position of the maximum shear stress. By determining the position, that is, the depth, the fatigue strength of the roll material or surfacing layer can be improved here, and the fatigue spalling life of the extruded roll can be improved. According to the pure elastic theory, the maximum fatigue shear stress distance from the depth of the roll surface should be 0.786 times of the arc length of the roll surface on the contact area between the material and the roll surface.

Contact roll surface arc length L = D π × (γ / 3600)-(1 ≤ 2)

D-Roller press diameter.

angle of bite

According to this method, the fatigue depth can be estimated roughly, but the error is very large, and the actual theoretical fatigue depth is much less than this value. The integral calculation of roll pressure to bite angle shows that 0.786b is about 5.2 mm. If the error of safety factor and mathematical model is taken into account, then the depth of the maximum shear stress of the roller press is about 10mm~15mm, which is exactly the depth of the transition layer, which may be one of the theoretical bases for the design of the surfacing layer of the roller press.

3 Conclusion

Through theoretical analysis, the theoretical fatigue depth of roller press is about 10~15mm.

1. The diameter of the roller is directly proportional to the fatigue depth, and the larger the diameter of the roller, the greater the depth of the fatigue can occur;

2. The bite angle is proportional to the fatigue depth. The larger the bite angle is, the greater the fatigue depth may occur. The size of bite angle γ depends on the type of material, particle size, shape and gap between the two rollers, roll pressure, roll surface state (stripe shape), roll diameter and rotating speed and so on. The biggest influence is obviously roll gap and roll pressure, the smaller the roll gap, the larger the roll pressure, will make the bite angle larger, make the fatigue depth deeper.

(3) controlling the appropriate roll gap, obviously, the smaller the original roll gap is, the smaller the material pass is. In order to achieve a higher output, it is necessary to increase the pressure of the silo and force the roll gap to increase the throughput. At this time, the material column rises, the bite angle γ increases, at the same time, the cake density in the roll gap increases, and the pressure value at the minimum roll gap also increases sharply, which will cause the roll surface pressure to be closer to the fatigue strength. It is proportional to the depth of the maximum shear stress, which is disadvantageous to ensure the fatigue life.

In addition, the density and hardness of metal foreign body are much higher than that of cement clinker, so entering metal foreign body will cause the sudden increase of roll pressure and even produce microcracks directly, so the material of roller press should remove iron as thoroughly as possible. In addition to the iron device on the feed belt, it is also necessary to set up a metal detector on the feeding belt of the powder separator. In addition, in the production process, it is necessary to ensure that the metal detector and pneumatic three-way valve are interlinked and smooth, and the reaction is fast, so as to remove the mixed metal foreign bodies in time and avoid the metal foreign bodies circulating continuously in the closed circuit system composed of roller press and V-shaped powder separator and repeatedly damaging the roll surface.

In a word, it is necessary to ensure that the roller press runs in stable and standardized operating rules in order to ensure the fatigue life of the roller press and maximize its effectiveness.

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