EFFECT OF ROLLER MILL ON THE GRINDING STABILITY OF WHEAT MILLING Edit by：www.immy.cn

EFFECT OF ROLLER MILL ON THE GRINDING STABILITY OF WHEAT MILLING

Edit by：www.immy.cn

The roller mill is the core host of wheat milling, and its stability directly affects the grinding stability of the mill, which is the fundamental guarantee of the overall flow balance and milling quality of the milling production line, and is an important technical index to measure the overall performance of the mill. The main factors affecting the grinding stability of the mill are not only the mechanical structure of the mill itself, but also the moisture, quality, particle size and equipment operation of the grinding material will affect the grinding stability of the roller mill.

With the continuous improvement of people's living standards in the new era, consumer demand has shifted to "eat well", "eat safely", "eat healthily" and "eat conveniently". The terminal market has higher and higher requirements for flour quality, the variety of special flour is increasing year by year, high-end flour is gradually seizing market share, the market competition is becoming more and more intense, the new contradiction between supply and demand effectively promotes the rapid development of the milling process in the direction of meeting the contemporary market demand, the longitudinal and horizontal flour road extension of the milling process, the light grinding fine grinding and fine grinding effectively improve the grinding rate and the processing accuracy of the flour, and also provides the basic conditions for the production of special flour, and the modern milling process also puts forward new requirements for the grinding stability of the mill.

1 Effect of Wheat Maize Flour on Grinding Stability

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The feeding system, grinding system and cleaning system of the mill have a great impact on the grinding stability of the mill, and the uniform and stable feeding is the basis of stable grinding, and the effective and reliable cleaning system is a necessary condition for grinding.

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1.1 Feeding system

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The feeding system, also known as the feeding unit, is mainly to stabilize the relative level height of the material in the feed barrel, and feed the material into the grinding area at a certain speed evenly and accurately along the entire roller length direction [1], and grind the material by the grinding system.

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(1) When feeding, it is necessary to ensure that the material is evenly spread throughout the feeding area, when the material can not be effectively spread over the feeding area and reaches the minimum material level setting value, the grinding unit should be able to leave the rolling and stop feeding in time to avoid damage to the surface of the grinding roller of the core components in the grinding system due to the empty feeding of the material.

(2) The thickness of the material feeding layer should be consistent to avoid the phenomenon of local thickness difference caused by thin one side and thick one side, broken flow of the material layer or superposition of the material layer.

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(3) According to the process requirements, sensitive and accurate adjustment of the feeding amount, when the material in the feeding bin fluctuates, the feeding amount can be automatically adjusted within a certain range to ensure the continuity of feeding and the relative balance of the overall process of materials.

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(4) Under the logic control of the automatic control system, it can realize the reliable sequential relationship of "feeding first and then rolling together, and first leaving and then breaking the material" with the clutch rolling mechanism of the grinding unit.

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(5) For the mechanical servo feeding system, the feeding door opening degree is controlled by the float type mechanical sensing servo to balance the material, so as to avoid the instantaneous increase or decrease of the feeding door opening, resulting in a sharp change in the thickness of the material and affecting the grinding stability.

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(6) For the electronically controlled automatic feeding control system, the level height (such as: capacitive level sensor, photoelectric level sensor, gravity sensor) is identified by the electronic level sensor and the speed of the feeding roller to establish a certain logical relationship, according to the material situation of the feeding barrel, adjust the feeding speed to balance the material, and when changing the feeding speed, it is necessary to avoid the instantaneous increase or decrease of the feeding speed, resulting in a sharp change in the feeding amount and affecting the grinding stability.

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1.2 Grinding system

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The grinding system is the key part that affects the grinding effect, is the core of the mill, also known as the grinding unit, in the grinding system, the middle convexity or taper of the grinding roller, the dynamic imbalance of the grinding roller, the radial runout of the grinding roller, the stiffness of the parts and structures have an important impact on the grinding stability.

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1.2.1 Medium convexity or taper of the grinding roller

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When grinding materials, the rolling distance between the rollers is generally 0.05~0.2 mm, and the roller has a large inter-roll pressure during grinding, and the bending strain will occur in the whole length of the roller body under the action of grinding force, and the strain is proportional to the pressure between the rollers[2] In addition, when grinding the roller, the maximum heat is generated near the bearing, and the outward expansion of the roller at both ends is slightly greater than that of the middle, and the microscopic deformation of the roller body occurs due to the influence of bending stress and thermal expansion, which has a greater impact on the grinding stability and production, so it is generally necessary to compensate for the influence of deformation on grinding by the convexity or taper in the prefabrication when grinding the grinding roller. As shown in Figure 1, the "grinding zone without medium crown or taper compensation value of the roller body" is in the shape of a jujube kernel with a large middle rolling distance and gradually decreasing at both ends, and the rolling distance between the rollers is kept uniform when grinding the material when grinding the material.

Obviously, the convexity or taper value in the grinding roller is too large or too small will cause the inconsistency of the grinding distance and affect the grinding, in order to ensure the overall powder yield, it will inevitably lead to the grinding area local (middle or both ends) rolling distance is too small, resulting in part of the grinding phenomenon, increasing the floating powder content and generating mechanical vibration, so the grinding roller must be ground in strict accordance with the process requirements, and the crown or taper value in the grinding roller must not be too large or too small. In actual production, it is necessary to continuously optimize the convexity or taper value according to the production situation, so that it is constantly close to the best compensation value in actual use.

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1.2.2 Dynamic imbalance of grinding rolls

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Factors such as casting defects or uneven materials of the grinding roller blank make the centrifugal inertia force generated by each microelement on the roller body not cancel each other when the grinding roller rotates, causing vibration, producing noise, accelerating bearing wear, shortening the life, and radial deformation of the roller body, so that when the roller mill grinding roller rotates and grinds the material, the grinding roller will inevitably have regular intermittent intermittent rolling distance changes in the grinding due to dynamic imbalance, and the larger the unbalance value, the greater the radial deformation of the grinding roller, as shown in Table 1 "the maximum deformation of the grinding roller in the state of unbalance level”。 When the deformation is at the unbalanced point, the rolling distance of the grinding roll will become larger, which will affect the stability of the grinding. To this end, it is necessary to reduce the radial deformation of the roller body due to dynamic imbalance as much as possible, which affects the grinding stability of the mill.

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In order to improve the service life of bearings and related parts and reduce the radial deformation of the roller body, it is necessary to dynamically balance the grinding roller to achieve the allowable balance accuracy level, or to reduce the radial deformation and mechanical vibration amplitude of the roller body within the allowable range, GB/T 34669-2017 requires that the dynamic balance of the grinding roller should not be less than G2.5, and the allowable eccentric wheelbase should be 39.9 μm[3].

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1.2.3 Radial runout of grinding rollers

The radial runout of the grinding roller is the main reason for the grinding performance of the roller mill, and it is also one of the main factors that cause the regular vibration of the roller mill, especially in the heart grinding process position of the small rolling distance between the rollers, the radial runout value of the grinding roller has a great impact on the stability of the grinding, the original Ministry of Commerce standard requires that the coaxiality of the grinding roller facing the bearing position should not be greater than 0.04 mm, with the continuous improvement of the equipment requirements of science and technology and pulverizing technology, the national standard GB/T 34669— In 2017, it is required that the radial runout of the grinding roller facing the bearing position should not be greater than 0.01mm, and the radial runout of the grinding roller surface in developed countries can be controlled within 0.005 mm, which has reached a fairly high accuracy level, and the highest manufacturing level in China can be controlled within 0.008 mm, which can fully meet the requirements of modern milling technology.

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1.2.4 Stiffness of components and structures

Stiffness refers to the ability of a material or structure to resist elastic deformation when subjected to force, the stiffness of parts and components is the basis of the overall stiffness of the structure, and the ability of the grinding unit of the mill to resist elastic deformation is crucial to the stability of grinding.

The rolling distance lag or change caused by the elastic deformation of the grinding unit of the roller mill grinding material or in the process of clutch and rolling distance adjustment has a certain impact on the grinding stability, when the rolling distance is tightened, the rolling distance is adjusted to the required tightness, after 10~20 min grinding, the rolling distance becomes tighter, and when the rolling distance is loosened, the rolling distance is adjusted to the required tightness, after 10~20 After min, the rolling distance will become looser, so in the actual production to adjust the rolling distance according to the equipment situation to master the rolling distance adjustment skills, do not habitually adjust the rolling distance of the mill frequently, often, the poor grinding stability leads to changes in the scraping rate and powder yield, and the frequent adjustment of the mill grinding distance and other improper operations have a greater relationship.

With the application of new materials and new processes, the stiffness and structural strength of parts have been greatly improved, and the changes caused by elastic deformation of grinding and rolling distance have been basically eliminated.

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2? Epilogue

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The grinding stability of the mill directly affects the overall process material balance, energy consumption and flour quality of the wheat milling process. The study of grinding stability will also be an eternal topic, and will be continuously improved in the development of mills and milling processes, so that they can meet the requirements of contemporary milling processes as much as possible.