ROLLER MILL DRIVE

ROLLER GRINDING AS A QUANTUM LEAP The crushing of cereal grains has historically been accomplished through pressure and shearing. Approximately 27,000 years ago, grinding stones were first employed for this purpose. Utilizing an oscillating motion of the grinding stone against a stationary surface, the grain trapped in between could be effectively processed. In many subsequent developments, the principle of maintaining one grinding surface stationary while the other moves relative to it was preserved. This is achieved, for example, through a horizontal or vertical pair of disks, a cone with a...

From the perspective of machine manufacturers, one aspect of roller grinding stands out: the „overdrive,“ which refers to the coupling of the two roll speeds. The technically straightforward design, consisting of two differently sized intermeshing spur gears at one end of the roll pair, represents an ingenious solution. In the configuration typical for grain roller mills, the slow roll must always be braked to maintain the desired speed ratio. However, if the grinding speed ratio remains constant, the considerable braking force applied to the slow roll can be efficiently transferred back to the...

DEGREE OF FREEDOM AND EFFICIENCY Pressure can be easily varied during operation by means of a variable grinding gap. Conversely, varying shear, generated by altering the speed ratio during operation, comes at a high cost. This can be achieved by feeding back the braking power with associated losses and/or through a technically complex machine design. As a result, the flexibility gained through a variable speed ratio has often been overlooked, optimized only for specific processes and kept constant for the majority of operations. EXPLANATIONS WITH EXAMPLES Given two typical, well-utilized passages:...

In 8-roller mills, which are often used in the first and second break passages, the product falls directly from the upper roll pair into the grinding gap of the lower rolls. VARIABILITY VS. ECONOMIC EFFICIENCY Nowadays, the simplest method to incorporate a variable speed ratio during operation is to equip each roll with its own motor and link the corresponding frequency converters in the intermediate circuit This individual roll drive can be configured as a direct drive or as a remote motor with belt drive. In such a system, the braking power from the generator-driven motor of the slow roll is...

ABSOLUTE POWER LOSSES Based on the previous examples, the power losses are now being estimated. It should be noted that even if very high efficiencies are deliberately assumed for the individual roll drive, a very clear picture still emerges: Roll pack with fixed overdrive Roll pack with individual roll drive IE3 asynchronous motor (mains operation) with belt drive on fast roll Belt overdrive – Two identical permanent magnet synchron synchronus motors with converters. – Converter coupled in intermediate circuit. Break passage required grinding power Total power loss* Reduction passage required...

VARIABILITY IN MILLS In a grain mill, there are numerous break and reduction passages where a variable speed ratio during operation may not be beneficial. However, variability can be advantageous for specific passages to enable the production of specialized products. For instance, this could include a grist passage where, in extreme cases, the fluting position (back/back to cutting edge/ cutting edge) is changed, or a smooth passage where a notably high shear is desired with a high-speed ratio. Roll pack with fixed overdrive Speed ratio Roll pack with individual roll drive Constant Variable within...

SUMMARY Energy efficiency can be achieved through the optimization of the mill diagram and the use of energy-efficient machinery. The recovery of energy, which must first be added to a system, leads to poorer energy efficiency. The power losses are greater with individual roller drive, as energy recovery for this application is not energy efficient. If the variability of the speed ratio is required for the production of special products, this can be easily implemented for selected passages with individually assigned motors. The technical simplicity and high efficiency of the traditional belt...