The machining centers of Alzmetall are characterized by their extremely high static and dynamic stiffness, which provides the required accuracy when turning or milling. The round-the axes Altenmarkter developers are relying entirely on the torque motors from IDAM who also bring a number of other Schaeffler Group products such as bearings or linear bearings with it.
If a customer says: "We were convinced from the outset of the outstanding performance compared to the competitive products", then this probably speaks for the services of his supplier. This statement comes from Ludwig Probst, authorized signatory and head of the development / construction of the machine tool factory and foundry Friedrich GmbH & Co. KG, short Alzmetall. The recognition went to INA - Drives & Mechatronics AG & Co. KG, IDAM for short of the Schaeffler Group.
The company is standing on two legs near the Chiemsee in Altenmarkt an der Alz: the foundry and the mechanical engineering. "In the foundry, we produce exclusively cast iron, and all grades ductile iron and gray cast iron weighing up to five tons," says the chief developer. In addition to the use of an automatic HWS molding machine, especially hand molding is produced. Mechanical engineering comprises three product groups: column drilling machines highly dynamic machining centers, which in turn are subdivided into CS, FS, GS and GX series, as well as special machines for series production.
High rigidity for accuracy"Our G-type series is unrivaled on the market in terms of dynamics, rigidity and thermal stability," says Probst. This unique selling point is due to the machining centers, which allow for absolute, symmetrical geometry in relation to the main spindle. "A machine must be able to absorb the reaction forces due to the high cutting speeds and feeds caused by the high cutting speeds and feeds so that they do not cause geometry errors." Everyone can accelerate and decelerate, but ensuring the geometry under these force ratios sets a corresponding one Stiffness of the machine ahead, "says Probst.
The "G" in the machine designation stands for gantry design: In this case, two autonomous axially parallel linear drives are matched to one another in the Y direction in such a way that the coordinate traveling gear achieves a high parallel movement accuracy and thus geometric perpendicularity to the X axis. The extremely high rigidity of the overall concept results on the one hand from the closed connection of the frame components with the coordinate traveling gear and, on the other hand, from the double-internal guidance systems of all linear axes. The associated static overdetermination is dominated by high manufacturing accuracy of the contact geometries due to purpose-oriented parallel clamping technology and integral design. In addition, we are able to install any technology-related main spindle ", says Probst on the advantages of the machines.
A manufacturer for four functionsDriving, guiding, storing, measuring: these are just four functions covered by various business units of the Schaeffler Group: From the GS650 to the GS1400, the machining centers have three to five axes. The five-axis version is divided into the three linear axes X-, Y- and Z, which are conventionally operated with ball screws, and the two rotary axes A and C connected in series, which are equipped with direct drives. Due to the many years of very good business relations with the Schaeffler Group, the design engineers at Alzmetall initially opted for the RE series of the Suhl drive specialists without a large selection process. A year ago, the record-breaking RKI series was added. "We had no other supplier of torque motors in our machines since their first use about six years ago," says Probst. The decision was made at that time due to the dynamics and power density in favor of IDAM drives. Alzmetall had mainly used external rotor motors, for example, in order to obtain a large clearance for the central power supply in the C axis. Since the establishment of so-called hybrid machines, which developed into the machining center with the introduction of turning and thus high speeds, the Oberbayern also use RKI engines.
The linear axes are also equipped with INA products: All axes feature RUE 45 E linear guides with two RUE shoes per rail. Various ZARF bearings are also used in the final drives. The C rotary axes of all GS machines incorporate the YRTSM INA rotary table measuring bearings, which are predestined for high speeds. As the name implies, here the measuring system is integrated in the bearing in such a way that the entire diameter range remains free for the energy supply of the machine.
500 percent more - a new dimensionAlzmetall already belonged to the selected customers for the test phase of the beta series of the RKI engines. The sales launch took place after a two-and-a-half-year development period about a year ago. "The motivation for developing the new functional principle came from using a direct drive for a hybrid car," says Marko Pfeiffer, Head of Technical Sales at IDAM. Requirements such as low mass, high efficiency, high torque at maximum speed and high-production suitability should be realized. The engine technologies of conventional direct drives, such as the predecessor engine series RI, were of limited use here. "After several simulations, prototypes, tests and design optimizations, we were able to develop a direct drive with completely new properties and up to 500 percent more mechanical performance - that was the birth of the RKI", summarizes Pfeiffer.
Structure and operation of a RKI engineWhen comparing a standard motor RI with the RKI series, it is noticeable that the surface magnets still used in the standard version have disappeared on the rotor. The RKI rotor consists of a laminated steel package in which a magnet system is embedded. This bundles the magnetic flux and increases it by about 30 percent compared to comparable surface magnets. These 30 percent are directly proportional to the generated force. One can assume the same ampacity and coil length.
Thus, only by replacing the rotor up to 30 percent more moment can be generated from a motor. This additional torque, in turn, has an influence on the reverse voltage and thus on the speed adaptation of the overall system.
Each time a magnet moves past a coil, the magnetic field induces a voltage in the coil. The amount of this voltage depends on the speed of the magnet. The higher the relative velocity between the two, the greater the induced voltage. The problem with this effect is that with high reverse voltages it is no longer possible to impress a current in the motor. It usually comes in this case to strong vibrations before the axis falls out of the scheme.
The point in time at which this effect takes effect depends largely on two values: the DC link voltage of the converter and the inductance / counter voltage constant of the motor. The DC link voltage is - depending on the inverter - usually at 540 to 600 V in Europe. The only way to change the speed of a motor is winding. By processing a stronger wire, the inductance of the motor decreases. To the same extent as the inductance decreases, the required current strength of the motor increases (half the inductance corresponds to approximately twice the current). With the winding adaptation, the speed can be quadrupled and the torque can be increased by 30 percent, even with the same installation space. This results in a fivefold higher mechanical performance.
Future development with obstacles"As far as the further development of the RKI is concerned, we are currently working on an eco version that uses cheaper magnets and is therefore no longer subject to the fluctuations in the price of magnetic products," says Pfeiffer. In the first half of last year, for example, the Price for the raw material neodymium, which constitutes in terms of cost, the decisive proportion of a neodymium-iron-boron magnet, increased by about five times. Even if it then came back to a significant price reduction, the Suhler want to be independent of such developments. "For example, in some applications, if one does not go for the performance limits of the engines but optimizes the magnet prices, 15 percent can save less torque than 20 percent of the magnet costs," says Pfeiffer. A corresponding product will be presented later this year.
Another topic for the IDAM developers will be the motor principles: "In most torque motors, we use the synchronous motor principle with permanent excitation with corresponding neodymium-iron-boron magnets, using so-called hybrid motors, the additional reluctance components within the motor and thus the typical stepping motor principle If you could reduce raw material costs and even increase speeds significantly, then there is another challenge: Standard converters in the industry do not currently support this motor principle, and it will probably take up to two years to get smaller manufacturers with correspondingly lower costs Converters come to the market, "suspects Pfeiffer.
The sales manager sees new fields of application in the current trend for the main drives for rotary tables to high-performance spindles, which provide very high moments. "I assume that someday a torque motor and a high frequency spindle will be very similar in their construction, where everything grows together a bit," concludes Pfeiffer.