Mondi Šte˘tí Chooses Direct Drive Technology


When Mondi began planning a major rebuild of the PM 5 sack kraft paper machine at its Štĕtí mill in the Czech Republic, one major goal was to eliminate the maintenance-intensive and trouble-prone wheel housing drive. In October 2015, the mill produced kraft paper for the first time with the help of a newly-installed direct drive system. Within the scope of the rebuild, the design speed for the drive was increased to 1200 m/min. AS Drives & Services GmbH (AS Drives), as general contractor for this turnkey project, modernized the complete drive and lubrication technology of PM 5.

Mondi Štĕtí belongs to South African-based Mondi Group, a leading international producer of sack paper and special kraft paper. Helmut Riesenberger, project director for the PM 5 rebuild at Mondi Štĕtí, made this comment after the first year of operation of the new components: “In spite of many parallel works—like changing the dryer hood, a new press section, and foundation works—the machine started up after all rebuild and modernization actions according to schedule. Concerning the new drive and the new lubrication technology, all our expectations have been fulfilled.”

Scope of the project

AS Drives carried out the engineering of the drive and lubrication technology, as well as the project and installation management. Within this major project, almost all drive points from the wet end to the reel have been modernized.

For the drive of the dryer group, Mondi again counted on direct drive technology from AS Drives. The complete FlexoDirect (Fig. 1) is developed and produced in Germany at AS Drives, and has been proven in paper machine installations since 2010. Altogether, 26 hollow shaft direct drives were delivered for the Štĕtí project for the drive of the dryer groups.

Next to 11 standard drives, AS Drives also delivered two fully-automated Cooliflex cooling aggregates, as well as 26 FlexoFlow Water intelligent flow controllers. With the Cooliflex and the FlexoFlow Water, an optimal temperature-management of the drives, even within the dryer group, is provided at all times.

In addition to the drive technology, AS Drives modernized the lubrication technology in the complete dryer section of the PM 5. Through the redesign of the framing-parts, the mill could completely dispense of oil within the gearbox.


Before the rebuild at Mondi Štĕtí, the dryer groups of the PM 5 were driven through a closed gearbox (Fig. 2 and Fig. 3) The necessity to charge the mechanic coupling point with fresh oil makes a gearbox difficult to manage with regard to leakages. This is a critical disadvantage, and not up-to-date with current drive technology.

The Štĕtí project demonstrates how an older gearbox drive can be modernized in an economical and sustainable way. The gearbox housing remains, as a carrying part of the machine frame. The expansion of gearwheels within the gearbox and the consequent elimination of the mechanical torque converter increase running smoothness and efficiency (Fig. 4). The old drive motors, including their pre-gearboxes, have been removed completely.

After the rebuild, only the roller bearings of the cylinder in the dryer group need to be supplied with fresh oil. The old gearbox drive was dried up.


In parallel with the dismantling of the old gearbox drive, the new FlexoDirect drive system was installed. During the Mondi Štĕtí project, the direct drive comes into operation for the drive of the dryer groups. This hollow shaft drive is installed directly to the cylinder journals. This setup drives the dryer cylinders nearly loss-free in a force-fitted, torsion-proofed way. In this process, the motor is carried from the journal—no additional base, console or frame is needed. A torque support with ball joint head protects the machine against the existing frame.

In 2014, AS Drives had realized a comparable drive project for the sister machine PM 6 at Mondi’s Dynäs (Sweden) mill. The advantages of this compact drive technique and the good results from that project prompted Mondi to again choose AS Drives for the Štĕtí project.

Yet the projects differ in the connection from the FlexoDirect onto the dryer groups. Previously, the FlexoDirect was used only for the connection to the driving side; for the project in Štĕtí, Mondi wanted a tender-sided connection with this drive. Therefore, the existing cylinder journal on the tender side was ground straight and was extended with an extracting link suitable for casting. A mounting set was integrated axial into the inside of the cylinder, so that the torque could pass force-fitted into the face of the journal. The support of the reaction torque was designed by AS Drives engineers, so that a good accessibility to the paper machine remains without tensioning of the drive.

The drive layout for the complete machine was layed-up by AS Drives per the specific characteristic of the felt guidance in the dryer section. The AS Drives engineers worked to ensure that only two different motor types are needed to equip the complete dryer groups, despite the different layouts of the dryer groups. This reduces inventory and the variety of the components. Due to the fact that synchronous machines come to operation, high total efficiency factors can be achieved.

The new system contains 26 FlexoDirect drives. In spite of the high power of the FlexoDirect, this motor has a compact structure and, compared to conventional drive solutions, fits well inside the dryer hood (Fig. 5.) With the installation of the FlexoDirect, no breakthrough of the dryer hood is necessary, which leads to better heat balance and a more beneficial hood design.

Because of the process-related higher temperature inside of the hood, the motor will be supplied with approximately 60-70°C warm water during operation. This water is provided by an external cooling unit within a closed cooling water circuit.

Due to the precise control of the FlexoDirect, the torsional rigid connection and the omission of mechanical components, Mondi Štĕtí got a smooth and dynamic drive, which is expected to meet future demands. The omission of the intermediate shafts, gearboxes and wheels reduce the maintenance effort and the noise emission as well as the harmful vibrations inside of the dryer group.


As part of the rebuild project, AS also modernized the complete oil lubrication system for the dryer section. The new oil lubrication system in the dryer section consists of a Lubriflex Type 5000 central lubrication unit, as well as 336 FlexoFlow intelligent flow controllers, allocated to 16 terminals. The Lubriflex lubrication unit involves all parts necessary to provide the oil. Additionally, a special degassing unit inside the tank of the Lubriflex degasses the oil. After the oil treatment, the FlexoFlow takes over the allocation of oil to the lube points.

Additional operating states like crawling, startup, and partial load with different values can be programmed—the FlexoFlow adjusts these values fully automatically on demand. Overflows of bearings due to cold oil will be avoided. All data is provided to the mill’s process-leading system. An adjustment of the oil flow values is also possible.

AS Drives installed all components for the systems. Besides the piping work, this included the setup of the Lubriflex as well as the commissioning of the complete system. Due to the optimal degassing features of the Lubriflex and the fully automatic adjustment of the FlexoFlow, Mondi Štĕtí received a reliable and environmentally-conscious lubrication technology with many advantages.

The control of the drive gave us enormous advantages in production, and with regard to our quality standards,” said Riesenberger. “The problems with the oil leakages at the closed wheel housings are 100 percent eliminated, so the maintenance effort was reduced drastically. The reduction of the vibration as well as the reduction of the noise level makes the new drive a real ‘silent drive.’ All in all, we are very happy with the rebuild.“

Ramazan Ataman is a sales director for AS Drives. Contact the author at [email protected], or learn more about AS Drives at

Fig. 1: The FlexoDirect direct drive.

Fig. 2: A conventional drive with electrical motor, pre-gear and shaft, outside the dryer hood.

Fig. 3: A closed gearbox on the PM 5 dryer groups, with steam heads already removed.

Fig. 4: Opened gearbox including gear pinions (partly dismantled).

Fig. 5: Post dryer section of the PM 5.