At Mondi’s Dynäs mill in Väja, Sweden, mechanical problems and high maintenance costs proved that the drive system on PM 6 needed a rebuild. Mondi Dynäs PM 6 produces sack kraft paper with basis weight ranges from 50–160 g/m2 and a web width of 6,500 mm. In 2014, the machine speed was 775 m/min.
Before the rebuild, the dryer groups on PM 6 were driven by a closed wheel housing drive. This drive system consisted of a central DC drive, which ran a number of downstream intermediate and drive wheels via a drive pinion. The drive wheels themselves were connected to the cylinder journal and turned the dryer cylinder. The high number of tooth meshes between the wheels created enormous friction losses and high wearout, which meant very high maintenance expenses (Fig. 1).
With a closed wheel housing drive, the mechanical drive system within the housing is influenced by high temperature fluctuation. The large number of coupling points and the contamination of the internal housing with oil makes the closed wheel housing drive difficult to manage concerning leakages. On PM 6, some groups of the dryer section were driven by a DRG-gearbox. This form-fit gear unit is driven by one central DC motor. This drive transfers motor torque via a pinion shaft to the intermediate gears and then to the drive wheels (Fig. 2).
The DRG-gearbox system could not solve the problems with the leakages and stress, but only shift them. Furthermore, the massive DRG-gearbox housing hindered access to the cylinder bearings as well as to the machine frame. A changing of a cylinder bearing could not be handled in a day’s shutdown.
THE REBUILD PROJECT
Mondi Dynäs planned to replace the old mechanical and electrical drive and install a new drive designed for a higher maximum production speed of 1,100 m/min. This drive was also to be designed to reach a creping speed of 15 m/min without any problems.
During the project, the susceptible DRG-drive, the old DC drives and the counter gearbox were uninstalled. The old, closed wheel housing drive was eliminated by dismantling the intermediate wheels. The housing of the old drive remained as a structural part of the machine frame.
AS Drives & Services (AS) was responsible for the rebuild of the mechanical drive components. Nearly all drive points of the PM 6 were rebuilt or renewed, from the wet end to the reel. AS handled engineering for the rebuild as well as the project and assembly management. The scope of supply included 20 standard drives, which came into operation in the wire and press section, at the Clupak, and at the reel. For the dryer section, AS delivered its newly-developed hollow shaft motor, FlexoDirect, with permanent magnet technology. AS also supplied the necessary cooling system: a Cooliflex demand-driven cooling unit, plus 26 FlexoFlow water control devices to automatically control the flow of the cooling water in response to the motor temperature.
“Overall, the project went very well,” says Jukka Linnonmaa, operations manager at Mondi Dynäs. “We have had no mentionable safety incidents and the expected additional production level is even better than expected.”
The Mondi Dynäs turnkey project marked the first use of the FlexoDirect direct drive system for a complete dryer section. This water-cooled, Permanent Magnet Synchronous Motor (PMSM) is the only drive solution available that allows the installation of the motor together with the steam head on the drive side. This is possible because the hot steam needed for the drying, as well as the condensate, can be led through the hollow shaft of the motor (Figs. 3 and 4.)
Unlike conventional drive solutions, the FlexoDirect can be installed within the dryer hood due to the compact design of the motor. A fixed connection of the FlexoDirect to the machine frame is not necessary; occurring reaction forces and tensions are not transferred in the frame, because the torque support is designed so that movements can be absorbed three-dimensionally. Because of the FlexoDirect’s high-precision control, the torsional rigid connection, and the elimination of the mechanical slack, Mondi Dynäs now has a smooth and dynamic drive. The elimination of all intermediary shafts, gearboxes and gear wheels also reduces the maintenance effort required for mechanical components.
“(The new drive technology) exceeds all expectations with regard to downtime improvements,” Linnonmaa says. “We also see enormous improvements by eliminating the old closed wheel housing drive. Maintenance expenses have gone down. The oil demand has decreased because we only have to lubricate the cylinder bearings. The problems with the leakages of the housing of the old drive are completely gone. Furthermore, the noise has been drastically reduced. That makes the work at the machine for all involved much better and more comfortable.”
One year after the Mondi Dynäs rebuild project in Sweden, AS rebuilt another complete paper machine in Czech Republic. The major objective was to modernize the paper machine and to eliminate the oil leakages caused by the closed wheel housing inside the machine frame.
AS equipped this machine with 26 FlexoDirect hollow shaft direct drives on the tending side of the paper machine (FlexoDirect can be installed at the drive side as well as on the tending side.) Beside 11 standard drives, AS also installed two Cooliflex units as well as the FlexoFlow intelligent flow meter. In addition, AS replaced the existing central oil lubrication system for the drying cylinder and guide roll lubrication. In total, AS supplied a 5000 liter tank with 368 FlexoFlow controllers to provide the oil for the 368 lubrication points.
Also in 2015, AS began a rebuild project in the US at Mondi Pine Bluff. The reason for the rebuild was to eliminate the outworn and obsolete paper machine drive. The old drive consisted of one long drive shaft, which transferred the power to each dryer group via pulleys and belts. Afterwards, each dryer group was driven by an open wheel drive. AS installed two FlexoDirect direct drives in the PM’s first dryer group and the cooling aggregate Cooliflex for the complete machine. AS also delivered one spare motor. The new drive system will be more reliable, more efficient, and have reduced maintenance expenses; the next step is to rebuild the rest of the machine.
Ramazan Ataman is sales director, AS Drives & Services GmbH.
Fig. 1: A view inside the housing,
with cog wheels.
Fig. 2: The DRG-gearbox.
Fig. 3: Two FlexoDirect with steam heads on the driving side of PM 6.
Fig. 4: Two FlexoDirect with steam heads with torque support in horizontal connection.
The AS Drives & Services assembly team.