Extruder gear
Extruder gears are used to convert torque and speed between the drive machine ( motor ) and the extruder . Depending on the type of extruder and its number of shafts, a distinction is made between single, double and multi-shaft extruder drives. Due to extruder-specific requirements, such as the need to absorb high axial loads and transmit very high torques , they stand out from standard gears and form a special type of gear.
Twin screw extruder gear
In the industrial production of plastics and foodstuffs, extruders are often used. In addition to the single-screw extruders, which are relatively simple in construction, the twin-screw extruders require a complex distribution gear to drive the extruder screws rotating in the same or in opposite directions . In particular, the parallel twin-screw extruders require a gearbox that transmits the highest torques evenly to both screw shafts in the smallest of spaces.
conditions
The gearbox is a central component of the extruder. It reduces the motor speed to the desired screw speed that is optimal for the extrusion process and transmits the required torque to the extruder screws. The drive power is evenly distributed to both worm shafts. In addition, the high screw back pressure from the extrusion process is absorbed and supported by the gear unit. The gearbox structure is essentially determined by the relatively small center distance of the output shafts and the required output torque. Both sizes are specified from the extrusion process. The gap width between the extruder screw flights limits the permissible relative rotation and the permissible elastic axial deformation of the gearbox output shafts to one another.
The torque factor f Md was defined as the size-independent dimension for the torque density of an extruder . It relates the output torque of an extruder screw shaft to the third power of the center distance of the screw shafts:
- f Md = T shaft / a³ [Nm / cm³] (1)
Table 1 shows the different requirements for counter-rotating and co-rotating extruder gears. Depending on the center distance, the possible output torque per worm shaft can be calculated from the specified torque factor according to equation (1).
| Counter-runner | Co-running | |
|---|---|---|
| drive | directly or via straps | directly |
| translation | 12 ... 80 | 0.8 ... 10 |
| Output speed | <5 ... 150 min-1 (in special cases also above) |
300 ... 1200 min-1 (in special cases also below) |
| Torque factor / shaft | up to 50 Nm / cm³ | up to 40 Nm / cm³ |
| Backprint from extrusion process |
up to 500 bar (in special cases also above) |
up to 250 bar (in special cases also above) |
Table 1: Requirement profile for twin screw extruder gear units
On the machine market, the trend can be observed that the output of the offered extruders is steadily increasing, which for extruder gearboxes means above all increasing required output torques and thus higher torque densities.
Constructive execution
Twin screw extruder gearboxes are mostly divided into two main units: the reduction gear unit and the downstream distribution gear unit.
The reduction gear represents the link between the drive motor and the transfer case. It is usually designed as a multi-stage spur gear reduction gear . Depending on the type of drive (direct drive or belt drive ), gear ratios in the range from 0.8 to 80 are common. The last reduction stage is often designed in such a way that the reduction can be adapted as required. Compared to counter-rotating extruders, co-rotating extruders are usually operated with much higher output speeds. The overall reduction of the gear unit is therefore lower in the case of gear units rotating in the same direction and the reduction unit can be designed in one stage or in some cases even be omitted completely.
The structural challenge in extruder gear manufacturing lies primarily in the design of the distribution gear unit, in which the power is split to the two output shafts and there must be space for a reliable axial bearing to absorb the back pressure forces.
Thrust bearing
In most of the gearboxes available on the market, one of the two worm shafts is supported by a multi-row axial cylindrical roller bearing (tandem bearing), which combines a small outer diameter with a high load rating. The axial bearing of the second output shaft is arranged further back in the transmission, since there is enough space available here. Due to this larger installation space, the bearing is not limited in its outer diameter and can be designed in a single row. Depending on the extrusion process and processing unit, a wide variety of back pressure conditions can be responded to. In this way, reliable operation can be guaranteed with low screw back pressures of approx. 100 bar (compounding operation) up to special applications of well over 700 bar.
Power split
The high output torque in combination with the back pressure forces require special design measures for the power split. In most systems, the torque is routed past the axial bearing of an output shaft (tandem bearing) via distribution shafts and is evenly distributed to both shafts in an output stage. There are a variety of power split systems. A rough classification can be made according to the number of shafts in the transfer case unit. Systems that work with two to seven shafts can be found on the market, cf. Picture 2.
The high loads that arise on the gears in twin screw extruders require very careful design and calculation of the gearing geometries as well as very high manufacturing quality and surface quality. In high-quality extruder gears, special corrective cuts are made on the toothing, which compensate the twisting of the pinion shafts due to the torsional stresses that occur and ensure perfect tooth meshing under load. The exact geometry for such correction cuts is determined with special calculation programs.
Applications
- Pipe and profile extrusion
- Plasticization of plastic during injection molding (usually with a heating jacket)
- Pellet production (food and non-food area)
Manufacturer
- Eisenbeiss GmbH
- Flender GmbH
- Henschel drive technology
- Hueber Getriebebau GmbH, Kirn
- Knoedler Getriebe GmbH & Co. KG, 73760 Ostfildern
- Koellmann Gear
- Leax Detmold
- PIV Drives Bad Homburg
- Wolfgang Preinfalk GmbH
- Renk AG
- RSGetriebe GmbH, 87527 Sonthofen
- SEW-Eurodrive GmbH & Co.KG
- STM spa / GSM spa, Italy
- Zambello
- Zollern GmbH & Co. KG
Individual evidence
- ↑ a b Henschel drive technology: parallel twin screws - counter-rotation. In: henschel.eu. Retrieved April 19, 2019 .
- ↑ plastics industry - LEAX Detmold. In: leax-detmold.de. Retrieved April 19, 2019 .