Machine element or mechanism – Gearing – Directly cooperating gears
Reexamination Certificate
1999-10-01
2001-11-20
Estremsky, Sherry (Department: 3681)
Machine element or mechanism
Gearing
Directly cooperating gears
C074S066000
Reexamination Certificate
active
06318202
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a transmission for driving the screws of a twin-screw extruder comprising two parallel output shafts of different lengths and rotating in opposite directions, the shorter one of which carries a drive gear meshing with one gearwheel each of two distributor shafts, which from the side of the longer output shaft can be driven by an intermediate shaft via gearwheels.
DESCRIPTION OF THE PRIOR ART
To be able to transmit the high torques for driving the screw shafts of a twin-screw extruder despite the predetermined comparatively small distance between the screw shafts, it is known U.S. Pat. No. 4,899,670 to use a transmission having two output shafts of different lengths, the shorter one of which carries a drive gear meshing with a gearwheel of the longer output shaft. In addition, the drive gear of the shorter output shaft is in engagement with a gearwheel of a parallel intermediate shaft, which from the side of the drive shaft is driven together with the longer output shaft. Since the drive gear of the shorter output shaft is thus driven via two gearwheels, only half the torque need be transmitted via these gearwheels to the shorter output shaft, which results in correspondingly smaller meshing forces. Moreover, since the drive gear of the shorter output shaft and the two gearwheels meshing with this drive gear are arranged in a common axial plane, the bending loads of the shorter output shaft remain small, which has an advantageous effect on the tandem bearing of the shorter output shaft, which is very sensitive to inclined positions. What is, however, disadvantageous in this known transmission is the fact that via the longer output shaft not only the torque for the attached screw shaft, but also half the torque for the shorter output shaft must be transmitted, i.e. a torque which corresponds to three quarters the entire torque to be applied, when the two screw shafts of the extruder should be driven with the same torque. The resulting larger diameter of the longer output shaft reduces the already limited distance between the two output shafts, which results in a design-related limitation of the transmittable torques, because the tandem bearing to be dimensioned sufficiently for the shorter output shaft must find room between the two output shafts.
To ensure that the longer output shaft merely has to transmit the torque required for the attached screw shaft, it is also known (AT 351,235 B) to drive the drive gear of the shorter output shaft not via a gearwheel on the longer output shaft, but only via two distributor shafts, which are meshing with the drive gear of the shorter output shaft via one gearwheel each. The drive of these two distributor shafts is effected via an intermediate shaft which is in drive connection with the longer output shaft via a pair of gearwheels. Since the two gearwheels of the distributor shafts cannot lie in a common axial plane with the drive gear of the shorter output shaft, which is driven by them, the engagement-related transverse forces are eliminated only in part, so that the shorter output shaft is exposed to a corresponding bending load, which in turn leads to a design-related limitation of the transmittable torque due to the resulting higher load of the tandem bearing. The increase of the transmittable torques, which is possible with such a transmission, in addition involves the disadvantage that the two output shafts are no longer in drive connection at the output end, so that a different torsional behavior of the longer output shaft and the distributor shafts has a disturbing influence, as the extruder screws to be driven change their axial position with respect to each other as a result of a different torsion of the output shafts. This change in axial position leads to different flank clearances of the extruder screws, which has an unfavorable influence on the extrusion process. In the extreme case, the flanks of the extruder screws might even converge, so that the extruder screws are destroyed.
SUMMARY OF THE INVENTION
It is therefore the object underlying the invention to provide a transmission for driving the screws of a twin-screw extruder such that despite the predetermined axial distance of the two output shafts an increase of the total transmittable torque becomes possible without endangering the tandem bearing for the shorter output shaft.
Proceeding from a transmission as described above, this object is solved by the invention in that on the longer output shaft a gearwheel meshing with the drive gear of the shorter output shaft is seated in a manner known per se.
Since as a result of this measure the drive gear of the shorter output shaft is driven via three gearwheels, namely the gearwheel of the longer output shaft and the two gearwheels of the distributor shafts, merely about a third of the torque to be transmitted to the shorter output shaft need be applied via the individual gearwheels, which not only involves a reduction of the load acting on the teeth of the drive gear of the shorter output shaft, but also provides for a substantial elimination of the transverse forces, because the gearwheels meshing with the drive gear of the shorter output shaft can be provided correspondingly distributed over the periphery of the drive gear. This means that as a result of the largely eliminated bending load of the shorter output shaft the tandem bearing for the shorter output shaft may be made comparatively small. Although the longer output shaft is exposed to a torque which corresponds to about two thirds the entire torque to be transmitted to both screw shafts, and must therefore be designed larger than for transmitting the torque required for one screw shaft, a considerable increase in performance can be achieved as compared to the known transmissions for a twinscrew extruder, and with comparable dimensions of the transmission. When the possible increase of the transmittable torque is not utilized, a corresponding prolongation of the useful life can be achieved by the inventive design of the transmission.
When the drive of the shorter output shaft is not effected by means of two, but by means of three distributor shafts, the torque to be transmitted by the longer output shaft on the one hand and by the three distributor shafts on the other hand is reduced to one quarter of the driving torque of the attached screw shaft, which involves a reduction of the torque to be transmitted by the longer output shaft, so that the mounting conditions for the tandem bearing of the shorter output shaft can be improved additionally.
Due to the additional drive connection of the drive gear of the shorter output shaft with the gearwheel of the longer output shaft great advantages are obtained for the uniform angular position of the two output shafts with respect to each other. Since the two output shafts lie in a common parting plane of the transmission casing, the same can be mounted comparatively easily.
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Collard & Roe P.C.
E. Eisenbeiss Söhne Maschinen- und Präzisionszahnräderfabrik Gmb
Estremsky Sherry
Lewis Tisha D.
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