The present invention relates to transmission systems. More particularly, it relates to a gearbox system for driving the screw shafts of a twin screw extruder.
BACKGROUND:
The Twin Screw Extruder consists of two screw shafts rotating inside a longitudinal bore thereof, compounding the materials for extrusion. The screw shafts further comprises processing elements called screw elements mounted on them. The screw shafts have to be externally powered to overcome the resistance provided by the materials being processed. This external power is provided directly by a motor through a gearbox.
Some of the Twin Screw Extruders known in the art were driven directly by a plurality of motors. Such systems were posed with the problem of ensuring the relationship between the torques transferred to the screw shafts and also the angular positions of the screw shafts. To overcome that problem, use of gearbox in conjunction with the Twin Screw Extruders came into being. The gearbox allowed the power to be transferred to the tv^^o screw shafts of the Extruder in a particular relationship.
The screw shafts powered through the gearbox face numerous limitations. The center distance between the two screw shafts of the Extruder is very small, typically of the order of 10 to 100mm. This puts an upper limit to the maximum diameter of the output shafts and hence, the torque carrying capacity thereof. Further, due to this restriction on the center distance, installation of radial bearing on both the output shafts of the gearbox becomes very difficult. With normal gear-trains, the output shafts are subjected to repetitive bending loads. Due to the repetitive bending stresses, the output shafts undergo fatigue which leads to early failure. Apart from the radial
loads, the extruder by its nature has to face large axial loads due to the extrusion
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pressure. This requires thrust bearings to be mounted on the shafts. Since the two shafts are too close together, a greater constraint in design is imposed due to this additional requirement.
The gearbox comprises various gears and shafts to form a gear train. All the shafts undergo twisting due to torque being transferred between the gears mounted on them. The extent of twist on a shaft depends on various parameters such as its effective length (length of the shaft between the two points of application of torque), diameter and the torque being applied to it. As a result, the twist shall be different for different shafts as all the shafts do not necessarily have the equal parameters. Therefore, there might remain a significant differential twist in the two output shafts of the gearbox which will lead to a mismatch in the arrangement of the screw elements. The mismatch in the arrangement of the screw elements leads to their breakage. So, there is need for a gearbox which takes care of the problem of large differential twist encountered in the shafts.
Several gear assemblies have been proposed to eliminate the problem of bending stresses in the output shafts of the gearbox. US patent 4399719 titled "Twin screw extruder with power branching gearing" assigned to 'Hermann Berstorff Maschinenbau GmbH' shows a gear train for branching power between the two screw shafts of the twin-screw extruder. However, the patent does not solve the problem of controlling differential twist in the two output shafts of the gearbox.
Another US patent 4586219 titled "Torque-splitting pinion gear for twin-screw machines" assigned to 'Blach et al.' talks about another geartrain for splitting the power into the two output shafts of Twin Screw Extruders. The gearbox disclosed in the prior art uses two gears to drive one of the output shafts, but the disposition of gears is not able to eliminate the bending loads completely. Further, the design of the
output shafts doesn't allow any differential twist between them. In such a case, there
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is no allowance for backlash, which may lead to the gear teeth getting jammed. So, there is need for a gearbox which controls the differential twist in the output shafts while taking care of the bending loads and without compromising on torque carrying capacity of the output shafts.
SUMMARY:
The present invention discloses a gearbox for providing driving power to the screw shafts of a twin screw extruder.
The gearbox branches the power from a source and provides it to the two output shafts of the gearbox. The gearbox ensures that angular position of both the output shafts are in a pre-determined relationship with each other at all the times during its operation.
In another aspect of the invention, the gearbox fijrther eliminates the bending loads on the output shafts of the gearbox. The bending loads are harmfixl for the output shaft in that they lead to fatigue which causes early failure of the shaft. Also, due to very small center distance between the two output shafts of the gearbox, installation of radial bearings becomes quite difficult.
In yet another aspect of the invention, the gearbox controls the differential twist between the two output shafts and keeps it within allowable limits. The resulting differential twist, while providing allowance for the backlash, is below the point at which breakage of the expensive screw elements mounted on the screw shafts takes place.
In yet another aspect the gearbox for transmitting torque to a co-rotating twin screw extruder, the gearbox includes a first output shaft driven by a driving means, an idler shaft driven by the first output shaft, and a second output shaft driven by the idler shaft, wherein the diameter and length of each of the first output shaft, the

second output shaft and the idler shaft are such that the difference in the angular twist of the first output shaft and the combined angular twist of the second output shaft and the idler shaft is not more than 1 degree.
BRIEF DESCRIPTION OF DRAWINGS:
FIG. 1 is a schematic diagram of an environment in which the invention can be incorporated, according to an embodiment of the invention.
FIG. 2A shows the orthogonal views of the gearbox, according to an embodiment of the invention.
FIG. 2B shows the front view and the top view of the gearbox, according to an embodiment of the invention.
FIG. 2C shows the input side view and the output side view of the gearbox, according to an embodiment of the invention.
FIG. 3A is the section view showing sections of the first output shaft, according to an embodiment of the invention.
FIG. 3B is the section view showing sections of the second output shaft and the idler shaft, according to an embodiment of the invention.
DETAILED DESCRIPTION:
DEFINITIONS:
Twin-screw extruders: Twin-screw extruders have multiple barrel sections with an 8 shaped bore from one end face to the other end face. These barrels sections are connected to each other with the bore axes coUinear. Twin-screw extruder fiirther has two extruder screw shaft assemblies comprising of extruder screw elements mounted on to the extruder screw shafts. These extruder screw shaft assemblies are housed in the longitudinal bore of the barrel sections. The material such as polymers,

elastomers, etc. fed through the intake barrel is processed inside the barrel bore, by the extruder screw elements.
Barrel section: A barrel section is typically a block with circular, square or rectangle cross section with a longitudinal through bore. This is a stationary part of the extruder processing zone. Inside the longitudinal bore of the barrel a pair of intermeshing screw shaft assemblies rotates.
Barrel assembly: Barrel assembly is formed by clamping a set of barrel sections one after another with the longitudinal bore properly aligned collinearly.
Backlash: Backlash is the amount of clearance between mated gear teeth. Theoretically, it should be zero, but in actual practice some backlash must be allowed to prevent jamming of the teeth due to tooth errors and thermal expansion. It is required to provide allowance for lubrication, manufacturing errors, deflection under load and differential thermal expansion between the gears and the housing.
DESCRIPTION OF THE INVENTION:
In the following description, for the purpose of explanation, specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent that the invention may be practiced without these specific details.
The present invention will be discussed hereafter in detail with reference to accompanying drawings and embodiments.
The present invention describes a gearbox for driving the pair of screw shafts of a twin screw extruder. The twin-screw extruder typically comprises a number of abutting barrel sections clamped to each other resulting in a long barrel. Each barrel section has a longitudinal through bore. A pair of extruder screw shafts is housed inside the resulting longitudinal bore after the barrel sections are abutted linearly to form a barrel assembly. In an extruder, different barrel sections are required to carry
out different processes such as conveying, kneading, mixing, devolatilizing, metering
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and the like. External power has to be provided to the screw shafts for carrying out these processes on the work material. This power is provided with the help of a gearbox which further receives fi*om a motor.
FIG 1 is a schematic diagram of an environment in which the invention can be incorporated, according to an embodiment of the invention. Twin screw extruder 102 comprises driving assembly 104 having a motor and a gearbox. Driving assembly 104 performs the task of transmitting power to pair of extruder screw shafts 106. The pair of extruder screw shafts 106 ftirther comprises processing elements called extruder screw elements mounted on them. The pair of extruder screw shafts 106 along with extruder screw elements rotates and processes the work material within the barrel assembly. The processed work material is ftirther utilized in die 108 to obtain the desired product.
FIG. 2A shows the orthogonal views of the gear train, according to an embodiment of the invention.
FIG. 2B shows the front view and the top view of the gear train, according to an embodiment of the invention. The gear train is driven by a motor (not shown) coupled to shaft 202. The power is transmitted to gear 212 via gear 206 and gear 204. Gear 212 divides the power equally between first output shaft 208 and second output shaft 210. Gear 212 engages gear 214, where gear 214 is mounted on idler shaft 216. Idler shaft 216 has gear section 218 (shown in FIG 2C) which drives simultaneously a first pair of gears 220. There is second pair of gears 222 arranged on either side of the horizontal plane passing through the second output shaft 210. Each of the first pair of gears 220 meshes with the corresponding second pair of gears 222. There is gear section 224 mounted on second output shaft 210. Each of the second pair of gears 222 meshes with gear section 224, with gear section 224 driving the second output shaft
210.
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FIG. 2C shows input side 200 view and output side 201 view of the gearbox, according to an embodiment of the invention. Second pair of gears 222 simultaneously drive second output shaft 210. Each of the first pair of gears 220 is on either side of the horizontal plane of the idler shaft 216. Further, second pair of gears 222, gear section 224 and second output shaft 210 lie in the same vertical plane. Therefore, the radial component of the force exerted by one of second pair of gears 222 is neutralized by the radial component of the force exerted by the other. Net resultant of the neutralization is pure torque acting on second output shaft 210. Hence, there are no bending loads due to zero resultant radial loads on second output shaft 210. Otherwise, due to bending loads, fatigue creeps in and leads to the unexpected and early failure of the shaft.
Further, due to the restriction on the center-distance of the two screw shafts (typically of the order of 10 to 100 mm) of the twin screw extruder, there is not enough space for the installation of the appropriate radial bearing on second output shaft 210 for completely taking care of the radial loads. According to the embodiment of the invention, first output shaft 208 is longer than second output shaft 210. As a result, appropriate radial bearings can be installed on the section of first output shaft 208 which extends beyond second output shaft 2] 0.
FIG. 3 A is the section view showing sections of first output shaft 208 and FIG. 3B is the section view showing sections of second output shaft 210 and idler shaft 216, according to an embodiment of the invention. Any torque acting on a shaft leads to its twisting, with the degree of twist depending on the Torsional modulus of rigidity of the material of the shaft, diameter of the shaft and the length of the shaft between the ends of the shaft on which torque is applied. The equations for the relationship between the various parameter of the shaft are as follows:

/TL\ /180\ \GjJ \ n )
I-
.32 /
where:
O = Degree of twist in the shaft (in degrees)
T = Torque applied on the shaft (in Nmm)
L = Effective length of the shaft (Length between the ends of the shaft where torques are applied) (in mm)
G = Torsional modulus of rigidity (in N/mm )
J = Polar moment of inertia of a shaft's section having a particular invariant cross-section (in mm'')
D = Diameter of the shaft (in mm)
The effective length of first output shaft 208 is different than the length of second output shaft 210. According to an embodiment of the invention, the material of first output shaft 208, second output shaft 210 and idler shaft 216 is same and thus has the same torsional modulus of rigidity. Further, the torque applied is same on first output shaft 208, second output shaft 210 and idler shaft 216, according to another embodiment of the invention.
The diameters of first output shaft 208, second output shaft 210 and idler shaft 216 are determined in such a way that a difference is provided between the twist in first output shaft 208 and the sum of the twists in second output shaft 210 and idler shaft 216. This difference allows for backlash for lubrication and to prevent jamming of the teeth due to manufacturing errors, deflection under load and differential expansion.

If there remains a large differential twist in the two output shafts of the gearbox, it will lead to a mismatch in the arrangement of the screw elements. This mismatch in the arrangement of the screw elements might lead to their breakage. Therefore, to avoid the breakage of expensive screw elements, backlash has to be minimized but it should not be eliminated altogether.
The difference in the twists should not be more than 1 degree, according to another embodiment of the invention. Hence, the gearbox ensures that the relative orientation of first output shaft 208 and second output shaft 210 remains, at every point of the operation, the same as their initial relative orientation.
The specific torque [T2/a^] of an extruder refers to the ratio between the torque in Nm [T2] and the third power of the center distance between the two axes [a] of the twin screw extruder. In accordance with an embodiment, the combined angular twist of the second output shaft and the idler shaft is not more than 1 degree at a specific torque of greater than 11 Nm/cin^3.
According to an embodiment of the invention, first output shaft 208, second output shaft 210 and idler shaft 216 are divided into various sections, as shown in FIG. 3A and FIG. 3B, having different cross-section along their length. The cross sections of the shafts are designed and machined so as to accommodate various thrust and roller bearings at various locations along their lengths. Further, since the twist depends on the dimension of the shaft, the twists calculated for each section will be different even though the torsional modulus of rigidity and the torque acting on each section of first output shaft 208, second output shaft 210 and idler shaft 216 are same.
Referring to the specific embodiment illustrated in the accompanying figures, the shafts are shown to comprise of various sections having different lengths and diameters. A typical calculation of twisting of shafts is as follows:
For first output shaft 208:
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v
As shown in FIG.4A, first output shaft 208 comprises of three sections having
lengths L|, i, Li, 2 and Li, 3 respectively. The diameters of the shaft at these sections
are D1,1, D1,2 and D1,3 respectively. The polar moments of inertia of the shaft at these
sections are Ji, 1, Ji, 2 and Ji, 3 respectively. The degree of twists of the shaft at these
sections are O 1^ 1, 01,2 and O j 3 respectively.
T - 582 * 10^ Nmm, G = 12 * lO'^ N/mm^
Li,i = 155 mm, L|^2 = 280 mm, Li^3 -20 mm
Di,i = 24.5 mm, Dj 2 = 23.8 mm, Di,3 - 22 mm
Putting the values into equations and calculating, we get
Ji, 1 - 33400 mm^ Ji,2 = 31500 mm^ Ji,3 - 23000 mm'^
2,1, O2,2, ^2,3 and O2,4 respectively.
T - 582 * 10^ Nmm, G - 12 * 10** N/mm^
L2,1 = 30 mm, L2,2 = 30 mm, L2,3 ~ 60 mm, L2,4 = 20 mm
D2,1 = 30 mm, D2,2 = 25 mm, D2,3 = 24 mm, D2,4 ^ 22 mm
Putting the values into equations and calculating, we get
h, 1 = 79500 mm^ h,2 = 38400 mm'*, J2,3 = 32500 mm^ J2,4 = 23000 mm*
2,3 + 02,4) + 03 = 3.33°
Difference in twist angles of first output shaft and second output shaft is equal to 0.67° (= 4" - 3.33°) which is within the maximum permissible limit of 1 degree. This difference in twist is to accommodate the backlash.
While the embodiments of this invention have been illustrated and described as above, it will be clear that the invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art without departing from the spirit and scope of this invention.
Specific embodiments are described below:
A gearbox for transmitting torque to a co-rotating twin screw extruder, the gearbox comprising a first output shaft driven by a driving means, an idler shaft driven by the first output shaft, and a second output shaft driven by the idler shaft, wherein the diameter and length of each of the first output shaft, the second output shaft and the idler shaft are such that the difference in the angular twist of the first
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output shaft and the combined angular twist of the second output shaft and the idler shaft is not more than 1 degree.
Such gearbox(s) for transmitting torque to a co-rotating twin screw extruder wherein the combined angular twist of the second output shaft and the idler shaft is not more than 1 degree at a specific torque of greater than 11 Nm/cm^3.
Such gearbox(s) for transmitting torque to a co-rotating twin screw extruder comprising, a first pair of gears placed on either side of the idler shaft and designed to mesh with the idler shaft, a second pair of gears designed to mesh with the second output shaft and placed on either side of the second output shaft, wherein each of the second pair of gears meshes with the corresponding gear of the first pair of gears, and the axis of the second output shaft is in the same vertical plane as the axis of each of the second pair of gears.
Such gearbox(s) for transmitting torque to a co-rotating twin screw extruder wherein the axis of each of the first pair of gears is in the same vertical plane as the axis of the idler shaft.
Such gearbox(s) for transmitting torque to a co-rotating twin screw extruder wherein, the first output shaft, the second output shaft and the idler shaft are parallel to each other.
Such gearbox(s) for transmitting torque to a co-rotating twin screw extruder wherein, the first pair of gears and the second pair of gears are helical.
Such gearbox(s) for transmitting torque to a co-rotating twin screw extruder wherein, the first pair of gears is identical to the second pair of gears.
Such gearbox(s) for transmitting torque to a co-rotating twin screw extruder wherein, the first output shaft, second output shaft and idler shaft are divided into two or more sections.
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Such gearbox(s) for transmitting torque to a co-rotating twin screw extruder wherein, the first output shaft, second output shaft and idler shaft have a different cross-section in each section.
Such gearbox(s) for transmitting torque to a co-rotating twin screw extruder, the first output shaft, second output shaft and idler shaft have different lengths in each section.
Such gearbox(s) for transmitting torque to a co-rotating twin screw extruder wherein the idler shaft has gear section which drives simultaneously the first pair of gears.
Such gearbox(s) for transmitting torque to a co-rotating twin screw extruder wherein the second output shaft has gear section which engages simultaneously with the second pair of gears.

a first output shaft driven by a driving means;
an idler shaft driven by the first output shaft; and
a second output shaft driven by the idler shaft; wherein the diameter and length of each of the first output shaft, the second output shaft and the idler shaft are such that the difference in the angular twist of the first output shaft and the combined angular twist of the second output shaft and the idler shaft is not more than 1 degree.
2. The gearbox for transmitting torque to a co-rotating twin screw extruder as claimed in claim 1 wherein the combined angular twist of the second output shaft and the idler shaft is not more than 1 degree at a specific torque of greater than 11 Nm/cm^3.
3. The gearbox for transmitting torque to a co-rotating twin screw extruder as claimed in claim 1 comprising:
a first pair of gears placed on either side of the idler shaft and designed to mesh with the idler shaft;
a second pair of gears designed to mesh with the second output shaft and placed on either side of the second output shaft, wherein each of the second pair of gears meshes with the corresponding gear of the first pair of gears; and
the axis of the second output shaft is in the same vertical plane as the axis of each of the second pair of gears.

4. The gearbox for transmitting torque to a co-rotating twin screw extruder as claimed in claim 3 wherein the axis of each of the first pair of gears is in the same vertical plane as the axis of the idler shaft.
5. The gearbox for transmitting torque to a co-rotating twin screw extruder as claimed in any preceding claim wherein the first output shaft, the second output shaft and the idler shaft are parallel to each other.
6. The gearbox for transmitting torque to a co-rotating twin screw extruder as claimed in claim 3 wherein the first pair of gears and the second pair of gears are helical.
7. The gearbox for transmitting torque to a co-rotating twin screw extruder as claimed in claim 3 wherein the first pair of gears is identical to the second pair of gears.
8. The gearbox for transmitting torque to a co-rotating twin screw extruder as claimed in claim 1 wherein the first output shaft, second output shaft and idler shaft are divided into two or more sections.
9. The gearbox for transmitting torque to a co-rotating twin screw extruder as claimed in claim 8 wherein the first output shaft, second output shaft and idler shaft have a different cross-section in each section.

c
10. The gearbox for transmitting torque to a co-rotating twin screw extruder as
claimed in claim 8 or 9 wherein the first output shaft, second output shaft and idler shaft have different lengths in each section.
11. The gearbox for transmitting torque to a co-rotating twin screw extruder as
claimed in claim 3 wherein the idler shaft has gear section which drives
simultaneously the first pair of gears.
12. The gearbox for transmitting torque to a co-rotating twin screw extruder as
claimed in claim 3 wherein the second output shaft has gear section which
engages simultaneously with the second pair of gears.
13. A gearbox for transmitting torque to a co-rotating twin screw extruder,
substantially as herein described with reference to and as illustrated by the
accompanying figures.