The invention, in general, relates to safety devices employed in torque transmission systems. More specifically, it relates to a torque transmission system for an extruder.

BACKGROUND
Torque transmission from one unit to another is common in mechanical systems. Typically, mechanical systems experience fluctuations in load and power supply that induce high levels of stress in torque transmitting members of the mechanical systems. Such high stresses may further lead to unexpected yielding or fracture of shafts that are used as torque transmitting members in the mechanical systems. Whenever a shaft yields or breaks, the entire mechanical system needs to be disassembled to replace the damaged shaft with a new one. In certain mechanical systems, for example in twin screw extruders, the extruder screw shafts are machined to specific complex profiles to meet certain requirements. These complex profiles of the extruder screw shafts demand special machining requirements resulting in high cost of manufacturing.

Apart from the damage caused to the shafts, other members of the torque transmitting system may also be damaged. For example, in the case of twin screw extruders, the extruder screw elements are mounted on to the extruder screw shafts. Under certain circumstances, the resistance to the rotation of the extruder screw shaft may increase suddenly causing possible breaking of the extruder screw elements mounted on to the extruder screw shaft. These extruder screw elements are housed inside the barrel of the twin screw extruder forming the extruder processing zone. Hence, whenever an extruder screw element breaks, the components of the extruder processing zone need to be disassembled to replace the broken extruder screw element. Further, the extruder screw elements are specially designed to meet certain functional requirements such as kneading, compounding etc and special manufacturing techniques need to be adopted to machine the extruder screw elements. Hence, the cost of replacing the extruder screw elements is considerably high.

In a twin screw extruder, two extruder screw shafts in proper orientation process the work material. The torque required for processing the work material is transmitted from a motor to a gearbox and from the gearbox to the extruder screw shafts.

Extruder screw shafts are designed on the assumption of uniform torque distribution between the two extruder screw shafts. However, the torque borne by each extruder screw shaft depends on the resistance it encounters. Hence, during actual working, torque imbalances may occur between the extruder screw shafts with one of the extruder screw shafts taking more torque than the other. This uneven distribution of torque results in excessive stress levels in one of the extruder screw shafts even though the sum of the torques taken in by the extruder screw shafts is less than the maximum designed torque for the extruder shafts resulting in breaking or yielding of the extruder screw shaft. Hence, it is desirable to prevent the damage to the expensive torque transmitting members in such twin screw extruders and similar mechanical systems.

With reference to figure 1, a conventional torque transmitting system 100 is illustrated connecting a driving means 102 to an extruder 104. The driving means 102 may be a gearbox, a motor, a turbine, an engine, a pulley, a gear train and the like. Torque is transmitted from the driving means 102 to the extruder 104 by means of a driving shaft 106, a coupler 112 and an extruder screw shaft 108. Driving shaft 106 may be an output shaft of a gearbox, a motor, a the, an engine and the like. Extruder screw shaft 108 is a driven shaft. Coupler 112 is typically used as an intermediate coupling means between driving shaft 106 and extruder screw shaft 108c In the conventional torque transmitting systems, when the rotation of the extruder screw shaft 108 is resisted, stresses are developed in driving shaft 106 and extruder screw shaft 108. Every extruder screw shaft 108 has a critical torque level, which is the level at which the extruder screw shaft is damaged or breaks. During extruder operation when torque transmitted to the extruder screw shaft reaches or exceeds the critical torque, the extruder screw shaft is liable to be damaged.

Conventional safety mechanisms for torque transmitting systems employ frictional force based clutches for torque limiting purposes. U.S patent US4944379 titled "Torque Limiter" discloses a system which employs frictional force based adaptor to limit the torque. The adaptor disclosed in the patent holds the members of the torque transmitting system by means of frictional force. The frictional force holding the torque transmitting members is overcome when the torque exceeds a certain limit. This causes the adaptor to slip over the torque transmitting members thereby preventing the damage to the torque transmitting members. Such mechanisms however cannot be easily deployed in twin screw extruders on account of space and design constraints.

Moreover, it is desirable that the safety mechanism be such that minimum or no alteration to existing extruder systems is required. The safety mechanism should also be such that the extruder system can function without it, if necessary.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The accompanying drawing illustrates the preferred embodiments of the invention and together with the following detailed description serves to explain the principles of the invention, where:

Figure 1 is a schematic diagram of a conventional torque transmitting system.

Figure 2 depicts one end of a shaft with external splines machined on it.

Figure 3 is a schematic diagram of a torque transmitting system in accordance with an embodiment of the invention.

Figure 4 depicts an adaptor in accordance with an embodiment of the invention.

Figure 5(a) represents the front, sectional and end views of an adaptor in accordance with an embodiment of the invention.

Figure 5(b) represents an isometric view of an adaptor in accordance with an embodiment of the invention.

Figure 6 depicts the assembly of an adaptor and an extruder screw shaft in accordance with an embodiment of the invention.

SUMMARY

The disclosure relates to a torque transmission system for coupling an extruder screw shaft having a critical torque level with the output shaft of a driving means, the torque transmission system comprising a coupler configured to engage with the output shaft of the driving means; and an adaptor configured to be engaged by the coupler; the adaptor further configured to engage the extruder screw shaft; the adaptor defining a pre weakened area designed to rupture at a torque level less than the critical torque of the extruder screw shaft.

The disclosure also provides for an extruder system comprising: a barrel having a bore with an extruder screw shaft located within the bore; the extruder screw shaft having a critical torque level; a driving means for driving the extruder screw shaft located outside the barrel; the driving means including an output shaft; a torque transmission system for coupling the extruder screw shaft with the output shaft of the driving means, the torque transmission system comprising a coupler configured to engage with the output shaft of the driving means; and an adaptor configured to be engaged by the coupler; the adaptor further configured to engage the extruder screw shaft; the adaptor defining a pre weakened area designed to rupture at a torque level less than the critical torque of the extruder screw shaft.

DETAILED DESCRIPTION

For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated system, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.

It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the invention and are not intended to be restrictive thereof. Throughout the patent specification, a convention employed is that in the upended drawings, like numerals denote like components.

Reference throughout this specification to "one embodiment" "an embodiment" or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrase "in one embodiment", "in an embodiment" and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

Referring initially to Figures 3 and 4, a schematic diagram of a torque transmitting system, according to an embodiment of the invention is illustrated. A torque transmitting system 300 connects driving means 102 with an extruder 104. The torque transmitting system 300 comprises of a coupler 110 and an adaptor 302. The driving means 102 may be a gearbox, a motor, a turbine, an engine, a pulley, a gear train and the like. The output shaft 106 of the driving means 102 is connected to an extruder screw shaft 108 of the extruder 104 by the torque transmitting system 300. Specifically, the output shaft 106 of the driving means 102 is connected to the coupler 110, the coupler 110 is connected to the adaptor 302, and the adaptor 302 is connected to the extruder screw shaft 108 of the extruder 104. Torque is transmitted fix)m the driving means 102 to the extruder 104 by means of driving shaft 106, coupling member 110, adaptor 302 and extruder screw shaft 108.

In the embodiment illustrated, the coupler 110 is a hollow tubular member with splines on its inner surface. The output shaft 106 of the driving means 102 as well as the extruder screw shaft 108 have external splines formed to engage the internal splines of the coupler.

Figure 2 illustrates the external splines formed on the output shaft of the driving means and the extruder screw shaft. Typically, splines on a shaft are used to transmit torque from one shaft to another shaft.

Figure 4 depicts a schematic diagram of an adaptor 302 in torque transmitting system 300, according to an embodiment of the invention. The adaptor 302 comprises a first end 404 that is configured to be engaged by the coupler 110 and a second end 406 that is configured to engage the extruder screw shaft 108. A pre-weakened area in the form of a rupture groove 402 is provided between the first end 404 and the second end 406 of the adaptor. The rupture groove 402 is designed to rupture at a torque level less than the critical torque of the extruder screw shaft 108. Rupture groove 402 may be a V-groove machined around the circumference of adaptor 302. Rupture groove 402 can be designed and machined on adaptor 302 by means of machining methods like turning process and the like. The rupture groove 402 may be any suitable profile including but not limited to a V-groove, a U- groove or any arcuate profile. The depth and dimensions of the rupture groove depend on the desired torque canning capability of the adaptor or the rupture threshold of the adaptor. One or more of such grooves may be provided on the adaptor.

Further, first end 404 of the adaptor 302 is configured to engage the coupler 110. The second end 406 of the adaptor 302 is configured to engage with extruder screw shaft 108. First end 404 and second end 406 are coupled to coupler 110 and extruder screw shaft 108 respectively using one of the coupling means which include but not limited to splines, glues, keyways, serrations and the like. In the embodiment illustrated, the first end 404 of the adaptor is provided with external splines that engage the internal splines of the coupler 110, and the second end 406 of the adaptor is configured as a hollow tubular structure configured to receive the extruder screw shaft 108. The hollow tubular structure of the second end 406 is provided with internal splines that engage the external splines of the extruder screw shaft 108.

Figure 5 (a) represents front, sectional, and end views of adaptor 302, in accordance with an embodiment of the invention while Figure 5(b) shows an isometric view of adaptor 302, according to an embodiment of the invention. Figure 5(a) and Figure 5(b) shows first end 404 of adaptor 302 machined with splines. The figures show second end 406 having diameter larger than the diameter of the first end 404. The larger diameter is configured to provide for the hollow tubular structure that receives the extruder screw shaft. Figure 5(b) also shows a zoomed-in view of the pre-weakened area in the form of the rupture groove 402 of adaptor 302.

Figure 6 depicts the coupling between adaptor 302 and extruder screw shaft 108, according to an embodiment of the invention. The adaptor 302 has one or more splines machined on the inner surface of its second end 406. Extruder screw shaft (driven shaft) 108 has similar splines machined on its outer surface. The adaptor 302 and the extruder screw shaft 108 are coupled using splines machined on their surfaces. A retaining screw 602 is used to hold adaptor 302 and extruder screw shaft 108 without any axial displacement.

The extruder 104 may be a single screw or a twin-screw extruder. The extruder may further be a co-rotating or counter rotating twin-screw extruder. The extruder comprises a barrel that has a bore and the extruder screw shaft is located within the bore. A driving means for driving the extruder screw shaft is typically located outside the barrel with the output shaft of the driving means connected to the extruder screw shaft to transmit torque.

In the case of a twin-screw extruder, the barrel has two parallel bores of equal diameter. The centre distance between the two bores lesser than the diameter of the bore. An extruder screw shaft is located within each bore. The extruder screw shafts may be configured for rotation in the same or opposite direction. The torque transmission system couples at least one extruder screw shaft with the output shaft of the driving means. A torque transmission system may also be used between each extruder screw shaft and the corresponding output shaft of the driving means.

SPECIFIC EMBODIMENTS ARE DESCRIBED BELOW

A torque transmission system for coupling an extruder screw shaft having a critical torque level with the output shaft of a driving means, the torque transmission system comprising a coupler configured to engage with the output shaft of the driving means and an adaptor configured to be engaged by the coupler, the adaptor further configured to engage the extruder screw shaft, the adaptor defining a pre weakened area designed to rupture at a torque level less than the critical torque of the extruder screw shaft.
Such torque transmission system(s), wherein the coupling member is a hollow tubular member with internal splines configured to engage external splines formed on the output shaft of the driving means.

Such torque transmission system(s), wherein the adaptor includes external splines at one end configured to engage with the internal splines of the coupling member.

Such torque transmission system(s), wherein the adaptor includes internal splines at one end configured to engage external splines formed on the extruder screw shaft.

An extruder system comprising a barrel having a bore with an extruder screw shaft located within the bore, the extruder screw shaft having a critical torque level, a driving means for driving the extruder screw shaft located outside the barrel, the driving means including an output shaft, a torque transmission system for coupling the extruder screw shaft with the output shaft of the driving means, the torque transmission system comprising a coupler configured to engage with the output shaft of the driving means and an adaptor configured to be engaged by the coupler; the adaptor further configured to engage the extruder screw shaft, the adaptor defining a pre weakened area designed to rupture at a torque level less than the critical torque of the extruder screw shaft.

Such extruder system(s), wherein the barrel has two parallel bores of equal diameter, the centre distance between the two bores lesser than the diameter of the bore; an extruder screw shaft located within each bore, each extruder screw shaft having a critical torque level; the extruder screw shafts configured for rotation in the same direction and wherein the torque transmission configured to couple at least one extruder screw shaft with the output shaft of the driving means.

Such extruder system(s), wherein the coupling member is a hollow tubular member with internal splines configured to engage external splines formed on the output shaft of the driving means.

Such extruder system(s), wherein the adaptor includes external splines at one end configured to engage with the internal splines of the coupling member.

Such extruder system(s), wherein the adaptor includes internal splines at one end configured to engage external splines formed on the extruder screw shaft.

INDUSTRIAL APPLICABILITY

The stress concentration effect occurs at structural discontinuities in the material. When load is applied on the material with structural discontinuities on its surface or in its interior, stress concentration takes place at these structural discontinuities. When the structural discontinuity is a crack on the surface of the material, the concentrated stress propagates the crack further into the material. Eventually, the material breaks due to excessive stress caused by a torque level equal to or higher than the critical torque level for the material.
Referring back to figure 4, the rupture groove 402 on the adaptor 302 is a structural discontinuity. Due to the stress concentration effect, the stress on the torque transmission system 300 gets concentrated at rupture groove 402 of adaptor 302. The rupture groove 402 of the adaptor 302 ruptures at a torque level less than the critical torque level of the extruder screw shaft 108. As a result of the stress concentration effect, fracture occurs at the rupture groove 402 at a pre-determined torque level less than the critical torque level of the extruder screw shaft 108.

The adaptor 302 disclosed in the invention can be employed in single screw or twin-screw extruders.

During extruder operation, the single screw extruder may encounter impurities or non-uniform densities in the work material. The impurities and non-uniform densities in the work material result in high resistance to rotation of the extruder screw shaft 108. Due to this sudden surge in resistance, the force applied by the extruder screw shaft 108 increases thereby increasing its toque level. The output shaft 106 of the driving means as well as the extruder screw shaft is subjected to additional stresses caused by the extruder screw shaft 108, and these stresses may be greater than the threshold of the rupture groove 402. Under these circumstances, adaptor 302 coupled to the extruder screw shaft will break at rupture groove 402. By breaking at rupture groove 402, adaptor 302 protects the extruder screw shaft 108 and other members of the extruder from breaking.

In the case of a twin-screw extruder, the work material resists the rotation of the extruder screw shafts. Stresses are developed in the extruder screw shafts due to this resistance and these stresses are typically not uniformly distributed between the two extruder screw shafts. This uneven distribution of the stresses results in very high stress in one of the extruder screw shafts. Under these circumstances, adaptor 302 coupled to the extruder screw shaft with higher stress will break at rupture groove 402. By breaking at rupture groove 402, adaptor 302 protects the extruder screw shaft 108 and other members of the twin-screw extruder from breaking.

The use of a two-piece torque transmission system for coupling the shafts and providing a torque safety limit ensures that the extruder can function even if the adaptor were to fail. Moreover, the two-piece system reduces overall cost of the replaceable component without requiring any changes in existing extruder systems.

While the invention has been illustrated and described with reference to the certain embodiments thereof, it shall be appreciated by those ordinarily skilled in the art that various modifications can be made in the formalities and the details without departure from the spirit and scope of the invention.

claim:

1. A torque transmission system for coupling an extruder screw shaft having a critical torque level with the output shaft of a driving means, the torque
transmission system comprising:

a. a coupler configured to engage with the output shaft of the driving means;
and

b. an adaptor configured to be engaged by the coupler; the adaptor further
configured to engage the extruder screw shaft;

the adaptor defining a pre weakened area designed to rupture at a torque level less than the critical torque of the extruder screw shaft.

2. A torque transmission system as claimed in claim 1 wherein the coupling member is a hollow tubular member with internal splines configured to engage external splines formed on the output shaft of the driving means.

3. A torque transmission system as claimed in claim 2 wherein the adaptor includes external splines at one end configured to engage with the internal splines of the coupling member.

4. A torque transmission system as claimed in claim 1 or 3 wherein the adaptor includes internal splines at one end configured to engage external splines formed on the extruder screw shaft.

5. An extruder system comprising:

a barrel having a bore with an extruder screw shaft located within the bore; the extruder screw shaft having a critical torque level;

a driving means for driving the extruder screw shaft located outside the barrel; the driving means including an output shaft;

a torque transmission system for coupling the extruder screw shaft with the output shaft of the driving means, the torque transmission system comprising:

a coupler configured to engage with the output shaft of the driving means;

and

an adaptor configured to be engaged by the coupler, the adaptor further configured to engage the extruder screw shaft;

the adaptor defining a pre weakened area designed to rupture at a torque level less than the critical torque of the extruder screw shaft.

6. An extruder system as claimed hi claim 5 wherein the barrel has two parallel bores of equal diameter, the centre distance between the two bores lesser than the diameter of the bore; an extruder screw shaft located within each bore, each extruder screw shaft having a critical torque level; the extruder screw shafts configured for rotation m the same direction and wherein the torque transmission system configured to couple at least one extruder screw shaft with the output shaft of the driving means.

7. An extruder system as claimed in claim 5 or 6 wherein the coupling member is a hollow tubular member with internal splines configured to engage external splines
formed on the output shaft of the driving means.

8. An extruder system as claimed in claim 7 wherein the adaptor includes external splines at one end configured to engage with the internal splines of the coupling
member.

9. An extruder system as claimed in claim 5, 6, 7 or 8 wherein the adaptor includes internal splines at one end configured to engage external splines formed on the
extruder screw shaft

10. A torque transmission system substantially as herein described with reference to
and as illustrated by figures 2 to 6.

11. An extruder system substantially as herein described with reference to and as
illustrated by figures 2 to 6.