Transmission components! How does the “heart” of a twin-screw extruder work?

For users, how should they choose and assess the extruder products become a tricky issue, what kind of extruder equipment is with high cost performance? From the purpose, the basic requirements for the equipment are mainly the following 5 points.

(1) Quality: quality is the most important to users, the quality of the extruded product is a commitment to customers, is the life of the enterprise, and the development of the enterprise is closely related, so whether the consistency of the batches of products can be guaranteed for a long time is an important indicator to assess the quality of an extruder.

(2) Efficiency: Efficiency is a further assessment indicator. To ensure the highest product yield per unit of time on the basis of quality is the goal of every enterprise and the path to rapid development.

(3) Costs: Improving efficiency itself is one of the means to reduce costs, improving efficiency directly reduces the energy consumption of water, electricity and gas per unit of time, which means lower running costs, while the most important concern of enterprises is to reduce the cost of manpower and management costs, and the demand for equipment and land is reduced.

(4) durable: efficiency and life is inversely proportional to the increase in efficiency may lead to a lower service life, in order to ensure or increase the service life under the premise of high efficiency, the need to use more durable materials, is the use of screw materials with high wear resistance, high strength mandrel.

(5) Practical: From the user’s point of view, whether the equipment is easy to operate and maintain, whether the equipment can run stably, with a high or low failure rate, is also one of the indicators to assess the quality of an extruder.

To sum up, quality and efficiency are two decisive and important indicators. How to “ensure quality” and “improve efficiency” is the goal that extruders have been constantly developing over the years.
Transmission components

The twin-screw extruder is divided into two key components: the “extrusion” and the “transmission”. The “extrusion component” is the key component for “quality assurance” and the “transmission component” is the key component for “efficiency” and “quality assurance”. The technological progress of the twin-screw is marked by the renewal of the torque distribution box, which shows its importance.

Today we will talk about the “transmission” component of the extruder.

The transmission component is the core component of the twin-screw extruder, just like the engine of a car, which can provide enough power to output enough output, so a strong transmission system is needed to provide power to improve the extrusion efficiency.

In twin-screw extruders, the power is reflected in the rotational movement of the screw element. The kinetic energy in the rotational movement is converted into torsional force, the greater the torque the more material is conveyed, i.e. a high-torque drive system is required to provide power in order to achieve high output.

There are two representative and typical configurations of the drive system: the parallel three-shaft gear structure and the double-side symmetrical drive gear structure.

(1)Parallel three-shaft drive structure

The advantages of this solution are simple structure, easy assembly, low manufacturing cost and increased torque of the system by means of reducing the load on the gear; however, the power of this structure will always be subjected to a larger load on the bearings on the B-shaft during parallel transmission ( as shown in Figure 3), and the B-shaft is always subjected to a force in the same direction during rotation, i.e. a combined bending and torsional action ( symmetrical cyclic stress), and the bearings on the B-shaft are subjected to The force on the B-shaft increases as its output torque increases, so that the bearing reaches the life limit of the gear shaft and bearing after 20,000h of force wear, so the gearbox life is governed by the B-shaft bearing.

Secondly, and crucially, as the B-shaft is always subjected to higher loads, the wear of the bearings is accelerated, causing the radial runout of the B-shaft to increase until it fails, making the system an unstable power system.

As the power source is unstable, it will lead to instability of the extrusion parts driven by it, i.e. the rotational trajectory of the screw in the barrel will also change with the increase of bearing runout, which will lead to uneven clearance between the screw and the barrel, resulting in uneven residence time of the material in the barrel, thus causing the consistency of the product quality to decrease.

(2) Double side gear symmetrical transmission structure

Figure 4 shows a schematic diagram of a double side gear symmetrical drive structure. The output shaft A of this structure, like the previous structure, has sufficient dimensional space and a large safety factor of strength and rigidity; the output shaft B is also limited by the centre distance and has the problem of size limitation. The solution to this structure is to divide the power into two groups of gears, and drive the B shaft symmetrically from the upper and lower directions, which not only reduces the load at the upper and lower gear meshing points by half and increases the torque load capacity, but also increases the amount of material to be fed. This not only reduces the load on the upper and lower gears by half and increases the torque load capacity, thus multiplying the filling volume and effectively increasing productivity, but also makes the radial forces on the B-shaft cancel each other out to zero. As shown in Figure 5, the gears of the B-shaft are only subjected to tangential forces, creating a perfect force couple drive, i.e. a pure torque output shaft, eliminating bending stresses on the B-shaft (which are only subjected to torsional forces, i.e. unidirectional cyclic stresses).

In addition, the structure is characterised by the fact that the radial bearing load on the B-axis is completely eliminated, so that the B-axis bearing theoretically never wears out, achieving a much longer service life of 72,000 h (approx. 10a) than in structure 1, and thus changing the history of the B-axis, where the rotational accuracy deteriorated over time.

The structure not only achieves high torque output, improves productivity and increases service life, but more importantly ensures the stability of the torque distribution system and the consistency of the product quality. The bearings on output shaft B are not subjected to radial forces and do not wear out, no changes in radial runout occur and no radial runout of the screw shaft is caused, which ensures that the whole system operates steadily and continuously over a long period of time, i.e. the screw and The gap between the screw and the barrel will not change unevenly and the residence time of the material in the barrel will not be uneven, thus ensuring the stability of the product quality. The exponential increase in torque allows for a significant increase in the filling volume and is a guarantee of exponentially higher production efficiency.

However, the disadvantages of this structure are: the relatively complex structure, the high machining accuracy required, the difficulty of assembly and the relatively high manufacturing costs.

The application of this drive structure is of great significance to the progress of extrusion technology as follows.

1) providing stable and reliable power for the extrusion system, so that stable extrusion has a reliable power guarantee.

2) With a higher torque output, higher dosing volumes can be achieved, also increasing the effective length of the screw for more uniform distribution and dispersed mixing.

3) Reliable power for precision extrusion and efficient compounding processes.

4)Higher output speeds and smaller screw clearances can be achieved.