Steadfast even under heavy loads

Hydraulic Shrink Discs in the Paper Industry

The paper industry places great demands on machines and components: They must not only withstand high speeds and forces, but also the adverse environmental conditions with high temperatures, humidity and chemicals that cause corrosion. Hydraulic shrink discs are ideal for these conditions: their quick installation reduces maintenance and downtime, and their intelligent design means that important functional surfaces are on the inside and thus protected from external influences.

Precision and power transmission for the highest demands in the paper industry

The primary function of hydraulic shrink discs is to safely transmit high torques between a gearbox or a drive and a shaft, through a frictional connection. This friction-locked connection is not only highly resilient and low-maintenance, but also backlash-free. In the paper industry, where machines often run at high speeds, the precise positioning of components is especially important. This requires a high level of precision in the manufacturing processes – one only has to think of the fine paper density, different thicknesses and materials of paper. If the connection is not good or has play, this results in production errors that can cause rejects and be expensive. In addition, the machines are subject to extreme wear. Hydraulic shrink discs are an indispensable component of drive and connection technology, especially at critical points in paper production. For example, on drive shafts, presses and rotating components, in transport systems, but especially on drums. These roll out the raw materials, join them together and bring the paper into the desired form. Hydraulic shrink discs are also suitable for the high stresses in pulp production.

Extreme conditions and corrosion

Paper production places further special demands on machines and systems. The conditions here are extreme: high temperatures, which are not a problem for hydraulic shrink discs in themselves, but when combined with high humidity or moisture, create an inhospitable climate; the additional presence of chemicals such as chlorine or bleach, which are necessary for the production of paper, makes this even worse. In the production of pulp, the raw material for paper, further chemicals are used; here the conditions are sometimes even harsher.

The high temperatures in combination with humidity and the effects of gases and chemicals prepare the ground and start for corrosion: this is the main problem in paper production and affects all components of the machinery, including electronics and control technology. Without protection, it occurs quickly – making it a major time and cost factor for the industry: Only components that can withstand corrosion can be installed. Mechanical engineers have to equip their systems accordingly so that they are and remain reliable, operational and safe in extreme environments.

Hydraulic function discs: Protection thanks to intelligent design

TAS Schäfer GmbH, the market leader in hydraulic shrink discs, has developed a protection concept for adverse production conditions that is based on two pillars: on the one hand, the outer surfaces of the shrink discs are sealed, and on the other hand, the shrink discs are constructed in such a way that important functional surfaces are on the inside.

Screws and washers are provided with a zinc flake coating, components are protected with varnishes or epoxy resin or are nickel-plated. Steels are also coated; in addition, acid-resistant and stainless steels are used for components in order to minimize problems with corrosion. In the case of hydraulic shrink discs, special plastics are used for seals and gaskets that can withstand the conditions.
In addition, the majority of the external surfaces that can be damaged by the conditions are irrelevant for the function of the shrink disc: this is because the surfaces that determine the function are moved to the inside of the structure by means of an intelligent design and thus structurally separated from the environmental influences. Elements such as rubber rings can ensure that external factors, including exposure to dust and sand, do not come into direct contact with the relevant component – in both the tensioned and untensioned state.

However, some points relevant to the function are unprotected when the hydraulic shrink disc is in the untensioned state. They must be protected with covers, for example, when the shrink disc is not mounted. During operation, they are no longer accessible due to the resulting tension; the movement of the pressure rings presses them against the inner ring.

During maintenance, which the customer can also carry out themselves, sensitive components such as seals, inner ring and screws are replaced – chipped material, rust and corrosion occur on the external parts despite the coating. The functional surfaces inside are subject to natural wear and tear due to their function. If the shrink disc is regularly serviced and refurbished and optimized for use in the paper industry, it can achieve a service life similar to that in a less hostile environment. The following always applies: All optimizations for greater durability do not lead to functional limitations of torque transmission.

Disassembly and tensioning: speed is of the essence

Unlike, for example, transmission test benches, the paper industry does not per se require the quick replacement of shrink discs, where the tension is often released and the disc re-tensioned. Nevertheless, setup processes must be easy to use and efficient: When drums are changed because new or reworked discs are to be used or because the system is being converted to other materials or paper formats, hydraulic shrink discs are advantageous – especially at critical points in the production line. Compared to mechanical shrink discs, they can be installed and removed much more quickly, reducing machine maintenance and downtime. It also helps if the hydraulic shrink disc can be quickly dismantled to give technicians quick access to the drive in the event of a complication or failure.

Although replacements are not often necessary, the time saved by the quick clamping with hydraulic system and the associated risk reduction play such a major role that the additional costs compared to mechanical shrink discs are acceptable. A major factor here is the environment in paper and pulp production, which also puts a strain on employees. Accordingly, when it comes to maintenance, the speed at which it can be carried out is an issue in order to keep exposure to substances such as sulfur or nitrogen oxides as low as possible.

In other areas, the speed advantage during assembly does not yet outweigh the higher price of hydraulic shrink discs compared to other coupling forms such as mechanical shrink discs. TAS Schäfer GmbH, for example, is working on being able to offer hydraulic shrink discs at a lower price by optimizing the design and components, and through simplification and standardization.

Conclusion

Hydraulic shrink discs are advantageous in the paper industry because, thanks to their quick installation, they reduce maintenance and downtime and thus reduce the associated stress on employees in the extreme production environment. With their robustness and long service life, they meet the requirements of the paper industry. Ideally, an intelligent design protects the functional surfaces from external influences, thus ensuring a long service life.

Autoren:
Rolf Gertner, Leiter Entwicklung / Konstruktion TAS Schäfer GmbH
Nadja Müller, Journalistin für Wordfinder