Rotary shaft seals: Masters of rotation

Stefa® RWDRs from Trelleborg Sealing Solutions within a rotating application, whose sealing lips press against the shaft, defining the boundary between the oil and air sides. (Image source: Trelleborg Sealing Solutions )

Energy consumption in systems with rotating components is significant. Trelleborg Sealing Solutions has achieved a reduction in this consumption of more than 20% with its latest generation of Stefa® rotary shaft seals (RWDR). These RWDRs also withstand the increased speeds and accelerations generated by modern electric motors. Trelleborg offers innovative sealing materials and various Stefa® RWDR types as standard products, but also develops customized solutions for gearboxes, pumps, robots and agricultural machinery through close partnerships to enable maintenance-free long-term operation.

At the heart of the rotating components of gearboxes, hydraulic pumps, robots and agricultural machinery is the rotary shaft seal (RWDR). These seals, which sit between two rotating machine parts, typically consist of a flexible sealing lip that presses against a shaft and a housing surrounded by elastomer or metal. The sealing lip keeps the lubricant inside the system on the oil side and protects against dust and other contaminants on the air side. Recently, Trelleborg Sealing Solutions introduced its new generation of Stefa® RWDRs, which effectively seal improved lubricants and reduce energy consumption by minimizing friction loss between the shaft and sealing lip.

Systematic representation of a Trelleborg Stefa® rotary shaft seal with its individual components. (Image source: Trelleborg Sealing Solutions)

Minimal elastomer wave interference

Trelleborg relies on advanced design standards for its Stefa® RWDRs, based on many years of experience in different applications. The sealing effectiveness of an RWDR depends largely on the radial force exerted by the sealing lip on the shaft. Although a higher contact pressure can improve the sealing effect, it also leads to increased friction and therefore unnecessary energy consumption. On the other hand, too little radial force can cause leaks, allowing lubricants to escape and dust and dirt to penetrate.

By using computer-aided finite element analysis (FEA), Trelleborg has determined the optimum balance between contact on the oil and air side to ensure the best possible seal. Simulations have also shown that many rotary seals have significant interference between the elastomer and the shaft, meaning that a significant part of the contact force comes from the elastomer itself. However, as elastomers change over time under various environmental influences such as temperatures, fluids or mechanical loads, the contact force also fluctuates. Trelleborg has therefore minimized the elastomer-wave interference to ensure constant performance of the RWDRs even after 10,000 hours of operation. A key element here is the spring around the sealing lip, which defines the contact force and makes it largely independent of the properties of the elastomer.

Distribution of forces within a Trelleborg Stefa® RWDR from dark blue to red. The contact force is strongest at the sealing lip towards the oil side (red) and in the lower area of the spring (green). (Image source: Trelleborg Sealing Solutions)

Constant contact force

The spring ensures that the contact force of the sealing lip on the shaft remains constant, as the spring force does not change over time. Classic spring steel is normally used for this, although Trelleborg uses stainless steel if corrosion resistance is required. Internal tests have shown that the contact force of the new generation of Stefa® RWDRs remains constant for more than 1,008 hours.

The design of these new Stefa® RWDRs has been standardized for shaft diameters from 20 to 200 mm, whereby the positive effect of the spring definition of the contact force is particularly striking for smaller diameters. The radial force is represented as a function of the preload of the elastomer and the force of the tension spring. While in conventional RWDRs the spring force component is typically between 50 and 60 percent, in the new Trelleborg design it reaches values of 80 to 90 percent, which almost completely reduces the preload of the elastomer.

In addition, Stefa® RWDRs are equipped with dust lips that protect the main sealing lip from dirt, water and other contaminants. Trelleborg recommends Stefa® RWDRs with dust lips especially for use in heavily contaminated environments. In the past, dust lips led to high torque consumption and therefore increased friction loss. Dust lips also wore out quickly and lost their functionality. By using suitable lubricants between the main sealing lip and the dust lip, the manufacturer minimizes wear and extends the service life of the dust lips.

Cross-section of a Trelleborg Stefa® RWDR with sealing lip and surrounding spring, which determines the contact force of the sealing lip on the shaft within the elastomer. (Image source: Trelleborg Sealing Solutions)

Lower energy consumption, longer service life

The improvements to the main sealing lip and the dust lip have reduced the energy consumption of the seals, which in turn reduces wear and heat generation. Heat generation at the sealing point in particular can accelerate wear, promote chemical reactions in the material and reduce the effectiveness of the lubricants. Lower energy consumption therefore leads to a longer service life for the entire tribological system, consisting of the seal, mating surface and lubricant.

When selecting the right sealing material, both the environmental conditions and the specific requirements of the RWDR must be taken into account. It is crucial to choose a material that is chemically resistant and at the same time remains robust against temperature fluctuations, ozone and other environmental influences. High wear resistance and elasticity with low friction loss and minimal compression set are also important. Trelleborg offers a wide range of materials, including acrylonitrile butadiene rubber (NBR), polyacrylate rubber (ACM), fluorinated rubber (FKM) and hydrogenated acrylonitrile butadiene rubber (HNBR), as well as other PFAS-free materials that are compatible with all modern hydraulic fluids. In the food sector, for example, the company provides materials for rotating applications such as gearboxes and mixers, always ensuring strict separation between lubricants and food or external chemicals. The suitability of the sealing materials is evaluated through analyses of tensile strength, elongation, volume and hardness changes, which Trelleborg carries out in its global laboratories through so-called swelling tests in a standardized manner.

Customization to customer requirements

Trelleborg uses different materials for the various types of its Stefa® RWDRs. These types are available as catalog products, but can be quickly and reliably developed into specific customer solutions from a modular system. To support customers in the early stages of product development, Trelleborg maintains close development partnerships that enable suitable seals to be designed in the virtual CAD-CAM phase and computer-simulated for size and performance.

For example, the type TRP identifies a Stefa® RWDR whose outer surface is completely coated with elastomer. This seal is designed for pressures up to 0.5 MPa, circumferential speeds up to 10 m/s and temperatures from -40 °C to +200 °C. An additional dust lip protects the main sealing lip from dust and other dirt particles, making this type particularly suitable for hydraulic pumps and motors operating under pressure in dirty environments such as agriculture or construction. To ensure a long service life, suitable grease should be used between the two lips. The TRP type provides a reliable static seal and effectively compensates for thermal expansion, showing little wear in low pressure operation.

In future, the standard range of Stefa® RWDRs will include all options in accordance with ISO 6149/1, including metal OD versions with a reinforced housing. In addition to the standard TRA and TRE types, which are manufactured with a corrugated elastomer outer sheath, customized designs are also possible. For example, customers can request a smooth or semi-rubberized outer casing as well as additional sealing lips. The spring force can also be specifically optimized for certain applications.