As a manufacturer of dynamic sealings for reciprocating and rotating applications, STASSKOL is always confronted with customer requirements that place a wide variety of demands on the respective sealing system.
Sealing elements for rotating shafts (so-called shaft seals) are subject to special stresses in agitators and conveyors. If solid particles or highly viscous media are processed, the sealing surface can be damaged - the sealing becomes contaminated with product particles and may lose its function. In contrast to gaseous media, liquid components or solids can also separate the sealing partners from each other and thus cause a large leakage path. In addition, an inhomogeneous distribution of the material to be mixed or conveyed in the process space also leads to localized radial loads on the shaft, which can lead to misalignment or uneven running of the shaft in the sealing area.
The SDF Series - STASSKOL DynamicSeal Faceseal - was developed to meet these special requirements (Figure 1). Two sealing rings are pressed axially against opposite housing surfaces by means of a barrier medium. An elastomer cuff ensures on the one hand the static sealing between the sealing ring and the shaft. On the other hand it is acting as anti-rotation device; thus, the rings are rotating with the shaft without slipping. At the same time, the sealing rings are pressed against the inner surfaces of the housing by the elastomer cuff – supported by sealing air – so that a high degree of sealing efficiency between the sealing ring and the housing is guaranteed. A special feature is the torque transmission between the silicone and the sealing ring via a Reuleaux triangle, which ensures a good form fit and thus, in addition to torque transmission, also good sealing.
SDF Series sealings are axially split, allowing for easy assembly around the shaft with the machine's bearings installed. In addition to the good mechanical connection and the high sealing effect, the puzzle joint in the sealing elements also ensures simple and tool-free assembly.
STASSKOL has always relied on its own materials to achieve good tribological performance, i.e. low heat generation and low wear. Based on the know-how in the development of high-performance polymers built up over many decades, new recipes were developed for the SDF Series and tested on the in-house tribometer. The focus was on minimizing friction and wear to maximize the service life of the SDF Series. In addition to the high wear resistance, special attention was paid to properties such as E-modulus, tensile strength and elongation at break to avoid damage to the sealing rings under heavy mechanical loads as far as possible. Product contamination must be avoided, especially in the food sector. To achieve this reliably, the sealing ring of the SDF Series can also be made of a special material that enables particles to be detected efficiently. The development of this recipe is based on experiments in which tiny samples were introduced into chocolate and identified using X-ray detection. Even the smallest samples could be reliably detected in the product (see Figure 2).
In this way, the customer can be offered a sealing system which, in addition to an efficient sealing effect, combines maximum wear resistance with high mechanical stability, and whose detectability of the central sealing material guarantees a high level of process reliability, especially in the food sector.
To confirm this in practice, the most promising material combinations were tested in the form of prototypes of the SDF Series on a shaft sealing test rig under process-related operating conditions. With variations in pressure and speed, wear and leakage rates as well as the temperature at the sealing surface were recorded (Figure 3). The test results showed that the material SK933 is the ideal solution for the sealing rings, as it offers the best tribological and thermal performance with high strength.
To be able to make a final assessment of the performance of the sealing, a field test was carried out on a twin-shaft mixer. In the industrial plant, 3.5 to 5 tons of grain flour are mixed with minerals, stones up to 4 mm in particle size and oil in batches to produce poultry feed. The mixing process takes about 4 to 5 minutes each batch and is carried out 24 hours a day, 5 days a week. The two shafts each have a diameter of 200 mm and rotate at 40 revolutions per minute. The previous sealing solution, a carbon ring seal, is no longer a suitable solution for the customer because product leaks of up to 25 kg per sealing and day occurred and there was a high consumption of sealing air.
With the aim of conserving resources through lower energy consumption for the sealing air and reduced feed loss through product leakage, the twin-shaft mixer was equipped with the SDF300. Improvements in the standard process conditions became apparent immediately after the sealings were replaced. Sealing gas pressure and sealing gas consumption could be reduced and product leakage was no longer detected (Figure 4 right). The run-in behavior of the seal has also been improved by changing the system to the SDF300. The optimal tribological properties in the form of low wear and tear and low friction guarantee low temperatures in the contact zone between the sealing partners of the SDF300, even during the first hours of operation. This avoids sticking as well as possible thermal damage to the product.
The new sealing concept SDF Series from STASSKOL combines high sealing efficiency and a long service life with easy handling, little cleaning effort and the possibility of mounting on an installed shaft without dismantling the system. The design of the SDF Series is kept simple to minimize costs while accommodating even the most difficult tasks such as efficiently sealing of high eccentric shafts.
The optimized materials are designed for a long service life, and special formulations make it possible to detect any contamination, especially in the food sector - a property that was not previously available for shaft sealings.
Practical use by customers shows that the universal and versatile SDF Series in the form of the SDF300 masters the challenges and quickly pays for itself by reducing leakage and energy consumption. This saves resources and increases the reliability and availability of industrial plants.