In industrial fields, hydraulic cylinders are perhaps the most effective and efficient method of pushing, pulling, lifting, and lowering almost anything. Sealing applications in hydraulic cylinders can be divided into three categories: light-, medium-, or heavy-duty. What differentiates them is the environment in which they operate. System pressure in more rigorous applications can reach around 400 bar, as opposed to the 160 bar threshold which applies light-duty application (lifters, for example). On top of that, there are variances in operating temperatures, heavy loads, and the level of external contamination local to certain environments. Selecting the right seal, one that is proven to perform in its respective setting, is therefore key to machine reliability.
Some typical industrial applications for hydraulic cylinders include construction vehicles, off-highway equipment, and agricultural machinery.
The value in testing
For SKF, the selection of high-performance materials combined with multi-faceted testing and failure analysis of seal designs are vital elements for hydraulic seal development. Large amounts of data gathered via sensors on in-house test rigs is used to evaluate seal material and design capabilities under different operating conditions.
Most of SKF’s hydraulic seal testing takes place in Judenburg, Austria. Here both, individual components and complete sealing systems are tested according to customer-specific requirements. Using SKF SEALJET Systems, which is also available for SKF distributors, seal prototypes can be created in-house in almost no time at all, which offers an extreme flexibility for internal test rig investigations.
Of course for customers, testing helps to guarantee a better chance that the seal will provide a suitable solution for their machinery and minimize the risk of having to redevelop a new design.
SKF Sealing Solutions in Judenburg (left), Austria is home to SKF’s hydraulic test rig (right)
Single product testing vs. seal package testing
It’s important to note that test rigs can accommodate single product testing and complete seal package testing. After all, sealing a hydraulic cylinder is a team effort, requiring all individual parts to interact as expected in unison. Sealing packages need to be carefully selected with the application and customer’s expectation in mind. If they are not, it could easily lead to early breakdown of the hydraulic cylinder.
Take a wiper seal, for example – the first barrier of defense in a hydraulic sealing system. It is well known that not only a wiper but the whole sealing system may have an influence on the dirt ingress behavior. Similarly, buffer seals perform their own unique duties. But testing them alone would not provide an accurate depiction of what might occur when installed in the system.
Complete rod seal packages include piston, rod, buffer, and wiper seals, as well as guide rings and O-rings.
Selecting the appropriate testing procedure
SKF carries out different testing procedures based on the exact needs of the given customer or to further develop seal designs or materials and offer ever-improving solutions. Design- and material evaluations are carried out as followed:
- Design evaluation
Two main attributes are examined during this special testing procedure: leakage and back-pumping ability of hydraulic seals. SKF testing team verifies if seals in a hydraulic system are interacting correctly with the moveable rod, according to their given operating parameters. Different to other procedures, during the design evaluation the seals are not stressed until they are destroyed and no longer useful. Rather, it’s important to verify the back-pumping behavior of seals when the lubrication film is increasing.
As the cylinder rod moves linearly back and forth, it is ensured that the velocity of the outstroke (left to right movement on the image above) is much greater than the instroke movement of the cylinder rod. The picture below shows a schematic graph of the friction and leakage performance of U-Cup seals as a function of the outstroke to instroke velocity ratio.
Influence of instroke and outstroke piston rod velocity on friction and leakage of U-Cups (schematic picture)
The higher the outstroke to instroke velocity ratio is, the higher is the fluid film thickness transported to the outside of the hydraulic cylinder. This can be seen as external leakage when the seal design, which is responsible for the back-pumping behavior, is not working properly. With this testing procedure it is possible to verify the leakage performance of e.g. U-Cup seals in a very short time frame without damaging the seals.
- Material evaluation
Equally important is the chosen sealing material. To help ensure that SKF and its network of distributors leverage the most appropriate materials, they are developed analyzed in great detail by material experts and product developers. In hydraulic seals, polyurethanes and PTFE are normally the chosen material.
During specific test procedures, seals undergo a challenging temperature cycle and are stressed with varying pressures and varying stroke speeds. And again, having several reference points in the form of competitor products can help provide a benchmark. While some of the material damages (e.g. wear and compression set, etc.) may be evident be simply observing the shape of the seal after testing, microscopic pictures can reveal material behavior in finer detail.
Preload loss caused by compression set is another typical failure mode for seals. Both the seal’s outer diameter and cross section are analyzed for preload loss and the data is recorded in a graph alongside reference seals. Generally, the lower the preload loss after testing, the better the suitability of the sealing material chosen for the application. Fortunately, and as a result of continuous testing and innovation, SKF sealing materials are known to outperform commercial and premium available materials on the market.
Summary and conclusion
As we have already mentioned, no internal seal test can replace a field test in the real application. However, evaluating single seals and their materials, as well as the complete seal package, can support to eliminate uncertainties and the likelihood of surprises.
These methods help ensure SKF customers can solve their sealing challenges. Our teams of experts are confident that – regardless of industry of application – seals that undergo rigorous in-house bench tests can improve machinery performance, reduce downtime, and even create a more sustainable operation by increasing efficiency.
Find out more about seal and bearing testing at skf.com or contact your local SKF subject matter expert.