Before we go into more details regarding leakage mechanisms, I would like to discuss the difference between the term leakage and the presence of an adequate lubrication film at the sealing lip surface. From a pure technical point of view, “absolute tightness” of a sealing system is neither possible nor desirable, because a thin lubrication film is necessary to lubricate the sealing lip. This is a prerequisite to reduce friction and wear, leading to a long seal lifetime.
A properly selected seal design ensures a hydrodynamic balance, conveying the lubricant under the sealing lip (so-called “back-pumping effects”) instead of creating leakage in one direction. Simply put, we are talking about a leakage, when we start to visibly recognize drops of the media to be sealed outside the sealing system. In the end, the level of “technical tightness” including all application related aspects as well as economic and environmental considerations are to be agreed between the end user and the seal supplier.
The first port of call to prevent seals from leaking actually comes before you install them. After doublechecking the correct seal design, size and material respectively designation, there’s a chance that the quality of your seals has been compromised, if the packaging isn’t intact. Never install a seal if the packing is damaged – it could not be fit for operation and, as a result, prematurely leak. When storing your own seals, make sure your warehouse provides a suitable environment. If they experience significant temperature fluctuations, humidity, UV irradiation or dirt and dust are present, your seals could deteriorate before they begin operation in your application – increasing the risk of leakage. It is also important to ensure your seals are installed correctly, which you can read more about here.
Are the materials you are using compatible?
One of the next critical factors to check to prevent seals from leaking is the operating conditions of your machinery. These should be compatible with the seals you are using – which may sound obvious, but I’ve often come across this issue. For example, certain materials such as PTFE compounds can handle wide temperature ranges. However, they cannot usually withstand dynamic runout (DRO) or shaft to bore misalignment (STBM) as effectively due to a lower level of flexibility compared with rubber materials or thermoplastic polyurethane elastomers. If you are using materials that don’t suit your machinery, your seals are likely to become damaged and potentially leak.
I would advise consulting manuals for both your applications and your seals. Speaking to an expert can also help, as they should have in-depth knowledge on the topic and can give you a quick answer on what is suitable. There is also plenty of informative material online, such as seal selection tables – both for general industrial applications and heavy industrial applications.
Approximately 40% of long-term gearbox performances failures can be traced back to poor interaction between the application’s seal and lubricant.
Source: Machine Design
And it’s not just compatibility between materials and operating conditions that can cause your seals to leak. It is crucial to properly select the sealing material in accordance to the media to be sealed. Furthermore, the compatibility to possibly used mounting lubricants, cleaning agents, and other media present around the seal at any stage needs to be considered. An example of this would be using any petroleum based lubricant for seal installation with a seal made of propylene rubber (EPDM or EPR), as this combination will cause the seal to swell and deteriorate in a short time.
Additionally, while the content of a lubricant may not damage a seal by itself, changes to temperature, for example, can have a negative effect. The temperature of grease rising in a heavy industrial application and altering the seal shape is an instance of this. The modern market offers what seems like an infinite number of lubricants – many of which are tailored to specific applications. If you can find a lubricant that was made for your application, I would definitely recommend using it – but make sure that the sealing material as well as the seal design are compatible with the application demand. By doing so, you basically eliminate the issue of accidentally using incompatible materials and lubricants.
Sometimes, it’s time for a change
So, what do you do if everything you’re using is compatible, but your seals are start leaking over time? In my experience this is normal in many situations. Like any other component in industrial machinery, seals only have a limited service life. It could be that you actually need to replace your seals because they are approaching the end of their lifecycle. Due to the complex tribological situation in a sealing system, it is difficult to accurately predict how long it will take for a seal to deteriorate. Planning maintenance in advance to avoid unexpected downtime is the best course of action. You can turn to an expert to rely on application expertise and get individual advice. Some companies even offer maintenance packages where they will take care of all these tasks so that you don’t need to act yourself.
Author: Heimo Rehschützecker