The challenges posed by large seals
27 June 2011
Phil Burge takes a look at some of the advances in large format bearing seals technology for heavy industrial applications
Large diameter seals, typically from 200mm to several metres in diameter, are needed for a diverse range of applications ranging from heavy manufacturing to power generation. In all cases, the seal materials must function under exceptionally high levels of stress (due to elevated temperatures, pressures and speeds – possibly poor lubrication), and for long periods of time without degradation; they must also be easy to fit and replace.
Clearly, it is important to select large diameter seals that are ‘up to the job’ in these applications, as the greater demands placed on conventional rubber or thermoplastic seals will cause them to fail prematurely, leading to sudden and potentially expensive equipment downtime.
Research conducted by SKF while developing its bearing Life Theory (adopted in ISO 281) indicated that up to 14% of all premature bearing failures can be directly attributed to contaminants entering bearings, either via poorly designed or specified seals, or because the materials used were unsuitable for the application.
The role of engineering plastics
To address these issues, leading manufacturers have been working to develop a new generation of bearing seals up to 4,000mm in diameter, that use the latest engineering plastics, such as PTFE (polytetrafluoroethylene) and HNBR (hydrogenated acrylonitrile butadiene rubber). Indeed, these materials offer outstanding characteristics that make them far more capable of withstanding the demands of modern industry than traditional materials.
PTFE, for example, is a thermoplastic polymer, with a chemical resistance which is far superior to that of any other material used in seal technology. The operating temperature of PTFE extends from -70°C to 200°C and in some applications can reach 260°C. Furthermore, the polymer has a smooth, dirt resistant surface and an extremely low coefficient of friction, with the ability to tolerate dry running and also provide insulation against the passage of electric current.
HNBR, on the other hand, is particularly suitable for applications where aliphatic hydrocarbons, such as propane or butane and mineral oils and greases, or sulphonated crude oils are present. It is also ideal for use with many diluted acids, bases, salt solutions and glycol water mixtures, even at high temperatures.
The new generation of sealing products is already proving its potential by reducing operating costs, through extended service life and lower lubricant consumption, increasing uptime in a range of heavy industrial applications.
Bearing seals put to the test
Large diameter seals from SKF are currently being used to protect bearings in China’s biggest vertical grinding mill, which was designed at the Tianjin Cement Development Centre. These seals are already helping to improve the lifetime and availability of bearings supporting the machine’s grinding rollers. With its 5.6m grinding table, the plant has considerably greater handling capacity than conventional models, while also delivering higher efficiency and energy savings. Realising that conventional seals would probably have a short lifecycle in this application, the senior design engineer worked closely with SKF’s team to develop an alternative approach.
The latter proposed the use of large diameter seals with PTFE wiper excluders. These excluders act as a first line of defence against the ingress of abrasive dust and particles. The low friction coefficient of this material also ensures that there is no temperature rise or premature material wear, despite the fact that it is in direct contact with the shaft. The chemical resistance of PTFE, moreover, provides additional benefits for applications that also require a ‘first defence’ exclusion of potentially corrosive substances from the bearing.
Phil Burge is with SKF
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