FEATURE PRODUCTION & ASSEMBLY
or such an inexpensive and seemingly simple component radial shaft seals can have a huge impact on equipment reliability, downtime and maintenance costs. A failed shaft seal on a manufacturing process line can contaminate product with spilled lubricant or create hazardous slippery conditions for operators. It can leave bearings and other moving parts starved of the lubricant they need, accelerating wear and eventual breakdown and allow dirt, water and other contaminants into the internal workings of critical machinery. Even before they fail seals can cause
costly damage. When dust, debris or wear particles get trapped under the lip of a seal they can mix with lubricant present to create an effective grinding paste, rapidly wearing both the seal lip and the shaft surface. The resulting groove in the shaft creates a space through which contamination can pass and lubricant can leak, even if the seal is replaced. Fixing this usually requires the machine to be dismantled so the shaft surface can be re-finished, a lengthy process potentially requiring grinding, metal spraying and heat treatment. You can try to reposition the seal onto a new part of the shaft but often space restrictions don’t allow this. Fortunately, many of these problems can
be prevented with proper selection, installation and maintenance of seals and the equipment on which they are installed. When these things are done right, a shaft seal can have an extremely long life, often as long as the bearings and other components it is designed to protect.
INSTALLATION PROBLEMS As with so many mechanical components installation problems lie at the root of many seal failures. If a seal is not installed carefully with the right tools there is an increased risk of damage to the shaft that can cause leakage. If the wrong size is selected for the shaft it can be deformed or the lip damaged. Similarly, the radial spring that keeps the seal lip in contact with the rotating shaft may be dislodged, sometimes without being noticed. Fast, automated assembly processes can create their own problems with radial seals. Sometimes the material can be deformed or stretched by rapid insertion into the housing. As the material contracts the seal can pop out of place. High quality shaft seals sometimes have ribs moulded
16 NOVEMBER 2016 | FACTORY EQUIPMENT
A TRICK UP YOUR SLEEVE F
Shaft seal failures can cause both significant downtime and equipment damage. Darron Henn from SKF describes how they can be prevented and fixed faster if they do occur
sealing system is to choose a seal that pumps the lubricant back to the oil side. A seal’s ability to pump is inherent in its
into their outer diameter to prevent this. Once a seal is in place its reliability depends on effective lubrication.
INSUFFICIENT LUBRICANT While most seals are designed to contain lubricant inside a motor, pump or gearbox, their effective operation relies on a small amount of lubricant forming at the lip to create a film just a few nanometers thick between the seal lip and the rotating shaft. If there is insufficient lubricant available to form the film, direct seal to shaft contact will lead to an increase in friction and raising under-lip temperature, accelerating wear leading to reduced life. If that temperature exceeds the rated
working temperature for the seal or the lubricant, the chemistry will start to degrade, eventually leading to failure. Preventing this problem is a matter of ensuring that the equipment is properly lubricated and maintained. As well as lubricant levels, temperatures should be monitored. If the lubricant itself regularly approaches or exceeds the design temperature of the seal, perhaps because of problems elsewhere in the machine, it can accelerate the degradation of the seal material. Where high operating temperatures are inevitable a seal made from a different, temperature resistant material should be selected. A sealing system is made up of multiple
inputs from several components including the shaft surface finish, shaft speed, lubricant viscosity and shaft misalignment. A sealing system can potentially leak when any one of these inputs are not in control. An important aspect of designing a robust
A thin flanged sleeve made of high-grade stainless steel can simply be pushed into position on the shaft, providing a smooth running surface for the replacement seal
design and can often prevent leakage when one or more of these inputs begin to contribute to leakage. A pumping seal can make up the difference for uncontrollable inadequacies in the system and prevent an imperfect situation from becoming a real problem. Some seal designs use an external secondary to lip to protect the primary one from contamination. This can seem like a useful extra layer of protection but as with the main lip a secondary lip requires a lubricating film, reducing the risk of thermal damage. Where double lip seals are used it is recommended to grease pack between the sealing lips at the time of assembly. Removal can distort or damage a seal. It is good practice to install a new seal every time equipment is dismantled to replace bearings or conduct other routine maintenance.
SMOOTH RUNNING SURFACE FOR REPLACEMENT SEAL If a seal failure or inspection during routine disassembly reveals damage to the shaft caused by insufficient or contaminated lubricant, machine owners can now avoid costly and time-consuming re-finishing by installing an SKF Speedi- Sleeve. This system uses a thin flanged sleeve made of high-grade stainless steel that can simply be pushed into position on the shaft, providing a smooth running surface for the replacement seal. The flange can be removed after installation or left in place if the machine configuration allows. The proprietary stainless steel material provides a durable engineered surface for the new seal while its excellent ductility helps it accommodate variations in shaft diameter and roundness. Microscopic pockets within the material
retain lubricant in the sleeve, helping to prevent dry running of the seal that might otherwise create excessive wear. An increasing number of manufacturers are choosing to install SKF Speedi-Sleeve on shafts as original equipment. Use of this sleeve system can cut manufacturing costs by reducing shaft surface finishing requirements, something that can be particularly expensive for shafts made of high performance materials.
SKF T: 0117 982 5729
www.skf.com/uk
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