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FEATURE BEARINGS, SEALS & GASKETS I


f you want to have the option of re-using a bearing after


inspection, or of examining it closely to help diagnose faults in your machinery, you need to remove it intact. Dismounting it gently will avoid damaging the shaft, housing and other associated components. After all, damage could reduce the whole system’s efficiency, shorten the replacement bearing’s service life and even lead to catastrophic failure. Good preparation for dismounting is therefore key. Clean the bearing unit, shaft and surrounding area thoroughly. Have a container ready to collect any lubricant, and a suitable solvent to clean up again before mounting the new or re-used bearing. Repair any damage on the shaft which


Dismount with care Although


largely constructed from robust steel, a bearing is a


precision-engineered component


might prevent the bearing from sliding out smoothly; and mark the bearing’s position in relation to the shaft, or within the housing. When re-mounting the same bearing, you can rotate it by 120˚ to 180˚ to place a different part of its raceway in the load zone. During dismounting, ensure that the shaft is well supported.


which can be damaged by inappropriate handling. You shouldn’t use a hammer to mount a bearing, but neither should you treat it disrespectfully when the time comes for dismounting, as Phil Burge, marketing and communications manager at SKF, explains


CHOOSE THE RIGHT TOOLS Roller bearings come in a variety of sizes, designs and types, with different arrangements such as cylindrical or tapered shaft seats, adapters or withdrawal sleeves. To meet their varying needs, SKF recommends the four following approaches to dismounting: manual; hydraulically assisted; oil injection; and heat. Manual methods are fine for small bearings with bore diameters up to about 80mm. These can be dismounted using a mechanical puller – its claws should engage with the inner ring or an adjacent structure. If necessary, you may withdraw the bearing via the outer ring, but in that case you should discard rather than re-use it. If the bore diameter is above 200mm, or if the bearing is mounted with


an interference fit, hydraulically assisted pullers can be used (see image below). Devices suitable for outside bearing diameters up to 500mm are available.


With the oil injection method, oil is injected under high pressure to separate the mating surfaces of the shaft seat and inner ring bore. Even the tight interference fit on a tapered seating is easily overcome. This technique is only possible in bearing assemblies designed with an oil supply duct and distribution groove. For some highly specialised cases,


such as cylindrical roller bearings in extremely harsh operating conditions, where frequent replacement is needed, heat-assisted dismounting is the only practical choice. Heating rings or induction heaters expand the ring


bores for easier removal. When applying hydraulic assistance, oil injection or heat to reduce resistance to


dismounting, beware of a sudden release of the bearing. A locking nut at the end of the shaft can be


used to limit its travel and avoid injury. Along with the bearings, components such as housings and seals


should be removed with equal care. Once removed, the bearing and housing


should be thoroughly washed with a suitable solvent, carefully dried and closely examined. If they are re- usable, protect their surfaces from corrosion with a coating of grease, oil or anti-corrosive fluid before re-mounting.


SKF www.skf.co.uk SELECTING THE RIGHT BEARING FOR OFFSHORE ENVIRONMENTS


Heat, contamination and unpredictable weather constantly threaten to disrupt the performance of offshore oil and gas facilities. But while bearings might seem a small thing considering the scale of many offshore rigs, failure of even the smallest bearing can bring operations to a halt. So, the first step to reducing maintenance


requirements is to ensure the right equipment is selected in the first place. Generally, the large-scale machinery found on offshore rigs are specifically engineered for such environments. Smaller components like bearings, however, are often purchased from suppliers that may not specialise in marine engineering, or understand how these environments would interact with certain components or materials. For marine environments, stainless steel


bearings are often selected, but which grade? 440 grade stainless steel, for instance, is known for its resistance to damp environments. Bearings made of this material are regularly used in environments subject to washdown and exposed to water. Despite this, 440 stainless steel has very poor resistance to salt water. In salt water or salt spray environments, 316


stainless steel bearings are a better option. In fact, these are commonly recognised as marine grade bearings. Despite this, they should only be used in marine applications above the water line, or in flowing, oxygenated water. Full ceramic bearings made from zirconia or silicon nitride can provide even higher levels of corrosion resistance and are often used fully submerged. When selecting bearings for such an


environment, designers should also enquire about other technical capabilities, such as the load rating and tolerances. Environmental responsibility, such as


adherence with the RoHS directive, should also be considered for electrical and electronic equipment at these sites. When checking this, it is worth investigating whether the bearings used comply with other standards, such as


30 DECEMBER/JANUARY 2019 | DESIGN SOLUTIONS


ISO9001 and ISO14001. It is also important to find out about the predicted lifespan of the bearing. In addition, are the bearing materials and lubrication suitable for a marine environment? Can the bearing run unlubricated with an acceptable lifespan as in the case of full-ceramic bearings? These are important considerations when specifying bearings that will run continuously with little or no maintenance. That said, the lubrication requirements will depend on the specific application of the bearing, its load and the conditions in which it will operate. Some bearing lubricants, for example, are not very water resistant and may eventually be washed out in a wet environment. Other lubricant specifications, such as viscosity and speed rating, can also have a knock-on effect on the lifespan of a bearing. However, requirements for this can only be determined according to the exact application in which a bearing will be used.


SMB Bearings www.smbbearings.com


Author: Chris Johnson, managing director, SMB Bearings


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