March 2019

60 n BEARINGS, BELTS AND CHAIN March 2019 www.drivesncontrols.com Correct bearing dismounting boosts machine performance D espite being made of steel, bearings are not immortal. While many will outlast the machinery in which they are fitted, others will need to be replaced. The initial mounting of bearings has a huge effect on machine performance, so is closely controlled. However, dismounting damaged bearings should be carried out just as precisely. Dismounting bearings incorrectly – or using the wrong tools – can harm vital machine components such as the shaft and housing. This may, in turn, damage the newly installed bearing – and have a detrimental effect on machine performance and longevity. Maintenance engineers, who must work fast to replace failed bearings, sometimes carry out this procedure too forcefully, using incorrect tools – such as hammers. However, there is no reason to do this, because there are many specialist tools (including hook spanners, axial lock nut sockets and impact spanners) and techniques to ensure correct dismounting. It is true that some bearings can be tricky to remove, but this makes it all the more important to use the correct mechanical, hydraulic or heat application method. SKF recommends four methods for dismounting bearings: manual; hydraulically assisted; oil injection; and heat. Some can be achieved using basic equipment – such as wrenches and hexagonal keys – while others can only be achieved using specialist tools such as pullers. Bearing size is the main factor that determines which method to use. Smaller bearings – those with bore diameters of around 80mm – can be handled with purely mechanical tools. Larger bearings, with bore diameters of 200mm or more, require more force, so should be removed using hydraulically-assisted or oil-injection methods, which apply much larger forces. For mechanical dismounting, it is vital to choose the correct puller. The puller type – such as a jaw or strong back puller – and its maximum withdrawal capacity will ensure that dismounting is achieved safely and easily. If possible, you should apply the withdrawal force to the ring with the interference fit. Many roller bearings – whether they are fitted on a cylindrical or tapered-shaft seat, adapter or withdrawal sleeve, or other arrangements – can be dismounted manually, or using the other techniques. For a bearing mounted on a cylindrical shaft seat, for instance, the shaft should be supported in a bench vice to avoid damage to the shaft or bearing. Small bearings can be separated from the shaft with a mechanical puller. Here, claws are applied to the inner ring or an adjacent component. If this is not possible, the bearing can be withdrawn via the outer ring – but in this case, the bearing should not be re-used. Bearings that are mounted with an interference fit need a larger dismounting force – which increases with bearing size. For this, SKF recommends a hydraulically-assisted puller – especially for large bearings. Pullers are usually available for bearings with outside diameters up to 500mm. These bearing arrangements can also be dismounted by injecting oil at high pressure between the shaft seat and inner ring bore until an oil film separates the surfaces completely. A smaller dismounting force is then needed if the bearing is removed quickly. Oil injection can only be used if an oil supply duct and distribution groove have been designed into the bearing. Another way of separating the surfaces is to use heat. Heating rings or induction heaters can be used in certain circumstances – such as for the inner rings of cylindrical roller bearings – but this is a specialised technique and usually only recommended as a last resort. As well as the bearings themselves, other components, such as housings and seals, also need to be removed. The way this is done can vary, but it can be achieved with kit as basic as wrenches, hexagonal keys and hook spanners. As with the bearings themselves, it is important not to use excessive force. A point to bear in mind with seals is that non-contact seals are often re- usable, so should be removed with care. However, contact seals are likely to be worn, so can be treated with less reverence. Although bearings are robust, they are highly engineered components and must be treated with care. Maintenance staff will find that if they dismount damaged bearings as carefully as they mount new ones, their machinery will last longer and perform more efficiently. n Proper dismounting of damaged bearings – using proven techniques and the correct tools – is as important as their initial installation to machine lifetime and performance, argues Phil Burge, SKF’s marketing and communications manager for the UK. The way that bearings are removed can affect their performance and operating lives