Mounting a campaign to cut bearing failure
Published: 03 March, 2016
Understanding the fundamentals of bearing mounting – and dismounting – and putting them into practice can have a massive effect in reducing failures. PWE spoke with Phil Burge, country communication manager at SKF.
An estimated 16% of all bearing failures – that’s almost one in every six – is caused by a very fundamental problem: incorrect mounting of the bearing on the shaft or housing. In statistical terms, it’s the same as having two entire months of machine downtime each year.
If such a basic error can cause so many problems, it is well worth taking a look at some of the fundamentals behind correct mounting – including the methods and tools that make it easier, and the consequences of bad practice.
Phil Burge, country communication manager at SKF, told PWE, that after studying any relevant drawings and instructions, there are several fundamental principles to consider when mounting a bearing: the correct order in which to assemble components; the correct bearing type, size and variant to use; the best lubricant for the application – along with the appropriate quantity; and, the most appropriate mounting method and tooling to use.
Burge explains that the tools and methods used usually depend on bearing size, and this splits into three categories: small (bore diameters below 80mm); medium-sized (diameters up to 200mm); and large (above 200mm).
He adds, that you must keep in mind that rolling bearings are precision components, and should be handled as such when mounting. Normally, the preservative applied to new bearings does not need to be completely removed. It is only necessary to wipe off the outside diameter and bore surfaces. However, it will need to be washed and dried if it is to be grease lubricated, or used at very high or low temperature. It goes without saying that – before mounting – bearings should be stored under optimum conditions, and kept in their packet until immediately before mounting.
Burge highlights that if possible, they should be mounted in a dry, dust-free area, away from machines that produce dust and swarf. If this proves impossible – such as for large bearings – the mounting position (and the bearing itself) can be protected from dust, dirt and moisture by plastic or foil.
Bearings should be handled with gloves – in part for cleanliness, but also for safety (such as protection against heat and oil).
If large bearings are to be moved, explains Burge, they should not be suspended at a single point as this could permanently deform the rings. Instead, use lifting tackle that supports the bearings from the bottom. A spring between the hook and lifting tackle can help to position the bearing onto the shaft. Threaded holes in the ring side faces can be requested, to accommodate eye bolts – but these should only be subjected to load in the direction of the shank axis.
When mounting a large, solid housing over a bearing that is already in position on a shaft, it is best to use three-point suspension for the housing – in which one of the slings is adjustable.
Fitted for purpose
Burge continues by highlighting that forcing a bearing into place is the most common reason for failure, and can have a permanent effect on system performance. Minor raceway damage can easily escalate into a major problem. In the best case scenario, he explains, the bearing and associated components can simply be replaced. However, the damage can have serious knock-on effects: the system runs less efficiently, so is more costly; or, it undergoes catastrophic failure that leads to lost production time. The trick is to mount the bearings without the use of ‘brute force’, using a variety of methods and tools.
Most bearings are fitted to their shaft or housing with one component having an interference fit. All mounting methods rely on obtaining this interference without excess effort, and with no risk of damaging the bearing. This can be done through mechanical, thermal and hydraulic means.
Burge comments that bearings with a cylindrical bore can be cold-mounted using a fitting tool like the SKF Bearing Fitting Tool Kit TMFT 36. The kit comprises impact rings, impact sleeves and a ‘dead blow’ hammer. It can also be used to mount other components such as bushings, seals and pulleys.
Bearings with a tapered bore can be fitted in the same way. However, Burge says that the SKF Drive-up Method is recommended for medium-sized and large bearings. It is based on a two-stage mounting procedure using a hydraulic nut fitted with a dial indicator. In stage one, a predetermined pressure is applied in the hydraulic nut, to give a reliable start position. Then, in stage two, pressure is increased in the nut to push the bearing ring further onto its tapered seat.
It controls the axial drive-up of the bearing (such as a spherical or toroidal roller bearing) from a predetermined position. The technique helps to reduce the use of feeler gauges and greatly reduces mounting time.
The SKF Oil Injection Method, explains Burge, allows bearings with an interference fit to be mounted and dismounted in a safe, controllable way. It does not require keyways to be machined on the shaft, which saves time and money.
A thin film of oil is injected at high pressure between the mating surfaces – virtually eliminating friction – to allow easy mounting and dismounting. As well as being appropriate for tapered shafts, this method can also be used for adapter sleeves and withdrawal sleeves, and for components such as couplings and gear wheels.
Both of these techniques can be applied with the help of specialist programmes from SKF, which further simplifies the task of mounting bearings correctly.
Large bearings (above 340mm diameter) can be mounted – without measuring radial internal clearance or drive-up distance – using a technique called SensorMount. A sensor, embedded in the bearing inner ring, indicates inner ring expansion, which can be read by a dedicated hand-held indicator. This shows the relationship between clearance reduction and bearing bore diameter.
Bearings can also be hot-mounted – by creating a temperature difference between the bearing ring and the shaft or housing. Traditional techniques such as oil baths are no longer recommended due to safety and environmental reasons. More modern methods include aluminium heating rings, electric hot plates (for small bearings), heating cabinets and infrared radiators.
SKF recommends using induction heaters, which quickly apply heat evenly to the bearings. These are safe because the heater and yoke never get hot. Induction heaters cause bearings to magnetise, so are equipped with a device that automatically demagnetises the bearing before it is installed.
Taking it off
While the main focus for technicians will be on mounting bearings, the reverse operation – dismounting – is no less important.
When dismounting, there is always the chance of damaging a good bearing. For this reason, Burge says an undamaged bearing should never be dismounted unless absolutely necessary. If it must be dismounted – and is intended to be re-used later – there are some golden rules to follow: never hit the bearing rings (or any other part) directly; do not allow the bearing force to be transmitted through the rolling elements; and do not heat the bearing with an open flame.
Burge adds that once a bearing has been dismounted, it should be cleaned with a suitable solvent then dried carefully. All bearing parts, especially raceways, rolling elements and cage, should be inspected for wear and damage. Small sealed bearings – or those that are very dirty – are probably not worth cleaning, should be scrapped and replaced. He explains that bearings can generally be dismounted using the same techniques as for mounting, using reverse procedures. Typical tools used for this include mechanical and hydraulically assisted pullers. Dismounting can also be achieved using oil injection – which reduces the force needed to remove the bearing – or using heating rings, which slightly expands the inner ring.
Bearing problems are generally identified through five main ‘symptoms’: excessive heat, noise, vibration, shaft movement or friction. Each symptom could be caused by a number of underlying problems: a bearing might be overheating, for example, because the seals are oriented in the wrong direction, or there is insufficient bearing clearance – both of which are caused by an error in mounting.
Burge concludes that the effects of poor mounting can be catastrophic, so it makes sense to revisit this most fundamental of skills – and try to shrink that 16% failure rate to zero.
For further information please visit: www.skf.co.uk