Rolling element bearings are a key constituent of virtually every rotating machine and consequently represent a significant cost for manufacturers – in the UK alone, the bearings market is worth £1bn per year. Extracting the maximum working life from each component is therefore essential to reduce costs and improve efficiencies.

 

When smooth rolling surfaces are damaged, through cracks, wear, lubricant deficiency, corrosion or plastic deformation amongst others, higher stress conditions are imposed upon the surface, which reduces the life of the bearing significantly. 

When roller bearings are used correctly, they will survive their predicted fatigue life. However, bearings often fail prematurely due to avoidable mistakes, such as improper mounting, mishandling, and poor lubrication, as well as entry of foreign matter or abnormal heat generation. A bearing in the early stages of failure may display some or all of the following symptoms:

• Leakage of lubricant, or visible contamination of lubricant
• Component vibration
• Irregular or unusual noises
• Increased temperature above norm when running
• Misalignment
• Unusual odours

These factors can easily be identified through manual inspection but some causes, particularly vibration, may be detected with condition monitoring devices. Some changes may be very slight and occur slowly over time, meaning it is important to keep a detailed inspection record so that patterns of gradual change can be identified. 

When damage is initially suspected, further investigation will be required to determine the exact nature, and importantly the cause, of deterioration. As most types of damage can be prevented by correct mounting, lubrication and sealing, it is vital that a damaged bearing is not simply replaced without uncovering the underlying reasons, as if these are not rectified, the cycle of damage will most likely continue. 

Bearing damage is often attributed simply to ‘wear’. Wear is surface deterioration caused by sliding friction between raceway and rolling element surfaces. In most cases there is an adequate oil film between these surfaces which will prevent damage, however this can change dramatically in the presence of contaminating materials. These typically enter a bearing through a faulty seal or contaminated lubricant, and can cause misalignment in the bearing, especially in tapered bearings where some surfaces will be subject to greater abrasion than others.

Fully cleaning the bearing before installation and effective lubrication management and filtration can prevent this type of wear. Slightly harder foreign particles such as larger pieces of metal offcuts, dirt or grit tend to cause pitting. These contaminants are usually caused by wear elsewhere in the machinery and, when carried through the machine in lubricating fluid, result in damage throughout the system. Again, replacing the bearing will not resolve the issue as particles will continue to travel through the system and possibly promote corrosion – of which there are several types. 

Rust corrosion is most common on ring surfaces or rolling elements, and can be caused by high temperatures and high humidity when stationary, as well as the presence of water droplets. Water in a system can often be attributed to incorrect storage of lubricants or faulty seals meaning storage conditions must be monitored. If machinery stands idle for a significant length of time, it is more susceptible to rust, especially in humid factory conditions. As a preventative measure anti-rust treatment can be run through the system. 

Corrosion can also be caused by improper handling so bearings should be kept in protective packaging until just before use and, where possible, lint-free gloves should be worn when handling bearings. Electrical corrosion, meanwhile, is caused by electric currents passing through the thin film of lubricating fluid found at the race and rolling elements of a bearing. The resulting burning can cause groove-like corrugations in the bearing surface, and can be prevented by fully insulating the bearing from surrounding instruments to protect against any electrical difference between the inner and outer rings, or electrical potential difference of frequency. 

There are several other types of damage which can be caused by inadequate lubrication, and these tend to manifest themselves through various stages of deterioration, meaning early identification could prevent a complete failure. The first stage is often discolouration due to a lubricant reaction at high temperatures (often caused by metal-to-metal contact).

This is followed by peeling or flaking as lubrication levels drop further and temperatures rise, causing small fragments of the raceway surface or rolling elements to become detached. If left untreated, cracks can develop, leading to irregular operating. If the issue of under-lubrication is allowed to continue, the most serious stages of damage – excessive roller end heat and total bearing failure – are liable to occur. 

Scoring and smearing occur when the lubricating film breaks down due to sliding rather than rolling, causing small seizures on the raceway surface. The first sign of a surface seizure is again discolouration of the bearing, followed by the softening and melting of element of the bearing, meaning early identification is essential. Similarly, fretting results from repeated sliding between two surfaces and is characterised by reddish-brown or black particles. Suitable lubrication will generally prevent fretting, but as it can also be caused by vibration or insufficient interference, engineers can also apply a preload and check the interference fit in order to prevent fretting.

False brinelling is another type of fretting which takes the form of hollow spots which resemble brinell dents. This is due to wear caused by vibration and swaying at the contact points between the rolling elements and raceway, and often occurs when the bearing is stationary and subject to vibration. False brinelling can be avoided by transporting bearings with the housing secured and the inner and outer rings packed separately, as well as preloading to avoid vibration and using the correct lubrication.  

A number of types of damage can be put down to misalignment or incorrect mounting. Straight-line scratches on the surface of raceways or rolling elements can be caused by shock during mounting or dismounting of the bearing. This can be avoided with the use of a press machine, using an appropriate jig or tool, and centring the relative mating parts during mounting.

Another type of mounting damage, fracturing, is caused by small pieces of metal breaking off due to excessive load or shock load which then erode the roller corner or rib of a raceway ring. This can again be prevented by installing the bearing using the correct tools and tension. Mounting flaws can also cause cage damage, which includes deformation, fracture and wear to the cage, cage pillar, side face, pocket surface or guide surface. These problems can also be caused by large moment load, sudden acceleration/deceleration, excessive rotation speed, poor lubrication or rises in temperature. Shock and excess vibration are a major factor in cage damage, and is itself frequently caused by incorrect mounting. 

Correct mounting, with the use of an appropriate shaft tolerance and geometry, can also regulate cracking – a type of bearing damage which can have severe consequences for other equipment in the system. Here, excessive heat generation due to sliding between the mating part and the face of the outer ring or excessive interference or load, can cause dangerous cracks within the bearing. As well as checking the bearing is mounted correctly, rectifying any mistakes in the shaft taper angle or cylindricality of the bearing can prevent cracking. 

Creep is the name given to a slippage at the fitting surfaces, which creates a clearance and leads to scoring and wear damage on both the inner and outer ring. Typically characterised by a shiny surface appearance, creep is usually caused by insufficient sleeve tightening or interference, or a loose fit, and can be prevented by tightening the raceway ring side face. Applying either an adhesive or a film of lubricant to the fitting surface can help avoid creep damage by minimising the possibility of slippage.

Most types of bearing damage are easily avoidable if installed carefully and maintained as part of a wider programme taking into account other system components. To simply replace a bearing that has failed without knowing the cause can prove to be very costly interim of both time and equipment. It is always best to examine the system as a whole and identify the root cause of the failure as this method will provide a greater payback in the long run.