Malcolm Watson and Tony Synnott discuss common causes of bearing damage and the effects that damaged bearings can have on machinery and production equipment, and provide some advice on how damage can be prevented.
Typically, when industrial bearings are handled and used correctly, they are able to last their predicted service life without any issues. However, problems begin to occur when avoidable mistakes and attention to detail are overlooked. Failure can be caused by improper mounting, mishandling, poor lubrication, entry of foreign matter or abnormal heat generation.
The inspection of bearings during periodic checks or when adjacent parts are being replaced can easily determine the condition of the bearing, and whether its continued service is advisable. A detailed inspection record should be kept, detailing dates of inspection, appearance, any maintenance actions undertaken, and most importantly, any signs of damage identified.
There are numerous tell-tale signs of bearing damage, all of which can seriously impact on service life, and ultimately on performance of the machinery containing the bearing. Many of the types of damage can be prevented by correct mounting, lubrication and sealing, and these should be at the heart of installation and maintenance regimes.
* Flaking. Flaking damage occurs when small pieces of material are split off from the smooth raceway surface or rolling elements. This can be attributed to rolling fatigue, which creates a rough, coarse texture on the raceway or ball surface.
This damage frequently results from poor or improper lubrication, or an excessive or moment load. Flaking can also be caused by the ingress of foreign debris such as hard particles or water. Further possible causes include poor mounting, or a progression from rust damage, corrosion pits, smearing and brinelling.
To counter the effects of flaking, the operator can reconfirm the bearing application, and ensure the load conditions are optimised. By using a lubricant with the correct viscosity and by improving the lubrication method along with better sealing, the initiation of flaking is minimised.
* Peeling. Dull or cloudy spots or light wear which appear on a bearing’s raceways can be signs of peeling. If peeling damage is not noticed and treated immediately, tiny cracks can develop and small particles can flake from the bearing.
Peeling damage can be caused by unsuitable lubricant, or the entry of debris into lubricant. Poor lubricant choice or application can result in rough surfaces, while surface roughness of rolling mating parts can also cause peeling.
The best way to eliminate the possibility of peeling damage is to ensure the correct lubricant is used and to optimise the sealing mechanism and the surface finish of the rolling mating parts.
* Scoring. One common cause of premature failure is scoring, which is triggered by an accumulation of small seizures caused by sliding rather than rolling, which can lead to a breakdown of the lubricating film. This can happen under excessive external load or preload or due to insufficient lubricating film or in the presence of contaminating debris that can lead to a breakdown of the lubricating film.
Scoring can be combated by improving both the lubricant method, and the lubricant itself, and also checking the size of the load and adjusting the preload.
* Smearing. Smearing is damage to the raceway surface, occurring when small seizures between bearing components occur due to oil film rupture and/or sliding. This causes the surface to roughen and can lead to localised melting.
Possible causes of smearing include the use of improper lubricant, water entry, sudden acceleration or deceleration or a combination of high speed and a light load.
Smearing can be addressed by ensuring there is sufficient preload and not too much bearing clearance, optimising the sealing mechanism and enhancing the lubrication method, ideally using a lubricant with good oil film formation ability. Fracture can also occur due to dropping or severe impact damage.
By using the correct lubricant, the effects of smearing can be nullified. It is important to understand the temperature and speed of the applications, and to use the required lubrication.
* Fracture. Another potentially severe type of damage, fracturing refers to the small pieces which break off due to excessive load or shock load acting locally on a part of the roller corner or rib of a raceway ring, usually during mounting.
Through the use of shrink fit and proper tools, fracturing can be easily avoided. The bearing rib should also be provided with sufficient back-up and support.
* Cracking. With the potential to cause severe damage to other equipment, cracking has numerous potential causes such as excessive interference or load, or shock load. Excessive heat generation due to creep or sliding between the mating part and the face of the outer ring can also result in cracking, as can poor shaft taper angle or cylindricality.
Cracking of the bearing can be avoided by checking the load conditions and correcting the interference. Correct mounting and use of an appropriate shaft tolerance and geometry will also minimise the risk of cracking.
* Cage Damage. Encompassing deformation, fracture and wear to the cage, cage pillar, side face, pocket surface or guide surface, there are numerous factors that can cause cage damage, most notably problems with mounting or handling, large moment load, shock and excess vibration, excessive rotation speed, sudden acceleration and deceleration, poor lubrication and rises in temperature.
To counter the effects of cage damage, vibrations in the equipment should be reduced. The mounting method must be checked, as should the surrounding temperature, rotation and load conditions.
* Denting. Denting can occur in one of two areas on the bearing. Denting of the bearing raceway or rolling element surface can be caused by shock damage during transport or mounting, excessive load impact, and the presence of debris such as metallic particles. If denting is at the ball or roller pitch, it is indicative of static impact damage.
To avoid the risk of denting, the housing should be thoroughly washed. Lubrication oil and grease should also be filtered while correct handling and mounting techniques, as recommended by the manufacturer, should be employed.
When the inner ring is fitted to the shaft, the push force should be through the inner ring face and never through the outer ring face. However, if the outer ring is being fitted into the housing, the push force should always be applied to the outer ring face and never through the inner ring.
* Pitting. Pitting creates a dull lustre on the rolling element surface or raceway surface on a bearing. It can be caused by poor lubrication or the presence of debris in the lubricant itself. Exposure to moisture in the atmosphere is also a frequent contributor to pitting.
To counteract the effects of pitting, the lubrication oil should be filtered thoroughly, and the sealing mechanism optimised.
* Wear. A surface deterioration due to sliding friction at the surface of the raceway, rolling elements, roller end face, rib face or cage pockets. It can be caused by either rust and electrical corrosion; debris entry; problems with lubrication or irregular sliding within the bearing.
The effects of wear can be countered by ensuring the correct lubrication medium and method is in place, and by filtering lubricant thoroughly to remove debris. Any misalignment can be eliminated by thorough checking of the bearing seating.
By taking preventative measures to ensuring the bearing is aligned properly, the effects of wear can be reduced or eliminated. Wear damage can also be prevented by making sure the bearing and its housing are kept clean and the sealing mechanism is optimised.
* Fretting. Occurring at the fitting surface and the contact area between the raceway ring and rolling elements, fretting results from repeated sliding between two surfaces. Fretting corrosion is another term used to describe the reddish brown or black worn particles.
The main cause of fretting is once again poor lubrication. This can be countered by using a proper lubricant for the operation, and applying a film of lubricant to the fitting surface.
Fretting on the raceway can also be caused by vibration or insufficient interference. In these instances, application of a pre-load and checking the interference fit are the key steps that should be taken.
* False Brinelling. One of the many types of fretting, false brinelling describes the occurrence of hollow spots that resemble brinell dents. These are caused by wear due to vibration and swaying at the contact points between the rolling elements and the raceway.
It is usually attributed to poor lubrication, and also oscillation and vibration during transport i.e. static vibration. The latter can be avoided by securing the shaft and housing during transporting, and transporting with the inner and outer rings packed separately. Vibration can also be reduced by pre-loading.
* Creep. Creep is the phenomenon where slipping occurs at the fitting surfaces, creating a clearance. Creep damage typically has a shiny appearance, occasionally with scoring and wear damage, which can occur on both the inner and the outer ring. The usual causes are either insufficient sleeve tightening, insufficient interference or loose fit.
To counteract the effects of creep damage, the installer must ensure that the raceway ring side face is tightened. Interference should be checked to prevent rotation and the sleeve tightening corrected, with the preload positioned in the axial direction. Applying either an adhesive or a film of lubricant to the fitting surface can help counter the effects of creep damage by minimising the possibility of slippages occurring.
* Seizure. Occurring when bearings suddenly overheat during rotation, the first sign of seizure is discoloration of the bearing. Elements of the bearing will then soften, melt and ultimately deform as damage accumulates.
This damage can be caused by debris or water entry, poor lubrication, excessive load, speed or preload, insufficient internal clearance, or imprecise installation of the shaft and housing, which can cause the shaft to bend excessively.
To ensure that seizures do not occur, the lubricant and lubrication method need to be assessed, as does the suitability of the bearing type which has been selected for the job. Precision of the shaft and housing should be verified, and the mounting optimised. The preload, bearing clearance and fitting must also be checked.
* Electrical Corrosion. Arching and burning can occur through the thin oil film at points of contact between the race and rolling elements of a bearing when electric currents pass through. This type of damage manifests itself in the form of visible ‘fluting’ or groove-like corrugations and is caused by electrical difference between the inner and outer rings or electrical potential difference of high frequency, generated by instruments used near a bearing.
To counteract electrical corrosion, the installer must ensure that the bearing is appropriately insulated. Electrical circuits should be designed to avoid current flow through bearings.
* Rust and corrosion. Occurring most commonly on the ring surfaces or rolling elements, rust and corrosion in the form of small pits can be caused a variety of factors - high temperature and humidity when stationary, formation of water droplets, and poor storage and handling.
To counter this, storage, handling, lubrication and sealing must be optimised, and an anti-rust treatment applied for any period of non-running. If these pits are noticed at ball or roller pitch spacing, it is indicative that moisture or water has entered the bearing while in the stationary position. It is important to check storage conditions to avoid this form of damage.
* Mounting flaws. 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, employing an appropriate jig or tool, and centring the relative mating parts during mounting.
* Discolouration. Discolouration of the cage, rolling elements and raceway rings occur due to a reaction with lubricant and high temperature. This is simply addressed by optimising the lubrication method.
Summary
Any of the above types of damage should be identified by regular checks to the bearing.
When evaluating whether a bearing can be re-used or not, the following points must be considered; the degree of bearing damage, machine performance, critical nature of the application, operating conditions and inspection intervals.
If the inspection reveals any of the above mentioned problems then the bearing must be replaced immediately.
If none of these are present, then employing the appropriate countermeasures will help to extend the bearing’s service life.
Malcolm Watson is Product Manager – Bearings, Brammer UK Limited, Wolverhampton, UK Tony Synnott is UK Engineering Manager, NSK Bearings Europe Ltd, Newark, Nottinghamshire, UK.
