Inspection and monitoring

Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

This section describes various aspects of inspecting and monitoring bearings in operation for the purpose of preventing problems. It also gives an introduction to troubleshooting and links to more detailed troubleshooting procedures.

Inspection during operation
Spotting early indications of bearing damage makes it possible to replace bearings during regularly scheduled maintenance. This avoids otherwise costly unscheduled machine downtime if a bearing fails. Important parameters for monitoring machine condition include noise, temperature and vibration.

Bearings that are worn or damaged usually exhibit identifiable symptoms (→ Troubleshooting, below). There can be a number of possible causes and this section helps identify some of these.

For practical reasons, not all machines or machine functions can be monitored using advanced systems. In these cases, trouble can be detected by looking at or listening to the machine. However, if deterioration can be detected by human senses, damage may already be extensive. Using objective technologies, such as advanced vibration analysis, means damage can be detected before it becomes problematic (diagram 1). By using condition-monitoring instruments and the SKF enveloped acceleration technology, the pre-warning time can be maximized.

Fig. 1 shows an example of how damage can progress, and diagram 1 shows this conceptually. A damage scenario may follow this sequence:
  1. Bearing starts to show abrasive wear.
  2. First spall, detected by SKF enveloped acceleration technology.
  3. Spalling has developed to an extent that the damage can be detected by standard vibration monitoring.
  4. Advanced spalling causes high vibration and noise levels and an increase in operating temperature.
  5. Severe damage occurs: fatigue fracture of the bearing inner ring.
  6. Catastrophic failure occurs with secondary damage to other components.

Monitoring noise and vibration

A common method used to try to identify deterioration or damage in a bearing is to listen. Bearings in good condition produce a soft purring noise. Grinding, squeaking and other irregular sounds usually indicate that the bearings are in poor condition, or that something is wrong. However, sound monitoring is of limited use. SKF recommends vibration monitoring. It is more thorough and allows better monitoring of bearings and rotating equipment.

Vibration monitoring is based on three fundamental facts:
  • All machines vibrate.
  • The onset of a mechanical problem is generally accompanied by an increase in vibration levels.
  • The nature of the fault can be determined by analyzing the vibration characteristics.

Monitoring temperature

It is important to monitor the operating temperature at bearing positions. If the operating conditions have not been altered, a sudden increase in temperature is often an indication of developed bearing damage and possible imminent failure of the bearing. However, keep in mind that a natural temperature rise can last up to one or two days immediately after first machine startup and after each relubrication when using grease.

Monitoring lubrication conditions

Bearings can only achieve maximum performance levels with adequate lubrication. The lubrication conditions of a bearing should therefore be monitored closely. The condition of the lubricant itself should also be assessed periodically, preferably by taking samples and having them analyzed.

SKF recommends the following general guidelines for lubrication-related inspection activities:
  • Check for lubricant leaks in the areas surrounding the bearing positions.
  • Keep protective collars and labyrinth seals filled with grease for maximum protection.
  • Check that automatic lubricating systems are functioning properly and providing the appropriate amount of lubricant to the bearings.
  • Check the lubricant level in sumps and reservoirs, and replenish as necessary.
  • Where manual grease lubrication is employed, relubricate according to schedule.
  • Where oil lubrication is used, change oil according to schedule.
  • Always make sure that the specified lubricant is used.
Inspection during a machine shutdown
When a machine is not in operation, it is an opportunity to assess the condition of bearings, seals, seal counterfaces, housings, and lubricant. A general inspection can often be done by removing a housing cover or cap. If a bearing appears to be damaged, it should be dismounted and thoroughly inspected.

Shaft and belt alignment, as well as a thorough inspection of the machine foundation and exterior, can also be done during a machine shutdown.

Any condition, whether it is a missing shim or a deteriorating foundation, can negatively affect machine performance. The sooner any problem is identified, the sooner corrective action can begin. It is far less costly to replace bearings and associated components during a regularly scheduled shutdown than during unscheduled downtime that unexpectedly takes the machine out of service.

Inspecting bearings

Bearings are not always easily accessible. However, when bearings are partially or fully exposed, visual checks can be made. The most practical time to inspect bearings is during routine maintenance.

When inspecting a mounted bearing, SKF recommends following these general guidelines:
  • Preparation
    • Clean the external surface of the machine.
    • Remove the housing cover, or the housing cap, to expose the bearing.
    • Take lubricant samples for analysis. For oil lubrication, take samples from sump/reservoir. For grease-lubricated open bearings, take samples from various positions within the bearing and surroundings. Inspect the condition of the lubricant. Impurities can often be detected by spreading a thin layer of the lubricant on a sheet of paper and examining it under a light.
    • Clean the exposed external surfaces of the bearing with a lint-free cloth.

  • Inspection
    • Inspect the exposed external surfaces of the bearing for corrosion. Inspect the bearing rings for any abnormal signs.
    • For sealed bearings, inspect the seals for wear or damage.
    • Where possible, rotate the shaft very slowly and feel for uneven resistance in the bearing; an undamaged bearing turns smoothly.

  • Detailed inspection of grease-lubricated bearings
    Grease-lubricated open bearings in split plummer blocks can be subjected to a more detailed in-situ inspection as follows:
    • Remove all grease around the bearing.
    • Remove as much grease from the bearing as possible using a non-metallic scraper.
    • Clean the bearing with a petroleum-based solvent by spraying the solvent into the bearing. Rotate the shaft very slowly while cleaning it, and continue to spray until the solvent ceases to collect dirt and grease. For large bearings that contain a build-up of severely oxidized lubricant, clean them with a strong alkaline solution containing up to 10% caustic soda and 1% wetting agent.
    • Dry the bearing, and surrounding parts, with a lint-free cloth or clean, moisture-free compressed air (but do not rotate or spin the bearing).
    • Inspect the bearing raceways, cage(s) and rolling elements for spalls, marks, scratches, streaks, discolouration and mirror-like areas. Where applicable, measure the radial internal clearance of the bearing (to determine if wear has taken place) and confirm that it is within the expected range.
    • If the condition of the bearing is satisfactory, apply the appropriate grease to the bearing and the housing and immediately and close the housing. If bearing damage is evident, dismount the bearing and protect it from corrosion. Then conduct a full analysis.

  • General recommendations
    • Take photographs throughout the inspection process to help document the condition of the bearing, lubricant and machine in general.
    • Check the condition of the grease at different places and compare with fresh grease (fig. 2). Keep a representative sample of the grease for further analysis.
    • Certain large and medium-size bearings are suitable for reconditioning. For additional information, refer to the SKF bearing maintenance handbook and publication SKF Remanufacturing Services.

Inspecting seal counterfaces

To be effective, a seal lip must run on a smooth counterface. If the counterface is worn or damaged, the seal lip will cease to function properly.

When inspecting the seal counterface, also check for corrosion, shaft wear, scratches, dents, lip wear, lip tears and so on. If corrosion is evident but not severe, use a fine wet/dry abrasive paper to remove it, and then make sure all remnants are also removed. Worn counterface parts of the shaft can be repaired using SKF Speedi-Sleeve.

WARNING

Avoid inhaling, ingesting or contacting solvents and alkaline solutions.  These can cause skin and eye burns or damage to respiratory or digestive tract.  If necessary, seek medical assistance.
Troubleshooting
Bearings that are not operating properly usually exhibit identifiable symptoms. The best way to identify these symptoms, and take corrective action at an early stage, is to establish a plant-wide condition monitoring programme.

In cases where condition monitoring equipment is not available or practical, the Troubleshooting section of the SKF bearing maintenance handbook presents some useful hints to help identify the most common symptoms, their causes, and whenever possible, some practical solutions. Depending on the degree of bearing damage, some symptoms may be misleading and, in many cases, are the result of secondary damage. To effectively troubleshoot bearing problems, it is necessary to analyze the symptoms according to those first observed in the application. This is dealt with in more detail in the publication Bearing damage and failure analysis.

SKF logo