Rolling bearing damage

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As a rule, rolling bearings are maintenance-free provided that the calculation and installation have been carried out correctly and the operating conditions calculated in advance are observed. However, a few things must be taken into account - in the selection, during installation and during operation.

Typical bearing failures and their consequences

The following parameters must already be taken into account when choosing a bearing: On the one hand, the correct dimensioning . The selection criteria include the speed , the radial and axial forces that occur, and any oscillations and vibrations that may occur . Secondly, the environmental conditions such as temperature , humidity and soil load , further directions of loading and installation conditions.

There are specially matched bearings for every application. Incidentally, the majority of bearing damage is due to inadequate lubrication ; the above-mentioned factors influence the lubricity considerably.

There are certain applications in which bearing damage is more common than average. As a rule, there are areas in which dirt or dust is present in significant quantities, for example in construction machinery, in mining, in the production of granules, powder, flour and the like. A., also areas with increased humidity or even sea water. Also relevant: an increased ambient temperature, this must already be added to the operating temperature when designing, because this can cause the permissible lubricant temperature to be exceeded.

A bearing damage always leads to economic impairment; Under unfavorable circumstances, the entire production line must be shut down for several hours to repair the machine in question. In the worst case, however, consequential damage has already occurred (for example to the impeller of a pump ), then the repair becomes correspondingly expensive and time-consuming.

Bearing failures: causes and damage

Various causes can lead to the failure of a rolling bearing:

  • Load: If the loads (axial, radial, speed, temperature, vibrations) during operation are higher than calculated, the service life is reduced accordingly. Consequences: The bearing overheats, the noise level increases and the lubrication fails.
  • Misalignment : For example, insufficient rigidity of the housing, design or assembly errors can result in unforeseen bearing forces and the load is unevenly. Damage as above.
  • Imbalance , vibrations : There are intentional (for example in piston engines ) and unintentional imbalances; the result is vibrations that reduce the service life in both cases. Damage as above.
  • Fatigue , wear and tear : Occur at the end of the service life. The greater the load, the shorter it is. The weakest link is usually the lubricant; as soon as this fails, material damage occurs within the bearing on the rolling elements and raceways.
  • Insufficient lubrication : If a machine is idle for a long time, this leads to a less than optimal distribution of lubricant. When starting up again, insufficient lubrication occurs in places until the lubricant has spread out enough again. The more often there are long breaks in operation, the more the bearing life is reduced. Overheating and increasing noise development are the consequences.
  • Insufficient sealing : Lubrication is particularly important for a long bearing life. If the seal is inadequate, the lubricant is lost and the system runs dry. Or dirt particles penetrate the bearing, they collect in the lubricant and thus get into the runway, which is damaged as a result. Damage as above.
  • Smearing due to sliding: This can have two causes: sliding of the rolling elements or tearing of the lubricating film . The consequential damage is usually caused by the ingress of water, poor lubrication or high speeds with low loads (= incorrect design)
  • Corrosion : penetration of moisture into the bearing inevitably leads to damage; the lubricant delays the process, but does not stop it.
  • Fretting corrosion : Often occurs as a result of a chemical reaction between the bearing shell and the shaft (tribo-oxidation), followed by destruction as a result of the relative movement of the components. Cause: too loose fit or moisture penetration.
  • Brinell effect : Also called standstill marking , depression or corrugation : caused by vibrations or plastic deformations. These micromovements create rough running surfaces in the bearing, resulting in increased noise, wear and tear and failure.
  • Electricity passage : Particularly in the case of roller bearings in electric motors controlled by a frequency converter, current passage or even flashovers can occur within the bearing. The lubricant burns locally, resulting in combustion residues that significantly impair the lubricant's effectiveness, and ultimately the bearing fails. Current-insulated bearings provide a remedy.
  • Crack formation : Occurs when the tensile strength or fatigue strength of the material is exceeded , usually caused by installation errors (e.g. too rough treatment of the bearing).
  • in extreme cases: exceeding the breaking strength

Detection of bearing damage

Excessive noise development is a sure sign of bearing damage. If the environment is too noisy to determine this, a stethoscope can help locate the source of the noise. If it is possible to turn the shaft manually, you may notice that it is running unevenly. In addition, if the bearing can be seen from the outside, bearing damage can be identified under favorable circumstances based on the temperature discoloration.

Assembly error

Well over 50% of all premature bearing failures can be avoided. Studies have shown that the causes and their frequency of all bearing failures (depending on the source) can be divided into four groups: 1. Inadequate lubrication (approx. 35% - 55%) 2. Contamination (approx. 14% - 18%) 3 Installation errors (approx. 16% - 17%) 4. Overloading or fatigue (approx. 10% - 34%) In practice, various factors as a combination of several individual factors often cause the failure.

The failure group of assembly errors can, however, be influenced very well by the user or repairer. The most common installation errors:

  • wrong or missing tools
  • improper installation, poor fastening
  • Tension, uneven wear, incorrect bearing fit ( tolerance , form defects or incorrect fit)

An incorrectly installed bearing can be damaged by these installation errors and fail quickly. The performance potential of modern rolling bearings can only be fully exploited if they are properly installed. Then the predicted bearing life should be reached and reliable operation of the system guaranteed. Incidentally, large bearings can be installed conveniently and effectively using the hydraulic method (oil pressure assembly).

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