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Before Considering Bearing Redesign

In the opinion of the senior author, the Original Equipment Manufacturers (OEMs) do a credible job of designing, building, and installing bearings. Within the industrial end user community there seems to be an overt tendency to rede-sign, and continually attempt to improve on the OEM bearings. In multiple cases, the bearing redesign has rendered the machinery inoperative. In other situations, the operating speed range of the machinery has been severely limited following the installation of presumably improved bearings. In all cases, the machinery diagnostician must approach bearing problems carefully, and conduct a methodical engineering analysis of the problem. Specifically, the following items should always be thoroughly examined and in some cases re-examined:

  • Check that the oil console or reservoir contains the correct lubricant.
  • Check the oil quality for proper density, viscosity, water content, etc.
  • Check the oil for the presence of any foreign materials.
  • Check for proper oil supply pressure, temperature, and system control.
  • Check the oil flow rate to each bearing, and verify that orifices are properly installed, and that orifice diameters are both reasonable and correct.
  • Check the oil drain temperatures, and relative flow rates.
  • Check that the bearing is properly installed with respect to shaft rotation.
  • Check that anti-rotation pins are properly installed with respect to rotation.
  • Check that the shaft to bearing clearance is correct.
  • Check that the bearing to housing clearance is correct.
  • Check that the bearing liner is not distorted or warped.
  • Check that the bearing splitline is not sealed with RTV, silicone, or other incompressible sealants. Use a thin grade of Permatex® sealant for this job.
  • Check for other mechanical changes in the train that would influence bearing load (e.g., changing a gear coupling to a large diaphragm coupling).
  • Check rotor balance records, and the last set of transient startup data.
  • Check coupling alignment for proper cold offset and hot running position.
  • Check for proper temperatures from imbedded thermocouples or RTD’s.
  • Check bearing temperature trends (day to night, week to week, etc.).
  • Check to be sure that shaft is level when hot and running.
  • Check bearings, seals, and couplings for evidence of electrical discharge.
  • Check pads and backing for evidence of wear, cracking, or fretting.
  • Check bearings for evidence of edge wear across bearings and machines.
  • Check for proper position of the journal within the bearing with prox probes.
  • Check shaft vibration for normal 1X running speed vibration vectors.
  • Check shaft vibration for any abnormal frequency components.
  • Check the attachment of the bearing housing to the casing and/or baseplate.
  • Check grout condition, and the attachment of baseplate to foundation.

If these checks are followed, and all identified problems corrected, the necessity to redesign or continually replace bearings will be greatly reduced. Bad habits seem to develop over time, and both operations and maintenance personnel have a tendency to get complacent. In many instances this will allow small oversights to turn into major problems. Hence, before jumping into a major rede-sign effort, the use and abuse of the current bearings should be examined.

There are situations when the bearings really do require an upgrade. If rotor or coupling changes are to be implemented, if the process loads or the lube and seal oil system are to be modified, or if greater reliability is required — then the existing bearings should be audited for potential areas of improvement. The addition of ball and socket bases for tilt pad bearings, the use of micro-babbitt, a change in bearing metallurgy for improved heat transfer, or providing directed lubrication are all common modifications that may benefit a particular bearing installation. In some applications, the installation of new bearing designs such as the Flexure Pivot Bearings described by Zeidan and Paquette may be highly beneficial. In other cases, an additional five gallons per minute of oil flow may be all that is required. Once again, the machinery diagnostician is advised to pro-ceed with logic, and proper engineering discipline.

Tip from Machinery Malfunction Diagnosis and Correction - Vibration Analysis and Troubleshooting for the Process Industries by Robert C. Eisenmann, Sr., P.E. and Robert C. Eisenmann, Jr.

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