Vibration 4

[SENGUTTUVAN]

Hi
Can anyone explain me about the Electrical Run out in a rotor shaft in simple terms.

 

[BobM3]

Do you mean mechanical runout?

 

[JJPellin]

For larger machines, there are often non-contacting proximity probes used to measure shaft vibration. These probes use a magnetic field across a gap between the probe tip and the surface of the shaft to measure the distance between the probe and the shaft. This signal can be used to determine the shaft movement (displacement), velocity or acceleration. In order for this signal to be accurate, the surface of the shaft has to have very low mechanical and electrical run-out. For our spare rotors, we require that the combined run-out is less than 0.00025". We measure this with a dial indicator for the mechanical run-out and a proximity probe for the electrical. Electrical run-out would suggest variability in the magnetic and electrical properties of the shaft. If the shaft material is not homogeneous, it will exhibit electrical run-out that will result in inaccuracy in the signal. Diamond burnishing the shaft can smooth out irregularities in the surface properties in some cases. Some materials are more prone to electrical run-out than others. If burnishing does not improve the run-out, the surface needs to be machined back to expose new material that may be more uniform in electrical properties. You can search for more information on this topic from the manufacturers of non-contacting proximity probes. Ours are made by Bently-Nevada which is now part of General Electric. I believe SKF owns another system that used be built under the name of Dymac.

Johnny Pellin

 

[WCFoiles]

Burnishing uniformly work hardens the surface, which can help. Some have tried micro-peening for a similar purpose.

Some materials can be come magnetized. Some machining operations can leave the target material non-uniformly magnetized. This requires de-Gaussing.

Hit the target with a hammer, remove the physical indentation, and it may leave residual stress (looking near heavy shrinks, too). The probes can be affected by this, too.

Regards,

Bill