Rotating Machinery Services

Measurement Techniques - At Standstill

Magnetic fields are measured with devices called gaussmeters. As the name implies, these meters measure magnetic flux density in units of gauss.

The process of measuring magnetic fields requires careful judgment as to where the most meaningful readings can be obtained. The most critical zones are generally those involving close clearances between rotating and stationary members. Hence, measurements should be obtained around the following areas:
  1. Around bearings and seals
  2. Shaft ends
  3. Coupling hubs
  4. Gear teeth
  5. Foot mountings
  6. Pipe flanges
Magnetic field levels are best measured with the machinery at standstill. As may be noted in the following section, a more useful practice is to conduct magnetic reading surveys on fully disassembled parts, recording readings taken before and after degaussing. Then as assembly takes place, magnetism should be measured on new and reworked parts as they are assembled, and of the assemblies themselves, especially in critical areas such as bearings, seals, gears, etc. High magnetism under these conditions may require downcycling degaussing to achieve acceptable levels.

Maximum Allowable Residual Magnetic Field Levels (As Measured in Open Air)
2 gauss: Bearing components, including pads and retainers, journals, thrust disc, seals, gears and coupling teeth.
4 gauss: Bearing housings.
6 gauss: Mid-shaft and wheel areas, diaphragms, etc.
10 gauss: Components remote from minimum clearance areas, such as casings, pipings, etc.

Measurement Techniques - When Operating

By far the greatest amount of information on magnetism and shaft currents in an operating unit is obtained by measuring shaft voltages and currents, utilizing shaft brushes and monitors. Shaft brushes are intended to ground the shaft, bypassing shaft currents and preventing them from flowing and damaging the bearings, seals and gears, etc. In the past, little attention was paid to the design and performance of brushes, with the consequence that various amounts of damage occurred, sometimes causing forced outages. Often, the brush current is passed to the casing or housing directly, with no means to intercept it for measurement and monitoring. Brushes employed are often not reliable in a hostile environment. Reliable shaft grounding brushes and a variety of monitoring equipment are available for protecting critical components and monitoring their performance. A suitable monitor is shown in Figure 15.

It is a good idea to perform shaft voltage and current tests, utilizing instruments intended for the purpose. Measurements should be made at strategic shaft locations before selection and installation of brushes. Such tests are useful in showing the character and power delivery capability of the shaft voltages and in determining whether the voltage is static or electromagnetic in nature.