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PLAIN BEARING FAILURE
8.0 Electrical Damage
- Electrical damage index
8.6 Electrical Damage: Summary
Characteristics
Both the passage of electric current and electrostatic discharge across a bearing oil film produces a characteristic matt appearance on the bearing surface (sometimes referred to as 'frosting'}and this damage is frequently replicated on the opposing steel surface(journal or thrust collar). As discharge occurs at the point of minimum film thickness, the damage is confined to the loaded region of the bearing, but is continuous on the opposing surface as this rotates and passes through the minimum film thickness. Examination at low magnification (5-10X) reveals shiny, rounded pits from which metal has been removed by melting.
The bearing surface is gradually degraded and, when it can no longer support a load-carrying oil film, breakdown occurs and a wiping failure follows (see Section 1). This is a secondary effect, but the problem is that it can remove the evidence of electrical discharge, though in the case of journal bearings some signs of this may remain in the unloaded half of the bearing and evidence of discharge should be found on the opposing steel surface.
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Electromagnetic damage is most frequently caused by leakage currents in electric motors, though it can also occur in large rotary machines (compressors, turbines) where the rotor has been handled with magnetic grabs. A potential of at least 250 mV is necessary for damage to be caused.
Electrostatic damage is most likely in large steam turbines and high-speed centrifugal compressors where particles are or droplets of liquid are present in the gas stream. It also occurs in compressors subject to on-line washing by the periodic injection of solvents. A potential built up of about 20V is required for damage to occur.
Actions
Electric motors should be provided with insulated bearing housings and fitted with earthing straps unless shaft voltages can be guaranteed to be less than 250mV.
The conditions required for the generation of electrostatic potential on a rotor - particle/droplet size and velocity - are difficult to define precisely. Thus, where there is a risk of liquid droplets or solid particles being present in the gas stream in turbines and compressors, the shaft should be grounded by earthing brushes if the bearing oil film can sustain a potential of 20V or greater.