Marine Maintenance Technology International - April 2016

    MARINE MAINTENANCE TECHNOLOGY INTERNATIONAL | APRIL 2016 ⠿ 71

CORROSION (^) ⠿
shaft and the propeller itself whose shaft is
grounded by means of a grounding system.
The potential between the propeller and
the hull can reach 700mV when the steel is
protected with only with sacrificial anodes.
Measurements on large container vessels
with big propellers have shown induction of
currents from the propeller of more than 40A.
However, by using the BAC Shaft Grounding
system, it is possible to reduce the potential to
close to 0mV.
To prevent galvanic or spark corrosion
on the shaft bearings, BAC produces a shaft
earthing device that is easy to install. This
system can be delivered with a measuring
cabinet with built-in amplifier. This feature
ensures that the system can be connected to
the vessel’s alarm system and will notify the
crew when a set threshold is exceeded. \
Free reader inquiry service
BAC Corrosion Control A/S
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Reader inquiry no. 103
LEFT: The BAC Shaft Grounding System
provides an effective solution to the
problem with corrosion of the propeller
shaft due to electrical potentials by
grounding the shaft to the hull
BELOW: Heavy-duty cables and
clamps are implemented to ensure a
very low electrical resistance in the
complete system

• s Serves a a short circuit between the
  rotating propeller shaft and the ship’s
  hull


• revent Can p the micro-pitting on the
  propeller, on the sliding surfaces
  of crankshaft journals and the main
  bearings of the engines, assumed
  to be spark erosion
  
  
  • Ensures excellent potential discharge
    and the contact brushes ensure
    lubrication and long life
  
  
  • ndicator Remote i system for continuous
    andffective e control of the whole system
  
  
  • Continuous slipring cleaning device to
    prevent an increase in residual potential
    during operation
  
  
  
  

The system consists of the following

main components:

• lloyilver A s a slipring is mounted on the
  intermediate shaft


• olders Brush h with silver/graphite
  brushes


• Grounding wire and grounding plate


• Monitoring box with millivoltmeter,
  ammeter and alarm output (optional)


• ilver/graphite The s brushes run on the
  slipring


• arthing The e brushes are connected to
  the hull through grounding wires and a
  grounding plate


• onitoring The m brush is connected to
  the monitoring box

SYSTEM CONFIGURATION

ADVANTAGES OF GROUNDING A PROPELLER SHAFT

1. Silver alloy band to be mounted, away from bearings, not
   exposed to water, and where it is easy to service


2. The shaft is ground to a clean surface, degreased so that the
   electrical resistance is minimized as much as possible


3. The silver alloy band is degreased on the rear side and laid on


4. The grounding plate is welded on the hull as close to the brush
   holders as possible


5. The shaft for brush holders is welded on in such a way that the
   least vibrations possible will occur and so that it is parallel
   with the propeller shaft.


6. The distance between the brush holders and the silver alloy
   band should be 2-3mm


7. The spring pressure on the brushes is 500-600g


8. The projecting ends of the shaft can be cut off