Ship Shaft Earthing

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Propeller-shaft earthing and

potential monitoring

- installation

- maintenance

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HANDBOOK- & OPERATING INSTRUCTIONS

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Slipring assembly

A turning propeller shaft on a ship becomes electrically insulated from the hull by the

lubricating oilfilm in the bearings and by the use of non-metallic bearingmaterials in the tail

shaft. When the shaft is insulated in this way an electrical potential can be measured between

the shaft and the hull and this can accelerate corrosion in the ship.

If the ship has a system of cathodic protection, whether it is sacrificial anodes or an

impressed-current system, the shaft-insulation will prevent the propeller and the boss from

receiving protection.

An electrical potential between the shaft and the hull can also cause a heavy current to flow in

bearings when the oilfilm breaks down or is contaminated with seawater.

This current can cause deep pitting of the bearingsurface.

Excessive wear on the shaftbearings can often be traced to this cause.

This potential trouble can be avoided and cathodic protection extended to the propeller if the

shaft is properly earthed with a propeller-shaft slipring.

The effectiveness of the shaftearthing system should ensure a contactresistance of less than

0.001 ohms for a waterfilled bearing and 0.01 ohms for an oilfilled bearing.

Tests indicate that only high-silver-content brushes running on a silver track has the

repeatable low conductivity needed to maintain these limits and to ensure a low resistance

contact is maintained even under dirty conditions.

The shaftearthing assembly comprises a pair of high silver content/graphite compound

brushes mounted in a balanced brushholder, running on a copper slipring with solid silver

inlay track.

Each brushholder has an adjustable springtensioner which is supplied preset to the minimum

and results in a pressure of approx. 790 grams on each brush.

At this pressure the expected life of the brushes is in excess of one year.

Handbook & Operating Instructions for Shaft Earthing and Monitoring Equipment Page: 1

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Shaftearthing – installation

Use a degreaser for cleaning the propellershaft from oil and grease before fitting the slipring.

The shaftslipring is supplied as two matched halves complete with band- and clamping-

arrangement and can be readily installed by competent engineering personnel.

All sliprings are manufactured slightly oversize to allow some degree of tolerance for final

fitting. All excess material should be removed by filing down at the joint faces before

securing with the bandclamping to form a tight close fit around the shaft diameter.

NOTE: To prevent brush 'bounce' and ensure maximum utilisation of the silver/

graphite brushes, it is essential that the joints form a smooth and flush profile

over the full extent of the slipring

Drawing no. SE 2100045 detail A refers.

The balanced brushholder is supplied ready for fitting to a 20mm diameter rod which should

lie with its center-line parallel to the shaft center-line. The brushholder should then be

clamped to this rod and aligned centrally over the silver track.

The rod and mountingbracket are supplied by the shipyard and can be mounted from either a

suitable frame or bearing-pedestal.

It is essential that the complete assembly should provide a good electrical contact between the

shaft and the hull, therefore the brushholder-rod support should either be welded or bolted to

the ship's structure and a short length of 25 mm² bondingcable should be connected between

the brushholder and ship's structure.

On completion of installation the complete assembly should be checked for trueness and

smooth running of the brushes in accordance with the attached checklist.

Cover the possible gap between the edges of the slipring and shaft with a suitable compound

preventing oil entering under the ring.

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Propellershaft potential monitoring

The advantages and benefits of installing shaftbonding-assemblies with hull cathodic-

protection are widely recognised and it is for this reason that many shipowners desire the

additional facility to monitor the effectiveness of the shaftbonding-equipment.

Measurement of the potential between the shaft and hull is achieved by means of a separate

monitoringbrush running on the main slipring but insulated from the hull.

This brush is then connected to the impressed-current powerunit or to a remote millivolt-

meter to display the shaftpotential with respect to the hull of the ship – drawing no.

SE 2400084 refers.

Installation

The shaft-monitoring-brushholder should be mounted on a 12 mm steel rod mountingpost

which should be positioned in a plane perpendicular to the center-line of the shaft.

The monitoring brushholder is insulated from the mountingpost by means of the insulating

bush provided – drawing no. SE 2200014 refers.

Before welding or finally fixing the mountingpost in position, the position of the brushholder

should be checked to ensure that it aligns centrally to the silver track and does not obstruct the

slipring-securing-clamps.

Once the mountingpost is secured in position, the brushholder is insulated mounted on the

rod and adjusted to ensure that the distance between the slipring and brushholder conforms

with the dimension shown on drawing no. SE 2200014.

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Adjustment of the brushholder is effected by releasing the clamp and sliding the brushholder

to achieve the specified distance. The brushholder is then clamped firmly in position and the

monitoringbrush inserted.

On completion of installation the monitoringbrush should be checked for trueness and smooth

running.

Connection to the required monitoringunit is made via ships internal wiring in accordance

with drawing no. SE 2400084.

Although the monitoring unit has been fitted-out with vibrationabsorbers it is essential that

this unit is mounted on a place where a minimum of vibration is to be expected.


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Slipring installation checks

1) Confirm slipring and brushgear are installed as per

drawing no. SE 2100045.

2) Is assembly clean and free from oil and grease? YES/NO

3) Is slipring a tight fit to the shaft over its whole length? YES/NO

Ensure no bumps or indentations can be felt over the whole

of the workingsurface

4) Are joints a good fit with no gap? YES/NO

5) Is brushholder secure on its rod? YES/NO

6) Confirm that brushholder and its mounting are solid and that it YES/NO

will not be affected by vibration

7) Confirm that brushfaces are tangential to slipring YES/NO

8) Confirm that brushes are free to move in their holders YES/NO

9) Is there electrical continuity between tail shaft and hull? YES/NO

10) Have the vibrationabsorbers been fitted between mV-meter-unit

and its wall-mounting? YES/NO


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Slipring maintenance

The grounding assembly should be checked every seven days for cleanliness. If there has

been a build-up of oil on the slipring face this should be removed with some degreaser.

Inspect and clean the brushes and brushholder to prevent blocking from dirt. Inspect the

brush- copper- leads (pigtails) to ensure they have not become loose or corroded. The brush-

wear-down should be noted and the compression of the holddown-springs on the brushes

should be adjusted to ensure good electrical contact.


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Drawings – Shaftearthing and Monitoring

PROPELLER-SHAFT EARTHING ASSY AND

BRUSHHOLDER INSTALLATION SE 2100045

PROPELLER-SHAFT MONITORING BRUSHHOLDER

INSTALLATION SE 2200014

SHAFTEARTHING & MONITORING

INTERCONNECTION DIAGRAM SE 2400084

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Ship Shaft Earthing

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