Propeller Shaft Bearings
Tides Marine International supply a range of water lubricated shaft bearings in imperial (¾” to 6” shaft diameters) and metric (25mm to 150mm shaft diameters) sizes, featuring a specially formulated rubber, moulded into a shell material. The rubber used is an extremely tough, chemical and oil resistant nitrile with a low coefficient of running friction, which offers outstanding resistance to abrasion and wear, even in the most adverse conditions.
Principle of Operation
Rubber and water make the perfect combination for a bearing material and lubricant. The natural resilience of rubber gives the bearing its shock, vibration and noise absorption properties whilst water is the perfect lubricating medium, particularly for marine craft, because of its natural abundance, non-compressibility and cooling properties.
The water enters the bearing through the longitudinal flutes and moves radially between the propeller shaft and the bearing face in a thin film. Once this film, or wedge, has developed the shaft does not actually come into contact with the bearing. The flutes are designed to maintain a water flow of 7.5 gallons per sq.in / 5.28 litres per sq.cm.
Three options of shell material are available through Tides Marine:-
- Corrosion resistant brass tube
- Phenolic bonded paper tube – brown in colour
- Glass Reinforced Plastic (GRP) tube – off white in colour
The Phenolic or GRP shell materials are designed to suit applications where galvanic corrosion is an issue, such as with aluminium hulled boats.
In addition GRP shell bearings do not swell in sea water and match all of the service parameters of a brass shell bearing, whilst being 60% lighter and impact resistant. A two part epoxy adhesive can be used to retain the bearing within the housing which removes the need to use grub screws.
Inner Lining Material
The Nitrile rubber lining material for standard application marine bearings is a 70 ± 3 Shore A compound, whilst a harder 85 ± 3 Shore A compound is available for bearings requiring a greater stiffness – such as for jet drive applications.
Optimum service temperature is 4º to 70º C, with an allowable operating temperature range of -25º to 85º C.
For operating temperatures above 30º C advice should be sought from our Technical Office relating to the clearance between the shaft and the bearing.
Bearings are designed to run on bright ground / polished shafting with a diameter of size to minus tolerance of h8 / h9.
Housings should have a bore tolerance of H7 – bearings are designed to be an interference fit.
The load that can be carried by a bearing is dependant upon the quality of the lubricating water and the tolerance or deflection of the shaft. A normal working load of 2.5 kgs/cm² is acceptable.