Bearings

Countrose offers an additional service to other manufacturers in the fact that we offer two different types of elastomeric lined bearings, polyurethane and nitrile rubber.
Polyurethane is a very tough, highly abrasion resistant elastomeric material. Its excellent physical properties allow it to operate at loads beyond that of other elastomers, whilst its high resilience and abrasion resistance make it ideally suited to the toughest working environments. Polyurethane is slightly absorbent and, when immersed in water, it will swell a small amount until equilibrium is reached. The bearings supplied by Countrose are designed with initial clearances to cater for the dimensional movements caused by changes in temperature and water content.
Nitrile rubber is softer and has slightly lower physical properties than polyurethane. Generally, nitrile has a lower load capacity and is less abrasion resistant. It is resistant to grease and oil and is dimensionally stable in water, allowing tighter clearances.
Under most normal conditions polyurethane and nitrile rubber lined bearings have been found to perform equally well, displaying the same wear rates and giving a similar co-efficient of friction under lubrication. However, where the vessel is operating in abrasive conditions, for example in shallow water containing sand or grit, polyurethane has proven to far outlast nitrile. In contrast, where vibration may occur in the shaft caused by misalignment, nitrile has proven to have superior noise dampening properties than polyurethane, because it is a softer material.
Countrose bearings are generally produced with straight flutes as, in most applications, the greater open flow area reduces the incidence of blockage. However, we can also offer non-standard bearings with special flute designs to suit the criteria of different applications, including where spiral flutes, either right or left hand start, are required.

Traditionally, rubber lined water lubricated bearings have a brass shell which offers structural stability, durablility and strength. Brass shell bearings are stable in the environment in which they operate and are relatively easy to fit and remove.
A non-metallic alternative is a phenolic shell, which is produced from a paper-based material to form a strong durable tube. Non-metallic shells are essential where corrosion or chemical reaction is possible which could lead to galvanic corrosion, for instance in aluminium and steel sterngear. Due to the nature of the phenolic material, these bearings swell when immersed in water. All phenolic shell bearings are initially designed with a formulated clearance to allow for this swelling, using our advanced computer system.
We can also offer, as standard, a Glass Reinforced Polyester shell (GRP) which offers the same advantages as the phenolic shell bearing but has minimal swell. GRP shell bearings can also be used to avoid galvanic corrosion.
Bearings can also be produced, to customer specification, using materials such as Stainless Steel 316, other stainless steel grades, higher grade bronzes, mild steels, cast irons and duplex alloys. We can work entirely to customer requirements and will source any specified shell material where possible.
We are able to produce bearings with an integral cast flange, as required to customer specifications, and can also supply bearings with additional features such as water inlet holes, lips, internal and external steps and milled slots.

Sizes
For correct operation of the bearing it is essential that, after fitting, a clearance exists between the shaft and the bore of the bearing, to provide for lubrication and dimensional movement due to changes in temperature and moisture content. When a bearing is press fitted into the housing, there will be a reduction in bore size approximately equal to the interference between shell and housing.
Countrose bearing sizes refer to the nominal size of the shaft. The actual internal size of the bearing includes a minimum clearance, calculated by Countrose, based on shaft and housing sizes machined to ISO286-2 tolerances. These sizes must be accurate if the minimum clearance is to be maintained after fitting.
Clearances
The clearances designed into our standard range of bearings are based upon the shaft being manufactured to ISO g6 tolerances. Plain bearings are supplied machined on the outside diameter generally to ISO p6 tolerances, to suit a housing which is within ISO H7 tolerances. This assumes that the bearing will be press fitted, as the reduction in bore diameter will be small enough to be accommodated within the designed clearances. Incorrect housing diameter will lead either to a loose bearing or the insertion force being too great, resulting in damage to the bearing and excessive bore closure.
Where deviations from the above shaft tolerances are necessary, the displacement should be towards a smaller shaft so that the minimum clearance is not reduced.
Fitting
It is important to ensure that the housings, bearings and shaft are all correctly aligned along the same axis. Any misalignment between the bearing bore and the shaft will effectively reduce the running clearance available to the bearing and may cause problems in operation, such as vibration or excessive wear.
Generally bearings are pressed into their housings with an interference fit. Alternatively, a light push fit can be used and the bearing retained by a radial grub screw, partially penetrating the shell. In this type of assembly care must be taken that the screw does not distort the shell or interfere with the lining.
Flanged bearings are similarly assembled with a light push fit to control concentricity, and retained by bolts or screws in the flange.
If it becomes necessary to establish the clearance prior to installation the bearing can be placed freely on the shaft with a dial test indicator at its TDC (Top Dead Centre) position. Lifting the bearing to its highest position relative to the shaft will indicate the clearance. To measure the clearance after interference fitting, a dial indicator may be used in a similar way, to measure free movement of the shaft.