The use of composite materials offers an alternative to traditional press and freeze fitting that ensures not only accurate alignment but also long-term secure location of the bearing. Alice Jucquois reports
The critical requirement with rudder bearings relates to the bush retention in the housing. Because the bush is manufactured from a different metal to the bearing block, local galvanic corrosion can occur, which causes the housing to become over-sized until eventually the bush can no longer be held in the correct alignment. This misalignment can result in the development of annular gaps between the housing and the bearing bush, resulting in excessive and premature bearing wear, a malfunctioning rudder and, consequently, costly repairs and loss of sea time.
Due to a combination of severe vibration, constant fretting and an aerated sea water environment, bearing installation can be a real challenge. The ideal installation would provide a perfect seal between the bush and the housing while offering a complete corrosion protection.
Traditional fitting techniques
When replacing rudder bearings on existing boats it is difficult to match the inside and outside dimensions of the old bearing. The three conventional techniques of fitting rudder bearings are press fitting, freeze fitting and heating the housing.
The first technique, press fitting, consists of pressing the bearing into the housing. The ease of fitting will vary depending on the finish of the housing and the size of the bearing. However, the pressing requires a large force, proportionate with the size of the bearing, and can damage a bearing that is not in line when fitting.
The freeze fitting or shrink fitting technique can be used as an alternative method. The two methods of freezing using liquid nitrogen are full immersion and vapour freezing. In full immersion, the bearing is placed in a container which is then filled with liquid nitrogen until reduction of the bearing to the required diameter. Once the required diameter has been achieved, it can be removed from the nitrogen and will then be held in place in the housing until the bearing returns to its original size. However, this method is very slow and time consuming because it takes a long time to reduce the bearing diameter and to generate enough clearance for fitting. The vapour freezing can also be used to fit the bearing by vaporising liquid nitrogen directly within the bearing until reduction. It will then be placed in the housing until returns to its original size. This method is more cost effective and safer than the immersion method but the rate of bearing contraction is a lot lower.
Using the same concept, another fitting technique consists in heating the housing in order to expand it. This technique involves hot work and can incur health and safety issues and thus requires an assessment of the risks before carrying out, which can be highly time consuming.
While these methods are very effective, there will be limitations in the level of corrosion protection they provide. Indeed, the bush and the housing are made of dissimilar material and they will remain in direct contact, causing the problem of local galvanic corrosion to re-occur. All the processes will require accurate machining of housing and/or bush to ensure clearances are within tolerance for installation.
Composite chocking for bearing fitting
Composite chocking offers an alternative to traditional press and freeze fitting that ensures not only accurate alignment but also long-term secure location of the bearing. Nowadays, polymeric composites and coatings are widely used in the Marine industry for the reseating or installation of new rudder bearings and pintle liners. Due to their excellent durability, high compressive and adhesive strength, low moisture absorption and electrical insulation characteristics, composite materials are ideal for the stable installation and permanent bonding of rudder bushes. Isolating the bearing into position, Belzona resins and paste grade composites provide a 100% contact between the bush and the housing, ensuring a permanent vibration-free location, and eliminating galvanic and crevice corrosion.
Application methods of composite materials
Belzona recommends two techniques for bearing installation with composite materials: the slip method and injection method. When installing the bearing, special attention should be given to the method used to align and centralise the bearing bush within the damaged housing. The techniques described below can be used to rebuild, realign and electrically isolate many types of bearing bush housings, including:
* Rudder bush housings
* Internal stern tube, bush housings
* A-frame bracket bush housings
* Stabiliser in shaft bearing bush housings
* Oversized cutlass bearing housings
a) Slip method
The slip method consists of simply sliding the bearing into the housing. A film of composite material is applied to the prepared inside surface of the housing and also to the outside surface of the bush. Further material is applied in excess to the internal surface of the housing to prevent air entrapment after assembly.
The bush is then jacked into position in the housing and correctly aligned. Any excess material extrudes during assembly will need to be removed immediately.
b) Injection method
Bearing bonding can be carried out by injecting the material between the bearing and the housing. The Belzona shim takes up any ovality or housing wear, thus creating a durable barrier with 100% surface contact, electrically isolating the bearing. This method has been applied to multiple situations including riser bearings and bushes amongst others.
The liner or bush should be aligned in the housing. The annular space between the bush and the housing is sealed where required to prevent loss of product during the injection process. This may be accomplished by mechanical means or by utilizing a fast curing paste grade product. Injection holes should be carefully positioned together with vent holes to prevent the formation of air traps; ideally, injection points should be spaced no more than 600 mm (24 inches) apart.
The selected material is loaded into disposable injection cartridges and injected using pneumatic equipment. The application progresses from the lowest to the highest injection point. Material is injected until it is exuded from the next highest injection point, at which time the first hole is sealed with a suitable bung and injection should begin at the next hole. This process continues until the annular space between the bush and the housing is completely filled with the chocking material. This is indicated by leakage of material through small vent holes drilled at the highest point available.
Case study – Belzona bearing installation in oversized bearing housing
In June 2010, a ship in a Bulgarian dockyard required installation of a new bronze bearing in an oversized bearing housing. Predominantly, severe corrosion and impact wear had led to the destruction of the old bronze bearing and irregular form of the housing leading to increased annular space between the new manufactured bearing and the housing. As a result, a new bronze bush bearing was manufactured and had to be reseated in the irregular bearing housing.
Bonding the bush in place with Belzona materials is a proven application in the Marine industry. The use of Belzona composite materials for installation of rudder bearings began in 1977 with collaboration between Belzona and Germanischer Lloyd. The project investigated the use of Belzona materials to eliminate the problem of galvanic corrosion between steel housings and rudder bushes, pintle cones and liners. It was proven to be a time and cost saving procedure in comparison with traditional repair, which would have involved welding and boring.
Belzona’s injection method was chosen as other alternatives would have incurred longer downtime. The injection of Belzona 1321 (Ceramic S-Metal), a two-part ceramic filled epoxy material designed to provide erosion and corrosion resistance of metal surfaces, was done through flexible hose placed into the four channels made in the back of the bearing.
Savings were made using this solution against expensive machining of the housing and supply of larger bronze bush. This standard repair procedure eliminates galvanic corrosion and provides long term location of the bearing.
Alice Jucquois is with Belzona Polymerics Ltd.