Freeze Fitting Rudder Bearings
As merchant ships get larger and faster there are increasing demands on steering systems and rudder bearing reliability, new freeze fitting methods are able to offer many advantages.
Although not normally considered when owners specify the makers list, there is a growing realisation that a malfunctioning rudder can cripple a vessel, with costly repairs and loss of sea time.
The critical requirement with rudder bearings relates to the bush retention in the rudder housing. If the bearing turns or jams on the rudder shaft, this can cause the bearing to move out of the housing if the retaining plate shears.
Orkot Marine Bearings, a member company of Trelleborg Sealing Solutions Ltd, has developed several advanced methods of fitting and retaining its bearings and devised sophisticated design programs that calculate the hoop stress required to retain the bearings under all conditions including Ice class vessels.
||Method 1: Freeze fit bearing with insulated blanket
||Bearing after fitting into stainless steel sleeve
The three normal techniques of fitting rudder bearings are press fitting, resin chocking and freeze fitting. The first of these requires a large force to press the bearing into the housing (at least 42 tonnes for a large container ship's 1200mm diameter bush) and can damage a bearing that is not in line when fitting.
Resin chocking is favoured by some shipbuilders as upper and lower pintle housings do not have to be accurately aligned during machining. Builders can either bore the housings in-line after welding, which is expensive and time consuming, or pre-machine them before assembly. Since this cannot guarantee alignment, bearings are assembled with a gap to the housing and aligned before pouring the chocking compound, which cures to form the housing and makes bearing removal on refit difficult by requiring it to be cut out.
Freeze fitting can be used most effectively with non-metallic bearings such as composite materials. Bearings can be frozen in solid carbon dioxide but liquid nitrogen (LN) is recommended to achieve the required diameter reduction with a composite bearing. The two methods of freezing using LN are vapour freezing and the more standard full immersion, which is where Orkot has developed some new thinking.
First, the vapour system uses the bearing as the freezing tank (method 1). Wooden end plates seal the bearing and LN is introduced using a thin lance, with an insulating blanket reducing heat transfer. This method reduces the amount of LN used and is safe as the operator is not in contact with it. However, this method is very slow because it takes a long time to reduce the bearing diameter and generate enough clearance for fitting.
Fully immersing the bearing in LN is more usual and can be done using a couple of existing container methods.
The method is to make a container able to withstand -197°C and accommodate the bearing. LN is poured in to cover the bearing and must be topped up to maintain this level since it will initially 'bail' off as the temperature reduces. When the LN reduces to a 'simmer' the bearing must be raised to check its diameter and make sure that there is sufficient clearance for fitting.
The bearing is then inserted into the rudder housing or into a machined metal sleeve which is then resin chocked into the rudder housing. While this method reduces refit problems it is costly and difficult to machine accurately. Another downside is a larger and more expensive rudder housing is required.
|Method 2: Bearing being
frozen in liquid nitrogen
|Bearing being removed
from liquid nitrogen
|The sleeve which will
accept the frozen bearing
The new LN freeze-fit method developed by Orkot uses a 3mm thin wall stainless steel housing rolled and welded without machining and offers an array of advanced benefits. The rolled sleeve is joined with an external axial stainless steel strip that is also used as the anti-rotation key after fitting the sleeve and bearing.
This sleeve is 20mm shorter than the bearing to reduce seawater corrosion after fitting to the vessel. It is re-rolled after welding to improve the roundness and then measured to allow the fitting calculation. The relative modulus of both the steel and bearing are taken into the calculation program, ensuring that the same bearing retention is achieved as in the conventional system.
The bearing is then frozen and when sufficiently reduced in diameter it is assembled into the sleeve. As the bearing expands on normalisation it rounds the sleeve as it is stressed.
||This particular bearing was produced for use in a large container ship and it is the largest ever fitted – 1200mm diameter
Because of the relative weight of Orkot TLMM bearing material to steel, the bearing can be finish machined after fitting. This reduces the maximum wear before refitting, thereby allowing for more wear-down before Approval societies require the bearing to be refitted. It is supplied in this condition to the shipyard ready for chocking.
Shot-blasting the sleeve/bearing assembly's steel housing with clean shot at the shipyard just prior to fitting is recommended to ensure getting a good bond between the sleeve and chocking compound. The gap between the sleeve and housing should be filled with a flexible sealant at both ends to protect the sleeve.
With this method refitting a new bearing is easily achieved by cutting two opposing axial slots in the bearing with a disc cutter to remove the hoop stress and the bearing halves can then be removed leaving the metal sleeve in place. New bearings should be freeze fitted to prevent the sleeve from moving in the housing.
When using a non-metallic bearing, such as Orkot TLMM or TXMM, it is essential for ship owners to have confidence in the material. TLMM has full class approval at 10n/mm2 from all class societies. Its compressive strength of >300N/mm2 is not seen as a particularly useful figure, with the material yield (elastic limit) of around 100N/mm2 being more important and giving a substantial factor of safety.
Other benefits of Orkot non-metallic bearing material are its sealing capabilities to prevent housing corrosion, increased elasticity over bronze, insulation against galvanic corrosion, self-lubrication and dimensional stability in seawater.
Over 900 vessels have been fitted successfully using Orkot Marine rudder bearing systems and methods.