Water for Submerged Bearing Lubrication 2

[MDMintern]

I only have limited knowledge about bearing systems but I am looking for a way to hold a long rotating shaft in place. I am looking to find out what kind of bearing would be best to run submerged in water that can also use water as a lubricant.

I am interested in finding out what experienced engineers think about this concept and if any one knows any info about this or how I can get info.

Thanks,
-Matt-

 

[unclesyd]

Here are two suppliers of water lubricated bearings. The Cutless is the most common form of these type bearings.

http://www.vesconite.com/index.htm

http://www.duramax-marine.com/

One of the oldest bearing materials is “Ligum Vita” a hardwood that was extensively used for many years and is still in use in some applications.

 

[rnd2]

Matt,
Long shafts, supported by water lubricated bearings are well known and find common application for high speed rotating shafts in the marine industry as propeller shaft bearings for conventional powered craft. Other common application are for rudder bearings. These are of course low speed.
In the mining, off-shore oil and process industries, water lubricated bearings support very long shafts that are used in vertical rise pumps.
High speed water lubricated bearings require a fluid film to be present in order to minimise contact other wise rapid overheating will result. Well designed water lubricated bearing systems aim to keep the overall bearing surface pressure as low as possible. It is common in the marine industry for the propeller shaft bearing length to be four times the diameter of the shaft, in fact many marine societies demand this.
The reason for maintaining long bearing lengths relative to the shaft is that unlike oil, water has very low viscosity and can only support a thin fluid film.
Nevertheless, a properly designed high speed water lubricated bearing system can efficiently produce fluid film conditions. Thus the shaft does not come into contact with the bearing and there is neglible wear.
Water immersed bearings on the other hand do not operate where fluid film conditions exist so to describe them as water lubricated is, in my opinion, inaccurate.
In these applications the shaft is always in contact with the bearing. In this case, depending on the composition of the bearing, water may actually be detrimental and will wash and leach away certain additives that may normally benefit a dry bearing. Selection of the correct water immersed bearing material is therefore very important.
Abrasive particles are detrimental to the life of all water lubricated and water immersed bearings as well as the shafts they suport. It is always advisable to design the system so that abrasive particles are excluded or at least are kept to a minimum. There are many ways to achieve this. In some instances it is not practical so elastomeric bearings may be employed. Elastomeric bearings, much like car tyres, momentarily spread around the particle and thus reduce bearing wear by deforming. While useful in some applications, where there are sharp particles and grinding abrasion and water, elastomeric bearings wear quickly. Another approach is to protect the shaft with a sacrificial sleeve and employ as hard a bearing as possible and design the bearing material so that as wear occurs, fresh extremely hard smooth particles are exposed.

 

[MDMintern]

Thank you both for the info and time, it is very useful.
-Matt-

 

[amorrison]

Wood bearing article in "machine Design" mag.

http://www.machinedesign.com/ASP/viewSelectedArticle.asp?strArticleId=56831&strSite=MDSite&catId=0