My question concerns the lubrication of a stainless shaft to a bronze bearing on a verticle turbine 1760 rpm. does pure water lub as well as raw lake water (filtered)?. Also what part of pure water does the lubricating.

Lubrication is generally a function of viscosity. Minimum viscosity required to support film lubrication increases with load and decreases with speed. Viscosity or water at room temperature is 0.9 cStokes. (note viscosity of VG32 oil at 100C is only ~ 5cStokes).

I believe due to the low load in typical vertical application, the water viscosity is above the minimum required viscosity for effective film lubrication. In the event that viscosity speed and load conditions are such that film lubrication cannot be supported, boundary lubrication occurs, there will be wear of the bronze bearing. Bronze of course is much softer than the shaft to prevent shaft damage.

Hydroplaning in a car is an example of low friction due to "fluid film" lubrication based on water viscosity, in a low-load, high relative speed environment.

Note cooling is another function performed by lubricants including water.

This message has been edited. Last edited by: electricpete,

If the viscosity of the two are +/- the same, another consideration in lubrication is additives and/or contamination.

Are there any minerals present in your raw lake water that should not be a desired "additive" in your application?

raw lake water was used as an example to pure water becaused I know that lake water works to lub vert turbines well,but i dont know how well pure water lubs, I thought raw water lubed better because of the impurities in the water. Seems I all wet with my thinking. Thanks electricpete & Mr Vader for your imput. Neal

There are two forces acting in a hydrodynamic bearing, radial and tangential. In the vertical turbine the force of the journal is mainly tangential so is appropiate to use the same work fluid as a lubricant.

Tangential force is directly proportional to viscosity and length of the bearing so, while lowest is the viscosity of the lubricant then higher has to be the length of the bearing. In the other hand, the tangential force is inversely proportional to the radial clearance, then you have to be carefull with the high radial clearance because the hydrodynamic pressure decrease dramatically at reason of the square.