Your Google fu has deserted you Michael. Here’s the link. No need to be a member

Tribology in mechanical watches

Should be fun! Questions about clock and watch oil tend to generate strong (religious) opinions on the  NAWCC forum.

Just put:

” Is finger popping cavitation?”

into Google and read some of the results, many from well established medical sources, and all appear to confirm that the popping of fingers is indeed cavitation of dissolved gasses such as nitrogen and carbon dioxide in the synovial fluid in the joint.

Maybe I wasn’t misled on this occasion.

I’m sorry Dave, there is only one correct definition of cavitation, it is not open to interpretation.  Cavitation is not the damage caused by the rapidly created and collapsing bubbles of gas in a rapidly pressure reduced fluid.  That is called cavitation erosion.  Cavitation is only the process of the gas bubbles forming and collapsing quickly.  This process can occur on the trailing face of a propeller blade and, frequently inside the casing and impeller of, in particular centrifugal sea water pumps, and may or may not lead to cavitation erosion.  Cavitation can also lead to loss of propeller performance without necessarily damaging the propeller.  There also seems to be generally a blurring of two quite separate issues, that have both seem to be described as cavitation.  The second one being when a propeller is turned so quickly that the cavitation that exists to a greater or lesser degree in most marine propellers on the negative pressure face becomes so excessive that the cavitation occurring on the rear face of one blade starts to affect the positive face of the next blade.  When this happens the positive face also becomes a negative pressure area and the slip jumps to 100%.  Basically the propeller spins in its own created volume of negative pressure and produces zero thrust.

Cavitation is also what is occurring withing the synovial fluid in a finger joint when you stretch the joint, creating the low pressure liquid and hence the sudden creation of the gas bubbles, which then collapse quickly.  Again this does not necessarily lead to cavitation erosion in the joint but it is cavitation that is creating the pop.  As far as I am aware cavitation is strictly only the creation and sudden collapse of the gas bubbles within a fluid as the pressure is very quickly reduced.

Frequently multiple definitions and confusion are created over time by those who refuse to accept that they may occasionally be wrong.

Dave, you are wrong, I have explained why, you refuse to accept it, hiding behind perceived multiple definitions doesn’t change anything, I have moved on.

For what little it is worth … My human biology lecturer 1968-1971 described the knuckle-cracking phenomenon thusly:

Some individuals have a tendency to develop bubbles within the synovial fluid, which are perceived as an uncomfortable pressurisation of the joint. The action of cracking the knuckle bursts these large bubbles back into solution, and relieves the discomfort.

Frankly, I don’t expect this [or rubber suckers] to feature in the Tribology Zoom on 19-Nov …

MichaelG.

No, SoD, it is you that has the wrong end of the stick. Cavitation is not the damage caused. It is the formation of bubbles of vapour (cavities) in a low pressure region. As the cavity moves to a region of higher pressure the bubbles implode suddenly and violently (cavitation collapse). If this happens adjacent to a solid surface it causes ‘cavitation erosion’ of the surface.

Another common place that suffers is the delivery cone throat in boiler feed injectors.

To add some drift to this topic, waterside erosion of wet cylinder liners can be reduced by changing the location of the gudgeon pin, slightly, within the piston, to reduce piston slap. This is a combination of gas pressures and mechanical impact shaking ferrite out of the liner material.

(Liners were often made by centrifugal casting of old rail chairs).

Not really cavitation erosion as such.

The excess fuel (Shuttle pump) version of the CAV DPA never made it into production because of the cavitation erosion of the hydraulic head. This produced a “shadow” impression of the port alongside it, leading to leakage (Which at 600 bar can be very significant – enough to prevent cold starting)

A huge problem, because the material was already double nitrided, so VERY hard.

The fuel cavitated and the bursting bubbles caused the erosion.

Howard