Ah I see Jose, I know what you mean, let me have a go at breaking down what Glyn has said, lubricants are a little like Chemistry to me I can't get my head around the "Physics" of it so I can't visualise it in me head. Glyn know more than the average person about oil (a lot more) he is a Tribiologist (look it up on Google, I had to, now it's my favorite new word til the next one comes up) so what is simple to Glyn is like brain surgery to me when it comes to oils. Anyway let me have a go :-


When grease is applied to surfaces in motion to one another (in this case rotation) it bleeds oil & additives onto the moving parts to lubricate them. When the motion stops the soap/thickener re absorbs the oil until the next requirement triggered by relative movement/motion.
Grease sort of becomes more fluid when moving parts pick it up and move it around, the slippery bits acts more like an oil and gets between the parts and lubricates as it works, but when the motion stops the grease does not run off like oil it returns to it's original state, until the parts start moving again.

Greases are designed for when reapplication of lubricant is infrequent or never as in the many sealed for life bearings, ball joints etc. etc. on the market. Grease is also designed (thickener dependent) to seal out water and contaminants.
Greases stay within the mechanism as they are designed to be thicker, sealed bearings stop contaminates entering the bearing, open race bearings often use grease that is thicker still so the grease actually seals the bearing up, my understanding of bearings that need regreasing through a grease nipple is to apply more fresh grease which pushes out the contaminated grease which can contain dirt and moisture depending on the use of the bearing, rather than the grease is no longer any use itself.

Grease can have a very long life in service. They are not just "rubbed off" ` They migrate straight back into the interface to be lubricated & provide a certain "film strength". Variable by type & design/formulation.
When say a gear in a mechanism starts moving, the "slippery" lubricating components of the grease are taken into the gears and do they job they are intended to do.

Certain surface active additives can be blended into grease that would require you to machine off the surface layer of the metal or whatever to get rid of them.
You will have heard of Molyslip additives I am sure, this is the sort of components that are within any grease, these components actually get into the surface of the metal and stay there, you cannot wipe them off, it's almost a chemical reaction that you cannot reverse and you would have to remove the surface of the metal to remove these components.

You almost have to think of grease and oil as a carrier for the other bits that do all the actual work , a bit like you and I carrying a virus, we are the Grease, the virus is the slippery stuff within the grease / oil, the main difference being that the virus is not good for us, but the slippery bits are.

Lubricants come in so many different forms, the grease or oil is used to carry these to where they are needed and keep them there or to allow them to flow around, it is the carrier that has different viscosities and temperature ranges more so than the additives as far as I am aware.

Glyn will probably correct me in my vastly poor adaptation of this very complex subject, this is not completely technically correct, but my best shot at my simple explanation, I learn more about lubrication as time goes on, but I will never understand it fully.

I hope this helps and does not get Glyn banging his head on his desk and pulling his hair out with exasperation ! I will probably get shot down for this explanation, but hey ho !