Dear All

Please tell me how can we define oil separation ? And how we can relate the to performance of any grease?

Romesh

Good question, I work with grease dispensing systems, and separation has often been a problem for my customers. Romesh, you are probably more interested in separation during operation (inside a bearing), I am interested in separation uder various conditions encoutered in a dispensing system.

For example,
A grease manufacturer's data sheet says the grease has 4.8% oil separation in 24 hrs at 100 Deg C.
Does that really mean it loses 4.8% of its oil content per day? In less than a week it would be unusable! But we don't store and dispense grease at 100 DegC. What affect does ambient temperature have?
We have found that storage under pressure (inside a pump or distribution header) causes faster separation. Also Velocity has an effect, increased flow velocity as grease flows through an orifice, valve etc. causes solids to be left behind.

So, is there any information available on the affects of temperature, pressure, and velocity on various greases? Are some base oils more likely to give up thier solids than others? Are some solids harder to keep in suspension than others?

If there is some manufacturer out there who could provide some useful data or an article on this, I would love to publish it on our web site.

Greases will separate some oil under high loads and temperature. Grease thickener may reabsorb part of the oil sometimes, but mostly, it will cause the grease to dry out. There are standarized tests for oil separation under temperature and pressure. If your application is affected by grease drying out and you think it is for oil separation, ask your grease supplier for separation test results, and choose a grease with a higher separation stability. Some thickeners are more stable, like polyurea or lithium complex, as I recall, but perhaps somebody else has more insight on which greases are more stable for oil separation. Couplings are affected by oil separation due to centrifugal forces and temperature. Research into coupling greases which of them is more stable for oil separation.

If oil separation is a problem buy a grease that has a ASTM D-1742 (Oil Separaton) rating of 0.00% .