Let’s talk about cavitation.
I have been trying to convince people that oil that is at a temperature less than that of normal operation is bad. People often say that oil when at 75 F and 200 cS thick is good for their engine. I say that oil starvation occurs. This is by the process of cavitation. The oil, even at 75 F is too thick to move through the oil ways at speed.
The system is designed for an oil of 10 cS thickness. A vacuum occurs in the pump and oil is not pumped. The gears simply rotate without oil movement. Furthermore there is a lot of heat generated that occurs locally that is detrimental to the metal parts and to the oil in the area causing oxidation, thickening and sludge formation. Many state that the difference in oil thickness between 3 and 4 cS is a BIG difference. Why then is the difference between 3 and 200 or 300 not a logarithmic difference for those small oil ways?
Our nuclear submarines have the same problem. The propellers can actually rotate so fast that cavitation and heat generation occurs. When needing to travel at very high speeds (60 MPH) they must be at a certain depth (deeper equals cooler water) so that the water can cool the propellers. Otherwise they melt (and become ineffective for propulsion as well). There are maximum speeds for the given temperature they are operating at.
People only think temperatures well below freezing are a problem for oil but it is also ROOM temperatures that are bad. It is my feeling that oil below 50 C (122 F) cavitate at more than 1,000 RPM and hence cause the sever wear that occurs at start-up. Only oils that are below 15 - 20 cS can be moved without cavitation in automotive applications.
This is why I pay more attention to pour point and viscosities at 40 C than all other people involved in the oils discussions here.
aehaas
I have been trying to convince people that oil that is at a temperature less than that of normal operation is bad. People often say that oil when at 75 F and 200 cS thick is good for their engine. I say that oil starvation occurs. This is by the process of cavitation. The oil, even at 75 F is too thick to move through the oil ways at speed.
The system is designed for an oil of 10 cS thickness. A vacuum occurs in the pump and oil is not pumped. The gears simply rotate without oil movement. Furthermore there is a lot of heat generated that occurs locally that is detrimental to the metal parts and to the oil in the area causing oxidation, thickening and sludge formation. Many state that the difference in oil thickness between 3 and 4 cS is a BIG difference. Why then is the difference between 3 and 200 or 300 not a logarithmic difference for those small oil ways?
Our nuclear submarines have the same problem. The propellers can actually rotate so fast that cavitation and heat generation occurs. When needing to travel at very high speeds (60 MPH) they must be at a certain depth (deeper equals cooler water) so that the water can cool the propellers. Otherwise they melt (and become ineffective for propulsion as well). There are maximum speeds for the given temperature they are operating at.
People only think temperatures well below freezing are a problem for oil but it is also ROOM temperatures that are bad. It is my feeling that oil below 50 C (122 F) cavitate at more than 1,000 RPM and hence cause the sever wear that occurs at start-up. Only oils that are below 15 - 20 cS can be moved without cavitation in automotive applications.
This is why I pay more attention to pour point and viscosities at 40 C than all other people involved in the oils discussions here.
aehaas
