Reasons for the decrease in the dynamic viscosity of lubricating oil

1. Rising temperature: Check the cooling performance of the lubricating oil cooler; if the tubes in the cooler are blocked, they should be cleared promptly. Increase the pressure difference between the inlet and outlet water of the cooler as well as the amount of water used for cooling. 2. Oxidation and deterioration. 3. Reduced viscosity due to the effect of shear forces. 4. Over time, the molecular chains of the viscosity agents shorten, resulting in a decrease in viscosity. Clean the crankcase and replace it with new lubricating oil. Mechanical friction within the crankcases of compressors and piston pumps, as well as in components such as crossheads, connecting rods, cranks, main bearing shells, big end bearings, small end bearings, piston rods, and wiper rings, generates mechanical impurities. There may also be debris that has broken off from the inner surface of the bearing housings. Have a mechanic inspect the situation and replace any components that are severely worn, while adjusting the clearances between various bearings and between the crosshead and its sliding tracks

To what extent does a decrease in the kinematic viscosity of lubricating oil affect equipment – does it impact the formation of the oil film? Does a value slightly below the standard range have a particularly significant effect on the equipment?

Should you conduct the inspection yourselves or have a professional company do it? It’s best to send the oil sample to a professional testing company for a comprehensive analysis.

It depends on what type of bearings are used in your device. In the case of rolling bearings, there is both rolling friction and sliding friction inside them; at high rotation speeds, poor lubrication can lead to wear, temperature rise, burning, and ultimately complete damage. If the viscosity of the oil is too low, when the bearings are under high pressure, it is easy for the lubricating oil film to break down, resulting in dry friction and increased wear ； If the viscosity of the lubricating oil is too high, it will increase the frictional resistance in the bearings, leading to an increase in oil temperature. In more severe cases, it will affect the flow of the oil, making it difficult to form an oil film, which is harmful to the bearings. For sliding bearings, the choice of oil viscosity depends on the diameter of the shaft journals, the rotation speed of the shaft, and the load per unit area. Its viscosity range is from L—AN15 to L—AN68. For bearings with high rotational speeds and large shaft journal diameters, the viscosity of the oil can be lower; conversely, a higher viscosity of the oil is required. The journal rotates within the bearing bush; due to the clearance in the bearing bush and the influence of external factors, eddy currents are generated in the bearing, causing the position of the journal inside the bearing bush to become unstable. Under certain oil pressure and viscosity conditions, lubricating oil accumulates to form an oil film. This oil film exerts a certain resistance; when this resistance is greater than the vortex force, the vortices are suppressed, causing the disturbed shaft journal to return to its original position. Such vortices are convergent, meaning that the rotation of the shaft journal within the bearing is stable. On the other hand, when the oil film force is less than the vortex force, the movement of the shaft journal becomes unstable. As the speed increases further, oil film oscillations occur, and these oscillations can cause significant damage to the shaft journal and bearing surfaces. It is necessary to select lubricating oil with the appropriate viscosity based on the equipment’s speed and the size of the shaft journals. Lower viscosity may lead to oil film vortices in equipment with lower speeds, resulting in increased vibration amplitude, while higher speeds can cause oil film oscillations that lead to severe vibrations and issues such as bearing damage. Therefore, when managing equipment, it is essential to pay close attention to every detail and not compromise on quality standards

Generally speaking, a viscosity reduction of 15% or an increase of up to 20% still allows for continued use, but it is necessary to increase the frequency of testing; once these values are exceeded, all or part of the components should be replaced immediately. The above statements are taken from the engine maintenance manual of a European brand. For your equipment, it is best to have the compressor manufacturer or the compressor bearing manufacturer confirm this. As for the decrease in lubricant viscosity, could it be caused by compressed medium entering the crankcase? In a case I encountered earlier, it was still the same engine; the viscosity decreased by 8%. It was later estimated that fuel oil may have mixed into the lubricating oil in the crankcase, resulting in a decrease in the viscosity of the lubricating oil. The manufacturer racked their brains trying to figure out how the fuel oil got into the crankcase, but after taking samples and leaving them there for 48 hours, stratification did occur, with the viscosity of the upper layer being significantly lower.

“Generally speaking, a viscosity reduction of 15% or an increase of up to 20% still allows for continued use, but it is necessary to increase the frequency of testing; once these values are exceeded, all or part of the components should be replaced immediately. " Learned: handshake. Thank you

We analyzed it on our own, and we conducted another test as well; the results were the same. We also sample the new oil purchased regularly, and there are no issues with it

Is there a mistake? The answer given by Bai Yunfan is the best one; I’m just learning*