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"My question is regarding lubricant selection for a heavily loaded, slow-speed gearbox that faces dropping viscosity and increasing wear issues. The gearbox is used in a steel melt shop and has a forced lubrication system with an operating temperature of 52 degrees C. The manufacturer's recommendation is a 680 viscosity grade (VG) mineral oil, but should we choose a 1000 VG polyalphaolefin (PAO) oil or a 1000 VG mineral oil with plastic deformation (PD) additives?"
Ideally, an inspection and analysis of the onsite equipment during normal operation would be preferred before a lubricant recommendation is made. In general, an oil viscosity for your application can be suggested based on the American National Standards Institute (ANSI)/American Gear Manufacturers Association (AGMA) 9005-E02 standard. This standard requires knowing the linear pitch speed for a single-reduction gearbox or the linear pitch speed of the lowest-speed gear match for a multi-stage gearbox. This information likely can be obtained from the original equipment manufacturer's specifications. Otherwise, it will be necessary to measure the diameter of the gears at the pitch line and then use the following formula: diameter (meters) x 3.142 x revolutions per minute (RPM)/60 = meters per second.
If the speed is 1-2.5 meters per second, an ISO VG 460 would be suggested for oil temperatures between 55 and 60 degrees C, while an ISO VG 680 would be preferred for oil temperatures around 65 degrees C.
Based on these numbers, it can be presumed that a higher viscosity is not needed at the indicated temperature ranges. However, other factors must be considered. For instance, it will be important to use an extreme-pressure (EP) formula. If the equipment has frequent shocks, start/stops or reverse operation, you may wish to increase by one viscosity grade. This also would be recommended if the pitch-line speed is less than 1 meter per second.
Switching from a petroleum-based oil to a synthetic PAO or polyglycol can provide good results. PAOs are very compatible with mineral oils, so an oil flush usually is not required when changing to this type of synthetic oil. On the other hand, polyglycols often are not compatible with mineral oils, which means a thorough flushing with the same polyglycol base stock would be necessary. Also, be sure to verify the compatibility of the synthetic lubricant with the gaskets or paint that will be in contact with the oil.
Finally, determine if the measured temperature was taken on the gearbox surface. If so, the oil inside the gearbox may be 5-8 degrees higher. You might need to install a cooling system to better control the lubricant temperature. Also, consider the possibility of combined failure modes in parallel. In this case, further root-cause analysis may be required.