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Before experts recommend any gearbox oil as such they would always analyze and inspect the onsite equipment when it is operating normally. Usually, suggestions regarding oil viscosity for the application could be made based on the AGMA (American Gear Manufacturers Association) 9005-E02 standard or the ANSI (American National Standards Institute) standard.
What does this standard need?
For this standard, you need to know the linear pitch speed of the lowest-speed gear match for a multi-speed gearbox or the linear pitch speed for a single-reduction gearbox. It is likely that you would get this information from the specifications provided by the original equipment manufacturer.
What else to do in this regard?
The abovementioned standard is an important part of the process of choosing the right gearbox oil. If you do not know it you would have to measure the diameter of the gears at the pitch line and then use this formula:
“Diameter (meters) x 3.142 x revolutions per minute (RPM)/60 = meters per second”
So, if your speed is between 1 and 2.5 meters per second, an ISO VG 680 would be suggested for oil temperatures around 65 degrees C (Celsius). In case the oil temperatures are between 55 and 60 degrees C you would need an ISO VG 460.
So, what can be presumed based on these numbers?
On the basis of the abovementioned numbers, one can presume that gearbox oil of higher viscosity would not be needed at the temperature ranges that have been indicated over here. However, you need to consider other factors over here as well. For example, it would be important to use an EP (extreme pressure) formula over here.
- Increasing the viscosity grade
If the equipment experiences frequent shocks, reverse operations, and stops or starts you may want to increase the viscosity grade by one. This would also be recommended in cases where the pitch-line speed is lower than a meter per second.
- Switching to PAOs
While choosing gearbox oil you can always switch over to a synthetic PAO over a polyglycol from a petroleum-based oil as that can provide you with desirable results. This is because PAOs are rather compatible with mineral oils which means that you do not need an oil flush when you are changing to this kind of synthetic oil.
- The issue with polyglycol
Polyglycols, on the contrary, are quite often not compatible with mineral oils. This means that if you start using them you would have to flush the equipment thoroughly with the same polyglycol base stock. Keep this in mind when you are choosing gearbox oil. It is also important in this context to verify how compatible the synthetic lubricant is with paint or gaskets that would be in contact with the oil.
This factor is perhaps of utmost importance when it comes to the context that we are talking about over here. This is because it plays a determining role in the formation of the lubricant film. It determines the film thickness within the interacting surfaces at a given load and speed. For most of the methods used to select the right viscosity, it is hard to determine that load though. In the case of gearbox oil, the extent to which viscosity changes with temperature is dependent on the base oil. Now the base oil can be mineral oil, ester, polyalphaolefin, or polyglycol. Apart from these, the VI improper additives, which the lubricant in question has, play an important regard in this context as well.
One of the most common methods of determining viscosity is a standard named ANSI/AGMA 9005-E02 created by both these bodies. The factors used in this method are load, pressure-viscosity coefficient, and viscosity index of the lubricant.
There are a lot of variations when it comes to gearbox oil but by and large, they can be classed into three categories – R&O (rust and oxidation), compounded, and antiscuff. Now, the conditions in which you are operating your device would determine the additive that you choose for your gear oils. If you want to use R&O, you will need to individually assess the anti-corrosion properties of the gear oils so that you can get corrosion protection in the case of the likes of copper and steel. Antiscuff is commonly known as EP (extreme pressure) lubricants, and they are better than R&O oils in some ways.
- Oil clotting and adrenaline effect
We all know about the clotting of blood and the role that it plays in restricting blood flow after any injury. This is important when it comes to choosing gearbox oil as well. If you do not want the oil to transport gummy and hard particles to critical orifices and clearances it is better to take a hygienic approach. This means that you need to keep the oil dry, healthy, and clean. This is rather common advice but still needs to be repeated from time to time.
- The type of oil that you need for lubrication
As far as gearbox oil goes synthetic oils normally have wider operating temperatures as well as the rate of oxidation. However, mineral oils are said to be suitable for the majority of these applications. You can also formulate oil in case of special applications such as beverages and food. There is some possibility that synthetic lubrication would have a greater life expectancy compared to mineral oil. You must also make sure that you consider factors such as the viscosity, oxidation rate, operating temperature, and application requirement of the oil when you are selecting the same.
Finally, you would have to determine if the temperature measured was taken on the gearbox surface. If it is indeed the case the temperature of the oil within the gearbox would be five to eight degrees higher. In that case, you may have to install a cooling system so that you can have better control over the temperature of the lubricant. You must also think of the possibility of using combined failure modes in parallel when you are choosing gearbox oil. In such cases though further root cause analysis would be needed.