Synthetic or semi-synthetic greases add performance advantages in terms of oxidative and thermal stability, according to Theisen. This is important for high-temperature applications, but synthetics also help somewhat in low temperatures.
Geyer points out that synthetics can do well in both extremes. "PAO-based synthetic oils give the grease much better flow properties in extreme cold than mineral-based oils." When used in conjunction with a lithium complex thickener, they also have higher heat capability. "If you have an application that is breaking down the mineral oil-based grease due to excessive heat, you could try a grease containing synthetic base oils as a correction to the problem."
Temperature variations also affect pumpability of the grease. Typically, a NLGI 2 grade is used in most construction equipment applications during the summer months. In cold climates, grease is then switched to a lighter viscosity NLGI 1 grade for the winter months.
Automatic lubrication systems require the use of even lighter grades of grease, typically NLGI 0 or NLGI 00 grades. NLGI 1 grade grease can sometimes be used, depending upon the environment. Automatic systems often will not tolerate the heavier NLGI grades normally used for manual greasing. "The central systems are not as friendly when it comes to using higher viscosity oils when you are trying to pump them greater distances," Geyer points out.
For example, say the automatic centralized system is pumping grease through a 1/4-in. line that runs up a 25-ft. boom and down a 25-ft. pivot boom on the other side to the bucket. "The system is trying to pump a thick product through a very small opening," Geyer explains. "You get a lot of resistance." Climate often plays a serious role here, as well.
Autolube systems compensate for the lighter viscosities by providing a continuous supply of fresh grease, which purges contaminants from the joints. "Keep in mind that a central system continually feeds grease to the application throughout the work day as compared to once or twice a day manually," says Betner.
With synthetic-based grease, you may be able to use a slightly heavier NLGI grade in the autolube system due to the grease's enhanced flow characteristics. However, use of an autolube system will require the stocking of separate greases.
Thickener affects compatibility
Greases are thickened by various types of soap; this thickness helps determine the NLGI grade. "If the grease is a NLGI 2 product, to make a NLGI 1, you reduce the percentage of thickener or soap content by adding more oil," Geyer explains. "In essence, the grease is getting softer."
The thickener type also determines compatibility. "Out of the three components of the grease - oil, thickener and additives - the thickener has the greatest impact on compatibility," says Betner. Poor compatibility can result in excessive softening or hardening of the grease.
"It's always best to err on the side of caution when changing out greases," Geyer emphasizes. "Ask the lubricant supplier if there will be any concerns."
Many different thickeners can be used. "Typically, the most common greases of choice in this area are lithium or lithium complex thickened greases," says Geyer. "The reason is that lithium and lithium complex soaps are generally fairly compatible with one another. You have fewer issues than with some of the other soap types that are out there."
Lithium-based products make up the largest percentage of greases used. Of these, lithium complex greases are the most popular. "The lithium complex with the proper additives would be your premium-type product. It has good high-temperature properties. It has good water washout resistance and it is a very stable product," says Theisen.
Lithium 12 hydroxystearate is a type of generic lithium grease, but it has a lower dropping point. It is therefore not well suited to higher temperature applications.
Poly urea-based greases are another product gaining in popularity. These greases also provide high temperature life and water resistance. "But if you are using lithium-based grease and go to poly urea-based grease, you really have to flush out the bearings in the system to make sure you don't run into compatibility issues," says Theisen.