- Buyer's Guide
Even in the most simple production environments, there is usually a reason for selecting several different greases to account for varying operating conditions, similar to using different oils. But time and time again, I find companies using one multipurpose grease, either through a lack of knowledge or misguided advice.
That's not to say that greases formulated and marketed as multipurpose greases are inherently bad; they are not. Unfortunately, the over-reliance on these products can cause real problems.
To understand this issue, consider the selection of the appropriate lubricating oils for a steam turbine operating at 3,000 rpm, and a multistage gear reducer with an output shaft speed of 20 rpm. For the steam turbine, the logical selection would be a rust and oxidation (R&O)-inhibited turbine, probably with an ISO viscosity grade of 32 or 46 centistokes (cSt).
The logic behind selecting the R&O type oil is to provide enhanced oxidation stability to achieve a long change-out interval coupled with the ability to withstand the deleterious effects of water in the event of moisture problems that are common in many steam turbine applications. In selecting an ISO VG 32 or 46, the lube engineer is looking at the speed of normal operation in conjunction with the dynamic load being supported by the shaft such that a full, hydrodynamic oil film is maintained at normal operating loads, speeds and temperatures.
Gear Oil Selection
In choosing the gear oil, the considerations may be more complicated and will depend on factors such as the gear geometry, reduction ratios and pitch line velocities. However, the bottom line follows the steam turbine example, which is the engineering aspects of the problem are considered before arriving at the appropriate choice. In this case, the choice would be a gear oil in the ISO VG 320 to 680 range, probably with some kind of extreme pressure (EP) additive package, given the low speeds and possibly high loads.
To use a gear oil in the turbine application would cause major problems. Among other issues, there would be inadequate flow to the bearings and high temperatures due to fluid friction. There would also be negative effects of the additives on the demulsibility and foam inhibition, and the potential for corrosive wear caused if an active EP additive is used.
Likewise, using turbine oil in the gearbox would be just as disastrous! Loss of oil film and inadequate EP protection would rapidly progress to severe wear and catastrophic failure in a matter of hours. Sufficient to say that few people would even think of using a high-viscosity EP gear oil in a steam turbine application.
Proper Grease Selection
Now apply the same logic to selecting the proper grease for a machine driven by an electric motor with grease-lubricated bearings operating at 1,700 rpm. This machine is also coupled to a gearbox with greased seals via a grease-lubricated coupling, driving a conveyor with double-tapered roller pillow block bearings turning at 50 rpm. Now let's consider the possible effects of using a single, multipurpose grease for all four (motor bearings, coupling, greased seals, pillow block bearings) grease points.
Generally, multipurpose greases are formulated from simple soaps, although some manufacturers use complex soaps, typically as an NLGI #2 grease. Additionally, they are often formulated with Group I mineral oils with an equivalent ISO VG around 150 to 220, and may contain antiwear (AW) or EP additives for mixed and boundary lubrication conditions. So how do these "average" properties affect this hypothetical machine?
For the electric motor, an ISO VG of 220 is a little high for most electric motor bearings. While this may not cause immediate failure, excess internal friction can cause an increase in temperature, possibly resulting in an increased energy consumption required to overcome fluid friction.
Likewise, depending on the chemical and structural stability of the thickener, there may be a change in consistency (either up or down) during the long time intervals between electric motor regreasing. In addition, some EP additives found in multipurpose greases have been shown to volatilize and migrate in the windings of the motor causing corrosion.
For the coupling, similar arguments apply. While higher point loading may indicate that the viscosity grade is fine, centrifugal force, combined with a long relubrication interval, may imply that the multipurpose grease thickener is unable to maintain its consistency.
For the gearbox shaft seals, the primary role of the grease is to seal out contaminants. In this instance, the multipurpose grease will likely perform admirably and is a good, economical choice.
As for the 50 rpm pillow block bearings, depending on the amount of loading (particularly if shock loading is present), a multipurpose grease with an ISO viscosity grade of 220 cSt may be too low. The location of the bearing in either high or low ambient temperatures may also dictate the use of a synthetic base oil rather than the Group I mineral oil, or a different NLGI grade in the event that the grease will be pumped for long distances.
Consider the Factors
The point of this exercise is not to give specific lubricant recommendations - there are always exceptions to any rule - but rather to suggest that diligence should be used when selecting greases. While there is no doubt that there are some excellent general-purpose greases on the market, consider whether the advantages of consolidation provided by multipurpose greases are outweighed by the potential negative aspects of a "jack of all trades, master of none" approach to lubricant selection.
As always, this is my opinion. I'm interested in yours.