The danger of cross-reference charts with greases
Cross-referencing greases is dangerous because just specifying the NLGI grade and EP additization doesn't guarantee they are equivalent.
There are industry standard specifications for engine oils, gear oils, hydraulic oils, turbine oils, etc. Standardized specifications are wonderful for the end user, simplifying product choice and ensuring a certain level of performance. This also facilitates cross-referencing, and ultimately, a degree of product commoditization.
Greases are different. There have been very few industry standard specifications, and as a result no two greases are exactly alike. Grease performance can vary not just due to formulation differences, but also as a result of the manufacturing equipment and processing used. The wide variety of grease applications creates a need for product differentiation, but makes it difficult to compare performance of the many greases in the marketplace.
What makes oils and grease so different?
Rheology: Grease stays put until there is sufficient stress (yield stress) by mechanical force, heat, centrifugal force, etc. at which point, grease flows.
The rheological behavior of grease is a function of many variables, including but not limited to thickener type and content, base oil type and viscosity, manufacturing processing differences, etc. The addition of structure modifiers (polymers) will further impact the rheology.
Oils are simpler
Oils are mostly defined by base oil quality and viscosity as well as additive performance. There is no thickener and no rheology differences to speak of. Oil lubricants do not have the challenges and variables in manufacturing that grease does. There are no chemical reactions, specified heating and cooling rates, milling and other processing variables to control as there are with grease. The result is more consistent properties from batch to batch.
As a consequence, there are numerous industry, as well as OEM specifications and standards:
Auto engine oils – API, ILSAC and ACEA service ratings (SL, SM, SN, etc.)
Auto gear oils – API service ratings (GL 4, GL 5, MT1, etc.)
Steam or gas turbine oils – D4304 standard spec for mineral steam or gas turbine oils
Hydraulic oils – D6158 standard spec for mineral hydraulic oils
Gear oils –AGMA 9005-E02
Grease specs and standards
Unlike the many specifications and standards for lubricating oils, there are relatively few for greases.
NLGI GC-LB (ASTM D4950) which specifies several levels of performance for automotive chassis and wheel bearing service (after-market) applications was developed in the 1960s and 1970s, before many of today’s test methods were developed and it is focused only on the automobile industry.
DIN 51825 provides a classification system according to performance levels and formulation components, but no certification system
Railroad standards EN1208, AAR M-914 and M-949
OEM specs also exist for some specific types of equipment
The new NLGI High-Performance Multiuse (HPM) grease standard and certification program was introduced in late 2020. This standard is more rigorous than the GC-LB certification, and is aimed at multiuse industrial applications. In addition to the “core” HPM standard, there are also 4 additional “tags” which provide enhanced performance in load carrying (HL), water resistance (WR), corrosion resistance (CR) and low temperature performance (LT). A grease can be certified to the core (HPM) standard, or the core plus any of the four “tags.”
Cross reference charts
The proliferation of standard specifications for lubricating oils has led to development and publication of many “cross reference” charts:
Read-across for greases is dangerous. Simply specifying the NLGI grade and EP additization does not guarantee these greases are in any way equivalent to one another. If their base oils are not the same type or viscosity, they are not equivalent. If they have different performance properties (anti rust, load carrying, water resistance, etc.), they are not equivalent. Even if these greases all had identical formulations, they would probably perform differently due to how each was manufactured.
For example, two electric motor greases appear very similar according to product data sheet claims, but perform very differently in mechanical stability and oil bleed tests:
NLGI High-Performance Multiuse (HPM) grease specification and certification program
The key to the HPM certification is that the specification is more extensive and challenging than typical grease specifications (including GC-LB), and requires validation of performance by NLGI through independent quality certification and testing agents. And while using HPM certified greases will provide a level of assurance of very similar performance with one another, which may facilitate consolidation, this still does not assure different HPM certified greases are compatible with one another. Since the HPM specification is “chemistry neutral”, different thickeners may be used to successfully meet the specifications, which introduces the potential for incompatibility. Compatibility testing between greases during product change overs is critical.