Frequently asked questions
While the NLGI HPM grease specifications are designed to ensure a high level of performance for many different applications (multiuse), they are not suitable for all applications. We consider that multiuse applications may include the following:
|Conveyor Bearings||Hinges||Electric Motor Bearings|
|Pillow Block Fan Bearings||Rollers||Thrust Bearings|
|Industrial Gearbox Bearings||Presses||Pump Bearings|
|Plain Bearings||Rolling Element Bearings|
What is critical to remember is that the HPM specification should not to be used as a short cut to grease selection. Each application should be evaluated for its requirements, based on speed, load, temperature and operating environment. These parameters will affect the choice of base oil viscosity, consistency, thickener type and additive needs. An HPM-certified grease may or may not meet these requirements. Consult an application engineer from your grease supplier to find out if an HPM grease is right for your application.
The enhanced performance tags which may be included with the HPM core specification (+WR, +CR, +HL, +LT) are designed to provide improved performance in the areas of water resistance, corrosion resistance, load-carrying, or low temperature performance compared with the core HPM specification. However, note that the HPM specifications may not ensure adequate performance for some specialized applications, such as those with extremely high loads, extremely high or low temperatures, extremely high speeds, extremely high water wash conditions, or any combination of the above.
Since greases in some applications experience certain conditions (e.g., low temperature or exposure to liquids), different sub-categories (tags) with additional performance requirements were defined as part of the overall specification. In these sub-categories, a grease must meet the performance of the core HPM grease specification plus the additional testing related to (a) water resistance (+WR), and/or (b) high load-carrying capacity (+HL), and/or (c) salt water corrosion resistance (+CR), and/or (d) low temperature performance (+LT).
NLGI continues to work on a “Long Life” sub-category but this specification will require a new grease life test method, which is not expected until the 2023-2025 timeframe. Just for reference, we often talk about the additional performance requirements as “tags” attached to the “core HPM grease specification.”
The limits chosen for the tests in the HPM specifications were selected in one of several ways:
- When the original drafts of the specifications were developed, limits were selected based on ASTM D4950, readily available data from public sources, and industry marketing specifications and claims. These sources were responsible for the majority of the original draft limits.
- During an extensive interview process conducted with end users, GC-LB certification holders and OEMs, we received feedback which led to adjustments to some of the proposed limits. In the process of gathering feedback on the HPM specifications, a philosophy emerged pertaining to limit setting: “challenging, but achievable.”
- If there was not good agreement on the proposed limits, the steering committee looked internally at data from their individual companies which could be shared among the committee, and based revised limits on consensus analysis of that data.
- A virtual workshop was held March 25, 2020 with participation from 45 stakeholders from the grease community. Through a lengthy process conducted by the steering committee, the feedback was distilled, evaluated, and finally used to further adjust limits in order to gain better consensus.
- And finally, where there was insufficient data or experience available within the steering committee or other stakeholders to feel comfortable with the proposed limits, we decided to conduct “validation testing”. In this program, volunteers from within the steering committee identified products in their portfolios which they felt would meet HPM, or HPM+ specs, and then ran tests to validate the draft limits. Examples of tests where “validation testing” helped to confirm limits:
- HPM: Elastomer compatibility (ASTM D4289)
b. HPM: Low temperature torque of ball bearing grease
@ -20 °C (ASTM D1478)
c. HPM+WR: Wet Roll Stability (ASTM D8022)
d .HPM+HL: 4 ball Wear (ASTM D2266)
e. HPM+LT: Grease mobility
@ -20 °C (U.S. Steel method LT37)
- HPM: Elastomer compatibility (ASTM D4289)
It should be noted that NLGI views the HPM and HPM+ specifications as living documents. In other words, as we gain experience and more data becomes available, test specifications may be adjusted, deleted, or added as deemed appropriate. One example is the D4170 fretting wear test, which currently does not require compliance until ASTM corrects bearing and method precision issues. Once ASTM completes its work, NLGI would add that test back to the HPM specification requirements.
The HPM grease specifications will not satisfy all applications or industries. Since specialized applications typically have their own specifications, the new HPM grease specification will not replace those specialized specifications. However, a user or manufacturer creating specialized specifications may build upon the HPM or HPM+ grease specifications for their individualized specification.
Since the HPM and HPM+ specifications are performance specifications, they are “chemistry neutral”. There are no formulation requirements for these specifications. Because they are “chemistry neutral”, there is a possibility for different HPM-certified greases to be incompatible, even if the greases are from the same supplier.
By definition, two lubricating greases are incompatible when a mixture of the products has physical or performance properties that are inferior to those of the individual greases. When two greases are incompatible, some mixtures will soften, leading to leakage through seals and away from lubricated surfaces. Some mixtures will cause the thickener to release the oil, and the separated oil may run from the bearing housing. Less often, mixtures may harden, and cause lubricant starvation and poor grease mobility. Grease incompatibility can lead to lubrication and equipment failure and is therefore an important issue.
Operators and end users may not always be aware of the risks involved when considering a change of grease in an application. This is true regardless of whether the grease(s) involved are HPM greases or any other greases. Therefore, it is incumbent upon the grease user to ensure compatibility if changing greases. Generally, grease suppliers can provide compatibility information, if asked. The use of publicly available grease compatibility charts is not recommended, as these generalize compatibility based solely on thickener types, whereas incompatibility can be the result of incompatible base oils or additives as well. It is always the safest choice to have the specific greases evaluated via laboratory testing.
IF NLGI were to include some sort of compatibility requirement in the HPM specifications, it would likely lead to exclusion of some types of grease thickener technologies, which goes against the HPM specification design principles. And since grease compatibility predictions should not be based on generic thickener-based compatibility charts, specific grease pairs should be tested. Given the wide variety of chemistries utilized in grease products, there is no way to include such a compatibility requirement in the HPM specifications.
Biobased grease is not addressed by the HPM grease specification. Users or specifiers can define their own requirements as to whether a grease must meet certain bio-based requirements. Early in the HPM grease project, it was noted that there are bio-based grease products that meet the GC-LB specification without any modification to the test procedures or limits. Therefore, the HPM grease specification does not differentiate between bio-based and other grease products. All greases must meet the performance limits defined by the core HPM grease specification and the applicable sub-categories.