Lubrication

How the NLGI HPM specification facilitates grease selection

Proper selection can be challenging as multiple criteria must be considered when selecting a lubricating grease.

By Dr. Gareth Fish August 10, 2021
Figure 1: Comparison of NLGI GC-LB and HPM core grease properties. Courtesy: NLGI

The basic property needed for a lubricating grease is to separate the moving surfaces. This reduces friction and energy consumption, prevents wear from occurring, and gives components excellent durability. All greases should offer this basic protection provided they have the correct base oil viscosity. Other necessary properties are oxidation resistance, rust and corrosion protection, and good structural stability. A grease that breaks down or collapses and runs out of the bearing will not offer long term protection, requiring shorter intervals between re-greasing and resulting in increased operating costs. The benefit of using a grease certified to National Lubricating Grease Institute’s (NLGI’s) new High-Performance Multiuse (HPM) grease specification is the grease will have other necessary properties built into the grease that are currently lacking in the GC-LB specification requirements.

Background

In 1989, the NLGI launched its new specification and industry certification program for automotive service grease used for chassis application and universal joints (LB), wheel bearings (GC), and multi-purpose (GC-LB) service greases. With the growth of sealed-for-life applications in vehicles, the need for automotive service greases for passenger cars and light trucks has declined dramatically. Grease marketers have used the GC-LB certification as a sign of a superior quality for higher performing greases and the specification has become more of sign of quality on any grease for industrial or automotive applications.

When launched, GC-LB approved greases had higher performance than the typical greases in use in the late 1980s. Over the next 25 years, it was slowly recognized that in many areas of performance, GC-LB greases lacked certain performance characteristics needed for more modern industrial machinery applications. In addition, better performing raw materials were used in grease formulations. GC-LB later became the minimum performance requirement with many desired lubricating property requirements unmet as shown in Figure 1.

HPM specification objectives

The aim of the new NLGI HPM specification, introduced in November 2020, is to fill the gaps in the performance profile that GC-LB greases lack. The current GC-LB specification does not adequately cover the structural stability of the grease and it does not cover higher temperature oil bleed. Concerns over softening due to shear and higher temperature oil bleed that can be present in current GC-LB grease are mitigated by the new HPM specification that includes both shear and roll stability requirements. In many pieces of industrial machinery, yellow metals such as brass or bronze can be used for bushings or bearing cages. The yellow metal components are prone to corrosion. The new HPM specification includes the requirement for the grease to have low copper activity that would prevent yellow metal corrosion from occurring.

The current GC-LB specification requires the grease to have a very wide temperature range from -40°C to 160°C (-40°F to 320°F). The wider range adds cost to typical grease formulations, requiring a high temperature thickening system and a base oil blend with low temperature capabilities. For most industrial applications, the required limit is -20°C to 120°C (-4°F to 248°F) and normal temperature thickeners and base oils can be used, along with appropriate specified testing requirements. The elastomer compatibility for the HPM specification is modified and compatibility is tested on a more representative elastomer (nitrile rubber) at a moderate and more realistic test temperature of 125°C (257°F) for a duration of 168 hours.

With respect to rust testing and water resistance, the new HPM specification has both requirements enhanced. The limit on water washout is tightened to a more realistic requirement for industrial applications and the grease is required to pass static and dynamic rust tests.

When running under higher load or in a stop/start condition where it is difficult to keep the mating surfaces fully separated, enhanced wear and load carrying are essential properties of any high-performance grease. The new specification requires passing results in both of these areas. The increased weld point as measured by the 4-ball extreme pressure test also brings the grease into line with international standards.

Looking ahead

Because a user must consider how the grease is to be delivered to the bearing or mechanism, a softer grease is typically chosen for use in a centralized grease distribution system. Assuming that the equipment is to be lubricated with grease from a hand grease gun or an automatic lubricator, a lubricating grease certified to the new NLGI HPM grease specification would be a good choice.

Adopting a grease meeting the new NLGI HPM grease specification should give the user the additional piece of mind that the gaps in the GC-LB specification have been plugged. Better performance, longer life, and extended re-greasing intervals should be delivered by greases meeting the new NLGI HPM grease specification. The use of the NLGI HPM specification does not eliminate the need to consult with a lubrication expert to make the correct base oil viscosity choice and other special requirements.


Dr. Gareth Fish
Author Bio: Dr. Gareth Fish is a technical fellow in the Industrial Additives Division of Lubrizol Corp., where he has worked since June 2007, employed initially as grease technology manager. He is a chartered scientist, Society of Tribologists and Lubrication Engineers (STLE)-certified lubrication specialist (CLS) and NLGI certified lubricating grease specialist (CLGS). Fish has written more than 60 technical papers on grease and tribology, three book chapters, holds four U.S. patents, and has held 80 public classes on grease and tribology.