Do you know how to gauge grease consistency and performance?

Lubricating grease is more than thick or viscous oil; it is thickened oil. The thickness of lubricating grease can vary greatly depending upon several factors. A thickener system turns lubricating oil into grease. Also, the fluid component could be watery like milk or viscous like honey. Likewise, base oil viscosities in greases vary. Further investigation shows the relationship between fluids and thickeners in lubricating grease formulations will influence field performance.

Proper grease lubrication involves careful selection of thickener chemistry, base oil type, base oil viscosity, National Lubricating Grease Institute’s (NLGI) penetration grade and additives to achieve the desired characteristics.

When troubleshooting grease failure modes, such as bearing leakage, a natural conclusion may be the application requires a harder grease. While manipulating thickener content and thus the consistency of the grease seems like the common-sense option, raising the grease’s base oil viscosity may provide a better technical solution by increasing film thickness.

Understanding which grease characteristic to adjust requires technical knowledge about the lubricated application including how it is pumped and applied.

Worked penetration is correlated to NLGI Grade

The NLGI grade of a grease is determined on a scale (see Figure 1) of the worked penetration (60 strokes) of a grease measured in cone displacement in 0.1mm increments taken at 25 ºC (77°F) in accordance with ASTM D217. The worked penetration value is then correlated to the NLGI grade.

The NLGI grade defines the actual consistency or hardness of the grease. For illustration, the most prevalent NLGI grade, 2, has a worked penetration range of 265-295 mm/10, like the consistency of peanut butter. NLGI 1 grade greases have a worked penetration value of 310-340 mm/10, with a similar consistency to mustard. The more thickener content a grease has, the harder its consistency will be, resulting in less cone penetration and a numerically higher NLGI grade.

NLGI 2- and 3-grade greases can possess improved water resistance, high-temperature stability, and better oil retention than their lower NLGI grade counterparts, but higher NLGI grade greases typically will exhibit poorer pumpability. Lower NLGI 1, 0 and 00 grade greases typically pump more readily and have better low temperature properties. For this reason, many applications using automated lubrication systems with piping less than one-quarter inch diameters are better suited for NLGI 1 grade or softer greases. This ensures proper lubricant delivery to the application point without overstressing distribution pumps or blocking lube lines which could cause failures (see Figure 1).

Figure1: Higher NLGI grade greases typically will exhibit poorer pumpability. Lower NLGI 1, 0 and 00 grade greases typically pump more readily and have better low temperature properties. Many applications using automated lubrication systems with piping less than one-quarter inch diameters are better suited for NLGI 1 grade or softer greases. Courtesy: JAX Inc.

Base oil is critical for industrial lubricating greases

Applications without pumpability concerns that require long-term retention and stability such as valves or seals, will need a grease with a higher thickener content (see Figure 2). NLGI 3 grade grease, which has a consistency like shortening, possesses enough thickness to lubricate in addition to providing a stable barrier from contamination.

Figure 2: Most industrial greases are more than 80% base oil, so it’s important to understand that the base oil component of grease is the most fundamental element protecting and separating lubricated contact surfaces. Courtesy: JAX Inc.

Because most industrial greases are comprised of more than 80% base oil, it’s important to understand that the base oil component of grease is the most fundamental element protecting and separating lubricated contact surfaces. The oil film thickness between moving surfaces is primarily linked to the base oil viscosity, so basic fluid lubrication principles apply.

Base oil viscosity is determined utilizing ASTM D445 which measures the length of time for a liquid to flow through a capillary tube under gravity. Viscosity is measured centistokes (cSt) and recorded at 40ºC to 100ºC (104°F to 212°F), with the viscosity in cSt at 40°C used to determine the ISO grade.

Base oils of low viscosity will provide thinner film thickness and flow more readily. Applications with high speeds will require a lower viscosity base oil (see Figure 3). This prevents unwanted churning, increased amperage draw and other failure modes caused by greases with an excessive base oil viscosity. Excessive viscosity forms a lubricating film too thick for the gap between the lubricated surfaces.

Figure 3: Applications with high speeds require a lower viscosity base oil to provide thinner film thickness and flow more readily. This prevents unwanted churning, increased amperage draw and other failure modes caused by greases with an excessive base oil viscosity. Courtesy: JAX Inc.

Greases used in lower temperature environments also benefit from lower viscosity base oil in the formulation. As temperatures decrease, the viscosity of the base oil will increase. If the increase is too large for the application, the same failure modes as described above will arise. A lower oil viscosity, typically ISO VG 68 or less, will possess a lower temperature pour point than higher viscosities of the same base oil type.

Lower speed and higher loaded applications can benefit from higher viscosity base oils providing a thicker and appropriate level of lubricating oil film within the contact zone. Also, applications with higher operating temperatures will require higher base oil viscosities to account for the expected loss in film thickness. Many elevated temperature applications utilize an ISO VG 220 oil or higher for this reason.

Just remember, increasing the base oil viscosity in a grease formulation from an ISO VG 68 to an ISO VG 460 will not affect the hardness or the consistency of the NLGI grade. An NLGI 2 grade grease can be manufactured with a thin ISO VG 22 base oil or a viscous ISO VG 1500 base oil (see Chart 1).

Chart 1: NLGI grade and worked penetration value ranges as compared to food items of similar consistency. Courtesy: JAX Inc.

While increasing the viscosity causes little to no change in the grease in terms of consistency, it will always yield application performance results which will affect suitability for specific temperatures and specific application speeds. The earlier illustration of higher base oil viscosity for greater film thickness to reduce oil leakage corrects a deficiency without having to manipulate the NLGI grade of the grease, possibly creating a different problem.

Industrial examples of grades and base oil viscosities on grease

With a better understanding of the functions and effects of varying penetration grades and base oil viscosities on grease and how each influences the performance characteristics of a grease, let’s explore some specific applications and their typical grease requirements.

Grease applications needing an NLGI 2 or 3 grade, including steel mill bearings and open gear applications, will typically require high-viscosity base oils in combination with a stiffer consistency. Steel mill applications such as continuous casters and rolling mills, can exhibit constant operating temperatures above 121ºC (250°F), making high-temperature stability a priority.

Because these applications are low speed as well, the correct grease will be formulated with oils having a viscosity typically in the ISO VG 220-460 range, providing ample film thickness even in elevated operating temperatures. Open gear applications such as on drawbridges are also relatively low speed and need the higher NLGI grade for good stability and adhesion, along with a considerably higher base oil viscosity to provide a thicker protective film between gear teeth.

When to use various grades and viscosities

A common application requiring a harder NLGI grade grease but lower base oil viscosity is electric motor bearings. Electric motor bearings are typically comprised of ball bearings moving at high speeds, resulting in numerous lubricated contact zones. It is important that the grease contains a lower base oil viscosity for the higher speed environment but is also stiffer, typically NLGI 2 or 3, for appropriate channeling characteristics. In this case, it is detrimental to fill the void left in the contact zone of the bearing with thickener. If the grease is soft enough to “flow back” and fill the void in the contact zone, the non-channeling grease will cause churning and fluid friction in the bearing. This results in excessive heat, damaging temperature increases and a shorter bearing life.

An example of a softer NLGI grade grease with a high viscosity is an oil field fracking pump plunger grease. This grease will typically have a soft consistency, NLGI 00 or 000, along with a base oil viscosity in the ISO VG 320-1000 range (see Figure 4). The high viscosity is necessary to provide a thick, long-lasting film that will provide protection from friction, corrosion and heat. The typical NLGI 00 and 000 grade are necessary for maximum flow and pumpability, acting as semi-fluid for the pump plungers, which will have a lower risk of leakage compared to utilizing a pure fluid lubricant.

Colder temperature applications, typically below -12.2ºC (10°F), will require a softer NLGI grade along with a lower base oil viscosity (see Figure 4). As mentioned previously, as operating temperatures decrease, base oil viscosity will increase, and the consistency of the grease will also stiffen. If the consistency of the grease becomes too hard and suddenly an NLGI 2 grease has thickened into the NLGI 4 range, the added thickness may cause application failure, especially in a bearing. In the food industry, chillers and freezers contain bearings that experience temperatures below -51.1ºC (-60°F). The correct grease will have a synthetic base oil that exhibits a low viscosity (ISO VG 46 or lower), along with an NLGI grade in the 0-1 range. This ensures the grease lubricant will have an acceptable consistency and film thickness in the bearing at low operating temperatures.

Figure 4: The correct grease for colder temperature applications will have a synthetic base oil that exhibits a low viscosity (ISO VG 46 or lower), along with an NLGI grade in the 0-1 range. This ensures the grease lubricant will have an acceptable consistency and film thickness in the bearing at low operating temperatures. Fracking pump plunger grease, NLGI00, ISO VG 1000. Courtesy: JAX Inc.

The effects of base oil viscosity on grease thickness

Having a better understanding of the working environment and conditions lubricating grease is experiencing and how it is applied is paramount to proper grease selection. If a “thicker grease” is recommended as a solution to a lubricant issue, it is important to understand how manipulating the base oil viscosity and penetration have effects on consistency and film thickness independent of each other. Performance and pumpability side effects arise from each change. Utilizing a grease with the appropriate combination of consistency and base oil viscosity will not only provide the short-term benefits of less maintenance and better conservation but will also achieve the long-term goal of maximizing machinery life.

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