Lubricating grease consistency

A grease consistency or penetration score indicates grease consistency or stiffness to help predict how it will migrate, stay, or flow in a typical grease application. Therefore, lubricating grease consistency or NLGI grade is the first and foremost requirement. During this test, a double tapered cone is allowed to sink freely into a grease cup for 5 seconds at 25°C under its weight. Finally, the cone’s distance traveled into the grease is measured and reported as penetration in a tenth of a millimeter (0.1 mm). Thus, penetration is the depth, measured to an accuracy of 0.1 mm to which a standard cone sinks into the grease. For example, a 28.5mm cone’s distance is a 285 penetration score (285x 0.1 mm). In short, the cone will penetrate more into a soft grease. Therefore, a higher penetration value indicates a softer lubricant.

As per NLGI, greases are available in nine consistency grades ranging from fluid (NLGI 000) to a semi-fluid to a firm block (NLGI 6). Therefore, grease penetration is calculated with the NLGI cone penetration chart using ASTM D217 Standard Test Methods for Cone Penetration of Lubricating Grease

NLGI Consistency # Worked Penetration in 0.1mm [*] Visual Characteristics Food Analogy
000 445-475 Very soft- just enough thickener to keep oil from running Ketchup
00 400-430 Very soft- just enough thickener to keep oil from running Applesauce
0 355-385 Soft- Still flowing Brown Mustard
1 310-340 Soft Tomato Paste
2 265-295 Normal grease (like a cream) Peanut Butter
3 220-250 Semi-solid Vegetable Shortening
4 175-205 Moderately Stiff Frozen Yogurt
5 130-160 Stiff Smooth Pate
6 85-115 Very stiff (Hard) Cheddar Cheese

[*] Penetration worked @25°C ASTM D217, DIN ISO 2137 in 0.1mm

Some grease suppliers use descriptions such as NLGI Number 1.5, which indicates that the grease is between NLGI Numbers 1 and 2.

Penetration procedures to measure grease consistency

  • Undisturbed penetration
  • Unworked penetration
  • Work penetration @60 double stroke
  • Prolonged work penetration
Grease Penetration
Grease Penetration

Undisturbed penetration

In undisturbed penetration, the penetration value is measured without any disturbance. It is not a standard procedure but used for evaluation of hardening or softening of grease during storage.

Unworked penetration

Unworked penetration is frequently reported along with worked penetration @60 strokes. A grease sample is transferred from the grease container to the test cup with minimum disturbance. It is not considered definite, but Less difference between unworked and worked @60 strokes is a desired property of grease.

Work penetration @60 double strokes

Worked penetration @60 double strokes is a standard reporting method. In this method, at the temperature of 25°C, grease is worked in a standard grease worker for 60 times (60 double strokes). Tested immediately using cone penetration, worked penetration @60 stroke is generally considered a standard.

Prolonged work penetration

This procedure tests the penetration after worked at 10,000, 100,000 or higher double strokes at the temperature of 25-30°C. Prolong work penetration is a common practice to predict the stability of grease. After worked with a grease worker, the grease sample is brought back to 25°C in one and a half hours. The grease sample is subject to worked again at 60 strokes at 25°C followed by penetration recorded.

Grease Worker

Motorized grease worker

Grease Consistency (grades) and applications

NLGI Consistency number 000

Consistency number 000 grade grease is like a liquid with the least amount of thickener. Excellent for the lubrication of sliding surfaces, guides, open-gear lubrication, and outdoor equipment with long lubrication pipes in centralized lubrication systems requiring low-temperature pumpability. It has many uses in single-line lubrication systems of commercial vehicles. In the same way, it is a preferred lubricant for wheel-flanges in stationary and mobile lubricating equipment.

NLGI Consistency number 00

NLGI 00 is also a liquid-like grease used for open gear, spur gears with friction surface lubrication, and centralized lubrication systems requiring low temperature and long-distance pumpability. Good for spur, bevel, and worm gears in open or enclosed gearboxes operating under normal and severe conditions. Preferred for the lubrication of sliding surfaces, guides, sliding bearings, and roller bearings required frequent relubrication.

NLGI Consistency number 0

A semi-fluid grease that is soft and still flowing with good low-temperature pumpability in centralized lubrication systems. Excellent lubricant for loaded roller sliding bearing. Recommended for the lubrication of needles and multiple row roller bearings, chains, and flexible chain couplings. Enclosed and open gearboxes or enclosed roller and plain bearings.

NLGI Consistency number 1

NLGI 1 grease has a soft, tomato-paste-like consistency. An excellent lubricant for the lubrication of needles and multiple row roller bearings. Similarly found uses in automotive wheel bearings and most electric drive applications. 

NLGI Consistency number 2

Number 2 has a peanut-butter-like consistency, and it is a widely used lubricating grease consistency. NLGI 2 grade is a standard grease for most industrial and automotive applications. In the same way, Lithium grease in NLGI 2 grade is the most common multipurpose grease for automotive, industrial, and marine applications. Excellent lubricant for the plain and antifriction bearings operating under moderate load and at medium speeds. It is a good lubricant for use in most flexible couplings.

NLGI Consistency number 3

NLGI 3 grease has a semi-solid appearance and is a common lubricant in automotive, industrial, marine, and agricultural applications. Excellent for antifriction bearings, wheel bearings, factory sealed ball bearings, and some flexible couplings. Grade #3 greases have good sealing properties to protect the equipment and surfaces against pollution, corrosion, and environmental impact.

NLGI Consistency number 4

NLGI Grade 4 grease has a moderately stiff consistency. Hence, it is a common lubricant for water pumps lubrication. Number 4 grease has uses in agricultural equipment, water pumps, and other high-speed, lightly loaded applications.

NLGI Consistency number 5

NLGI grade number 5 grease has lower bleeding or oil separation. Hence, it is a stiff grease with a consistency like smooth pate. Lithium grease in NLGI 4 grade has less than 0.4% oil separations and a further lower bleeding score in NLGI-5 grade. Lubricating greases in NLGI 5 grades are not very common but have an excellent performance in applications where a low score of bleeding or oil separation is required. For example, in overhead conductors and high-speed applications.

NLGI Consistency number 6

NLGI 6 is not a common grease grade. Lubricating grease with NLGI-6 has a very stiff consistency like a block of Cheddar Cheese. A lubricating grease with extremely low oil separation has found its uses in Pillow-block lubrication. It is also an excellent lubricant in high-speed applications where extremely low oil separation is a must.

References:

ASTM D217

NLGI Grade

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