NLGI Consistency Grades for Bearing Grease

Grease is the one bearing component most buyers never specify consciously. It arrives sealed inside the bearing, or it comes from a gun at the end of a routine, and yet the choice of consistency and chemistry decides friction, leakage, the re-lubrication interval and, in a sealed-for-life part, the entire service life. The classification systems behind grease - NLGI consistency numbers, base-oil viscosity, thickener families and performance approvals - form a compact language any buyer can learn in one sitting, and that language is the first step toward a specification that survives contact with a real machine.

The cone penetration scale

The NLGI number grades how stiff a grease is, measured by how far a standard cone sinks into it under defined conditions. The scale runs from 000, a semi-fluid grease close to pourable, through 0 and 1, up to 2, the classic butter-like consistency of most bearing greases, and on to 6, which is block-hard. The measurement itself is standardised in ISO 2137, so a grade means the same thing across suppliers even when their products differ in every other respect. Because the reading is taken on a worked sample, a grease that has been churned in a bearing can measure softer or firmer than its box grade, which is one reason a laboratory penetration figure and a field observation do not always agree.

• 000 to 00. Semi-fluid, for sealed gear cases and slow units that will not leak.
• 0 to 1. Very soft to soft, for central systems, cold starts and high-speed spindle greases.
• 2. Butter-like, the default for rolling bearings in general duty.
• 3. Firm, for high vibration, vertical shafts and washdown sealing.

Consistency is a property, not a grade

The number says nothing about quality. A softer grease feeds better to distant points and flows at low temperature; a stiffer grease stays in place under vibration and resists leakage at gaps. That is the whole of the trade-off at this axis. Treating the NLGI number as a ranking - assuming that a higher number is a better grease - is the most common misreading, and it leads to stiff greases in cold systems and soft greases thrown out of fast bearings.

Stiffness is not a ranking

Two greases of the same grade and different chemistry can behave completely differently in the same bearing, because the number describes only stiffness. The grade narrows the field; it does not rank the products within it.

The four axes behind a grease name

Consistency alone specifies almost nothing about performance. A complete grease description has four more axes, and each answers a different question. Base-oil viscosity, graded in ISO VG numbers, must match the bearing's speed and load - high-speed bearings want lower viscosity, heavily loaded slow ones want higher. Thickener family sets the temperature ceiling, water resistance and mechanical stability. Additives tune extremes such as extreme pressure, wear, rust and oxidation. And performance approvals are the only part of a datasheet that reports proven behaviour rather than a claim.

Two of those axes, base oil and thickener, do most of the work on real machines. A high-speed position wants a low-viscosity base oil so that churning stays low, while a heavily loaded slow position wants the thicker film a higher viscosity provides; getting that relationship wrong shows up as either heat or wear. The thickener then decides whether the grease stays where it was put, how it behaves when water arrives, and how much temperature it can take before it oxidises. A grade-2 grease with the wrong base oil is still the wrong grease.

Reading a grease line in a specification

A complete grease line reads compactly once the vocabulary is familiar. An entry such as "lithium-complex, NLGI 2, ISO VG 220, minus 30 to plus 140 degrees C, water-resistant, EP" describes a real industrial grease in a dozen words and, more usefully, excludes most of the wrong alternatives before anyone places an order. Buyers who specify at this level turn lubrication from folklore into procurement, and they gain the ability to hold a supplier to a stated property. The classification of greases into families is set out in DIN 51825, so a specification can cite both the penetration number and the family with confidence, and the technical library applies the same vocabulary to real bearing duties.

Matching thickener chemistry to the environment

The environment usually picks the thickener. Lithium and lithium-complex chemistries dominate general service because they cover a wide temperature band and resist water reasonably well. Calcium sulphonate excels where water and washdown are constant. Polyurea is the electric-motor staple, prized for low oil separation over long intervals. PTFE and a few specialty thickeners serve food-grade and extreme-temperature duties. Matching the thickener to the real environment, rather than to habit, is what keeps a grease in the bearing instead of on the floor.

Water is a thickener decision

Where washdown or condensation is constant, the thickener's water resistance becomes the limiting property, and no amount of base-oil quality compensates for a thickener that emulsifies.

Heat is a base-oil decision

Where the duty is hot and dry, the base oil and its oxidation stability set the ceiling, and complex or synthetic chemistry earns its place.

The habit worth building is to read a grease the way a specification reads it: family first, grade second, then base-oil viscosity and the temperature window, and finally the approvals that prove the behaviour. A grease chosen that way can be defended; a grease chosen by colour, brand or habit cannot be, and when a bearing fails the question of whether the right grease was used will have an answer instead of an opinion.

Mixing greases and the purge rule

Grease mixing deserves more attention than it usually gets, because its consequences are asymmetric: the wrong blend can fail slowly, through leakage or starvation or oxidation, so the error is found months after the gun was used. Thickeners based on different soaps can soften or harden when combined, and occasionally react chemically. The safe rules are simple.

The failure is asymmetric

A wrong blend rarely fails on the day it is made. It leaks slowly, starves a contact gradually, or oxidises at its own pace, so the mistake surfaces months later, long after the gun that caused it has been forgotten. That delay is why the rules are worth enforcing even when they feel fussy.

• Continue a known grease. Stick to a documented product where one is already in service.
• Purge before you change. Remove the old grease completely before introducing a different chemistry.
• Do not trust the grade alone. Two greases of the same NLGI number are not interchangeable.
• Label every machine and gun. A small grease list, written down, prevents most mixing errors.

The signals that a grease choice is wrong

Grease errors announce themselves in ways worth cataloguing, because each signal maps to a specific correction and the mapping is exactly what the vocabulary is for. Reading the symptom correctly usually names the correction before anyone reaches for a grease gun.

Relubrication as a calculated quantity

Re-lubrication is a step-by-step calculation from speed, temperature and grease volume, not a calendar ritual, and over-greasing remains the most common lubrication error in industry, heating bearings and bursting seals. Modern practice adds ultrasonic interval tools and quantity-controlled guns, but the principle predates them: grease is a metered engineering material, not a topping-up fluid. A workable interval balances the calculated life of the grease against the cost of the downtime it prevents, and a condition-based top-up - driven by temperature, vibration or ultrasonic listening - often beats a fixed calendar even when that calendar was chosen well. The re-lubrication interval and grease selection guides carry the calculation, while the oil lubrication guide sets out when the medium should change altogether. For bearings that live in dust, humidity or washdown, the thickener choice meets the harsh environment and temperature questions directly, and the storage and shelf life notes cover how grease ages before it is ever fitted. The deep groove ball range and self-aligning roller range show where each consistency class appears on a maker's shelf, and a NSK or comparable datasheet names the thickener so the choice can be checked against the duty.