What is the difference between NLGI 2 and NLGI 3?
The NLGI grade describes consistency: the grease’s resistance to deformation. It is based on worked penetration at 25°C: 265–295 tenths of a millimetre for NLGI 2 and 220–250 for NLGI 3. Lower penetration means a stiffer grease, so NLGI 3 is stiffer than NLGI 2.
This is not a base-oil viscosity scale or a quality rating. It does not say whether the grease will handle a particular temperature, load or water exposure. Two products with the same NLGI grade may have different base oils, thickeners, additives and application limits.
When is NLGI 2 a sensible starting point?
SKF names NLGI 2 as the most common grade in normal rolling-bearing applications. It recommends softer NLGI 1 greases for low ambient temperatures and oscillating applications, and NLGI 3 for the conditions described in the next section.
If the grease is delivered by an automatic lubrication system or operates at low temperature, check its pumpability and behaviour under the actual conditions. According to SKF, lower-consistency greases pump more easily, so NLGI 2 is not always the best choice for such a system.
When does NLGI 3 make sense?
SKF recommends NLGI 3 greases for large bearings, vertical shafts, high ambient temperatures and vibration. On a vertical shaft the housing design also matters: a good seal, shield or baffle plate below the bearing is needed to keep the grease from leaving it. NLGI 3 then helps reduce grease leakage.
Under vibration, stiffness alone is not enough. SKF points out that the grease needs good mechanical stability so that it does not harden or soften under vibration and shear; a higher consistency may help but does not guarantee this. A higher NLGI grade also does not mean a higher base-oil viscosity. At low temperature a stiffer grease can restrict bearing rotation and oil bleed. NLGI 3 therefore makes sense when the operating conditions and the bearing or machine manufacturer’s recommendations support it.
Why is the NLGI grade not enough?
Check the base oil and thickener
The base oil helps form the lubricating film, while the thickener helps keep the product at the lubrication point. Their properties affect behaviour at temperature, under shear and load, and in contact with water. The NLGI grade does not replace this information.
Before changing product, check compatibility with the grease already in the bearing. The same NLGI grade does not guarantee it. SKF warns that mixing incompatible greases can change the consistency significantly, and the resulting leakage or hardening can damage the bearing. If the documentation does not confirm miscibility, plan the changeover with cleaning or purging of the old grease according to the plant procedure.
Consider temperature, speed and position
Selection requires at least the bearing type and designation, speed, load, operating temperature, sealing, shaft position, vibration and grease delivery method. At high temperature, compare the actual bearing temperature and the base-oil viscosity with the catalogue data. SKF notes that the temperature limits given by grease manufacturers are based on the grease’s chemical properties and do not mean that the grease will lubricate the bearing properly across that whole range.
A practical decision example
Suppose grease has to be replenished at two points. In a horizontal bearing running at a normal temperature with a typical delivery method, NLGI 2 may be the sensible starting point. In a large bearing on a vertical shaft exposed to vibration and higher ambient temperature, NLGI 3 may be considered, but only after checking the housing design, manufacturer guidance and delivery capability.
If grease is being pushed out of the second bearing, changing from NLGI 2 to NLGI 3 is not yet a diagnosis. First check quantity, interval, temperature, seal condition and the mechanical behaviour of the grease. If the grease softens under vibration, SKF recommends a grease with better mechanical stability or a stiffer grease, up to NLGI 3.
How to check the choice before changing product
- Identify the lubrication point. Record the bearing type, designation, dimensions and position, together with the machine data.
- Collect operating conditions. Measure or confirm temperature, speed, load, vibration, moisture and possible contamination.
- Compare the product data sheet. Check NLGI, base-oil viscosity, thickener, temperature range, water resistance, mechanical stability and delivery method.
- Confirm miscibility. Check the documentation of both products to see whether the new grease can be mixed with the current one.
- Set quantity and monitoring. Base amount and interval on bearing, machine or dispensing-system documentation, then observe temperature, vibration and leakage after the change.
How to read grease data in the Molyguard catalogue
In the Molyguard 2022 catalogue, IVORY HT has NLGI grade 2 and a synthetic base oil listed as ISO 460. It is intended for bearings operating at high temperature, under extreme pressure and in the presence of chemicals, water or steam. The catalogue attributes its suitability for heavy loads to a special soap that withstands high pressure. When comparing products in the bearing greases category, read the NLGI grade together with the base oil, the thickener and the conditions at your lubrication point.
How to prepare a grease-selection enquiry
When asking about grease selection, provide the bearing designation, shaft position, speed, load, temperature, delivery method, current product, quantity and interval, and the symptoms that led to the review. You can send this information through the TC Kolor contact page. With this information, it is possible to assess whether the difference between NLGI 2 and NLGI 3 matters at that point.
