Grease selection by base oil, thickener and NLGI grade: reading a grease listing

A grease is mostly oil, and the rest decides whether that oil stays where it is needed. The listing for every grease gives the same few facts — base oil, thickener, viscosity, grade, temperature — and each answers a different question about the job. Read in the right order, they narrow a range of sixty greases to two or three.

/01Base oil: temperature and compatibility.

Mineral and PAO greases cover most plant duty; PAO keeps working colder and lasts longer at temperature. Silicone greases hold their consistency across the widest temperature span — Molykote 33 Light is listed from −73 °C — but silicone oil carries less load between steel surfaces, so they suit light loads, rubber and plastics better than heavily loaded metal bearings. PFPE and fluorosilicone greases are the high-temperature and chemical-resistance end of the range.

/02Thickener: what holds the oil.

The thickener is the sponge that holds the oil and releases it under load. Simple lithium soap is the general-purpose standard; lithium complex, aluminium complex and polyurea are built for higher temperatures and longer life, and aluminium complex is the thickener behind the FM food-machinery greases on our listings (G-4500 FMS, G-1502 FM, G-0062 FM). Silica and PTFE are non-soap thickeners, used with silicone, fluorosilicone and PFPE oils. Mixing greases with different thickeners can soften or harden them, so purge rather than top up when changing product.

/03Base oil viscosity: speed and load.

The viscosity of the oil inside the grease matters as much as the grease itself. Slow, heavily loaded bearings and open gears need a thick oil to keep a film between the surfaces; fast bearings and cold starts need a thin one that does not churn or stiffen. Our listings show the full span: PG-65 at 17 cSt and BG-555 at 26 cSt at 40 °C at the thin end, G-4500 FMS at 100 cSt and G-1502 FM at 220 cSt in the middle, and EM-50L at 1,050 cSt and EM-D110 at 9,850 cSt at the thick end. The 1122 chain and open-air grease is listed at 1,500 mm²/s.

/04NLGI grade: consistency.

The NLGI grade measures how stiff a grease is, from semi-fluid grades through 2, the usual bearing grease, to stiffer grades above it. Our Molykote greases run from NLGI 0 (G-0060 FM, G-0061 FM) to 3, with many at 1 or 2. Softer greases flow and pump more easily, which is why centralised lubrication systems and cold climates favour them; stiffer greases stay put in vertical shafts, loose seals and vibrating equipment. The grade is not a quality rating — the same base oil and thickener can be made in several grades.

/05Check what the grease touches.

A grease that suits the bearing can still attack the seal, swell a plastic gear or contaminate a product. PAO greases such as PG-65 Plastislip are formulated with plastics in mind; silicone greases such as Molykote 55 O-Ring Grease (−65 °C to 175 °C) for rubber seals; 3451 and 3452 are listed as chemical-resistant bearing and valve greases with fluorinated thickeners. For food plants, the FM greases are the food-machinery grades — confirm the registration on the datasheet. The other Molykote types are covered in Molykote lubricant types explained.