Greases

NLGI indicates consistency, from 000 (nearly fluid) to 6 (very hard). Grade 2 is the most common for rolling bearings; grade 1 or 0 is used in centralised lubrication systems and grease-filled gearboxes; grade 3 for large bearings, vertical shafts or heavy vibration.

A code such as ISO-L-XBCEB 2 encodes, in turn, the lowest operating temperature, the highest, water resistance, extreme pressure (EP) capability, and finally the NLGI grade. It lets you compare greases from different brands by operating conditions instead of trade names.

Best avoided. Lithium, lithium complex, polyurea and calcium sulfonate greases may be incompatible, and a mix can soften and run out or harden. When changing grease, purge the old grease from the bearing and shorten the first relubrication interval.

Overgreasing is a common fault: churning heats the bearing and can push out the seal. The relubrication quantity is usually given by the bearing maker's formula, for example about 0.005 x outside diameter x width (grams, dimensions in mm). If there is a drain plug, open it while greasing so the excess can escape.

Lithium grease is the general purpose type, usually good to about 120 °C. Lithium complex has a higher dropping point, handles around 150 °C or more, and suits hot, heavily loaded bearings. Polyurea gives long life at high temperature and is common in electric motor bearings greased for life.

Yes. Grease stored for long periods can bleed oil, seen as a free oil layer on the surface, or change consistency. A thin oil film on top of the tin can be stirred back in, but grease that has hardened, lumped or clearly changed colour should not be used. Keep lids closed and take grease out with clean tools to keep dust and water out.

Use the speed factor n.dm, the rotational speed multiplied by the bearing mean diameter, to select the grease. The higher the speed, the more a low base oil viscosity with suitable consistency is needed, often NLGI 2 or 3 with a synthetic base oil. Grease with a thick base oil generates heat from churning at high speed.

Grease with MoS2 or graphite is used for heavy load, slow motion, shock or oscillation, where an oil film is hard to keep, such as pins, bushes and slideways. The solids plate onto the surfaces and keep protecting when the oil film breaks down. It should not be used in high-speed rolling bearings, because the particles raise friction and heat.