Lubrication

Why it matters

Lubrication is the cheapest maintenance there is and one of the biggest causes of failure when done badly. Too little lubricant and metal touches metal; too much and bearings overheat; the wrong type, or the right type contaminated with dirt and water, and parts wear fast. Many plants have a lubrication technician role for exactly this reason.

What lubricants do

A lubricant separates moving surfaces with a film, reduces friction and wear, carries away heat, protects against rust and, in the case of grease, helps seal out dirt. In a rolling bearing the oil film is thinner than a human hair, so a few particles of grit can break through it.

Oil

Oil is used where speeds or temperatures are high, where heat must be carried away, or where a circulating system already exists: gearboxes, hydraulic systems, compressors and large plain bearings.

Viscosity is the oil's resistance to flow, the single most important property. Industrial oils are graded by the ISO VG system: the number is the viscosity in centistokes at 40 °C. ISO VG 32 and 46 are common hydraulic oils; VG 220 and 320 are common gear oils. Higher loads and slower speeds need thicker oil; higher speeds need thinner oil. The viscosity index tells you how much viscosity changes with temperature.

Additives matter too: anti-wear, extreme pressure (EP) additives for gears, rust and oxidation inhibitors, anti-foam. Oils are not interchangeable just because the viscosity matches.

Grease

Grease is oil held in a thickener, like a sponge holding water. It stays in place, so it suits bearings that cannot have an oil bath, open gears and parts that are hard to reach.

• NLGI grade measures stiffness. NLGI 2 is the most common general-purpose bearing grease. NLGI 0 or 00 are soft greases for centralized systems; 3 is stiffer.
• Thickener type (lithium complex, polyurea, calcium sulfonate, aluminum complex and others) affects water resistance, temperature range and compatibility.
• Base oil viscosity still matters, just as it does for oil.

Compatibility: mixing greases with incompatible thickeners can make the mixture soften and run out or harden. When changing a grease type, purge the old grease out and check the manufacturer's compatibility chart.

How much grease, how often?

Bearing makers publish formulas and charts. A widely used starting point for the amount when regreasing a rolling bearing from the side is:

grease (grams) = 0.005 × D × B

where D is the bearing outside diameter and B its width, both in millimeters.

Worked example. A 6310 bearing is 110 mm OD and 27 mm wide.

grease = 0.005 × 110 × 27 ≈ 15 g

A typical grease gun gives roughly 1 to 1.5 g per stroke, so weigh a few strokes of your own gun. Here that is around 10 to 15 strokes, not "until it comes out the seal".

Intervals depend on bearing size and speed, temperature, load and contamination. Manufacturer charts give a base interval, which you shorten for heat (a common rule halves it for every 15 °C above about 70 °C), vertical shafts, vibration, dust or water. Then you adjust with experience: condition monitoring such as ultrasound can tell you when a bearing actually needs grease.

Good lubrication practice

• Clean before and after: wipe the grease fitting and the gun tip. Fit caps to fittings.
• Label everything: color-coded tags on fittings and matching containers stop mix-ups.
• Store properly: sealed containers, indoors, first in first out. Use filtered transfer containers for oil, not open buckets.
• Check, do not just add: sight glasses on gearboxes, oil condition (color, smell, water), breathers and seals.
• Oil analysis on important machines finds wear metals, water and degraded additives long before failure.
• Record it in the CMMS: what, where, how much, when.

Common mistakes

• Over-greasing: the most common error. Excess grease churns, heats the bearing and pushes past seals. In electric motors it can get into the windings.
• Mixing incompatible greases.
• Using a general grease where a high-temperature or food-grade (NSF H1) one is specified.
• Topping up a gearbox with whatever oil is nearby.
• Greasing a sealed-for-life bearing that has no fitting by drilling it.

Safety

Lubrication routes often take you close to moving machinery. Many plants let trained staff grease some points while running, through extended lines outside the guards, but only where the site procedure says so; everything else needs lockout. Grease guns develop very high pressure: never point one at anyone, and treat injection injuries as emergencies. Read the safety data sheet for each lubricant.

Recap

• Oil for speed, heat and circulating systems; grease where the lubricant must stay put.
• ISO VG = viscosity at 40 °C; NLGI grade = grease stiffness.
• Regrease amount ≈ 0.005 × D × B grams; intervals shorten with heat and contamination.
• Clean, labeled, recorded and not overdone.

Try it

Open the Do tab and take the quiz on choosing a lubricant and interval.