Bearing Lubrication: Grease, Oil, and Relubrication Intervals

Why Lubrication Matters

Bearings fail most often because of inadequate lubrication — wrong lubricant type, wrong amount, or relubrication that comes too late. A thin film of lubricant separates rolling elements from raceways, carries heat away, and blocks contaminants. Getting that film right is one of the most cost-effective maintenance tasks in any plant or marine facility.

Grease vs. Oil Lubrication Systems

Grease is the most common bearing lubricant. It is a base oil (mineral, synthetic, or biodegradable) thickened with a metallic soap or other agent so it stays where you put it. Grease suits sealed or shielded bearings, moderate speeds, and applications where relubrication access is infrequent.

Oil bath lubrication submerges the lower portion of the bearing in an oil reservoir. It works well for gearboxes, slow-to-moderate speed bearings, and wherever continuous cooling is needed. The oil level should reach the center of the lowest rolling element; overfilling causes churning, heat, and energy loss.

Oil mist systems atomize oil and carry it through piping to bearing housings. They suit high-speed bearings and large numbers of points fed from a central system. Oil mist requires disciplined setup and environmental controls because mist can escape to the atmosphere.

Circulating oil systems pump filtered, cooled oil continuously through the bearing housing. They are used on large, high-load, or high-speed machines where heat removal is critical.

Viscosity

Viscosity is a lubricant's resistance to flow. For bearing oil, viscosity is expressed as ISO Viscosity Grade (ISO VG) — for example, ISO VG 68 has a kinematic viscosity of 68 mm²/s (cSt) at 40 °C. The correct viscosity depends on bearing speed and load: high speed calls for lower viscosity; high load or low speed calls for higher viscosity. Using oil that is too thin starves the film; oil that is too thick increases drag and heat.

For grease, the base oil viscosity inside the thickener must also match the application. A grease labeled NLGI 2 can contain a low or high base oil viscosity — always check the technical data sheet.

NLGI Consistency Grades

The National Lubricating Grease Institute (NLGI) rates grease firmness on a scale from 000 (semi-fluid) to 6 (very hard block grease). Most industrial bearings use NLGI 2, which is smooth, spreadable, and stays in place across a wide temperature range. NLGI 1 is used in cold environments or centralized grease systems where pumpability matters. NLGI 3 is used in high-temperature or vertical-shaft applications where runout is a concern.

| NLGI Grade | Consistency | Typical Use |

|---|---|---|

| 000–00 | Semi-fluid | Enclosed gear cases |

| 0–1 | Very soft | Centralized lube systems, cold climates |

| 2 | Smooth paste | General industrial bearings (most common) |

| 3 | Firm | High temp, vertical shafts |

| 4–6 | Hard to block | Specialty, high-temperature plain bearings |

Base Oil Types

Mineral oil greases are economical and compatible with most seals. They suit general-purpose industrial bearings.

Synthetic base oils (polyalphaolefin, ester, polyalkylene glycol) extend service life, perform at temperature extremes, and reduce friction. They are specified for electric motor bearings operating at high speed, food-processing equipment (with the right additive package), and marine propulsion shaft bearings that must tolerate wide temperature swings.

Biodegradable greases use vegetable-based or synthetic ester base oils. Regulations for many marine environments — especially where grease can contact water — require environmentally acceptable lubricants (EAL). Always verify the vessel's regulatory requirements before substituting lubricants on propeller shaft or stern tube bearings.

Grease Quantity and Housing Fill

Overgreasing is as damaging as undergreasing. Excess grease raises operating temperature, churns, degrades, and can force its way past seals into motors or brakes. A standard guideline is to fill the bearing housing one-third to one-half full for moderate-speed bearings. High-speed applications may call for one-third or less. Follow the equipment manufacturer's specification first; use the general guideline only when no specific value is given.

Relubrication Intervals

Manufacturers publish formulas or tables to estimate how long grease will remain effective. The interval depends primarily on bearing bore diameter, operating speed, operating temperature, and load. A common published approach expresses the interval in operating hours. As a general pattern:

• Larger bore → shorter interval (more surface area, more grease needed)
• Higher speed → shorter interval (grease works harder, oxidizes faster)
• Higher temperature → significantly shorter interval (a sustained temperature rise of roughly 15 °C above the rated base temperature can cut grease life in half)
• Heavy load or shock load → shorter interval
• Contaminated or wet environment → shorter interval

Always use the specific manufacturer's lubrication guide (SKF, Timken, NSK, or equivalent) for the formula applicable to their bearing series. The worksheet in this lesson uses one such published formula approach.

Safety During Lubrication

Lockout/Tagout (LOTO): Before opening a bearing housing to inspect or regrease, follow your facility's energy control program (29 CFR 1910.147). Isolate all energy sources — electrical, pneumatic, hydraulic — and verify zero energy state before touching rotating equipment. The only exception is a purpose-designed running relubrication fitting on a machine that the manufacturer has explicitly designed and labeled for that procedure.

PPE: Wear safety glasses or a face shield when using grease guns under pressure. Grease injection injuries (high-pressure injection through skin) are a medical emergency. Wear chemical-resistant gloves when handling oil or grease for extended periods.

Housekeeping: Wipe excess grease from fittings and housings immediately. Grease on floors creates slip hazards. Dispose of used lubricants according to local regulations; never pour oil into floor drains.

Compatibility: Never mix greases from different thickener families (lithium, calcium, polyurea, etc.) without confirming compatibility — incompatible greases can liquefy and lose all load-carrying ability.