Bearing Lubrication: Grease Selection, Quantity Formula G = 0.005×D×B, Intervals & Common Mistakes

Bearing lubrication is the process of supplying lubricant (grease or oil) to the contact zone between rolling elements and raceways to reduce friction, dissipate heat, prevent corrosion, and extend bearing service life.

According to SKF Lubrication Management, 43% of industrial bearings fail prematurely due to improper lubrication — including starvation, over-greasing, wrong grease type, or mixing incompatible greases. Correct lubrication is the simplest and most effective way to extend bearing life 2–5×. This article provides an in-depth analysis of lubricant types, grease quantity and interval formulas, common mistakes, and application-specific grease selection — based on data from SKF, FAG/Schaeffler, NSK Lubrication Guide, and DIN 51825 standard.

Two Primary Lubrication Types

Grease Lubrication — Most Common (90% of applications)

Approximately 90% of industrial bearings use grease lubrication. Grease consists of three components: base oil (70–90%), thickener (5–20%), and additives (1–10%). The base oil provides actual lubrication; the thickener holds oil at the contact point; additives provide anti-wear (EP), anti-oxidation, and anti-corrosion properties.

Grease advantages: good sealing (blocks dust/water), no complex pumping systems required, minimal leakage, suitable for most low-to-medium speed applications.

Oil Lubrication — For High Speed or High Temperature

Oil lubrication is used when: the speed factor ndm > 500,000 (n = rotational speed in rpm, dm = mean diameter in mm), operating temperature exceeds 120°C, or active heat dissipation is required. Common in: CNC spindles (angular contact bearings), industrial gearboxes, steam turbines.

Oil viscosity must achieve a kappa ratio (κ) ≥ 1 at operating temperature — meaning actual viscosity ≥ minimum required viscosity per the SKF viscosity chart. Kappa < 0.4 causes direct metal-to-metal contact and rapid failure; κ = 1–4 is the optimal range.

Bearing Grease Classification

The most widely used grease in Vietnam: lithium complex EP2 (NLGI grade 2). Each bearing manufacturer offers recommended greases: SKF LGMT 2/LGMT 3, FAG Arcanol L135V, NTN Multemp SRL.

Critical warning: never mix greases with different thickener types. Lithium mixed with polyurea forms hard precipitate, completely losing lubricating ability. When switching grease types, the bearing must be removed, cleaned of all old grease with solvent, and repacked with new grease.

Calculating Grease Quantity

Initial Fill (New Installation)

When installing a new bearing, initial grease fill should occupy approximately 30–50% of the free volume inside the bearing. Over-filling causes:

• Temperature rise from grease churning
• Pressure that destroys seals, causing leakage
• Accelerated grease oxidation from heat

Re-greasing Quantity

Empirical formula per SKF:

G = 0.005 × D × B

Where:

• G = grease quantity per application (grams)
• D = bearing outer diameter (mm)
• B = bearing width (mm)

At a cement plant in Hai Phong, applying the G = 0.005 × D × B formula instead of "pump until it overflows" reduced grease consumption by 35% and extended ID fan motor bearing life by 40%.

Calculating Re-lubrication Intervals

Re-lubrication intervals depend on bearing size, speed, and temperature. Simplified SKF formula:

tf = K × (14,000,000 / (n × √d)) × ft

Where:

• tf = re-lubrication interval (operating hours)
• K = bearing type factor (1.0 for DGBB, 5.0 for SRB, 10 for tapered rollers)
• n = rotational speed (rpm)
• d = bore diameter (mm)
• ft = temperature factor (1.0 at 70°C, 0.5 at 85°C, 0.25 at 100°C)

Example: 22 kW electric motor using 6308 C3 (d = 40 mm), speed 1,460 rpm, temperature 70°C:

• tf = 1.0 × (14,000,000 / (1,460 × √40)) × 1.0 = ~1,517 hours ≈ re-lubricate every ~6 months (running 3 shifts)

SKF DialSet software or the FAG ARCANOL App provides more precise calculations, factoring in load and contamination levels.

Common Lubrication Mistakes in Vietnam

1. Over-greasing

The most common error. Technicians pump "to be safe" — grease overflows seals, raises temperature, and reduces life faster than starvation. Rule: grease cavity should only be 30–50% full.

2. Mixing Incompatible Greases

Using lithium grease then adding polyurea (or vice versa) causes precipitation. Before switching grease types, clean thoroughly.

3. Contaminated Grease Guns

Pump tips covered in dust and grit — pumped directly into the bearing. Hard particles of 10–20 μm are sufficient to cause raceway abrasion. Always wipe the pump tip and nipple clean before greasing.

4. Fixed Schedule for All Machines

A 5 kW lightly-loaded motor and a 200 kW heavily-loaded motor cannot share the same lubrication interval. Calculate individually for each bearing.

5. Greasing Sealed Bearings (2RS/2Z)

Bearings with double seals (2RS) or shields (2Z) are factory-packed with grease — adding more creates pressure that destroys seals, defeating the purpose.

Grease Selection Guide by Application

Grease Compatibility Matrix

Mixing greases with different thickener chemistries can cause catastrophic results. The table below provides a quick-check reference before switching or topping up grease.

"NO" indicates chemical incompatibility — mixing causes the thickener structure to collapse, releasing base oil and losing consistency. "OK" indicates general chemical compatibility, though mixing different brands of the same thickener may still alter performance.

Safe Grease Switching Procedure

1. Remove the bearing from the housing if accessible
2. Clean all old grease using petroleum-based solvent (mineral spirits, kerosene)
3. Dry completely — use compressed air, ensuring no moisture enters
4. Apply new grease at 30–50% of free volume
5. Run at reduced load for 30–60 minutes, monitoring temperature
6. After stabilization, apply final quantity per G = 0.005 × D × B

If removal is not possible (e.g., back-to-back pairs in a gearbox), flush by pumping new grease through until old grease is fully displaced — typically 5–10× the normal re-greasing quantity. Monitor discharged grease color until it matches the new grease.

Automatic Lubrication Systems

Single-Point Lubricators

Small devices mounted directly on the grease nipple, automatically dispensing grease at a preset interval (1–12 months). Common products: SKF LAGD, FAG CONCEPT, perma STAR.

Advantages: no manual labor, clean grease (fully sealed), precise quantity. Disadvantages: $20–60 per unit, cartridge replacement needed. Ideal for: high-mounted fan motors (difficult access), 24/7 continuous-running electric motors, hazardous locations (high temperature, chemical exposure).

Centralized Lubrication Systems

A central pump delivers grease through piping to multiple lubrication points simultaneously. Standard equipment on construction machinery (excavators, cranes) and large production lines.

At an animal feed plant in Dong Nai, installing a progressive centralized system for 28 bearing housings on the pellet line reduced manual greasing labor by 80% while ensuring correct quantity and timing at every point.

Kappa Ratio (κ) — Evaluating Lubrication Adequacy

The kappa ratio indicates whether a bearing is receiving adequate lubrication. Calculated as:

κ = ν_actual / ν_minimum

Where:

• ν_actual = actual kinematic viscosity of the base oil in the grease at operating temperature (mm²/s)
• ν_minimum = minimum required viscosity, read from the SKF viscosity chart based on mean diameter dm = (d + D)/2 and speed n

Kappa Calculation Example

Bearing 6310 C3 (d = 50, D = 110 mm) on a centrifugal pump at 1,450 rpm, operating temperature 60°C. Grease: SKF LGMT 2 (mineral base oil, viscosity 110 mm²/s at 40°C).

1. dm = (50 + 110) / 2 = 80 mm
2. From SKF chart: at dm = 80, n = 1,450 → ν_minimum ≈ 9 mm²/s
3. From base oil table: LGMT 2 at 60°C → ν_actual ≈ 40 mm²/s
4. κ = 40 / 9 = 4.4 → slightly above optimal but acceptable

If temperature rises to 80°C: ν_actual drops to ~18 mm²/s → κ = 18/9 = 2.0 — still in the optimal range.

Oil Lubrication Methods

Oil Bath

The bearing runs partially submerged in an oil reservoir — the lowest rolling element dips into the oil. Common for gearboxes and rolling mills. Correct oil level: at the center of the lowest ball or roller. Too high causes churning, increasing temperature.

Oil Mist

Oil is atomized into a fine aerosol using compressed air and delivered to the bearing. Suitable for very high speeds (ndm > 1,000,000). Used for CNC spindles, gas turbines.

Oil Jet

High-pressure oil sprayed directly into the raceway through precision nozzles. Provides maximum heat dissipation. Used for extreme-speed or combined heavy-load/high-speed applications — turbine shafts, aircraft gearboxes.

Grease Life Estimation — L50 Formula

Grease has a finite life — even under ideal conditions. The estimated L50 grease life (50% probability grease still functional) per FAG/Schaeffler:

log(L50) = -2.6 + 2830 / (273 + T)    [hours]

Where T = operating temperature (°C).

Every 15°C temperature increase halves grease life. This is why electric motor bearings running at 80–100°C need more frequent re-lubrication than fan bearings operating at 40–50°C.

Additional Real-World Scenarios

Scenario — Coal Conveyor at a Thermal Power Plant

An 800-meter coal conveyor at a thermal power plant in Quang Ninh with 120 UCP 212 pillow block housings using 6212-2RS bearings. Previous manual greasing: 2 technicians × 4 hours = 8 labor-hours per cycle, routinely missing 10–15 points. After installing a dual-line centralized lubrication system, labor dropped to zero (fully automatic), no points missed, and average bearing life increased by 60%.

Scenario — Screw Air Compressor

A 75 kW screw compressor at a plastics factory in Binh Duong using tapered roller bearing 32213 for the rotor shaft. Maintenance used standard lithium grease (NLGI 2, max 120°C) — but discharge temperature reached 95°C, conducted through the housing to the bearing. Grease degraded rapidly; bearing failed after 6,000 hours. Switching to lithium complex grease (SKF LGMT 3, rated 150°C) and reducing the re-lubrication interval from 2,000 to 1,000 hours extended bearing life to 15,000+ hours.

Scenario — Vibrating Screen at Aggregate Plant

A vibrating screen at a crushed stone aggregate plant in Binh Dinh using 22222 E1 C3 spherical roller bearings. Original grease: standard lithium EP2 applied every 500 hours. Problem: bearings consistently failed from water washout during the rainy season — water from wet aggregate sprayed directly onto the bearing housing. Solution: switched to calcium sulfonate complex grease (EP2 grade) with superior water washout resistance per ASTM D1264 test. The team also installed additional splash guards on the housing. Result: bearing life doubled from 4,000 to 8,000+ hours, and the grease maintained its consistency even after direct water contact.

Grease Selection for Vietnam's Climate

Vietnam's tropical climate (25–40°C ambient, 70–95% relative humidity) presents specific challenges for bearing lubrication:

• Standard indoor applications: lithium complex EP2 (NLGI 2) handles 90% of factory equipment. Good water resistance and thermal stability to 150°C.
• Outdoor and construction equipment: calcium sulfonate EP2 provides the best rust protection and water washout resistance — essential for construction machinery operating in rain, mud, and dust.
• Food and beverage: NSF H1-certified greases are mandatory for any equipment in direct or incidental food contact zones. SKF LGFP 2 and Kluber NH1 11-222 are common choices.
• Coastal environments: salt spray corrodes standard lithium greases within months. Calcium sulfonate or barium complex greases maintain protection in marine and port applications.
• High-temperature positions (dryers, kilns, exhaust fans above 120°C): synthetic base oil with polyurea or PFPE thickener. Standard mineral-based grease degrades rapidly above 120°C per the L50 formula.

Key Takeaways

• 43% of premature bearing failures stem from lubrication errors — the most controllable factor in bearing maintenance
• Grease quantity: G = 0.005 × D × B (grams) — over-greasing is as dangerous as starvation
• Never mix different thickener types (lithium + polyurea = disaster)
• Calculate re-lubrication intervals individually based on bearing size, speed, temperature
• C3 clearance is mandatory for electric motors due to tight fits and operating temperature
• Sealed bearings (2RS) do not need re-lubrication — replace when grease is depleted