Lubrication: Oils, Greases & Machinery Reliability

Start with the idea

Lubrication is one of the simplest-looking maintenance activities and one of the easiest to misunderstand. The objective is not to add as much lubricant as possible. It is to maintain the right lubricant, in the right place, in the right condition.

Rub your palms together. You feel friction and warmth because the surfaces resist sliding against each other.

Now imagine two metal surfaces operating under significant load, speed and temperature. Without suitable lubrication, surface irregularities interact, generating friction, heat and wear.

A lubricant helps manage these interactions by forming protective films and, depending on the system, carrying heat or contamination away.

The basic principle is straightforward:

A machine requires enough of the correct lubricant to maintain suitable contact conditions. More is not automatically better.

There is an important distinction between lubrication and simply making something slippery. A successful lubrication system must work under the equipment's actual:

• Load.
• Speed.
• Temperature.
• Operating duty.
• Environmental conditions.
• Bearing or gear geometry.
• Material and seal compatibility.

Why it matters in real machines

Lubrication affects nearly every rotating or sliding industrial system. Examples include:

• Electric-motor bearings.
• Pump and compressor bearings.
• Gearboxes.
• Conveyors.
• Chains and sprockets.
• Sliding guides.
• Machine-tool spindles.
• Hydraulic machinery.
• Certain couplings and mechanisms.

Consider a gearbox that begins running hotter than normal. Possible causes include:

• Incorrect lubricant viscosity.
• Low lubricant level.
• Excessive lubricant quantity.
• Contamination.
• Poor gear alignment.
• Bearing damage.
• Increased operating load.
• Restricted cooling.

Changing the oil without understanding the cause may not solve the problem. Lubrication is part of a mechanical system, not an isolated maintenance task.

Core concepts

3.1 How lubrication protects surfaces

A rotating shaft and its bearing are never perfectly smooth at the microscopic level. Without adequate separation, their surface asperities can interact.

Depending on geometry, load, speed and lubricant conditions, a machine may operate in different lubrication regimes.

These regimes are not interchangeable, and a particular machine may experience more than one during operation.

3.2 Oil versus grease

Both lubricate, but they suit different equipment arrangements.

When oil is generally useful

Oil is common where lubricant must circulate, carry heat or reach multiple components through a designed system. Examples include industrial gearboxes, hydraulic systems and circulating-oil bearing arrangements.

When grease is generally useful

Grease is often selected where lubricant retention, sealing arrangement and compact delivery are important. Examples include many rolling-element bearings and selected moving joints.

However, bearing manufacturers may specify oil for certain high-speed, high-temperature or heavily loaded applications. The application determines suitability.

3.3 Understanding industrial oil viscosity

Viscosity describes resistance to flow. A high-viscosity oil generally resists flowing more than a low-viscosity oil at the same temperature.

Industrial oils often use an ISO Viscosity Grade designation.

ISO VG indicates nominal kinematic viscosity at 40°C.

Common grades include:

One mm²/s equals one centistoke (cSt).

For example: ISO VG 68 = nominally 68 cSt at 40°C.

Under the viscosity classification, the permitted grade range is approximately ±10%, so ISO VG 68 covers 61.2–74.8 cSt at 40°C.

The ISO VG number is not a quality rating, a maximum operating temperature or a complete lubricant specification. Two ISO VG 68 products may have different additive systems, base oils, viscosity indices and suitability.

3.4 Why viscosity changes with temperature

For most common liquid lubricants, viscosity decreases as temperature rises.

An oil selected using its 40°C viscosity must also be evaluated at the operating temperature. A lubricant that provides adequate film thickness at one temperature may perform differently at another.

The viscosity index (VI) is a way of describing the relative sensitivity of an oil's viscosity to temperature changes. It does not independently establish suitability.

Oil selection must consider the equipment manufacturer's viscosity requirements, expected working temperatures, loading and operating conditions.

3.5 Understanding grease

Grease is not simply unusually thick oil. A typical lubricating grease contains:

1. Base oil.
2. Thickener.
3. Additives.

The base oil provides much of the lubricating action. The thickener influences how the grease retains and releases that oil, its structure and its consistency.

Additives may provide oxidation resistance, corrosion protection, antiwear or extreme-pressure characteristics.

Different thickeners include lithium, lithium-complex, calcium sulfonate, polyurea and other systems. The thickener name alone does not determine the complete performance or compatibility of a grease.

3.6 NLGI consistency grades

The National Lubricating Grease Institute classifies grease consistency using worked-penetration measurements.

NLGI 2 is a common grade for many general-purpose applications. However, consistency alone does not describe:

• Base-oil viscosity.
• Maximum operating temperature.
• Bearing-speed suitability.
• Water resistance.
• Additive compatibility.
• Lubrication performance.

A grease labelled "NLGI 2, ISO VG 220 base oil" has a grease-consistency grade of 2 and a nominal base-oil viscosity of 220 cSt at 40°C. These are separate characteristics.

3.7 Grease compatibility

Mixing greases can cause:

• Softening.
• Hardening.
• Oil separation.
• Changes in mechanical stability.
• Reduced lubrication performance.
• Seal or material compatibility problems.

A basic compatibility review considers:

A compatibility chart is preliminary guidance, not a guarantee. ASTM D6185 provides a testing framework for grease compatibility [VERIFY applicable edition].

Never identify a grease solely by its colour. Two red greases may have completely different formulations.

3.8 Relubrication intervals

Relubrication is not simply a fixed calendar activity. The appropriate interval depends on:

• Bearing type and size.
• Rotational speed.
• Temperature.
• Load.
• Grease characteristics.
• Operating hours.
• Water and dust exposure.
• Seal design.
• Housing configuration.
• Lubrication path.

High temperatures and contamination may shorten useful lubricant life. A relubrication plan should establish:

What: Approved lubricant product.

Where: Correct lubrication point.

How much: Approved quantity.

When: Specified time, operating hours or condition-based trigger.

How: Approved equipment-specific procedure.

Verification: Evidence that the intended lubricant was supplied and abnormal conditions were not introduced.

3.9 The problem with over-greasing

Over-greasing is a real failure mechanism. When excess grease occupies a space intended to provide suitable lubricant distribution, the rolling elements may churn through it.

This can produce:

• Higher operating temperature.
• Increased resistance and energy loss.
• Damage to lubricant structure.
• Pressure against seals.
• Grease leakage.
• Accelerated deterioration in unsuitable conditions.

The comparison does not imply universal fill percentages.

An important point: a bearing with grease leaking from a seal is not necessarily suffering from insufficient lubrication. Leakage can also indicate an excessive fill, incorrect grease, damaged seal or another problem.

3.10 Grease-quantity formulas [VERIFY]

Published manufacturer references sometimes provide approximate replenishment formulas. For example, SKF describes particular bearing-relubrication arrangements using:

Side replenishment:

Gp = 0.005 × D × B [VERIFY]

Replenishment through suitable ring lubrication holes:

Gp = 0.002 × D × B [VERIFY]

Where:

• Gp = approximate grease replenishment quantity, g.
• D = bearing outside diameter, mm.
• B = bearing width, mm.

These relationships are arrangement-specific guidance, not universal formulas. They do not establish that a particular bearing should be relubricated, that grease can enter the bearing, or that a housing has an adequate relief path.

Before any practical use, confirm the exact bearing designation, lubrication path, housing design, manufacturer guidance, grease product, interval and applicable service procedure.

3.11 Lubricant contamination

Lubricants can become contaminated through:

• Dust.
• Metal wear particles.
• Water.
• Dirt entering during maintenance.
• Incorrect lubricant mixing.
• Degraded seals.
• Poor storage.
• Process chemicals.

Contamination can cause abrasive wear, corrosion, reduced film performance and other degradation mechanisms.

For oil systems, analysis may include:

• Viscosity.
• Water content.
• Particle contamination.
• Wear metals.
• Oxidation or other condition indicators.
• Relevant additive or chemical condition.

Particle cleanliness is commonly communicated using ISO 4406 codes, where applicable. The appropriate testing and response criteria depend on the equipment.

3.12 Automatic lubrication systems

Automatic lubrication systems supply lubricant through a designed delivery arrangement. Common configurations include:

Single-point lubricators: Serve an individual lubrication point.

Centralized systems: Deliver lubricant to multiple points through distributors, metering devices or progressive systems.

Oil circulation systems: Move oil through equipment for lubrication and, where designed, cooling and filtration.

Advantages may include:

• More consistent delivery intervals.
• Reduced missed lubrication points.
• Lower manual intervention.
• Better documented lubricant consumption.
• Access to condition and fault monitoring.

But automatic does not mean maintenance-free. Potential faults include:

• Empty reservoir.
• Blocked lubrication lines.
• Leaking connections.
• Failed pump or distributor.
• Incorrect metering.
• Incorrect lubricant.
• Delivery to the wrong location.
• Faulty level or pressure indication.

A system reporting that a pump has cycled does not necessarily prove that every bearing received the required lubricant.

3.13 Storage and handling

Good lubrication begins before the lubricant reaches a machine.

For oils:

• Keep containers correctly labelled and sealed.
• Protect against moisture and debris.
• Use suitable, clean transfer equipment.
• Follow storage limits and shelf-life guidance.
• Prevent mixing with incompatible products.
• Maintain traceability to the correct product.

For greases:

• Keep containers closed when not in use.
• Use dedicated, clearly identified transfer equipment.
• Avoid introducing dirt through handling.
• Prevent cross-contamination between grease products.
• Follow manufacturer storage conditions and stock-rotation requirements.

A clean lubricant introduced with a dirty grease gun or transfer container can immediately lose part of its contamination-control advantage.

Worked example

Problem: Interpret an industrial oil grade and estimate a hypothetical bearing grease-replenishment quantity.

These are two separate calculations intended to demonstrate lubricant specifications.

Part A: ISO VG 68 oil

Suppose a gearbox documentation package specifies ISO VG 68 oil at the applicable viscosity classification.

The nominal viscosity at 40°C is: 68 mm²/s.

Using the standard grade tolerance of ±10%:

Minimum = 68 × 0.90 = 61.2 mm²/s.

Maximum = 68 × 1.10 = 74.8 mm²/s.

Therefore: ISO VG 68 grade range at 40°C = 61.2–74.8 cSt.

This range describes viscosity classification only. It does not determine the correct additive package, lubricant compatibility or service suitability.

Part B: Grease replenishment

Consider a hypothetical open rolling-element bearing with:

• Outside diameter D = 80 mm.
• Bearing width B = 21 mm.
• A conceptual side-replenishment arrangement.
• An assumed need for periodic replenishment, subject to manufacturer confirmation.

Using the published style of relationship:

Gp = 0.005 × D × B [VERIFY]

Gp = 0.005 × 80 × 21

Gp = 8.4 g [0.296 oz].

This is a mathematical illustration, not an approved application quantity.

If a suitable bearing-ring replenishment arrangement were instead specified, the alternate expression would give:

Gp = 0.002 × 80 × 21 [VERIFY]

Gp = 3.36 g [0.119 oz].

Comparison

The calculated difference arises from different assumed delivery arrangements, not from a change in bearing size.

Independent check

8.4 / 3.36 = 2.5.

The ratio agrees with the factors 0.005 / 0.002 = 2.5.

Engineering interpretation

The calculation is incomplete until the following are established:

1. Is the bearing actually designed for relubrication?
2. Is the lubrication point connected to the intended cavity?
3. Is a relief or discharge arrangement provided where required?
4. Is the lubricant appropriate?
5. Is the relubrication interval known?
6. Are grease delivery and quantity verified?
7. Does the equipment manufacturer approve the method?

The manufacturer's procedure takes precedence over generic teaching formulas.

Common mistakes

In the field

Imagine a generic conveyor drive whose bearing temperature has increased over several weeks. A technician notices grease near the housing seal.

The tempting response is to add more grease, assuming that the bearing needs lubrication. However, the available observations support several possible explanations.

A structured investigation would review:

Trend: Is the increase gradual or sudden? Under what operating loads and speeds was temperature recorded?

Lubricant: Is the specified product being used? Has the product or supplier changed?

Quantity: Has the relubrication quantity or interval changed?

Delivery: Are the grease line, fitting and lubrication point functioning correctly?

Housing: Is there a designed relief route? Are seals damaged or pressurized?

Mechanical condition: Is there unusual vibration, misalignment, bearing movement or loading?

Environment: Has water exposure, dust or ambient temperature changed?

The outcome may be a lubrication correction, mechanical repair, revised inspection practice or further engineering evaluation.

Observation first. Diagnosis second. Authorized corrective action third.

Do not approach moving or guarded equipment or perform lubrication work outside the approved procedure.

Quick checks

Free resources & references

1. NLGI — Grease consistency grades — Official worked-penetration grades and consistency classification.
2. NLGI — Frequently asked lubrication questions — Compatibility, mixing, handling and practical concerns.
3. STLE — Lubrication Fundamentals — Lubricating grease composition, viscosity and application considerations.
4. SKF — Bearing Installation and Maintenance Guide — Bearing lubrication, relubrication arrangements and manufacturer-specific guidance.
5. Petro-Canada Lubricants — Industrial viscosity classification — ISO VG reference values.
6. Anton Paar — ISO viscosity classification — Explanation of the ISO VG system.

Some linked technical references describe particular equipment configurations or products. Verify the applicability and current editions of relevant standards before practical use.

Photograph sources and reuse licences are linked beneath each image. Third-party photographs retain their respective licences.

Mini-project

Project: Create a lubricant-selection and contamination-control worksheet.

No operating machinery is required. Start with three hypothetical systems:

For each system, create the following fields:

1. Bearing or gear type.
2. Operating speed.
3. Estimated load condition.
4. Operating temperature.
5. Environmental exposure.
6. OEM lubricant specification.
7. Lubricant type.
8. Relevant viscosity or consistency designation.
9. Compatibility considerations.
10. Relubrication requirements.
11. Contamination-control measures.
12. Missing information requiring verification.

Use manufacturer literature to research a suitable approach, but do not invent product requirements where the equipment specification is unknown.

Add a second worksheet for a hypothetical centralized greasing system with five lubrication points. Identify possible faults such as an empty reservoir, blocked line, failed pump and incorrect product.

For each fault, record the possible consequence and the evidence needed for investigation.

The objective is to learn that lubrication reliability depends on product selection, delivery, cleanliness, monitoring and documented decisions—not just on adding lubricant.

Next topics

• Bearings — Connect lubricant selection to bearing loads, speed and service life.
• Power-Transmission Hardware — Explore the lubrication needs of gears, chains and couplings.
• Failure & Safety Factors — Understand wear, fatigue and component failure mechanisms.
• Measurement Tools — Learn how reliable observations support maintenance decisions.