Lubricants and Lubrication System Selection: The Invisible Film Between Survival and Catastrophe

The Invisible Film Between Survival and Catastrophe

A machine doesn't scream before it dies. It just gets hot, seizes, and takes your production schedule — and your budget — down with it.

That's exactly what happened to the practitioner Patel, a maintenance engineer at a mid-size manufacturing plant running three shifts of CNC equipment. One Thursday morning, a spindle bearing on their highest-revenue lathe locked up mid-cycle. The workpiece was destroyed. The spindle required a full rebuild. Twelve days of downtime. The total cost, including lost production, replacement parts, and emergency labor, exceeded 85,000 units of value.

The root cause? Someone had packed the bearing housing with the wrong grease — and too much of it.

the practitioner's story isn't rare. It's practically universal. And the lesson buried inside it is the foundation of everything you're about to learn.

Why Lubricants Exist — And Why Most People Get Them Wrong

A lubricant serves six fundamental purposes:

• Reduce friction between moving surfaces
• Prevent wear of mating components
• Prevent adhesion (galling and metal transfer)
• Distribute load across contact zones
• Cool moving elements by carrying away frictional heat
• Prevent corrosion of internal surfaces

Most people think of lubrication as "making things slippery." That oversimplification is where failures begin.

The range of materials used as lubricants has expanded enormously. In addition to traditional oils and greases, modern lubricants include plastics, solids, and even gases. The available categories span:

• Petroleum fluids
• Synthetic fluids
• Greases
• Solid films
• Working fluids
• Gases
• Plastics
• Animal fats
• Metallic and mineral films
• Vegetable oils

The only universal limitations on any lubricant are its ability to replenish itself, its capacity to dissipate frictional heat, its reaction to high environmental temperatures, and its stability in combined environments.

Key Insight: Because of the massive selection of lubricating materials now available, choosing the wrong one — or applying the right one incorrectly — is more common than having no lubricant at all. The material and the method of application both matter.

Lubricating Oils — The Most Versatile Lubricant on Earth

The Workhorse: Mineral Oil

The most versatile and best-known lubricant is mineral oil. When applied in well-designed applications that account for the limitations of both mechanical and hydraulic elements, oil is recognized as the most reliable lubricant available.

Mineral oils are offered in a wide selection of stocks, carefully developed to meet the requirements of specific applications. But here's the critical detail most engineers overlook:

Lubricating oils are seldom marketed without additives blended for a narrow range of applications. These "additive packages" are developed for particular use cases, so consulting with qualified lubrication specialists on proper selection is essential.

The Seven Most Common Oil Additives

A more recent development is a series of ashless organic compounds that leave no residue when heated to a temperature high enough to evaporate or burn off the base oil. Originally produced for internal combustion engine applications, these additives have found wide acceptance in applications where metallic or mineral trace elements would promote catalytic, corrosive, deposition, or degradation effects on mechanism materials.

The Hidden Danger of Additives

Here's where the practitioner's plant — and thousands of others — run into trouble.

Additives are not stable over the entire temperature and shear-rate ranges considered acceptable for the base stock oil application. This means additive-type oils must be carefully monitored to ensure they are not continued in service after their principal capabilities have been diminished or depleted.

Consider detergent-dispersant additives as a case study:

• They function excellently to reduce and control degradation products that would otherwise deposit on operating parts and oil cavity walls
• Because they cause the oil to carry a higher than normal amount of breakdown products in fine suspension, they may cause accelerated deposition or foaming when the additive has been depleted or degraded
• Ingestion of water by condensation or leaking can cause markedly harmful effects

Viscosity index improvers present another trap:

• They modify oils so viscosity change is reduced over the operating temperature range
• The original stock will revert to its natural state when the additive has been depleted or degraded due to exposure to high temperatures or high shear rates
• In heavy-duty installations, selecting a heavier or more highly refined oil is generally advisable rather than relying on a less stable viscosity-index-improvement product

Viscosity-index-improved oils should generally be used only in applications where the shear rate is well below 1,000,000 reciprocal seconds, as determined by:

$$\text{Shear Rate } (s^{-1}) = \frac{DN}{60t}$$

Where:

• $D$ = journal diameter (inches)
• $N$ = journal speed (rpm)
• $t$ = film thickness (inches)

Types of Oils — Choosing the Right Base Stock

Beyond additives, you must understand the wide variety of oils — natural and synthetic — available. Each has its own features that make it suitable for specific applications and limit its utility in others.

Properties of Commercial Petroleum Oils

Group application summary:

• Group A — Automotive. With increased additives: diesel and marine reciprocating engines
• Group B — Gear trains and transmissions. With E.P. additives: hypoid gears
• Group C — Machine tools and other industrial applications
• Group D — Marine propulsion and stationary power turbines
• Group E — Turbojet engines
• Group F — Reciprocating aircraft engines

Critical Takeaway: The interrelation of shear rate, load, and temperature variations often prohibit precise predictions of fluid durability. Prototype and rig testing are frequently required to ensure the most satisfactory fluid selection.

Viscosity — The Single Most Important Property of Any Oil

What Viscosity Actually Means

Absolute viscosity is defined as a fluid's resistance to shear or motion — its internal friction. This property is the force required to move a plane surface of unit area with unit speed parallel to a second plane at unit distance from it.

In the metric system, the unit of viscosity is called the poise. In the English system, it is called the reyn. The relationship:

$$1 \text{ reyn} = 68{,}950 \text{ poises}$$

One poise is the viscosity of a fluid such that one dyne of force is required to move a surface of one square centimeter with a speed of one centimeter per second, the distance between surfaces being one centimeter.

Kinematic vs. Absolute Viscosity

Kinematic viscosity is related directly to the flow time of a fluid through a viscosimeter capillary. By multiplying the kinematic viscosity by the density of the fluid at the test temperature, you determine the absolute viscosity.

Because in the metric system the mass density equals the specific gravity, the conversion from kinematic to absolute viscosity is generally made in this system and then converted to English units where required.

Viscosity Unit Conversion Table

Where ρ = specific gravity of the oil.

Additional conversions:

The Golden Rule of Viscosity Selection

Here is the principle that could have saved the practitioner's plant 85,000 units of value:

As a general rule, the lightest-weight oil that can carry the maximum load should be used.

This runs counter to intuition. Most people assume "thicker is safer." But in reality, a higher viscosity than necessary to maintain the oil film results in wasted power — energy expended to overcome the internal friction of the oil itself.

Where load carrying capacity feels borderline, lubricity improvers should be employed rather than an arbitrarily higher viscosity fluid. A thicker fluid may increase friction losses sufficiently to lower operating viscosity into the range provided by an initially lighter fluid — a self-defeating cycle.

Lubricant Selection by Speed and Load

The relationship between journal speed, bearing load, and appropriate lubricant follows a clear pattern:

Rule of thumb: Heavier oils for high loads; lighter oils for high speeds.

Specific Gravity of Oils — The Density Factor

API Gravity and Specific Gravity

The standard practice in the oil industry is to obtain a measure of specific gravity at 60°F on an arbitrary scale, in degrees API, as specified by the American Petroleum Institute.

The relationship between API gravity and specific gravity (grams of mass per cubic centimeter) at 60°F:

$$\rho_{60} = \frac{141.5}{131.5 + \rho_{API}}$$

The specific gravity at some other temperature, $T$, is found from:

$$\rho_T = \rho_{60} - 0.00035(T - 60)$$

Where $T$ is in degrees Fahrenheit.

Normal values of specific gravity for sleeve-bearing lubricants range from 0.75 to 0.95 at 60°F. If the API rating is not known, an assumed value of 0.85 may be used.

Application of Lubricating Oils — The 10 Factors That Make or Break Your Selection

In the selection and application of lubricating oils, careful attention must be given to the temperature in the critical operating area and its effect on oil properties. Analysis should include detailed attention to cooling, friction losses, shear rates, and contaminants.

A common mistake: oil selections that result in excessive operating temperatures because the initial viscosity was too high, raising friction losses. Improved cooling — such as increasing oil flow — can improve fluid properties in the load zone. In many gear trains, reducing churning and aeration through improved scavenging and direction of oil jets also produces significant improvements.

The 10 Critical Factors in Oil Selection

These factors are generally modified by additives. Since additives are used in limited amounts, blended oils are not as durable as the base stock and must be used in carefully designed systems. Maintenance procedures must be established to monitor the oil and replace it when additive effectiveness degrades.

• In large systems supervised by a lubricating engineer, sampling and laboratory analysis can be relied upon
• In customer-maintained systems (like automobiles), the design engineer must specify a safe replacement period that accounts for variations in service and utilization

Lubrication Systems — Delivering Oil Where It Matters

Getting the right oil is only half the battle. Delivering it to the right place, in the right amount, at the right time is equally critical.

Oil Bath Lubrication

The bushing is submerged in oil. This is the most reliable of all methods except pressure lubrication. Requirements:

• The housing must be oil-tight
• The shaft speed must not be so great as to cause excessive churning

Oil Ring Lubrication

Oil is supplied to the bearing by a ring in contact with the shaft. Within reasonable limits, the ring brings enough oil to maintain hydrodynamic lubrication. Critical parameters:

• If shaft speed is too low, insufficient oil follows the ring to the bearing
• If shaft speed is too high, the ring speed can't keep pace, and oil is lost by centrifugal force
• Optimal peripheral speed: 200 to 2,000 feet per minute
• Safe load for hydrodynamic lubrication: one-half that of pressure-fed bearings
• Unless the load is light, safe load drops to one-quarter of pressure-fed bearings

Wick or Waste Pack Lubrication

Delivers oil to a bushing by capillary action of a wick or waste pack. The amount delivered is proportional to the size of the wick or pack.

Oil-Conducting Capacity of Wicks:

• With oil level maintained 3/8 to 3/4 inch below the top of the oil tube, each strand of clean worsted yarn carries slightly more than one drop of oil per minute
• A 24-strand wick feeds approximately 30 drops per minute — sufficient for large bearings at high speed
• Remove wicks when machinery is idle — they continue delivering oil until the supply is exhausted, flooding the bearing

Drip-Feed Lubricators

A simple cup or manifold connected by pipe to each bearing. The rate of feed is regulated by a needle or conical valve. A loose-fitting cover prevents foreign matter from contaminating the oil.

Wiper-Type Lubricators

Used for out-of-the-way oscillating parts. A wiper consists of an oil-cup with a central blade attached to a moving part. A fibrous strip fed with oil from a source is placed on a stationary part — the cup scrapes along the material and wipes off oil, which passes to the bearing surfaces.

Centralized Lubrication Systems

Various forms of centralized systems simplify and improve lubrication efficiency. In general:

• A central reservoir provides the oil supply
• Oil is conveyed to each bearing through individual lines or a single line with branches
• Pumping is manual (single lever movement) or automatic (mechanical drive from a revolving shaft)
• All bearings in the system are lubricated simultaneously
• Centralized force-feed lubrication is adaptable to lathes, planers, milling machines, and many other types
• It permits the use of a lighter grade of oil, especially where complete coverage of moving parts is assured

Gravity Lubrication Systems

Usually consist of a small number of distributing centers or manifolds from which oil is taken by piping directly to the lubricated surfaces. Each bearing point has its own independent pipe and connections. The means of maintaining steady supply include drip feeds, wick feeds, and wiping oilers.

Maintenance Tip: Oil manifolds, cups, and pipes should be cleaned occasionally with steam conducted through a hose or with boiling soda water. When soda water is used, disconnect the pipes so no soda water reaches the bearings.

Oil Mist Systems — Precision Lubrication and Cooling

A very effective system for lubricating and cooling elements that require a limited quantity of fluid uses a device that:

1. Generates a mist of oil
2. Separates out the denser and larger (wet) oil particles
3. Distributes the dry mist through a piping or conduit system

The mist is delivered into the bearing, gear, or lubricated element cavity through a condensing or spray nozzle, which also meters the flow.

Advantages of Oil Mist Systems

• Cooling — the mist provides effective heat removal
• Clean lubricant — the atomization process filters the oil
• Pressurized cavities — prevents entrance of contaminants
• Efficient application — very limited lubricant quantities required
• Near-automatic performance — minimal operator intervention

System Specifications

• Fluid reservoirs range from a few ounces up to several gallons
• Can accept shop air or work from a self-contained electrically powered compressor
• With proper fluid temperature control, these units can atomize and dispense most motor and many gear oils

Limitation: Not ideal for very low temperature environments or applications where monitoring accumulation of solid oil in visual devices is impractical.

Lubricating Greases — When Oil Won't Stay Put

Why Grease Exists

In many applications, fluid lubricants cannot be used because of:

• Difficulty of retention (no effective housing or seal)
• Difficulty of relubrication (inaccessible locations)
• Danger of churning (high-agitation environments)
• Need for simplification (fewer maintenance touchpoints)

Greases are formulations of petroleum oils thickened by dispersions of soap, but may also consist of synthetic oils with soap or inorganic thickeners, or oil with silaceous dispersions.

How Grease Actually Works

In all cases, the thickener — which must be carefully prepared and mixed with the fluid — is used to immobilize the oil, serving as a storehouse from which the oil bleeds at a slow rate.

Though the thickener very often has lubricating properties itself, the oil bleeding from the bulk of the grease is the determining lubricating function.

Critical Threshold: When the oil has been depleted to 50% of the total weight of the grease, the lubricating ability is no longer reliable. In some applications requiring an initially softer and wetter material, this level may be as high as 60%.

Greases for Plain Bearings

Where full-film lubrication is not possible or impractical for slow-speed, fairly high-load applications, greases are widely used. Key facts:

• Full-film lubrication with grease is possible but requires an elaborate pumping system
• Bearings supplied with grease are usually lubricated periodically
• Grease lubrication implies boundary lubrication conditions — design accordingly
• Lubricating greases are essentially a combination of mineral oil + metallic soap thickener
• Coefficients of friction for grease-lubricated bearings range from 0.08 to 0.16, with an average design value of 0.12

Grease Classifications — The NLGI System

Consistency Grades

To classify greases by mobility and oil content, they are divided into grades by the NLGI (National Lubricating Grease Institute). Grades range from 0 (softest) through 6 (stiffest), determined by penetrometer testing.

Understanding Thixotropic Behavior

Many greases exhibit thixotropic properties — they soften with working, as they often do when agitated by working elements or acceleration. This means:

• The worked penetration value should be used as a guide to compare the material's original manufactured condition
• Many greases stiffen when exposed to high shear rates at moderate loads (as in automatic grease dispensing equipment)
• Final application of a grease must be determined by a carefully planned cut-and-try procedure

Grease Application: The Cavity Contact Principle

Grease is normally applied so that the material in the cavity contacts the bearing in the lower quadrants, ensuring that the excess originally packed into it impinges on the material in the reservoir.

If the space between the bulk mobile grease and the bearing is too large, a critical delay period occurs before lubricant can be resupplied. Since most lubricants undergo attrition due to thermal degradation, evaporation, shearing, or decomposition, this delay can be fatal.

With proper grease selection (one that does not slump excessively) and reservoir construction to prevent churning, the initial bearing action will purge excess grease and establish a flow path for bleed oil. For this purpose, most selected greases are Grade 2 or 3, falling into the "channelling" variety.

Types of Grease — Matching Chemistry to Application

Commonly Used Greases

Synthetic Greases

Composed of normal types of soaps but use synthetic hydrocarbons instead of normal mineral oils:

• Available in a range of consistencies
• Both water-soluble and insoluble types
• Can accommodate a wide range of operating temperatures
• Consult the lubricant manufacturer for special-purpose recommendations

Specialty Grease Thickeners

For certain gears and slow-speed journal bearings, greases are thickened with:

• Carbon
• Graphite
• Molybdenum disulfide
• Lead
• Zinc oxide

These materials are also used to inhibit fretting corrosion or wear in sliding or oscillating mechanisms and in screw or thread applications.

Modern Grease Selection Guidance

• Lithium soap greases — best for moderate temperature applications (up to 225°F)
• Soda-soap greases — work well up to 285°F
• High-temperature applications (250°F and above) — finely divided dyes and other synthetic thickeners
• Greases vary in volatility (affecting useful life) and viscosity (affecting load carrying capacity) — both must be considered

The "Crater Compound" Warning: An asphaltic residual compound used as "gear grease" — extremely stiff with an extreme temperature-viscosity relationship. Its oxidation resistance is limited and low mobility in winter temperatures makes it a material to use with care. However, it is used extensively in the railroad industry where its ability to adhere to gear and chain contact surfaces outweighs its limitations.

Temperature Effects on Grease Life — The Rule That Halves Everything

Since most grease applications are made where long life is important and relubrication is impractical, operating temperatures must be carefully considered and controlled.

The Half-Life Rule

Being a hydrocarbon and normally susceptible to oxidation, grease is subject to this fundamental principle:

Above a critical threshold temperature, each 15- to 18-degree Fahrenheit rise in temperature reduces the oxidation life of the lubricant by half.

This means that a grease with a 10-year life at 150°F might last only:

Bearing Cavity Fill Guidelines

Care must be taken not to fill the cavity too full:

• The bearing should have a practical quantity of grease worked into it with rolling elements thoroughly coated and the cage covered
• Housing should be no more than 75% filled — with softer greases, no more than 50%
• Excessive packing causes overheating, churning, aerating, and eventual purging with final failure due to insufficient lubrication

This was the practitioner's exact mistake. His technician filled the bearing housing to 100% with a Grade 1 grease. The bearing churned the grease into foam, ran hot, degraded the lubricant in days instead of years, and seized.

Never add "a bit more for good luck." Hold to the prescribed amount and determine it with care on a number of representative assemblies.

Monitoring Bearing Temperature

Most satisfactory control can be established by measuring bearing temperature rise during a controlled test at a consistent measuring point. Once a baseline and limiting range are determined, all deviating bearings should be dismantled, inspected, and reassembled with fresh lubricant for retest.

General guideline: A well-lubricated grease-packed bearing will have a temperature rise above ambient, as measured at the outer race, of 10 to 50°F. In applications where heat is introduced through the shaft or housing, add that temperature to the frame or shaft temperature.

Relubricating with Grease — The 8-Step Procedure That Prevents Failure

In applications where sealed-grease methods are not applicable, addition of grease at regular intervals is required. Follow this procedure precisely:

The 8-Step Relubrication Protocol

Where laboratory access is available, samples of purged material may be analyzed to determine lubricant deterioration and to search for foreign material that may indicate contamination or bearing failure.

Grease Renewal Periods for Anti-Friction Bearings

Grease should not remain in a bearing for longer than 48 months — or if service is very light and temperatures low, 60 months maximum — regardless of operating hours. Separation of oil from soap and oxidation continue whether the bearing is in operation or not.

The Cleaning and Repacking Procedure

Before renewing grease in a hand-packed bearing:

1. Remove the bearing assembly and wash in clean kerosene, degreasing fluid, or other solvent
2. Immediately wash again in clean light mineral oil (preferably rust-inhibited)
3. Do not spin the bearing before or while it is being oiled
4. For hard, difficult-to-remove grease, soak for a few hours in light mineral oil warmed to ~130°F, then wash in cleaning fluid
5. Avoid chlorinated solvents
6. When replacing grease, force it between the balls or rollers with fingers, dismantling the bearing if convenient
7. Fill the available space inside the bearing completely, then spin by hand — wipe off any grease thrown out
8. The space on each side of the bearing in the housing should be no more than half-filled

The Over-Lubrication Paradox: Unlike any other kind of bearing, anti-friction bearings more often give trouble due to over-lubrication rather than under-lubrication.