Lubricants and Lubrication System Selection: Solid Film Lubricants

Solid Film Lubricants — When Liquids Can't Survive

When Solids Become the Only Option

Solids such as graphite, molybdenum disulfide (MoS₂), polytetrafluoroethylene (PTFE), lead, babbitt, silver, and metallic oxides are used to provide dry film lubrication in:

• High-load, slow-speed conditions
• Oscillating load conditions
• Extreme temperature environments

Though most are employed in conjunction with fluid or grease lubricants, they are often applied as the primary or sole lubricant where their inherent limitations are acceptable.

Solid Lubricant Properties

Three Inherent Limitations of Solid Lubricants

1. Cannot carry away heat — no cooling function
2. Cannot replenish themselves — though they generally lay down an oriented film on the contacting interface
3. Relatively immobile — must be bonded to the substrate by a carrier, plating, fusing, or chemical/thermal deposition

Coefficient of Friction Range

Solid lubricants provide coefficients from 0.4 down to 0.02, depending on the method of application and the material against which they rub.

PTFE exhibits a coefficient as low as 0.02 in normal atmospheres after establishing a film on both surfaces. However, it is subject to cold flow and must be supported by a filler or matrix. Since it can now be cemented in thin sheets and is often supplied with a fine glass fiber filler, it is practical where speed and load do not combine to melt the bond or cause sublimation.

Bonded MoS₂ films (using resin and ceramic combinations as binders) are deposited over phosphate-treated steel, aluminum, or other metals. Since the action produces gradual wear, life is limited by the thickness that can be applied — not over one or two thousandths of an inch in conventional applications.

The Temperature Breakdown Effect

Solid lubricants provide the most effective boundary films in terms of reduced friction, wear, and metal transfer. However:

• As operating temperature approaches the melting point of the solid film, friction may increase by 5 to 10 times
• The rate of metal transfer may increase by as much as 1,000 times
• As temperature increases further, metal transfer accelerates by an additional factor of 20 or more
• The final effect is equivalent to metal-to-metal contact without any lubricant

These changes, due to the physical state of the lubricant, are reversed when cooling takes place.

Surface Preparation Is Everything

The performance of any solid film lubricant is limited by the care and preparation of the surface to which it is applied. If the film can't adhere properly, it will come off in flakes — often jamming under flexible components.

Surface requirements:

• Surfaces should be finished to no more than 24 microinches AA
• Waviness should be no greater than 0.00002 inch
• Surfaces should be smooth and flat for long life and minimum friction

Professional Advice: Seek the assistance of the supplier's field engineer and establish close control of both surface preparation and solid film application procedures. Good solid film lubricant performance cannot overcome the need for better surface finishing.

Anti-Friction Bearing Lubrication — The Speed Factor System

The Speed Factor Concept

The limiting factors in anti-friction bearing lubrication are the load and the linear velocity of the centers of the balls or rollers. Since these are difficult to evaluate directly, a speed factor is used:

$$S_i = \text{Inner Race Bore Diameter (inches)} \times \text{RPM}$$

$$S_m = \text{Inner Race Bore Diameter (mm)} \times \text{RPM}$$

Grease Requirements for Anti-Friction Bearings

To be suitable, grease must have:

1. Freedom from chemically or mechanically active ingredients — no uncombined metals, oxides, or solid contaminants
2. Minimal tendency toward consistency change — thickening, oil separation, evaporation, or hardening must be negligible
3. A melting point considerably higher than operating temperatures

Standard Grease Selection

For almost all applications, use:

• No. 2 soda-base grease or mixed-base grease with up to 5% calcium soap
• Blended with an oil of 250 to 300 SSU at 100°F
• For high speeds ($S_i$ of 5,000 or over), a grease made with oil of ~150 SSU at 100°F
• Temperature range capability: −40°F to +250°F with standard greases

Speed and Temperature Limits for Grease

• Grease is usually not suitable for $S_i$ over 12,000 (or $S_m$ over 300,000), though successful applications have been made up to $S_i$ of 50,000
• Practical upper temperature limit: 210°F for standard greases; 300°F is the absolute extreme even with synthetics
• Above 210°F, grease renewal periods become very short

Oil Lubrication for Anti-Friction Bearings

Oil lubrication is adopted when speeds and temperatures are high or when a central oil supply is desired. Requirements:

• Well refined with high film strength
• Good resistance to oxidation and good corrosion protection
• Anti-oxidation additives are helpful but not critical below 200°F
• Anti-corrosion additives are always desirable

Oil Viscosity Selection for Ball Bearings

Key notes:

• Within a given temperature and speed range, use oil toward the lighter end of the grade as speeds increase
• Roller bearings usually require oil one grade heavier than ball bearings for the same speed and temperature range
• Cooled oil is sometimes circulated through an anti-friction bearing to carry off excess heat from high speeds and heavy loads

Aerodynamic Lubrication — When Air Becomes the Lubricant

The Concept

A natural extension of hydrodynamic lubrication consists of using air or some other gas as the lubricant. The viscosity of air is 1,000 times smaller than that of a very thin mineral oil. Consequently, the viscous resistance to motion is vastly reduced.

However, the distance of nearest approach — the closest distance between the shaft and bearing — is also correspondingly smaller, demanding special precautions.

Requirements for Aerodynamic Lubrication

If all conditions are fulfilled, extremely successful bearing systems can run at very low coefficients of friction. They may also operate at very high temperatures since chemical degradation of the lubricant need not occur. And if air is the lubricant, it costs nothing.

Primary Applications

• Oil-free compressors and gas turbines
• Data recording heads for computers — air separates the recording head from the magnetic disc by a thin air film
• High recording density requirements necessitate the smallest possible film thickness — a typical thickness is around 1 µm

Analysis

The analysis of aerodynamic bearings is very similar to liquid hydrodynamic bearings. The main difference is that gas compressibility becomes a distinctive feature and must be incorporated into the analysis.

Elastohydrodynamic Lubrication (EHL) — The Physics That Shouldn't Work, But Does

This is where lubrication science becomes genuinely extraordinary. And this is the section that, once understood, will change how you think about every gear, cam, and bearing in your facility.

The Paradox

In traditional bearing analysis, surfaces are assumed to be perfectly rigid and to retain their geometric shape during operation. But what happens when elastic deformation of the surfaces occurs?

Consider a steel shaft resting on a rubber block. The shaft deforms the block elastically, creating an approximation of a half-bearing. When a lubricant is applied, it is dragged into the interface and, under the right conditions, forms a hydrodynamic film. But the pressures developed in the oil film must now match up with the elastic stresses in the rubber — changing its shape.

This is elastohydrodynamic lubrication (EHL).

  Dry Contact              Lubricated Contact (EHL)
  ┌──────────┐             ┌──────────┐
  │          │             │    ↻     │  ← Rotating Shaft
  └──────────┘             └──────────┘
  ╔══════════╗             ╔════╗  ╔══╗  ← Deformed Rubber
  ║          ║             ║    ╚══╝  ║     (shape changed
  ║  Rubber  ║             ║  Rubber  ║      by oil pressure)
  ╚══════════╝             ╚══════════╝

Where EHL Occurs

• Between rubber seals and shafts
• Between a windshield wiper blade and windshield in rain
• In gear teeth contacts (pressures of 700 × 10⁶ N/m²)
• In ball and roller bearings
• In cam and tappet systems

The Pressure Spectrum

The Grubin-Crook Explanation

The explanation, first provided by A.N. Grubin in 1949 and later expanded by A.W. Crook in 1958, reveals something remarkable:

With most mineral oils, application of high pressure leads to an enormous increase in viscosity. At a pressure of 700 × 10⁶ N/m², the viscosity may be increased 10,000-fold.

The oil entering the gap between gear teeth is trapped between the surfaces and, at the high pressures in the contact region, behaves virtually like a solid separating layer. This explains why many mechanisms operate under much more severe conditions than classical theory would allow.

The Unsolved Mystery

The great success of EHL theory raises a paradox that has not yet been satisfactorily resolved: Why do lubricants ever fail, since the harder they are squeezed, the harder it is to extrude them?

Two leading theories:

1. High temperature flashes may locally destroy the film
2. High rates of shear can actually fracture the lubricant film — since when trapped, it is instantaneously more like a wax than an oil

EHL Film Thickness Equations

For Line Contact (gears, cam-tappet):

$$h_o = \frac{2.65 \cdot \alpha^{0.54} \cdot (\eta_o U)^{0.7} \cdot R_e^{0.43}}{w^{0.13} \cdot E_e^{0.03}}$$

For Point Contact (ball bearings):

$$h_o = 0.84 \cdot \alpha \cdot \eta_o U^{0.74} \cdot 0.41 \cdot R_e \cdot \left(\frac{E_e}{W}\right)^{0.074}$$

Where:

• $\alpha$ = pressure-viscosity coefficient (typical value for mineral oil: $1.8 \times 10^{-8}$ m²/N)
• $\eta_o$ = viscosity at atmospheric pressure (Ns/m²)
• $U$ = entraining surface velocity, $U = (U_A + U_B)/2$ m/s
• $W$ = load on the contact (N)
• $w$ = load per unit width of line contact (N/m)
• $E_e$ = reduced Young's modulus (N/m²)
• $R_e$ = reduced radius of curvature (m)

Reduced Young's Modulus

$$\frac{1}{E_e} = \frac{1}{2}\left(\frac{1 - \nu_A^2}{E_A} + \frac{1 - \nu_B^2}{E_B}\right)$$

Where $\nu_A$, $\nu_B$ are Poisson's ratios and $E_A$, $E_B$ are Young's moduli of the contacting bodies.

Reduced Radius for Ball Bearings

• Contact between ball and inner race: $R_e = \frac{r \cdot R_1}{R_1 + r}$
• Contact between ball and outer race: $R_e = \frac{r(R_1 + 2r)}{R_1 + r}$

Reduced Radius for Involute Gears

At a distance $s$ from the pitch point:

$$R_e = \frac{(R_1 \sin\psi + s)(R_2 \sin\psi + s)}{R_1 + R_2}\sin\psi$$

Where $R_1$, $R_2$ are pitch radii and $\psi$ is the pressure angle.

The Specific Film Thickness (λ)

The most commonly used parameter for relating film thickness to surface quality:

$$\lambda = \frac{h_o}{\sqrt{R_{m1}^2 + R_{m2}^2}}$$

Where $R_m = 1.11 R_a$ is the root-mean-square height of surface asperities and $R_a$ is the centre-line-average height.

Design Rule: If λ > 3, it is usually assumed that there is full separation of contacting bodies by an elastohydrodynamic film.

Critical EHL Design Requirements

This type of lubrication becomes apparent only when the film thickness is less than about 0.25 to 1 µm. To exploit it successfully:

• Surfaces must be very smooth
• Surfaces must be very carefully aligned
• If these conditions are met, gears, cams, and tappets can operate at very high contact pressures without metallic contact

Coefficient of friction in EHL:

• Lightest pressures: µ ≈ 0.01
• Highest pressures: µ ≈ 0.1

Mineral Oil vs. Synthetic Oil in EHL

A critical finding: mineral oils have reasonably good pressure-viscosity characteristics for EHL applications. Synthetic oils, however, do not appear to have satisfactory pressure-viscosity characteristics for this regime.

Viscosity-Pressure Relationship — The Foundation of EHL

The Barus Equation

Lubricant viscosity increases with pressure. For most lubricants, this effect is considerably larger than the effect of temperature or shear when pressure is appreciably above atmospheric. This is of fundamental importance in the lubrication of highly loaded concentrated contacts such as rolling contact bearings, gears, and cam-tappet systems.

The best-known equation for calculating viscosity at moderate pressures:

$$\eta_p = \eta_o \cdot e^{\alpha p}$$

Where:

• $\eta_p$ = viscosity at pressure $p$ (Ns/m²)
• $\eta_o$ = viscosity at atmospheric pressure (Ns/m²)
• $\alpha$ = pressure-viscosity coefficient (m²/N)
• $p$ = pressure of concern (N/m²)

The value of $\alpha$ can be obtained by plotting the natural logarithm of dynamic viscosity $\eta$ measured at pressure $p$. The slope of the graph is $\alpha$.

Dynamic Viscosity and Pressure-Viscosity Coefficients for Common Lubricants

Engineering Insight: Note that glycerol has the lowest pressure-viscosity coefficient despite having a high base viscosity. This means it would be a poor choice for EHL applications — its viscosity doesn't increase significantly under pressure. Mineral-based machine oils and cylinder oils, with their higher α values, are far better suited for highly loaded contacts.

Improvement method and result

Six months after the catastrophic bearing failure, the practitioner had rebuilt his plant's lubrication program from the ground up. Here's what changed:

Before the failure:

• Technicians grabbed "whatever grease was on the shelf"
• Bearings were packed "until grease came out the sides"
• No temperature monitoring after relubrication
• No documentation of lubricant types or application quantities
• Oil changes happened "when the oil looked dirty"

After the failure:

• Every bearing had a documented lubrication specification — lubricant type, NLGI grade, quantity, and interval
• A centralized lubrication system replaced manual greasing on 60% of the equipment
• All anti-friction bearings were packed to no more than 75% housing capacity
• Temperature rise was measured and documented after every relubrication event
• Oil analysis samples were pulled monthly and sent for laboratory testing
• The half-life temperature rule was posted on every machine — in bold

The result? In the 18 months following the program overhaul, the plant experienced zero bearing failures attributable to lubrication — down from an average of four per year. Maintenance costs dropped by approximately 30%, and unplanned downtime was cut by more than half.

The invisible film between survival and catastrophe isn't just oil. It's knowledge, discipline, and respect for the physics that govern every moving part in your facility.

Your Next Step

Look at one machine in your shop — the one that runs the most hours or carries the heaviest loads.

Ask these three questions:

1. What lubricant is specified, and is that what's actually being used?
2. How much is being applied, and does it match the manufacturer's recommendation?
3. When was the lubricant last changed, and what was the bearing temperature rise after relubrication?

If you can't answer all three with confidence, you've found your starting point. The most expensive lubricant failure is always the one you could have prevented with information you already had.

This guide covers lubricating oils, types of oils, viscosity, specific gravity, oil application, lubrication systems, oil mist systems, lubricating greases, grease classifications, types of grease, temperature effects on grease life, relubrication procedures, solid film lubricants, anti-friction bearing lubrication, aerodynamic lubrication, elastohydrodynamic lubrication, and viscosity-pressure relationships. Bookmark it. Reference it. Share it with anyone who touches a grease gun or oil can in your facility.