Lubrication Fundamentals Series — Week 6
Last week we established that friction is not an abstract force — it is a physical event happening at the microscopic peaks of every metal surface in your facility, millions of times per second. This week we look at exactly how lubrication manages that event — and why the answer is not one film, but five.
Machine surfaces are lubricated by interposing pressurized fluid, semi-fluid, or solid lubricating films between moving surfaces. There are five distinct types, each operating under different conditions, each with different requirements, and each capable of either protecting your equipment or failing it depending on whether the right conditions are met.
Understanding which film is active in your equipment — and whether it is adequate for the conditions — is one of the most practical diagnostic skills in lubrication engineering.
Film 1: Hydrostatic Lubrication
Forced Protection Before Movement Begins
Hydrostatic lubrication is the simplest film to understand and one of the most important to get right. It is the film that exists before your equipment moves.
In hydrostatic lubrication, metal surfaces are separated by a film of pressurized oil or grease where the separating pressure is supplied by an external source — a pump, a grease gun, or a pre-lubrication system — before any relative motion between the surfaces begins.
Why does this matter? Because startup is the most dangerous moment in any machine’s operating cycle. Before the shaft turns, before the bearing rotates, before any hydrodynamic film has had a chance to build, the surfaces are in static contact. That first revolution — and the first several revolutions — are the moments of highest wear risk in every cycle.
A grease gun generates 3,000 to 10,000 psi of pressure. That is enough to force lubricant into the contact zone and establish a protective film before movement begins. That window — between the moment lubrication is applied and the moment the machine starts — is when hydrostatic film is doing its most important work.
Equipment that is started dry, or that sits idle long enough for lubricant to drain away from contact surfaces before the next start, pays a wear penalty with every cycle. Pre-lubrication systems on large rotating equipment exist precisely to address this — ensuring the film is established before the first revolution, not after.
Film 2: Boundary Lubrication
Chemical Protection When the Film Is Challenged
Boundary lubrication is fundamentally different from every other film type. It is not a fluid film at all — it is a chemical coating.
When conditions push beyond what a fluid film can manage — extreme loads, very slow speeds, the transition moments of startup and shutdown — the fluid film thins to the point where asperity contact becomes inevitable. Boundary lubrication is what protects surfaces when that happens.
Boundary films are chemical coatings applied to and adhering on the surface asperities themselves, provided by the additive content of the lubricant. These additives — anti-wear agents, extreme pressure additives, friction modifiers — react with the metal surface under heat and pressure to form a sacrificial layer. That layer has less shear resistance than metal-to-metal contact, so when asperities do touch, they shear through the chemical coating rather than through the metal itself.
The operating temperature range of boundary films is critical and often overlooked. Different additive chemistries are effective at different temperatures:
- Grease soaps and polymers — lower temperature ranges
- Anti-wear additives — mid-range operating temperatures
- Extreme pressure (EP) additives — high temperature, high load conditions
- Organometallic and sulfur/phosphorus compounds — the highest temperature boundary protection available
Selecting a lubricant without understanding which boundary film chemistry is appropriate for your operating temperatures is one of the most common formulation mistakes in industrial lubrication. An anti-wear additive package that performs well in a moderate-temperature bearing application may be completely ineffective in a high-temperature gear application that requires EP chemistry.
Film 3: Mixed Film Lubrication
The Transition State Most Equipment Operates In
Mixed film lubrication is exactly what the name implies — both a thin fluid film and a chemical boundary layer are active simultaneously.
The bulk of the opposing surfaces are separated by a fluid lubricating layer, but the higher asperities still make contact with each other and depend on the boundary film for protection. Mixed film is a transition state — the condition between full fluid film lubrication and boundary lubrication — and it is where most industrial equipment spends more time than most operators realize.
Every startup is a mixed film event. Every shutdown. Every load spike that exceeds the capacity of the fluid film to maintain full separation. Every moment when operating temperature pushes viscosity below the level needed to maintain a full hydrodynamic or elastohydrodynamic film.
The practical implication is this: a lubricant operating in mixed film conditions needs to perform in both regimes simultaneously. The fluid viscosity must be adequate to maintain as thick a film as possible, and the additive chemistry must be effective enough to protect the asperity contacts that the fluid film cannot prevent. A lubricant optimized for one requirement but deficient in the other will underperform in mixed film conditions — which is precisely the condition it will most frequently encounter.
Film 4: Hydrodynamic Lubrication
Full Film — The Goal State
Hydrodynamic lubrication is the film regime that lubrication engineering is designed to achieve and maintain. It is also called fluid film or thick film lubrication — a pressurized film of lubricant fully interposed between sliding surfaces in relative motion, with surface asperities completely separated.
In hydrodynamic lubrication, sliding mechanical friction is replaced by fluid friction. The surfaces are no longer in contact with each other — they are floating on the lubricant film. Wear is essentially eliminated. Heat generation drops dramatically. Energy consumption falls. This is the operating condition that delivers full designed equipment life.
Hydrodynamic films depend entirely on correct fluid viscosity. The film builds as a function of speed — as a shaft rotates, it drags lubricant into the converging gap between the shaft and bearing, building pressure that lifts the shaft off the bearing surface. The faster the rotation and the higher the viscosity, the thicker the film. The heavier the load, the more viscosity and speed are required to maintain full separation.
This is also why the hydroplaning analogy is useful: a car tire at highway speed on a wet road rides on a pressurized water film exactly as a hydrodynamic bearing rides on a pressurized oil film. The physics are identical. And just as a tire loses its water film at low speed — which is why hydroplaning risk actually decreases below a certain speed — a hydrodynamic bearing loses its film at startup and shutdown, which is why those transitions are so critical.
Film 5: Elastohydrodynamic Lubrication
The Film That Defies Intuition
Elastohydrodynamic lubrication — EHD or EHL — is the most complex and counterintuitive of the five film types. It is also the film that protects some of the most highly loaded components in your facility: rolling element bearings and gear sets at the pitch line.
Under the extreme contact loads present in rolling element bearings and gear tooth contact, something remarkable happens. The lubricant viscosity increases by a factor of 10⁶ to 10⁸ times under pressure — transforming from a liquid into an effectively solid film just 0.1 to 0.5 microns thick at the contact zone. That solid film separates the metal surfaces under conditions that would squeeze any conventional fluid film completely out of the contact zone.
Simultaneously, both the lubricant and the bearing surface itself behave elastically. The top and bottom of the bearing flatten slightly under load, spreading the contact stress over a larger area. As the bearing exits the contact zone, it returns to its original round shape — exactly like a rolling tire on pavement that flattens at the contact patch and rounds again as it rolls forward.
The critical requirement for EHD film formation is correct viscosity at the inlet of the contact zone. The lubricant must have sufficient viscosity to enter the bearing contact zone at the operating speed, and sufficient resistance to being squeezed out under the operating load. Get the viscosity wrong — too thin — and the EHD film cannot form at the required thickness. The result is metal-to-metal contact under the very conditions the EHD film was designed to prevent: extreme load, rolling contact, high stress.
This is why rolling element bearing and gear lubrication requires such careful viscosity selection — and why viscosity index matters so much in applications where operating temperatures vary. A lubricant that has the right viscosity at operating temperature but thins excessively as temperature rises will progressively lose its EHD film protection as the equipment warms up.
Reading the Five Films as a System
The five lubricating films are not a menu from which one is selected. They are a system, and most equipment cycles through multiple regimes in the course of normal operation.
A rolling element bearing starts in hydrostatic film — the grease packed at last service providing the initial protection. As speed builds, it transitions through mixed film into elastohydrodynamic lubrication at operating speed. Under load spikes, it may drop back into mixed film or boundary conditions momentarily. At shutdown, it transitions back through mixed film as speed decreases, returning to static contact on whatever hydrostatic film remains.
Every one of those transitions is a lubrication event. Every one of them either goes well — with the right viscosity, the right additive chemistry, and adequate lubricant quantity — or it extracts a wear penalty that accumulates over the life of the component.
The Stribeck-Hersey Curve maps all five regimes onto a single graph, plotting the coefficient of friction against the ratio of viscosity, speed, and load. It makes visible what is otherwise invisible: which film your equipment is operating in at any given moment, and how far it is from the conditions that would push it into a higher or lower regime. Understanding that curve is the foundation of intelligent lubricant selection — which is exactly what we will cover next week.
Next week: Viscosity — The Single Most Important Property Nobody Fully Understands. We will look at what viscosity actually is, how it changes with temperature, what Viscosity Index really measures, and why getting viscosity wrong is the single most common — and most costly — lubrication error in industrial maintenance.
Danny Stephens is a Certified Lubrication Specialist, recognized by the Society of Tribologists and Lubrication Engineers (STLE), specializing in reliability-led lubrication programs across multi-site manufacturing operations.
The views expressed in this article are my own and do not represent those of my employer or any affiliated organization.
© 2026 Danny Stephens, CLS. All rights reserved.
#Lubrication #LubricatingFilms #ReliabilityEngineering #MaintenanceExcellence #LubricationEngineering #AssetCare #Manufacturing #CMRP #PredictiveMaintenance #ContinuousImprovement
