08/06/2026 by Daniel Tait
Lubrication is defined as the process of decreasing friction and wear by applying a lubricant to moving surfaces working in proximity.
Once added, an effective lubricant from a quality brand like Mobil or Morris can prevent direct metal-on-metal contact, protecting against heat build-up and damage.
Inadequate lubrication is often the root cause of a wide range of negative outcomes. Premature component failure, unscheduled downtime, productivity loss and higher energy bills impact the success of operations, but correct lubricating practices can counter them.
The condition of the lubricant film is the determining factor affecting the protection level an oil or grease offers, and it is influenced by multiple factors like load, speed, viscosity and temperature. As a result, understanding the way lubrication films behave under different conditions is key to solving these problems and optimising equipment performance.
There are three main types of lubrication regime: boundary, mixed and full film. These break down into two sub-categories, hydrodynamic (HD) lubrication and elastohydrodynamic (EHL). In this in-depth guide, we take a closer look at these regimes, their characteristics, applications and maintenance requirements.
What is a lubrication regime?
The term “lubrication regime” is used to describe the nature of the lubricant film produced between two moving machinery surfaces in contact. It can define how much these surfaces physically touch, but also the type of wear that they experience, dependent on the thickness of the lubricant, the load and the speed the machinery is working at.
Boundary lubrication
The first point to note about boundary lubrication is that it should be avoided whenever possible. Lubricant experts agree that during this regime, friction is typically at its highest level. Boundary lubrication generally occurs under high load and low speed conditions, but also when machinery starts up and shuts down.
The regime is strongly associated with metal-on-metal contact, when two sliding machine surfaces meet. While metal surfaces appear smooth to our eyes, at a microscopic level, they feature many imperfections like peaks (asperities) and valleys. If a machine were to run with zero lubricant, the metal asperities on opposing surfaces connect and grind together, causing wear, friction and damage.
During the boundary lubrication regime, there is little to no lubricant, and the oil and grease film is likely not thick enough to cover the surface roughness. This results in the maximum metal-to-metal contact during a lubrication cycle, with statistics showing that around 70 per cent of wear occurs during boundary lubrication regimes.
Unwanted outcomes are high friction levels, accelerated wear, surface fatigue, scuffing, adhesive wear and excessive heat generation.
To prevent these problems, operators must select high-performance lubricants that have beneficial additives, like extreme pressure (EP) and anti-wear agents. Viscosity is another key consideration, with lubricants chosen thick enough to protect surfaces but thin enough to reach all contact points.
Mixed lubrication
As sliding speeds rise, boundary lubrication is reduced dramatically, and a wedge of lubricant film forms between the metal surfaces in motion. With film thickness increased and the potential for asperities to make contact is reduced, the friction coefficient also falls dramatically, creating the mixed lubrication regime.
As a result, mixed lubrication is considered a transitional phase, occurring between boundary lubrication and full-film lubrication. During the regime, the load is partly supported by surface asperities, as in boundary lubrication, but also with hydrodynamic fluid pressure present in full film lubrication.
In line with being a transitional phase, mixed lubrication involves variable wear and friction rates. There is partial surface contact, but it’s dependent on operating conditions. For lubrication to remain effective, stability under dynamic conditions is necessary.
To manage this regime, whenever possible, operators should optimise the way they operate, paying attention to rotational speeds and loads. Lubricants selected should be able to maintain performance while working under fluctuating conditions. Systems should be monitored during operations for shifts between boundary, mixed and full-full film regimes, noting noise, vibration and temperature changes and adjusting as required.
Full-film lubrication
Regarded as optimal operating conditions, the full-film lubrication regime sees the lubricant film separate the moving machine surfaces completely, effectively preventing all metal-to-metal contact. As noted earlier, there are two sub-categories of full-film lubrication: hydrodynamic and elastohydrodynamic.
Hydrodynamic lubrication
During hydrodynamic lubrication, moving surfaces are kept apart by a thick and continuous film, so no contact occurs. It happens between sliding and conforming surfaces, where fluid pressure solely supports the load.
As a result, a protective barrier is naturally formed by the shape and motion of the machine surfaces themselves. As surfaces slide against each other, they draw the oil into a narrowing and converging gap.
As viscous lubricant is squeezed into the space, fluid pressure is generated. As it rises, it pushes the surfaces apart, with a fluid cushion between them.
Elastohydrodynamic lubrication
A crucial lubrication regime, elastohydrodynamic lubrication occurs in highly stressed and non-conformal machine contacts. Examples include rolling element bearings, cams and gears, where extreme pressure can cause substantial elastic deformation in the contact surfaces, leading to temporary viscosity spikes in the lubricant. The lubricant creates a protective film that prevents friction, wear and overheating.
During both these regimes, machinery operates with higher energy efficiency levels and minimal wear occurs. However, lubricants in service must still maintain film integrity even under pressure and fluctuations in temperature.
Consequently, high-quality, high-performance lubricants that can retain their viscosity at a wide temperature range and those with extreme pressure additives are of benefit. Beyond lubricant selection, operators should regularly monitor the oil or fluid’s condition and ensure its viscosity is fit for purpose. When applying lubricant, it is important to make sure machinery receives an adequate and consistent supply in all load conditions.
To sum up, effective lubrication involves far more than just applying grease, fluid or oil to equipment. Each individual lubrication regime puts specific demands on which lubricant is selected, how it is applied and the maintenance strategy best adopted.
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