Boundary Lubrication

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The energy efficiency of machinery has become more and more important to businesses. Lubricant manufacturers do their part to improve efficiency by lowering lubricant viscosity to lessen hydrodynamic friction losses. However, this means that more of your assets are operating in the boundary lubrication regime for prolonged periods. Boundary lubrication is not ideal, as the more time spent in this regime, the greater your component wear rate will be. So, it is crucial to better manage friction and wear in the boundary lubrication regime. However, what exactly is boundary lubrication and why is it a concern?

What is Boundary Lubrication?

Boundary lubrication occurs when your lubricant is not supporting the applied load. Instead, metal-to-metal component contact is supporting the load. This is because the lubricant film thickness is smaller than the composite surface roughness. Furthermore, this is particularly common for components with rough surfaces when they operate at high pressure and low sliding velocity.

Why is Boundary Lubrication a Problem?

There will always be asperities (peaks) and valleys on bearing surfaces, regardless of how well they are machined, finished, and cleaned. When two bearing surfaces are in contact, their asperities interfere with one another causing wear. If you properly lubricate your bearings, you can significantly lower friction and wear by decreasing the asperities’ contact.

Boundary lubrication is a problem, especially during stationary or slow moving conditions. The pressure pushes out a large amount of lubrication from between the surfaces. In this case, there is significant contact between the two surfaces despite the thin lubricating film remaining.

Time spent in boundary lubrication conditions greatly impacts the life and performance of bearings. Interactions between surfaces determine friction, heat, and wear of components during boundary lubrication. With heat generation, oxidation and viscosity changes can occur, further negatively affecting film strength.

What are the Other Stages of Lubrication?

A mixed lubrication stage occurs after boundary lubrication. When the bearing surfaces move at a faster speed, a thicker lubrication film forms between them. Due to the increased pressure on the oil film, the viscosity of the lubricant also increases. This is called the pressure-viscosity coefficient. During this period, the equipment is increasing its velocity up to its full operating speed. Only some surfaces separate during this transition, while others remain in contact with each other.

Lastly, there is elastohydrodynamic lubrication. Once your equipment reaches a sufficient velocity, your lubricant will form a sufficient layer between the two surfaces. This lubrication regime is important, as your asperities will no longer be in contact. Furthermore, your lubricant will reach the right viscosity to provide strong film strength for your load.

How to Reduce the Impact of Boundary Lubrication

Lubricant additives and surface coatings are the two main strategies for reducing boundary friction and wear.

Anti-Wear Additives

To protect metal surfaces during boundary conditions, anti-wear (AW) additives are a great option for your lubricant. These polar additives bond with frictional metals and form a ductile, ash-like film. The heat of contact activates these additives, allowing them to form a protective film that minimizes wear. If boundary conditions are met, the AW film shears rather than the surface material. Furthermore, they prevent oxidation of the base oil and corrosion of the metal.

Lubricating machinery requires additives, whether they enhance, suppress, or impart new properties to the base oil. However, the additives become “used up” by performing their function. If you rely on additives to reduce wear, then wear damage will increase when you deplete the additives. Make sure you check your additive package before the additives run out because once they’re gone, they’re gone.

Continuous Improvement of Lubrication Management

Improper lubrication causes 43% of mechanical failures, 50% of damage to roller bearings, and 70% of equipment failures. So, continuous improvement of your lubrication management is essential. Understanding the various stages of lubrication is a step in the right direction. Next, you just have to take action to reduce the impact of boundary lubrication on your assets.

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