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What happens when solid surfaces are so close together and the solid asperities dominate the contact?
What is the primary purpose of introducing lubricants between two moving surfaces?
What occurs when load increases, velocity decreases, or the fluid viscosity decreases?
What is the primary function of boundary lubricants?
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What type of lubrication occurs during start-stops of self-acting bearing surfaces?
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What is the result of lubrication in a moving contact?
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What type of lubrication occurs in a starved contact?
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What are the benefits of lubrication?
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What is the typical thickness of the lubricating film in Hydrodynamic lubrication?
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What type of lubrication occurs when two surfaces are mostly in contact with each other even though a fluid is present?
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What is the range of the coefficient of friction in Hydrodynamic lubrication?
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What type of lubrication occurs when the elastic deformation of the contacting solids plays a significant role in the lubrication process?
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What is the typical range of film thickness in Elastohydrodynamic lubrication?
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What type of bearing is characterized by the creation of pressure within the system due to shaft rotation?
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Which mode of lubrication is characterized by partial metal-to-metal contact and a relatively thin lubricant film?
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What type of wear can occur during start-stop operations and corrosive wear of the surfaces as a result of interaction with the lubricant?
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What type of lubrication occurs when two surfaces are partly separated, partly in contact?
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Under what conditions can a hydrodynamic bearing operate under boundary lubrication?
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What is the typical range of film thickness in the transition between Hydrodynamic/Elastohydrodynamic and boundary lubrication regimes?
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What is the term for the phenomenon where welding of two metallic surfaces occurs due to extreme temperature conditions?
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Which type of lubrication is characterized by the separation of two surfaces by a fluid film in conformal contact?
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What is the term for a condition of lubrication where the lubricant film is completely ruptured, and direct contact between metallic surfaces occurs?
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Which type of lubrication is a subset of hydrodynamic lubrication?
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What is the primary cause of welding between metallic surfaces in extreme boundary lubrication?
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Study Notes
Lubrication and Friction
- Solid contacts between unprotected metal surfaces can lead to a cycle of adhesion, metal transfer, wear particle formation, and eventual seizure.
- Chemically reacted film prevents adhesion during most asperity encounters in liquid lubricated systems.
Stribeck Curve
- Load increase, velocity decrease, or fluid viscosity decrease can lead to a sharp increase in COF and approach high levels.
- This condition can also occur in a starved contact.
Boundary Lubrication
- Solid surfaces are so close together that solid asperities dominate the contact.
- In the absence of boundary lubricants and gases, friction may become very high (>1).
- All self-acting bearing surfaces operate in the boundary lubrication regime during contact start-stops.
- Failure of boundary lubrication occurs by adhesive and corrosive wear.
- Boundary lubricants form easily sheared films on bearing surfaces, minimizing adhesive wear and corrosive wear.
Function of Lubricants
- Lubricants reduce friction and wear between moving surfaces, improving efficiency and reducing wear.
- Lubricants fill surface cavities, making surfaces smoother, and reducing friction and wear rates.
- Lubricants reduce instant failures, fatigue failure, surface failures, and stress concentration.
Lubrication Regimes
- Hydrodynamic lubrication (HD): two surfaces are separated by a fluid film (conformal contact).
- Mixed lubrication: two surfaces are partly separated, partly in contact.
- Boundary lubrication: two surfaces are mostly in contact with each other, even with a fluid present.
Elastohydrodynamic Lubrication (EHD)
- EHD is a subset of HD, where elastic deformation of contacting solids plays a significant role in the HD lubrication process.
- Film thickness is typically 0.5-5µm.
- In heavily loaded contacts, high pressures can lead to changes in lubricant viscosity and elastic deformation of the bodies in contact.
Wear Mechanisms
- Adhesive wear occurs during start-stop operations and corrosive wear of surfaces can occur as a result of interaction with the lubricant.
- Fatigue wear is most common in well-designed heavily loaded bearings.
- Transition between HD/EHD and boundary lubrication regimes can occur, with both mechanisms functioning.
Self-Acting Bearings
- Self-acting bearings operate under hydrodynamic lubrication, where pressure is created within the system due to rotation of the shaft.
- This type of bearing is seen in bearings mounted on engines and centrifugal pumps.
Extreme Boundary Lubrication
- Extreme boundary lubrication occurs under extreme conditions of load and temperature, where the fluid film gets completely ruptured, and direct contact between metallic surfaces takes place.
- This phenomenon is based on the theory of hot spots, where welding of surfaces takes place due to extreme temperature conditions, resulting in severe damage to physical properties.
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Description
This quiz covers the principles of friction and lubrication, including the Stribeck Curve and its effects on friction and wear. Learn about the importance of lubrication in preventing metal seizure and wear.
