Understanding Lubrication and Friction

Lubricants

• The millwright's duties include maintaining lubrication systems and maintaining machinery in proper working condition.

Types of Lubrication

• Friction causes wear and reduces efficiency of machinery, lubrication is the primary method to reduce friction.
• Types of lubrication include:
  • Oil
  • Grease
  • Air
  • Dry solid lubricants
  • Self lubrication

Oil

• Oil and other liquid lubricants comprise the majority of lubricants used today.
• They can be inexpensive and readily available, fluid properties make them easy to move for lubrication.

Grease

• Grease is suited for applications where oil does not stay where it is needed.
• The thickener holds the lubricant in place directly where it is needed.

Air

• Air can be an effective lubricant when pressurized and pushed between moving components.
• It is not commonly used because it requires an expensive supply mechanism, like a compressor.

Dry Solid Lubricant

• Dry lubricants are small particles of materials that have a low co-efficient of friction.
• They are suitable as lubricants and used as an additive to give grease and oils superpowers.

Self-Lubrication

• Cast iron, Teflon, and some plastics are examples of materials that have some self-lubrication properties.
• Self-lubricating means the base material has some lubricating properties.

Functions of a Lubricant

• A lubricant reduces friction by separating moving surfaces with a film that has less friction than the moving surfaces, the main function of a lubricant.
• A lubricant also
  • Protects against corrosion of the surfaces
  • Carries away heat
  • Carries away contaminants
  • Helps seal

Reducing Friction

• Lubricating oils are primarily used to reduce the friction between moving surfaces.
• Fluid friction resists movement the least of all types of friction.
• Oils are commonly used to reduce friction in components such as
  • Machining,
  • Forming metal,
  • Threaded fasteners,
  • Plain bearings,
  • Anti-friction bearings.

Protect Against Corrosion

• Corrosion occurs when oxygen reacts with metals, rust on steels is a common example of corrosion.
• A simple way to prevent corrosion involves coating the surface in a film of oil to limit oxygen contact with the metal.
• Metal surfaces are splashed with oil in the lubrication process to help reduce corrosion in the machine.

Remove Heat

• In an engine, oil can be pumped through a heat exchanger (cooler) before it travels to bearings and other components.
• Some components do not have coolant pumped through them like the crankshaft, so engine oil is the primary cooling method.
• Ribs on the oil pan help transfer and cool the oil to repeat the cooling cycle.

Carry Away Contaminants

• It is impossible to perfectly clean machinery, and abrasion and wear occur inside equipment.
• Contamination is not conducive to long machine life, but oil helps wash contaminants out of critical areas.
• Contaminants settle in the oil reservoir or are collected by a filter if there is filtration.

Sealing

• Inside a reciprocating compressor, oil is commonly added to lubricate the cylinder.
• It can help form a barrier that bridges imperfections in the surfaces, because of the molecular attraction between the oil and surfaces.
• Oils with a higher viscosity seal more effectively compared to oils with lower viscosities.

Key Terminology

To understand how lubrication works, some fundamentals of physics must be understood:

• Friction
• Fluid film types
• Oil wedge theory

Friction

• Friction is the opposition or resistance to movement between two surfaces that touch each other.
• The best way to mitigate friction is to use lubricants.

Effects of Friction

Cons:

• Resistance to movement increases power consumption
• Causes wear
• Generates waste heat

Pros:

• Provides traction between the rolling elements and the raceways in anti-friction bearings to ensure rolling instead of skidding
• Promotes accurate tracking in bearing raceways
• Provides a means of starting, stopping and accelerating most machines
• Provides a means of polishing

Types of Friction

• Sliding,
• Rolling, and
• Fluid.

Sliding Friction

• Sliding friction is the resistance to movement that is produced when one surface slides across another.
• The resistance is produced by interlocking, tearing, and momentary molecular welding of the peaks of the asperities of 2 surfaces.
• Sliding friction offers more resistance than rolling or fluid friction.

Rolling Friction

• Rolling friction is the resistance to movement of one surface rolling over another.
• Rolling friction results mainly from resistance of the material to being deformed.
• Rolling friction offers less resistance than sliding friction, more than fluid friction.

Fluid Friction

• Fluid friction is the resistance to flow within the fluid is the result of electromagnetic bonds (attraction) between the molecules of a fluid.
• These bonds resist the movement of molecules sliding when fluid begins to flow.
• The stronger the attraction between the molecules, the slower the fluid flows.
• Fluid friction offers less resistance than rolling or sliding friction.
• Fluids with higher viscosity have higher fluid friction than those with lower viscosity.

Types of Lubricant Films

• Fluid film is a term used to describe the thickness of lubricant between moving surfaces.
• Because fluid friction opposes movement the least, there must be enough fluid to allow full film lubrication.
• Otherwise, boundary or partial film lubrication will occur.
• The thickness of the fluid film required determines the lubrication regime that will need to be followed, or the type of lubrication that will occur.
• The three basic regimes of fluid film lubrication are full film lubrication, elasto-hydrodynamic film lubrication, and boundary film lubrication.

Full Film Lubrication

• Full film lubrication has a film thick enough to separate the surfaces that are working and moving with a continuous film.
• This type of film:
  • Offers the least friction resistance
  • Eliminates wear on the moving surfaces that it separates
  • Generates the least amount of heat.

Elasto-Hydrodynamic Film Lubrication

• An elasto hydrodynamic film completely separates two surfaces and is therefore a type of full film lubrication.
• However, in this case, the two surfaces are loaded to the extent that deformation takes place in the loaded area.
• In an anti-friction bearing the deformation occurs on the rolling elements and the raceways.
• The anti-friction bearing lubrication is an example of elasto-hydrodynamic film formation.
• An elasto-hydrodynamic lubrication film forms under the loaded contact between the rolling element and the raceway.
• Load is high enough to cause deformation on the rolling elements and the raceways.
• Viscosity increases due to the extremely high pressures, thus a pseudo solid that ensures maintenance of surface separation.

Boundary Film Lubrication

• A boundary film does not completely separate the moving surfaces, so some of the tips of the asperities touch, as in.
• Normally boundary film does not allow lubrication to prevent wear or damage to moving surfaces.
• Partial film lubrication describes a fluid film amount between boundary and full film lubrication.
• Although it is better to have a partial film, than boundary film, it is still not as effective a preventing wear as full film lubrication is.

Boundary film lubrication:

• Forms at low speeds
• Occurs when the viscosity of the lubricant is too low to support the load
• Forms if there is not enough lubricant
• Results in wear, heat, and higher friction than in full film or elasto-hydrodynamic film.

Oil Wedge Theory

• An oil wedge is a type of full film lubrication that occurs between a moving component and a stationary one.
• It is caused by hydrodynamic principles where the shaft's rotation creates an oil wedge that supports the shaft and relocates it within the bearing clearances.
• A formation of the oil wedge is a means of separating the shaft from the bearing with a film of oil.

How the Oil Wedge is Formed

• Hydrodynamic lubrication uses the rotation of the shaft to form an oil wedge that forces a full film of fluid into the loaded area.
• The formation of an oil wedge is one means of separating the shaft from the bearing using film of oil.
• Demonstrated the four steps of oil wedge formation.
  • The oil sticks to the unloaded surface of the shaft and bearing while the loaded area remains dry.
  • As rotation begins, the shaft starts to climb up the side of the bearing on the dry area, until it rolls onto the surfaces coated with oil (boundary lubricated).
  • If Shaft rolls onto the oiled surface and skids back down, a layer of oil is trapped under the loaded area.
  • Shaft continues to turn drawing a wedge of oil under loaded area, this wedge lifts the shaft off the bearing surface.

Lubricity

• A lubricant reduces friction when applied as a surface coating to moving parts, which reduces the heat generated when the surfaces move.
• The property of reducing friction is known as lubricity.

Viscosity

• Viscosity measures fluid friction (resistance to flow).
• Fluids that flow easily, have low viscosity.
• Fluids that flow very slowly, have high viscosity.

Viscosity Units:

• Pascal-seconds (Pa.s)
• Centipoise(cP)
• Centistokes (cSt)
• Saybolt universal seconds (SUS)

The Importance of Viscosity

• Viscosity is considered the most important property of a lubricant and affects a range of factors involved in lubrication:
  • Affects film strength, which determines film's load capacity
    • The higher the viscosity, the higher the load capacity of the film
  • Affects the flow's rate
    • The higher the viscosity, the slower the flow rate.
  • Since viscosity is a measurement of fluid friction, the higher the viscosity, the higher the internal friction of the fluid.
  • Internal friction affects the heat generated in the lubricant as it flows.
    • The higher the viscosity, the more heat generated as the fluid moves.
  • Affects the power consumption of a machine
    • The higher the viscosity, the higher the internal power consumption, due to increased internal friction.
  • Affects the sealing properties of oil
    • A higher viscosity lubricant has a greater sealing effect than a lower viscosity lubricant.

Factors that Affect Viscosity

• Viscosity is affected by the following three conditions:
  • Temperature
  • Pressure
  • Speed

Centistokes

• The centistoke is a British unit of viscosity that has been accepted by industry worldwide.
  • 1 centistoke (cst) = 1 square millimetre/second.
  • The centistoke measures a type of viscosity called kinematic viscosity.
  • Kinematic viscosity is measured by timing the flow of oil through a capillary tube at a controlled temperature.
  • The viscosity number that appears on most industrial oil containers is the viscosity in centistokes, measured at 40 °C.
  • Oil manufacturers mayprovide viscosity measurements at other temperatures.
    • Measurements at 100 °C are useful to predict the oil's high-temperature performance.
  • The American Standards of Testing and Materials (ASTM) and the International Organizationfor Standardization (ISO) use centistokes in their standards.

Saybolt Universal Seconds

• Saybolt universal seconds (SUS or SSU) is another kinematic unit of viscosity.
• It is popular in the United States. 60ml of lubricant is timed to flow through a calibrated tube held at a specific temperature.
  • These units are determined with a Saybolt viscometer.
  • The SUS viscosity is measured at 100 °F (38°C).
  • The American Gear Manufacturer's Association (AGMA) uses SUS units to set their standards for gear oils.

Absolute Viscosity

• Absolute viscosity is measured in centipoises
• It is a direct measurement of the force of resistance to flow in a fluid.
• For industrial applications, viscosity is measured in centistokes (cst) or converted from absolute units by dividing the absolute viscosity in centipoises by the specific gravity of oil.

Viscosity Index

• Viscosity index (VI) identifies the amount of change in the viscosity of a lubricant over a temperature range.
  • An oil that shows very little change as heated from 40 °C to 100 °C would have a high viscosity index (VI number).
  • The viscosity index is not an absolute number because it is a relative rating for the oil.
  • It compares the oil with two standard oils with arbitrary VI ratings of 0 (poorest oil produced in 1939) and 100 (best oil produced in 1939).
  • Today, an oil with a viscosity index of 150 would be considered a good oil.
• The viscosity index tells how stable a lubricant is over a operating temperatures.
• You can predict whether the oil is too thick for low temperature applications or too thin for high temperature applications.

Pour Point

• Pour point is the lowest temperature at which an oil is observed to flow.

Flash Point

• Flash point is the lowest temperature at which the vapour above an oil ignites when a flame is passed over it.
• The flash point indicates the fire or explosion hazard of the lubricant.
• A key Workplace Hazardous Materials Information System (WHMIS) criterion is to classify products as combustible or flammable.

Types of Oils Characteristics and Applications

• The formulation of lubricating oils varies significantly depending on its application due to the various functions of oils in their application.

Petroleum-Based Oils

• Petroleum-based oils are derived from the base stock produced at refineries.
• They owe their properties to the type of crude oil they are refined from.
• They originate from two types of crude oil: paraffin and naphtha.
• Each type of base stock has its own characteristics and can handle particular conditions with less need to modify with additives.

Naphtha Base Stock Characteristics of Oils

• The oils have several characteristics:
  • They are mainly used as the base oil in cold temperature grease and in refrigerationsystems.
  • They are more reactive than paraffin based oil which means they are less stable.
  • They have the greatest change in viscosity over a wide temperature range (low viscosity index) and this limits their use to indoors or in climates where there is little temperature change. -They flow at lower temperatures than paraffin-based oil.

Paraffin Base Stock Characteristics of Oils

• This oil has the following characteristsics:
  • Highest wax content
  • Least reactive base oils (most stable)
  • They have the least change in viscosity over a wide temperature range (have a high viscosity index)
  • Less capacity to dissolve additives than naphtha-based oils (less solvency)
  • Less costly than synthetic oils The oils do not wear out. They can be re-refined to remove contaminants putting fresh additives in. Independent laboratory testing shows they are of top quality. They are stable up to 55C The operating temperature range can be extended with additives. Good demulsibility which refers to meaning separate from water. Free or corrosives in their pure state.

Synthetic Oils

• Synthetic oils have been developed to solve lubricating problems that petroleum oils are unable to overcome.
• They are used if they can reduce operating costs enough to offset the higher initial cost of this lubricant.
• They're made from at least 10 chemical bases and manufacturers mixed with with various additives to produce lubricants for suitable conditions.
• Synthetics such as some types are stable up to 250C and have the widest operating temperature range of any oil.
• They can be blended with petroleum oils are more expensive than petroleum-based oils. They dont contain waxes to hinder a low temperature flow.

Biodegradeable Oils

• Biodegradable oils were developed in response to enviromental concerns to reduce contamination.
• They are normally vegetable oil and are easily broken down with bacteria but since they can be broken down in the environment, they are less stable than petroleum or synthetic oils.

Society of Automotive Engineers Viscosity Grades

Society of Automotive Engineers(SAE) is a viscosity grading system for engine oils for manual transmission. The information in an SAE grade contains two viscosity measurements to be: At low temperature for winter months to ensure starting ability or at High Temperature. Low temperatures are labeled with the W and show cold weather starting ability and a cold cranking simulation is measured and converted to centistokes. High temperatures are viscosity readings measured in centistokes with a capillary tube in summer.

Additives

• Additives enhance the performance of oil under specific operating conditions, amounts of additives carefully formulated.
• An additive to make up about 22% of the oil does wear out or get used up and is rated their effectiveness is rated by standard tests which are useful depend on the application.
• In particular, there comes:
  • Oxidation inhibitors which inhibit the oxidation process, also called anti-oxidants.
    • Oxidation occurs when oil reacts with oxygen and breaks over time due to temperature or contaminants.
    • Additives help slow this process.
  • Rust/Corrosion Inhibitors prevent damage caused by water and acid by forming polar molecules protective or chemical layers onto the oil and are rated by corrosions protection tests.
  • Foam Depressants released at the surface of the oil reduce tension with small amounts of organic copolymers or in emergencies an alerosal can to avoid reduced load capacity, increase oxidation, overflow, high operating temperatures and wear.
  • In some other wear tests such as metal to metal contact one can use Anti-Wear additives that form low temperature and high pressure that is also a result of EP or some other Molybdenum Disulphide.
  • Then comes the Detergents that reduce the ash content and the ashless dispersants to maintain performance while providing good cleanliness of the oil and the Demusifiers also play a factor as its water mixed and emulsified or deulsified depending on it.

Oil Analysis

• It involves a small sample from the machine to determine conditions such as contamination level, types of materials, condition and wear rate.
• Have oil analysis done by a technician to get the best sample of oil according to machine parameters.
• Take a sample when machine is at normal temp.
• Seal sample, and send with machine information.
• Check for silicone, water, and particles in results.

Explore the principles of lubrication, including the properties of dry, fluid, and self-lubricating lubricants. Learn about the types and functions of lubricants in reducing friction and wear between moving surfaces. Understand the causes and effects of friction, including its applications in both reducing and enhancing machine operation.