Lubrication Methods & Purpose

Lubrication

• Lubrication is applying materials between moving objects for smooth operation
• Oil or grease are used for rolling bearings to prevent noise, wear, and heat, solid lubricants are additionally used
• The amount and type of lubricant depend on speed, temperature, and conditions
• Lubricants need periodic replacement/oiling due to service life or pollution

Purpose of Lubrication

• Prevents wear and fatigue by forming a lubrication film
• Enhances driving characteristics by reducing noise/friction
• Prevents bearing overheating and lubricant deterioration by radiating heat
• Prevents foreign material penetration, rust, and corrosion

Lubrication Methods

• Grease or oil is used for bearing lubrication
• Choosing the appropriate method depends on operating conditions and purpose
• Oil lubrication is generally better, grease lubrication is widely used due to space availability and simplicity

Grease Lubrication

Lubricating Grease

• Grease is a solid or semi-solid lubricant containing thickener and special ingredients
• Different kinds of grease have distinct characteristics, requiring careful selection

Base Oil

• Base oil is the main lubricating component, forming 80-90% of grease
• Mineral and compound base oils are the two main types
• Mineral oils with higher viscosity are for high load, low speed, and high-temperature
• Lower viscosity mineral oils are for low load, low speed, and low-temperature applications
• Compound base oils are expensive and used for extreme temperatures, wide ranges, and high-speed applications
• Ester, poly-olefine, or silicon series compound base oils are used, fluorine compound oils are becoming more popular

Thickener

• Thickener is key in deciding grease properties, thickness depends on the amount mixed

• Metal soap, non-organic non-soap, and organic non-soap are the three main types of thickeners

• Metal soap thickeners are mostly used, non-organic non-soap thickeners are for special cases like high temperature

• Grease with a high dropping point can be used in high temperatures

• The water-resistance of grease depends on the thickener

• Na soap grease or grease containing Na soap should not be used in wet or high humidity environments due to deterioration

Additives

• They enhance the grease performance and meet customer demands
• They improve physical or chemical properties, and minimize wear, corrosion, or rust
• Various additives prevent oxidization, wear and tear, or rust

Worked Penetration

• Represents grease hardness shown as penetrated depth (1/10mm) by a pendulum
• The greater the value, the softer the grease

Polymer Grease

• Hardened lubricant mixed with polyamid that allows long-period grease supply
• Widely used in bearings under strong centrifugal force or prone to leaking/pollution

Life Span of Grease

• The period from bearing operation start to failure due to insufficient lubrication, with F10 denoting 10% failure possibility
• F10 Life Span Curves are obtained by experiments
• Lubrication interval tf is the minimum value, a refilling interval is shorter for stability; reliability increases with specification

Oil Lubrication

Lubricants

• Lubricants are divided into mineral oil base and synthetic types
• Viscosity is key when selecting, it should not cause abrasion/burning or temperature/friction rise
• Low viscosity lubricants are for high speed, low load; high viscosity for low speed, high load
• Lubricants should be selected with viscosity specified by ISO, viscosity index can be used for references
• The viscosity reduces by half when the temperature of lubricant increases by 10, although it depends on viscosity indices

Oil Lubrication Methods

Oil Sump Lubrication

• The most common method, especially for low to medium speed operations
• The oil surface should be at the center of the lowest rolling element, and oil level can be confirmed by using the oil gauge

Drip Feed Lubrication

• For small bearings operating at high speed, oil supply is controlled by adjusting drip volume

Throwaway Lubrication

• Uses gear or circulation ring to supply oil to bearings, used in transmissions or gears

Circulation Lubrication

• Cools bearing parts revolving at high speed or with high surrounding temperature
• Oil is fed through feed pipe, recovered through recovery pipe, cooled, and re-fed
• The recovery pipe diameter is bigger than the feed pipe to prevent back pressure

Jet Lubrication

• Used for high speed revolution bearings, oil is jet-sprayed through nozzles under constant pressure
• Jet stream speed should be faster than 1/5 of inner ring speed to weaken air wall, a greater number of nozzles makes total volume of lubricant smoother and increases the cooling effect

Spray Lubrication

• Vaporizes lubricant by blowing air to be sprayed into the bearing
• Low lubricant volume reduces churning resistance, good for high speed revolution bearings
• It minimizes pollution, and extends bearing life with fresh lubricant, used for various machining

Lubricants

• Moving surfaces in all machines rub against each other, causing resistance known as friction and wear
• Lubricants reduce friction by keeping surfaces apart, minimizing material destruction; this process is called lubrication

Function of Lubricants

• Reduces wear by preventing metal contact
• Reduces metal expansion from frictional heat
• Acts as a coolant
• Avoids unsmooth motion
• Reduces maintenance cost
• Reduces power loss in internal combustion engines

Mechanism of Lubrication

Thick-Film Lubrication (Fluid-Film/Hydrodynamic Lubrication)

• Surfaces are separated by a thick fluid film, preventing direct contact and reducing friction
• The lubricant should have minimum viscosity and remain in place

Thin Film Lubrication

• Preferred when a continuous film cannot persist, lubricated by materials adsorbed on metallic surfaces through physical or chemical forces
• Vegetable and animal oils and their soaps form a metallic soap film, but break down at high temperatures
• Mineral oils are thermally stable, adding vegetable/animal oils increases oiliness, graphite and molybdenum disulphide are suitable

Extreme Pressure Lubrication

• Occurs when moving surfaces have high pressure and speed, causing high temperatures, liquid lubricants fail to stick/vaporize.
• Special extreme pressure additives are added to mineral oils, forming durable films on metal surfaces
• Important additives contain chlorine, sulfur, or phosphorus, and react with metallic surfaces to form chlorides, sulfides, or phosphides

Classification of Lubricants

Liquid Lubricants/Lubricating Oils

• Liquid lubricants are classified into animal and vegetable oils, mineral or petroleum oils, and blended oils

Characteristics of Good Lubricating Oils

• High boiling point
• Low freezing point
• Adequate viscosity
• High resistance to oxidation and heat
• Non-corrosive properties
• Stability to decomposition at operating temperatures

Animal and Vegetable Oils

• Extracted from crude fat and vegetable oils, possessing good oiliness and sticking to metal surfaces at high temperatures/loads
• They are costly, oxidize easily, and hydrolyze with moist air/water, but are used as blending agents

Mineral or Petroleum Oils

• Lower molecular weight hydrocarbons with 12-50 carbon atoms
• These are cheap, abundant, and stable, but have low oiliness that can be increased by adding compounds like oleic/stearic acid.

Blended Oils

• No single oil has all required properties; additives are used and examples are oleic/stearic acid, palmetic acid, coconut oil, castor oil

Semi-Solid Lubricants/Grease

• Combining lubricating oil with thickening agents, consists primarily of special soaps of Li, Na, Ca, Ba, Al
• Non-soap thickeners include carbon black, silica gel, polyureas, are able to support heavy loads at low speed
• The Internal resistance is much higher than lubricating oils, cannot dissipate heat effectively

Solid lubricants

• Preferred when (1) a lubricating film cannot be secured by the use of lubricating oils or grease (2) contamination(by the entry of dust particles)is unnacceptable (3) the operating temperature or load is too high, even for grease to remain in position and (4) combustible lubricants must be avoided
• Used in dry powder form or with binders to stick firmly to metal surfaces during use
• Dispersions in nonvolatile carriers like soaps, fats, waxes, soft metal films, graphite, molybdenum disulphide, tungsten disulphide, and zinc oxide
• Graphite is soapy to touch non-inflammable, stable up to 375° C, has a flat plate structure and the layers of graphite sheets are arranged one above the other and held together by weak Vander Waal’s forces
• Molybdenum Disulphide has a sandwich-like structure with a layer of molybdenum atoms in between two layers of sulphur atoms and is stable up to 400° C.

Properties of Lubricants

• (1) Viscosity (2) Flash Point and Fire Point (3) Cloud Point and Pour Point (4) Aniline Point and (5) Corrosion Stability

Viscosity

• Property of liquid that offers resistance to flow, unit is poise, determinant of operating characteristics
• If the viscosity is too low, a liquid oil film cannot be maintained
• If the viscosity is too high, excessive friction will result
• Viscosity decreases with temperature, a good lubricating oil's viscosity should not change much, measured by Viscosity Index (V. I).

Flash Point and Fire Point

• Flash point is the lowest temperature at which the lubricant oil gives off enough vapors that ignite for a moment, Fire point is the lowest temperature when a tiny flame is brought near it
• The fire points are 5° C to 40° C higher than the flash points, safeguard against risk if fire, during the use of lubricant

Cloud Point and Pour Point

• Cloud point is the temperature when the cloud appears, pour point is the temperature where the oil ceases to flow
• indicate the suitability of lubricant oil in cold conditions, it possess a low pour point for use in a machine

Aniline Point

• The minimum equilibrium solution temperature for equal volumes of aniline and lubricant oil samples
• Indicates the possible deterioration of the lubricant oil in contact with rubber sealing, a higher aniline point means a higher percentage of paraffinic hydrocarbons

Corrosion Stability

• Stability of the lubricant oil is estimated by carrying out corrosion test
• If the copper strip has tarnished, it shows that the lubricant oil contains any chemically active substances which cause the corrosion of the copper strip

Essential requirements of a good lubricant

• High viscosity index
• Flash and fire points higher than the operating temperature of the machine
• High oiliness The cloud and pour points should always be lower than the operating temperature of the machine
• Low volatility of the lubricating oil
• Least amount of carbon during use
• High aniline point
• Higher resistance towards oxidation and corrosion
• High good gergent quality

Additives

• They falls just a few parts per million to serveral percentage, depending on the function that these products have to carry out
• Substances to improve the intrinsic characteristics of the base oils
  • Viscosity index modifiers
  • Pour point improvers
• Lubricant protective substances
  • Antioxidants
• Substances giving new properties and protecting the metal surfacces of engins
• Detergents, dispersants, friction modifiers, anti-wear/Extreme Pressure (EP) additives, rust and corrosion inhibitors

Viscosity index modifiers

• They polymers with a variable molecular weight
• Ethylene-propylene copolymers
• Linear, partly branched or star-shaped hydrogenated polyisoprenes
• C12 to C18 polymetacrylates of long chain alcohols
• Linear, partly branched or star-shaped hydrogenated styrene-isoprene copolymers
• Polyisobutenes

Pour point improvers

• Improve the pour-point characteristics of the lubricant at a low temperature
• Main types
  • polymethacrylates
  • ethylene-vinyl acetate copolymers
  • polyfumarates

Antioxidants

• oxidation inhibitors
  • alkylated aromatic amines
  • sterically hindered phenols
  • zinc dialkyl dithiophosphates
• derivatives of dialkyl dithiocarbamic acid

Detergents and dispersants

• Reduce the formation of deposits and keep the insoluble substances produced by them in suspension
• preventing any further evolution towards more harmful species

Metallic detergents

• Neutralize the acid products of combustion (organic acids and sulphur oxides), to reduce sludges and deposits on the pistons and prevent problems for the piston rings
• colloidal dispersions in carbonaceous lubricant bases of alkaline or alkaline-earth metals, stabilized by an adsorbed layer of surfactant molecules
  • Sulphonates
  • ulphophenates
  • Salicylates

Dispersants

• Control the state of aggregation of sludge and, in diesel engines, of soot consist of amphiphilic molecules in which the lipophilic portion usually consists of polyolefinic chains, while the polar group is, in general, the derivative of a polyamine or of a polyol Important classes
  • Succinimides
• Succinic esters Alkyphenolamine

Friction modifiers

Influence the friction coefficient under boundary lubrication conditions chemical species, may consist of very long amphiphilic organic molecules or of metal-organic compounds

Anti-wear/EP additives

• Reducing wear under boundary lubrication conditions they react with the metal surfaces forming protective tribo-chemical layers.
• They consist of zinc dialkyl dithiophosphates, There are also wear-prevention additives

Anti-corrosives/rust-inhibitors

• Protects the metals against corrosion Acts by creating a physical barrier
• ex: etoxylate alcohols, long-chain carboxylic acids, phosphoric esters, amines, imidazoline and thidderivatives