Lubricating Oil Flashcards
Lubricating oil
Used in machinery to reduce friction between fixed and moving parts. All lubricating oil must be free from impurities and have good resistance to oxidization and to deterioration. Must not allow bearings and parts to corrode, whether the machine is at rest or running. Should not readily break down and should retain the desired properties over its lifespan.
Two basic categories
Mineral: refined from naturally occurring petroleum or crude oil
Synthetic: chemical compounds that are artificially made, can be manufactured using chemically modified petroleum components but can also be synthesized from other raw materials
Why mineral oils are the most common?
- The supply of crude oil has rendered them inexpensive and a large body of data on their properties and uses already exists
- Can be produced in a wide range of viscosities for different applications
- Different viscosities can be blended together to improve their performance in a given application.
Mineral oil manufacturing process
Crude oil is transported from the oil well to the refinery by pipeline or tanker ship. At the refinery, the oil undergoes sedimentation to remove any water and solid contaminants. During this process, the crude is pumped into large holding tanks, where the water and oil are allowed to separate and the contaminants settle out of the oil.
After sedimentation, the crude oil is pumped into fractionating towers. Within the tower, the thousands of hydrocarbons in crude oil are separated from each other by a process called fractional distillation. The crude oil is heated to about 700 F. At this temperature, it breaks down into a mixture of hot vapor and liquid that is then pumped into the bottom of the first of two fractionating towers. Here, the hot hydrocarbon vapor float upward. As they cool, they condense and are collected in different trays installed at different levels in the tower. Normal atmospheric pressure is maintained continuously and about 80% of the crude oil vaporizes. The remaining 20% is then reheated and pumped into a second tower where vacuum pressure lowers the boiling point so it can vaporize at a lower temperature.
The lube oil that has been collected passes through several ultrafine filters which remove the remaining impurities such as aromatics that affect the oil’s viscosity in a process called solvent extraction. When lube oil is treated with the solvent, the aromatics dissolve. Later, the solvent is removed and the aromatics can be recovered from it.
The next step is called chill dewaxing - removing hydrocarbons that solidify readily at low temperatures. These are referred to as wax and are undesirable as they will cause clogging of filters, incomplete lubrication and lead to engine wear.
The final step is called hydrofinishing, done to improve the color and oxidation stability by removing the last of the non-hydrocarbon components. Also improves ability to be stored for long periods of time.
After the oil is refined, it is mixed with additives to give it desired physical properties. The lube oil is also subjected to a variety of QC tests to asses viscosity, specific gravity, color, flash and fire points. The results allow the oil to be classed by grade, packaged for sale and distributed.
Synthetic oil manufacturing
Base stocks are made from organic compounds or synthetic hydrocarbons using a process that rearranges the structure so that all the molecules are uniform in size, shape, and weight. Synthetics are custom-designed to produce the ideal lubricant for specific applications.
Manufactured for a specific purpose: high temp, extreme low temp, high loads, resistant to fire.
Lubricating Oil Properties
1a. Viscosity: kinematic - fluid’s tendency to flow as the time it takes to pour out of a container, measured in flow volume over a period of time. Measured in centistoke (cSt) or Sabolt Universal Second (SUS or SSU)
1b. Viscosity: dynamic - tendency to flow as indicated by measured resistance, energy required to move through a fluid. Measured in centipoise (cP).
In both cases, higher = more viscous (thicker) = greater internal friction and resistance to flow.
2. Viscosity Index : high VI indicates a small change with temperature. Above 95 would be considered high. Low VI indicates large change, below 60 is considered low.
3. Total Base Number : measure of the oil’s alkalinity. When engines are running, acids are a by-product of combustion. The lube oil must be capable of absorbing these acids
4. Pour Point: lowest temperature at which the oil can still be poured out of a container
5. Flash Point: temperature at which the vapor of the oil will start to combust but not continue to burn when mixed with the air. Synthetics have a much higher flash point than mineral.
6. Shear stability: expression of how well the oil stands up to mechanical shear loads. Oil with poor shear stability will shear out and lose viscosity. Oils that don’t have a high shear stability will eventually fail under high loads.
Lube oil grades
Common engine oils are classified by their viscosity and performance according to specifications.
Straight grade oil (single grade oils): designed for use when operating temperatures are relatively constant (tight temperature range). Viscosity index is very low so the oil can only be used in machines with a small variation in temperature.
Multigrade oils are concerned with the performance of both high and low temperatures - they provide low temperature fluidity as well as high temperature stability required for operation in a changing environment. Created by selecting a base stock that possesses the desired low temperature properties. Chemical compounds known as viscosity index improvers are added that minimize the thinning of the oil as temperatures increase. Performance at low temp will be similar to that at high temp.
Lubricating Oil additive
Lube oil alone cannot provide all the characteristics required to perform the needed functions of a lubricant but the performance can be enhanced by the addition of certain chemical compounds referred to as additives.
Contribute a significant percentage of the oil’s makeup : 2-15% for gear lube; 7.5-25% for motor oils.
The concentration of additives in the oil is critical: more additives don’t provide benefits and performance may actually deteriorate. Creating the proper mix is critical
Purpose of oil additives
- Enhance existing base oil : antioxidants, corrosion inhibitors, anti-foam agents, demulsifying agents
- Suppress undesirable base oil properties: pour point depressants, viscosity index improvers
- Impart new properties to the base oil : extreme pressure additives, detergents, tackiness agents
Criteria used to select additives to be blended
- What is their intended function
- Will they mix easily with the selected base oil
- Are they compatible with other additives in the formulation
- Are they cost effective
Types of additives (12)
- Oxidation inhibitors
- Pour point depressants
- Viscosity Index improvers
- Anti-wear (AW) and Extreme Pressure (EP) additives
- Rust and corrosion inhibitors
- Detergants
- Dispersants
- Friction modifiers
- Anti-foaming agents
- Demulsifiers and emulsifiers
- Biocides
- Tackifiers
- Oxidation inhibitors
Oil heated in the presence of air oxidizes. As a result, acidic material forms and the viscosity increases. As the oil temperature increases the oxidization increases exponentially. Large amounts of air increase the oxidation rate and certain metals can act as a catalytic agent and oxidization promoter. Deposits such as varnish and lacquer will form on hot metal surfaces and may further oxidize to form sludge.
Oxidization inhibitors are used mainly to improve the life expectancy of a lubricant at elevated temperatures. Also reduces thickening of the oil and minimizes the formation of sludge and other deposits
- Pour point depressants
Used to improve low temperature fluidity in stocks containing paraffinic materials. As these oils are cooled, wax crystals begin to collect and form into large structures. If allowed to continue, the oil becomes extremely viscous and will eventually solidify. Pour point depressants are polymers that inhibit the formation of these wax crystals and their agglomeration.
Pour point depressants can only extend the lower operating temperature range of a base fluid by 5-25F.
Only for mineral oil
- Viscosity Index improvers
Can be changed by blending different base stocks or by adding VI improvers. Used to reduce the thinning effects caused by the oil’s operation at elevated temperature and are the key component that allows for the production of multi-grade oils. However, negative property being that the molecules are large, which makes them susceptible to degradation when the oil is subjected to excessive shearing forces
- Anti-wear (AW) and Extreme Pressure (EP) additives
Used to provide wear protection when the oil film alone is not capable of preventing contact between components, typically the case in boundary layer lubrication. Work by providing a sacrificial wear surface or by changing the surface metallurgy of the components.
Anti-wear: form thin, tenacious films on loaded parts to prevent metal-to-metal contact, assist in the reduction of friction, wear, scuffing and scoring under boundary layer conditions.
Extreme pressure: form thin, tenacious films on heavily loaded or shock loaded components and prevent wear by resisting the tendency of the sliding surfaces to weld to each other on the microscopic level.
- Rust and corrosion inhibitors
Form protective films on metal surfaces, protecting them against attack by water and/or corrosive contaminants. Necessary because the oil itself tends to offer very little protection.
- Detergants
Minimize deposit formation in the high temperature areas of an engine and aim to neutralize acids that form in the oil.
- Dispersants
Help keep solid contaminants in suspension within a lubricant to prevent sludge, varnish and other carbon deposits from forming on engine parts. Also prevent contaminants from collecting and forming into larger, potentially dangerous particles.
- Friction modifiers
Alter the coefficient of friction, commonly used in automotive engine oil to improve fuel economy. Suspended solids: teflon, graphite, and molybdenum.
- Anti-foaming agents
When a fluid is agitated, air can be trapped forming bubbles. Because air is compressible, the ability of the fluid film to prevent contact is reduced, and because the mixed air contains oxygen, oxidation of the fluid is increased.
Silicone compounds can be used to reduce the surface tension of the air bubbles which will result in a more rapid breakdown of surface bubbles.
However, excess amounts of these agents will promote foaming
- Demulsifiers and emulsifiers
Demulsifiers prevent the formation of an oil and emulsion from forming and are found in lubricants exposed to steam or water.
Emulsifiers perform the opposite function and are found in cutting oils and fire resistant fluids
- Biocides
Often added to water based lubricants to prevent the grown of bacteria
- Tackifiers
Stringy material to prevent the lubricant from flinging off of the metal surfaces during rotation
Depletion of additives
- Decomposition : additives break down
- Adsorption: additive clings and mixes with metal or particle surfaces
- Separation: additive settles out of the oil by settling or through filtration.
When an additive package weakens, the viscosity of the oil will increase, sludge begins to form, corrosive acids start to attack the surfaces of bearings, and wear becomes more prevalent.
