Lubrication Flashcards
The resistance to movement between two surfaces that touch each other is what?
Friction
What are the negative effects to friction? (3)
- Resists movement so increases power consumption
- Causes wear
- Generates heat = reduces life
What are the positive effects of friction? (4)
- Provides traction
- Promotes accurate tracking (bearings)
- Change of motion (start, acceleration and stopping)
- polishing
What are the 3 main types of friction?
- Sliding
- Rolling
- Fluid
The resistance to movement that is produced when one surface slides across another is what friction?
Sliding friction
Which type of friction produces the most amount of resistance?
Sliding
The resistance to movement of one surface rolling over another is an example of what friction?
Rolling friction
The resistance to flow within the fluid is which friction type?
Fluid friction
What is friction?
The resistance of movement between 2 surfaces
What type of friction exists within a lubricant?
Fluid friction
What is a fluid?
Anything that flows.
Can be liquid or gas
A substance that reduces friction when applied as a surface coating is considered what?
A lubricant
The property of reducing friction is known as what?
Lubricity
What are the 5 functions of lubricants? (Liquids)
- Reduce friction
- Anti-corrosion
- Remove heat (liquids)
- Remove contaminants (liquids)
- Help seal
How do lubricants help reduce friction?
By separating the moving surfaces with a film that has less friction than the moving surface.
What determines the lubrication regime or type of lubrication?
The thickness of the fluid
What type of lubrication is thick enough to completely separate the surfaces with a continuous film? (No damage)
Full film lubrication
What are the 3 characteristics to full film lubrication
- Offers least friction resistance
- Eliminates rust on moving surfaces
- Generates the least heat
Which type of lubrication completely separates 2 surfaces to the extent that deformation takes place?
Elasto-hydrodynamic film
Which lubricant film type does not separate the surface so some of the tips of asperities touch?
Boundary film lubrication
What are the characteristics of boundary film lubrications?
- Forms at low speeds
- Forms when the viscosity of the lubricants is too low to support the load
- Forms if there is not enough lubricant
- Results in higher friction
What lubrication type relates to the rotation of a shaft forming an oil wedge that forces a full film of fluid into the loaded area
Hydrodynamic lubrication
Relating to hydrodynamic lubrication, what does the minimum speed required to form a full film depend on?
The load pressure and viscosity of the fluid
Relating to hydrodynamic lubrication why is it important to rotate the shaft above the minimum speed for full film lubrication as soon as possible?
Destructive wear occurs on the bearing and the shaft if not at speed
How does total clearance of a bearing affect the load support from fluid film?
Large clearance area = small load support
Small clearance area = large load support
What are the 3 factors that affect the thickness of the hydrodynamic film in the loaded area?
- Speed
- Viscosity
- Load pressure
How does speed affect the thickness of hydrodynamic film?
The height of the film increases with speed
How does viscosity affect the thickness of hydrodynamic film?
The height of the film increase with viscosity
How does load pressure affect the thickness of hydrodynamic film?
The height of the film decreases with load pressure
Why does hydrodynamic film height change with load pressure?
The film acts like a spring
How does lubricant reduce friction?
- protective film between components
- replacing sliding friction with fluid
What type of lubricant film forms under a loaded rolling element?
Elastro-hydrodynamic lubrication
Does elasto-hydrodynamic lubrication provide a complete separation of the rolling surfaces?
Yes
Does boundary film provide complete separation of surfaces?
No
What type of film has the most friction?
Boundary film
Under what conditions does boundary film form? (3)
- low speeds
- viscosity too low
- insufficient lubrication
Why does a reduction in clearance increase the load capacity of a journal bearing?
Reduction in clearance increases the arc of the load supporting film
(Less area to fill)
What are the (2) conditions necessary to ensure full film lubrication?
- shaft is above the minimum speed
- viscosity must be high enough to support the load
The measurement of fluid friction (resistance to flow) is know as what?
Viscosity
Fluids that flow easily have what viscosity?
Low
Fluids that flow very slowly have what viscosity?
High
What is the most important property of a lubricant?
Viscosity
What are the 3 factors that affect viscosity?
- Temperature
- Pressure
- Speed
How does temperature affect viscosity?
Higher temperature = lower viscosity
Why does temperature affect viscosity?
As temperatures rise the molecular activity does too, breaking some bonds and allowing more free movement
How does pressure affect viscosity?
Large increase in pressure = increase in viscosity
Why does pressure affect viscosity?
Molecules forced together increases the strength of attraction between them
How does speed affect viscosity?
Depends on the liquid.
What is non Newtonian liquids?
Liquids that viscosity decreases as speed increases.
What happens to the viscosity of liquids containing polymer additives when speed is increased?
Viscosity normally decreases
What happens to the viscosity of Newtonian liquids when speed is increased?
Viscosity remains constant
What are the units of viscosity? (2)
- SUS (or SSU)
- centistoke
What is the centistoke?
British unit of viscosity
What is the measurement of a centistoke?
1 centistoke = 1 square millimetre/second
What is kinematic viscosity?
A type of viscosity
What does a centistoke measure?
Kinematic viscosity
How is kinematic viscosity practically measured?
By timing the flow of oil through a capillary tube at a controlled temperature
The viscosity number that appears on most oil containers is measured at what temperature?
40 degrees Celsius
What does SUS stand for?
Saybolt universal seconds
What temperature is the Saybolt universal seconds (SUS) measured in?
100 degrees Celsius
What is absolute viscosity?
Direct measurement of the force of resistance to flow in a fluid
What is the direct measurement of the force of resistance to flow in a fluid called?
Absolute viscosity
What is absolute viscosity measured in?
Centipoises
How is absolute viscosity determined?
Measuring the force required to move one surface, of a specified area, across a film of fluid of a specified height, at a specific rate of speed, and a specific temperature.
Measuring the force required to move one surface, of a specified area, across a film of fluid of a specified height, at a specific rate of speed, and a specific temperature. Is the method to find what?
Absolute viscosity
For industrial purposes viscosity is measured in what unit?
Centistrokes (cst)
A number that identifies the amount of change in the viscosity of a lubricant over a temperature range is called what?
Viscosity index (VI)
Viscosity index is what?
A number that identifies the amount of change in the viscosity of a lubricant over a temperature range.
Why is viscosity index (VI) important?
Tells you how stable a lubricant is over a range of temperatures.
The lowest temperature at which an oil is observed to flow is called what?
Pour point
What is pour point?
The lowest temperature at which oil is observed to flow
How is pour point determined?
3 degrees Celsius above the temperature at which oil shows no movement.
3 degrees Celsius above the temperature at which oil shows no movement is called what?
Pour point.
The lowest temperature at which the vapour above an oil ignited when a flame is passed over is called what?
Flash point
How are low temperature engine grades indicated is SAE?
By a W after the number
(Example 10W30; 10 is the low temp)
How does an increase in viscosity affect the load carrying capacity of a lubricant?
Increase the load carrying capacity
What is the most important property to consider when selecting oil
Viscosity
What (5) things does viscosity affect in the performance of a lubricant?
- Load capacity
- Flow rate
- Fluid friction
- Heat generation
- Sealing effect
Does an increase in speed affect the viscosity in many commonly used oils?
No
What is the viscosity in centistrokes of Telus 68 oil at 40 degrees Celsius?
68 centistrokes
What units of viscosity are used by the AGMA?
SUS (Saybolt universal seconds)
What does the viscosity index of an oil indicate?
How much the viscosity will change between 40-100 degrees Celsius
What viscosity index is considered good in todays society?
150
What is the practical importance of knowing the pour point of an oil?
To determine the lowest temperature a n oil can be used for start ups
What do the SAE grade numbers indicate?
Viscosity at 30 degrees Celsius and 100 degrees Celsius
What were the SAE standards originally developed for?
Automotive
What is the purpose of multi grade oils?
For start up at low temps, and adequate film strength for running temperatures
What temperature is 20W oil tested at?
-5 degrees Celsius
What temperature is 0w tested at?
-30 degrees Celsius
Derived from the base stock produced at refineries, describes what type of oil?
Petroleum-based oils
What are the 2 types of crude oils (base stock) ?
Paraffin
Naphtha
Do petroleum based oils wear out?
No
What Temperature are based oils stable until?
55 degrees Celsius
How can petroleum oils operating temperatures be extended?
With suitable additives
What is the term used to describe oil separating well from water?
Demulsibility
Is petroleum corrosive in its free state?
No
What is the cheapest type of oil?
Petroleum based
What is the most commonly used base stock used in oil?
Paraffin
What base stock causes the highest wax content in oil?
Paraffin
What is the least reactive base oils? (Most stable)
Paraffin
What base stock has the least change in viscosity over a wide range of temperatures?
Paraffin
What base stock has the highest viscosity index?
Paraffin
What base stock is commonly used for lubricants in refrigeration systems?
Naphtha
What base stock is commonly used in cold temperature grease?
Naphtha
What base stock has the greatest change in viscosity over a wide range of temperatures?
Naphtha
What base stock has a greater capacity to dissolve additives?
Naphtha
What base stock has a greater solvency?
Naphtha
What base stock has a lower pour point?
Naphtha
Where are petroleum based oils most widely used? (2)
Bearings
Gears
Why were synthetic oils developed?
To solve lubrication problems that petroleum oils couldn’t over come
When are synthetic oils used?
If they can reduce operating costs enough to offset their higher price
How many chemicals are synthetic oils made from?
Min 10
What are the 3 main additives to synthetic oil?
Alkylated aromatics
Fluorocarbons
Silicate esters
Some types of synthetic oils are stable up to what temperature?
250 degrees Celsius
What oil has the widest operating temperature range?
Synthetic
What oil can be sometimes blended with petroleum oils?
Synthetic
What oil can sometimes be biodegradable?
Synthetic
What oil can sometimes be fire resistant?
Synthetic
What oil can sometimes be water soluble?
Synthetic
Petroleum based oils have presence of waxes which hinder what?
The ability to flow at low temperatures
What are two problems that occur with petroleum based oils at high temperatures?
Thinning
Oxidation
Synthetic oils are used to operate in what conditions?
Extreme
Why were biodegradable oils developed?
In response to environmental concerns
What is the main issue with biodegradable oils?
Very unstable
What are biodegradable oils normally made from
Vegetable oil
What are oil additives?
Chemicals added to a base oil from the manufacturer
What percentage of oil do additives make up?
22%
Do oil additives wear?
Yes
How are oil additives effectiveness rated?
Standard tests
What is another term for oxidation inhibitors?
Antioxidants
What are additives in oil that work to inhibit or slow down the process of oxidation called?
Oxidation inhibitors / antioxidants
What is occurring when oil reacts with oxygen and breaks down?
Oxidation
What happens with oxidation when temperatures increase?
Oxidation increases
What is the rate that oxidation doubles in petroleum based oils?
Every 8 degrees after 55 degrees Celsius
What does oxidation produce?
Sludge
Acids
Varnish
What happens to the viscosity of oil with oxidation?
Viscosity increases
What are the two categories of rust/corrosion inhibitors?
Chemical
Physical
How do chemical rust/corrosion inhibitors work?
They form a protective layer on the metal surface but reacting with the metal
How do physical rust/corrosion inhibitors work?
Polar compound that are attracted to the metal surface to form a protective layer.
What are some harmful effects of foam in oil?
-Reduces load capacity
-increased oxidation of the oil
-loss of oil due to over flow
-increased operating temperatures
-cavitation
-poor oil circulation
What is cavitation?
Gas bubbles that collapse violently in the loaded areas causing putting on the bearing surfaces
The theory of ________ forces causes unlike materials to stick together.
Adhesive
___________ forces cause molecules of like materials to stick together
Cohesion
Where does adhesive forces occur with metal and oil?
Between the oil and metal surfaces
Oil molecules that are attracted and held more strongly to metal than others the adhesive forces are strong. Oils rich in these molecules are high in what? (2)
Oiliness
Lubricity
What factors go into choosing the best viscosity of oil? (5)
-speed (rpm)
-load
-Temperature
-pressure
-environment
When viscosity of oil goes up what happens to it’s ability to carry load?
Also increases
How does the journal speed affect oil viscosity?
Higher speed = lower viscosity
To minimize power losses the oil should have what viscosity?
Lowest
What are the two ways that viscosity is measured?
Dynamic
Kinematic
What is determined by measuring the force required to over come fluid friction in a film of known dimensions?
Dynamic / absolute viscosity
What is another name for dynamic viscosity?
Absolute
Where is dynamic/absolute viscosity more commonly used? (2)
Bearing design
oil flow calculations
What is a measure of viscosity that is affected by the density of the oil
Kinematic viscosity
What is kinematic viscosity most common measurement value?
Centistrokes ( cSt )
What are the 3 main groups of additives and inhibitors?
-air
-water
-load
Does a higher or lower viscosity index number (VI) mean a greater change to viscosity with temperature
Lower = more change with temperature
What direction does oil move in a bearing when it first begins to rotate?
Opposite direction of the bearing
At full speed what direction does oil move in a bearing?
With the direction of bearing rotation
The process of oil forcing shaft up to the centre of a bearing until pressure is equalized and shaft position is maintained is called what?
Oil wedge theory
Efficiency of oil wedge depends on what?
-load
-rubbing speed (rpm)
- temperature
- clearance
- oil or grease type (viscosity)
- volume of lubricant
- shaft surface finish
- where lubricant is supplied
What is it called when the film of oil fails for a short time to prevent metal on metal contact?
Boundary lubrication (marginal lubrication)
Why does boundary lubrication (marginal lubrication) happen?
-shaft is under pressure at rest so oil wedge isn’t maintained
-overload of the bearing
-newly machined surface isn’t broken in (too rough)
How is oxidation of oil controlled? (3)
-Refining process
-oxidation inhibitors
-anti corrosion additives
How is air (foam) controlled in oil? (2)
-foam depressants
-foam inhibitors
How is water controlled in lubrication systems?
-help it separate (demulsibility)
-help it mix (emulsibility)
-prevent rust (rust inhibitors)
What is the ability of petroleum oil to separate from water upon standing called?
Demulsibility
What is the ability for petroleum oil to mix with water called?
Emulsibility
What are the 2 groups of load capacity additives in oils?
-anti-wear additives
-extreme pressure (EP) additives
What oil additive minimizes surface wear under normal circumstances?
Anti-wear additives
What are the properties of anti-wear additives? And what do they do?
Phosphorus
Active sulphur
Zinc
They polish the contacting surfaces
What oil additives are used for heavy load or shock load conditions?
Extreme pressure (EP) additives
What is the common chemical properties of extreme pressure (EP) additives?
- compounds of sulphur and/or phosphorus
- lead soap
- long chain elastomers
Why should you not mix extreme pressure (EP) oils?
There may be a chemical reaction
What do detergent additives do?
They clean the surfaces of components
What do dispersant additives do?
Keeps contaminants in suspension
What are the two main types of oil lubrication systems?
-once through (wasted oil)
-enclosed system
What quality oil is normally used in a once through oil system?
Relatively low grade (low cost)
Typically No additives
What type of oil is typically used with an enclosed oil system?
Highly stable
Typically has additives required
What method of lubrication involves a ring around a shaft that rests in an oil bath?
Ring oiler lubrication
How does a ring oiler lubrication system work?
A ring sissy on the shaft, the ring has a larger ID than the shafts OD allowing it to move through an oil bath and transfer oil to the shaft
Which type of lubrication delivery involves a large gear picking up oil from an oil bath carrying it to a mesh point?
Splash lubrication
How does oil in a splash lubrication system get to the bearings?
Through a trough that the gear splash oil into, the trough than drains to the bearings
What happens if a splash lubrication system has too much oil in it?
Excessive foaming and rising temperatures.
Decreased viscosity
Lowered service life
Possible early lip seal failure
What is the oil ring used in a oil bath lubrication system called?
Flinger
EP (extreme pressure) additives tend to corrode what?
Copper and bronze
What is the ASTM EP 4 ball test used for?
To test extreme pressure capabilities of an oil
Why are detergents not normally recommended for high temperature applications?
They leave a residue called ash when oil is burned
