Lubrication Flashcards
What is roughly the maximum temperature to operate oils at?
250 degrees Fahrenhite.
What does total base number (TBN) measure?
The ability of the oil to neutralize acids.
What do oxidation inhibitors do?
The reduce oxygen attack on the lubrication base oil.
What do corrosion inhibitors do?
Protect non-ferrous metals from corrosion by forming a barrier between their environment.
What do rust inhibitors do?
Protect iron and steel from oxygen attack by protecting the metal’s surface.
What do friction modifiers do?
Reduce friction by physical adsorption of polar materials on metal surfaces.
What do anti-wear agents do?
Stop wear of components.
What does an extreme pressure additive do?
Forms a protective film on the metal that increases the load at which scuffing or seizure occurs.
What do pour point depressants do?
Give oil better low temperature fluidity.
What does viscosity measure?
What is the lowest temperature at which a liquid gives off enough vapours to form an ignitable mixture with air?
Flash point.
What does ISO VG stand for?
International Organization for Standardization Viscosity Grade.
What NLGI rating is the grease most commonly used in grease guns?
Grade 1 or 2.
What do the values in the ISO Cleanliness Code represent? (eg. 19/17/13)
The first number (19) measures the amount of particles greater than 4 microns.
The second number (17) measures the amount of particles greater than 6 microns.
The third number (13) measures the amount of particles greater than 14 microns.
How does overfilling a lubricating system affect its operation?
Overfilling the lubricating system can result in excessive foaming and a temperature rise of the unit.
What does the dropping point of a grease refer to?
The temperature at which a grease becomes fluid enough to drip.
What is a neat cutting oil?
Neat oil has no water added.
What is sliding friction?
Sliding friction occurs when two surfaces slide over each other, such as in journal bearings or pistons sliding in a cylinder.
What is rolling friction?
Rolling friction takes place when a spherical or cylindrical body rolls over a surface. Common examples are ball and roller bearings.
What is fluid friction?
When lubricating oil is applied to two surfaces in contact, a film of oil is formed, filling up the depressions and covering the projections on both surfaces. Because there is no metal-to-metal contact, sliding occurs between the layers of oil within the film This is called fluid friction.
What are adhesive forces?
The theory of adhesion states that adhesive forces cause unlike materials to stick together (polar attraction). Examples are the forces of adhesion between grease and metal or between oil and metal.
What are cohesive forces?
The theory of cohesion states that cohesive forces cause molecules of like materials to stick together. Examples are the forces of cohesion between grease molecules (or between copper molecules).
What is oiliness?
Oiliness or lubricity is the extent to which molecules attract to and form a film on metal. Lubricants high in oiliness have strong adhesive forces.
What is viscosity?
Viscosity is the resistance to flow. Fluids with higher viscosity have stronger cohesive forces.
High viscosity oils have less active molecules, what effect does this have on pressure and ability to carry loads?
High-viscosity oils, with their less active molecules, exert greater pressures and can carry greater load.
What must be considered when determining the best viscosity oil to use is a system?
The ideal choice of oil viscosity depends on speed (rpm), load, temperature, pressure, and environment.
What effect does journal speed have on the oil viscosity needed to carry a specific load?
The higher the journal speed, the lower the oil viscosity needed to carry a specific load.
What viscosity oil should be chosen to minimize power losses?
To minimize power losses, the oil should have the lowest viscosity able to carry the load on the bearing.
What effect does temperature have on oil viscosity?
As temperature rises, oil “thins out” and its viscosity drops.
What does dynamic or absolute viscosity measure?
Dynamic or absolute viscosity is determined by measuring the force required to overcome fluid friction in a film of known dimensions.
Because it depends only on fluid friction, dynamic viscosity is used most frequently in bearing design and oil-flow calculations.
What does kinematic viscosity measure?
Kinematic viscosity is a measure of viscosity that is affected by the density of the oil.
The most common units of kinematic viscosity, are metric centistokes, abbreviated cSt.
1 cSt = 1 mm2/s
What does viscosity index measure and what do the values mean?
Viscosity index measures the reaction of an oil’s viscosity to temperature.
The higher the VI number, the smaller the change in viscosity due to temperature.
How is an oil wedge formed?
- 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).
- When the shaft rolls onto the oiled surface it skids back down and traps a layer of oil under the loaded area.
- As the shaft continues to turn, it draws a wedge of oil under the loaded area. At first this wedge lifts the shaft off the bearing surface with a partial film and then lifts it higher to form a full film.
What is boundary lubrication?
Under many circumstances it is not possible to prevent metal-to-metal contact entirely. When the film of oil fails for a short time, this is called boundary or marginal lubrication.
When does boundary lubrication occur?
- Where bearings are under pressure.
- Frequent overloads from shock loading.
- Newly machined surfaces may be so rough that an oil wedge sufficient to prevent all metal-to-metal contact is difficult to maintain.
What is hydraulic lock?
If you over-lubricate a threaded hole or the screw that threads into it, excess oil drains to the bottom of the threaded hole. As the bolt is tightened, it presses the liquid causing a hydraulic lock that may crack the casing.
Why is air bad for oil and how is it controlled?
The oxygen in air reacts with the oil and an oxidation reaction takes place. This creates varnishes and sludge, as well as acids.
Air is controlled by the addition of oxidation inhibitors, anti-corrosion additives, and foam depressants or foam inhibitors.
Why is water bad for oil and how is it controlled?
Water can cause oxidation of the oil and rusting of machine components.
Water is controlled by demulsifiers, emulsifiers, and rust inhibitors.
How is load capacity controlled?
Chemicals are added to the oil to increase its load capacity.
Anti-wear additives minimize surface wear under normal circumstances.
Extreme pressure (EP) additives are used for heavy load or shock load conditions.
What are detergents and dispersants?
A detergent cleans the surfaces of components.
A dispersant keeps the contaminants in suspension.
Why are detergent motor oils not usually added to industrial oils?
- Contamination of the oil, causing excessive foaming.
- Deposits in critical clearance spaces.
- Filter problems.
What are the three types of once-through oil systems?
- Hand oiler or squirt can.
- Wick-feed oiler.
- Drip feed oiler.
What are the downsides to hand oiling?
This method leads to extremes of over- or under-lubrication.
How does a wick-feed oiler work?
It uses the capillary action of a strand Or strands of wool to transfer the oil from the reservoir to the shaft.
The flow of oil depends on the number and length of wool strands and on the height of the oil in the reservoir.
How does a drip-feed oiler work?
It provides a visual check and a means of controlling the flow of oil by adjusting a needle valve. To minimize oil loss, shut the valve off when the machine is not used.
What are the downsides of wick and drip feed oilers?
- The capillary action of the wick filters the oil, so after a time the wick gets dirty and must be replaced.
- The needle valve is easily fouled by a small piece of dirt.
What is an enclosed oil system and what type of oil is needed?
- An enclosed system is one in which the oil is circulated and used over and over again.
- If the oil is used over a period, a highly stable oil with additives is required.
How does a ring oiler work?
A ring oiler is a mechanical means of oiling a shaft. The ring has an internal diameter (ID) larger than the outer diameter (00) of the shaft. It rests on top of the shaft with the bottom of the ring in the oil at the bottom of the housing. As the shaft turns, friction pulls the ring around with oil clinging to its surface.
How does splash lubrication work?
Splash lubrication is the most common method of lubrication in enclosed gear systems. In most units, the larger gear picks up the oil and carries it to the mesh point. It also splashes oil to a trough which drains to the bearings.
The oil level must be kept high enough to ensure that the gear will pick up sufficient oil. What happens if the oil is over filled?
Too much oil in the housing results in excessive foaming and a marked temperature rise of the unit. This decreases the viscosity of the oil, resulting in a less effective oil wedge and a marked rise in the operating temperature. This reduces the service life of the oil and causes early lip-seal failure.
How does oil bath lubrication work?
Worm-wheel gears and moderate-speed chains are lubricated by passing them through an oil bath. Worm-wheel units with the worm on the bottom are lubricated by the worm passing oil to the wheel from an oil bath. An oiler ring is attached to the shaft, and is called a flinger.
* The flinger picks up oil from the bath and transfers it to the worm bearings and to the gear.
* As the gear rotates, it contacts two scrapers with a clearance of 0.0 I 0” to 0.015”.
* The scrapers remove the oil from the gear and direct it to troughs which are cast in the housing and lead to the gear’s bearings.
When is an enclosed recirculating oil system used?
- Enclosed circulating lubrication is used mainly when a large number of bearings all use the same grade of oil.
- Another general application is running bearings at a high temperature and pumping cool oil from the reservoir over the shaft and bearings to control the heat rise.
When is oil mist lubrication typically used?
Air and oil mist lubrication is frequently recommended for high-speed bearings and high-velocity roller chain. The volume of oil supplied is sufficient to provide an oil wedge, but not enough to create foaming or oil churning that will lead to a heat rise.
What is the characteristic of grease called directional fluidity?
When moving in a bearing, the grease tends to “shear” into thin layers that move in the direction of rotation.
Under shearing stress, the apparent viscosity of the grease falls rapidly until it approaches the viscosity of the oil used in its manufacture.
For low speed applications with extreme conditions of moisture, dirt, or poor sealing, how should the housing be packed?
Fill the housing with grease. Excess grease works out past the seal to form a secondary seal to keep out contaminants.
How is a spring-compression grease cup used?
- The cup is attached directly to the bearing assembly.
- Pressure is applied to the grease by screwing the cap down by hand, or by an automatic spring compressor.
- Adjust the screw near the attachment point to allow for flow adjustment.
What type of lubricant is used for open gears?
Open gears are lubricated with a
grease or very heavy oil.
What is an asperity?
Asperities are high points on a surface.
What is full film lubrication?
Full film lubrication is thick enough to completely separate the surfaces with a continuous film?
What is 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.
The film drops back to its original viscosity as soon as the rolling element moves past the loaded area of contact.
Examples are:
* Hydroplaning car tires.
* Load point between the meshing teeth in gear boxes.
* The fluid between the body joints in humans.
What is boundary film lubrication?
A boundary film does not completely separate the surfaces, so some of the tips of the asperities touch.
What is partial film lubrication?
A thicker film than boundary lubrication where the peaks of asperities can contact.
What is full film (hydrodynamic film) lubrication?
A full film of lubrication exists between the surfaces and asperities do not touch.
How is friction affected by speed in hydrodynamic lubrication?
The shaft must reach a minimum speed to overcome boundary lubrication.
Friction decrease as hydrodynamic lubrication increases.
How does total clearance affect the load capacity of a bearing?
Larger clearance decreases the load support area.
The three factors effecting the thickness of a hydrodynamic film are speed, viscosity, and load pressure. How does each factor effect film thickness?
- Speed - The height of the film increases with speed.
- Viscosity - The height of the film increases with viscosity.
- Load Pressure - The height of the film decreases with load pressure.
What is the basic operation a viscometer?
A sample is added to the viscometer, which is in a constant temperature page, and the time for the sample to flow is measured.
What is pour point?
Pour point is the lowest temperature at which an oil is observed to flow.
What are SAE W grade oil used for?
W grades are for low temperatures.
W grades are determined by testing at low temperatures ranging from -5°C for 20W oil to -30°C for 0W oil.
What is the purpose of a multigrade engine oil?
Multigrade engine oils are oils that have viscosity index improvers added to them. The additives help maintain the oil’s viscosity at higher temperatures.
For example, 10W-30 is a multigrade oil that flows like 10W oil at -30°C. However, 10W-30 maintains its film strength of a 30 oil at 100°C.
How does the oxidation rate of petroleum based oils change above 55-degrees Celsius?
The oxidation rate of petroleum-based oil doubles for every 8 degrees Celsius temperature rise above 55 degrees Celsius.
How do rust inhibitors work?
They form a protective layer on the surface of the metal surface.
How do foam depressants work?
Silicone polymers, or organic copolymers, are finely dispersed in very small amounts to reduce the surface tension of the bubbles, causing them to burst.
How do EP additives work?
The EP additives react with the metal to form a very weak coating on the asperities. This results in a very weak bond that is easily sheared without scoring or much heat.
In an oil bath why is it important that the level of the lubricant is maintained between one-third and one-half of the way up the lowest rolling element?
If the oil level is too high, the rolling elements churn the oil and generate excessive heat.
Why is it important to mount a constant level drip cup oiler on the correct side of the housing with respect to the normal direction of shaft rotation?
If the bottle is installed on the wrong side, the rotation at start-up draws oil from the bottle and floods the bearing.
If the machine rotates in a clockwise direction, mount the bottle on the left side. If the machine rotates in a counter clockwise direction, mount the bottle on the right side.
How does a pressure lubrication system work?
A pressure lubrication system forces oil into the engine bearings.
How does an oil-mist lubrication system work?
Oil mist systems create an aerosol mixture of oil droplets suspended in air.
How does an air-oil lubrication system work?
Air-oil systems deposit fine droplets of oil on bearings without creating a potentially dangerous aerosol.
When are block lubrication systems typically used?
Block lubrication systems are generally used when elements are difficult to access.
During storage, how are lubricant drums placed in order to prevent dirt and water from being drawn in whenever there is a temperature drop?
Store lubricant drums upside-down or on their sides with the bungs at 3 and 9 o’clock.
How do grease thickeners work?
- When the fibers are forced to flow above a certain speed, they align themselves. This has the effect of lowering the viscosity of the grease so that it flows much more easily.
- The base oil held in the thickener is released as the pressure of the grease is increased at the load point (like liquid being squeezed from a sponge). The oil then provides the film to separate the moving surfaces.
- When the movement of the grease stops, the fibers in the thickener return to their weblike structure.
