Lubrication Flashcards
Why is lubrication used?
Lubrication is used to reduce the effects of friction between mating surfaces, prevent wear and corrosion, and guard against solid and liquid contamination.
What are the 6 main function of lubricant?
- Form a film between mating components which move relative to one another so that metal to metal contact is prevents. The film must be sufficiently thick in order to prevent contact even under conditions of heavy load, high speed, low and high temperature extremes, and variations in machinery vibration.
- Reduce the effects of friction and eliminate unnecessary wear and erosion.
- Protect against the adverse effects of corrosion.
- Seal the rotating equipment components against contamination such as dirt, dust, water, or other liquids.
- Cool the moving parts by directing the heat created by friction and operation through the lubricant to other parts of the system.
- Dampen noise
What are the effects of friction?
Whenever two mating surfaces move against each other, friction is produced. Excess friction causes heat, wear, and will reduce the efficiency of the machine.
Sliding friction
Two parallel surfaces move relative to each other.
Found in journal bearings and gear systems, pistons/rings, gears (high pressure)
Rolling friction
Occurs in parts that are designed to roll together on mating surfaces with theoretically no sliding motion.
Occurs in roller, ball, and needle bearings.
Boundary Layer Lubrication
Metal-to-metal contact between the rotating and stationary surfaces. If two surfaces are magnified, it could be seen that they appear rough in contour. In low spots, a finite layer of lubricant exists, but at the high spots, the film can be as thin as a molecule.
When the two surfaces are separated by a thin film, boundary lubrication is said to exist. This exists at the start up and shut down of rotating machinery. The parts don’t become separated until the lube oil pump supplies enough oil to do so.
This type of lubrication also occurs when the relative movement between surfaces is low or when there is insufficient lubricant to adequately separate the surfaces.
Prolonged boundary lubrication will cause high friction, heating, and eventually may cause the parts to seize together
Hydrodynamic Lubrication
AKA fluid film lubrication
Happens when the fixed and rotating surfaces are completely separated by a film of lubricant. The thickness of the lubricant film is sufficient to ensure that there is no metal to metal contact between the surfaces. Friction is at a minimum, mechanical efficiency is at a maximum, and wear is virtually eliminated.
Occurs when the rotating part has sufficient speed after start up.
Hydrodynamic lubrication : loaded zone
In any type of rotating machinery, clearance is required between the fixed and rotating shaft. As the shaft rotates, a force is developed that causes the center point of the shaft to be offset from the center point of the bearing. This force acts on one point along the circumference of the bearing (located at the point of maximum pressure) and is known as the loaded zone. The rotating shaft now acts like the impeller of a pump and drags oil into the loaded zone, forming a wedge between the parts, keeping them separate.
Partial Hydrodynamic Lubrication
AKA mixed film lubrication, elastro-hydrodynamic lubrication
Occurs in heavily loaded contact surfaces, normally occurs in ball bearings. When a ball under load rolls along the raceway of a bearing race significant high pressure occurs at the point of contact between the ball and the race. The surfaces are pressed together and flattened out for a very brief moment and when the balls rolls on, the contact surfaces return to their original shape. The lubricant between the surfaces is almost totally forces away from the point of high pressure contact and the surfaces almost come into direct contact with each other
Requirements for hydrodynamic lubrication
- Viscosity: measure of fluidity (resistance to flow) of an oil at a given temperature. Must be adequate to separate the moving parts at the operating temperature of the machine. If too high, power will be lost due to the need to overcome the internal friction of the oil. If too low, the oil will not remain in place and wear will occur. Most suitable oil the lowest viscosity that will maintain an unbroken film between two moving parts at operating temperature
- Speed: since the rotating shaft acts similar to a pump impeller, sufficient speed must be present to deliver oil into the loaded zone so an oil wedge can be created.
- Load: bearing load determines the thickness of the oil wedge. If too high, oil will be squeezed out and the oil layer thickness will be reduced causing boundary layer conditions
- Length of bearing vs diameter of shaft: long bearings have less tolerance for misalignment than short ones and leakage is important for bearing cooling and flushing. Bearing length should be no greater than twice the journal diameter.
- Supply of lubricant: an ample continuous supply of clean lubricant is required for cooling and flushing and the temperature must be such that the viscosity is within the desired limits.
- Clearance: there must be adequate clearance between the journal and the bearing so that an oil wedge can be formed. Generally 1 thou clearance per inch diameter of the journal
- Surface finish: must be smooth so that metal to metal contact will not happen through the oil wedge.
Lubricant selection considerations
- Size of the machine and rotating parts
- Operating temperature range
- Load placed on the bearings or gears
- Speed of rotation
- Type of environment in which the machine operates
- Maintenance intervals
Grease lubrication
Bearing operates under normal (low) speeds and temperature conditions
Oil lubrication
When the speed of rotation or the operating temperatures make it impossible to use grease, and when heat needs to be removed from the bearing unit by a lubricant
