Lubrication
Purpose of Lubricant:
1. Form film between fixed + moving parts
a. Reduces friction under all:
i. Temp ranges
ii. Speeds
iii. Loads
b. Must be sufficiently thick
2. Reduce friction (metal-metal)
a. Reduce wear
b. Wear is a compounding effect since debris from wear causes more
3. Protect against corrosion + its effects
4. Seal rotating parts from dirt, contaminants + water
5. Cool moving parts
a. Directs heat caused by friction + operation to other parts of system
b. Ex: Crankshaft
6. Dampen noise
Friction:
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When 2 surfaces move against each other
Causes:
o Heat
o Wear
o Reduces efficiency
2 types of friction in machinery:
1. Sliding friction
a. 2 surfaces moving parallel to eachother
2. Rolling friction
a. 2 surfaces roll over each other
Sliding occurs in:
1. Bearings + journals
a. Journal = polished + hardened surface that sits inside bearing (crankshaft journal)
2. Piston/rings
3. Gears
Rolling occurs in:
1. Rolling element bearing
2. Ball bearing
3. Needle bearing
Machinery must be lubricated
3 Types of Lubrication
1. Boundary layer
a. Surface of bearing + journal are not completely smooth
b. Small peaks + valleys exist
c. Lubricant sits in valley, but little to none on peaks
d. When sliding, there will always be metal-metal contact at peaks
e. Occurs at start up + shut down
f. Prolonged causes parts to seize together + increased wear
2. Hydrodynamic
a. Fixed + rotating parts completely separated by oil film
b. Rotating part needs sufficient speed at startup+
c. Oil wedge forms in “loaded zone”
i. As journal rotates, force moves it over + down (left if CW)
ii. Loaded zone is point on bearing with maximum pressure
iii. Lubricant introduced into space between bearing + journal
iv. Rotation of journal draws lubricant around space (like pump impeller)
v. Wedge completely separates fixed + rotating parts
d. Friction + wear is at a minimum, wear at maximum
3. Partial Hydrodynamic (Mixed) (Elastro-hydrodynamic)
a. Occurs on heavily loaded surfaces
i. Normally in ball bearings
1. As ball rolls along raceway of bearing, high pressure occurs
between race + ball and surfaces are pressed together and flattened
for a brief moment
b. When load increases, it presses oil wedge out transitioning from hydrodynamic to
boundary layer
c. Increased wear
Oil Wedge Factors:
1. Viscosity of lubricant
a. Too high, not drawn in to make wedge
b. Too low, leaks out + no wedge
i. Desire lowest viscosity possible (ROT)
c. Must operate at correct temp
2. Speed of rotation of journal shaft
a. Sufficient speed to draw in oil + form wedge
3. Load
a. Determines thickness of oil wedge
b. Too high, oil wedge is pressed out + mixed or boundary occurs
4. Length of bearing vs diameter of shaft
a. Bearing should be as short as possible
i. Never longer than twice diameter of journal (ROT)
b. Used to believe that bearings should be as long as possible
i. Now known that the journal deflects under load causing bearing to be
loaded on both ends
c. Longer bearings require have less tolerance for misalignment
5. Supply of lubricant
a. Clean
b. Correct temp
c. Correct viscosity
6. Clearance
a. Adequate between journal + bearing for wedge to form
b. 0.001” of clearance for every 1” of diameter (ROT)
7. Surface finish
a. Bearing + journal should be smooth
Lubricant Selection:
1.
2.
3.
4.
5.
6.
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Size of machine + rotating parts
Operating temperature range
Load placed on bearings + gears
Speed of rotation
Type of operating environment
Maintenance intervals
Grease lubricant used:
o Low speed
o Low temp
Oil lubricant used:
o High speed
o High temp
o When heat needs to be removed
o When impossible to use grease