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Terms in this set (141)
What is the journal?
The part of the shaft in contact with the bearing.
Do plain bearings have speed restrictions? Why?
No, Because plain bearings are fixed.
What are the 2 main functions of a journal bearing?
1. Hold the journal in their required location
2. Support the intended radial load.
What are the 3 types of radial loads on a plain friction bearing?
1. Stationary load
2. Oscillating load
3. Reciprocating load.
How is the load distributed on most plain bearings? Where does most of the wear occur?
Down. Lower part of the bearing.
What are the 2 types of radial bearings?
1. Split bearing
2. Bushing (is solid)
Is the shell of a split bearing flexible?
What are some features of the split bearing (Thick-wall construction)?
1. Used in gearboxes, turbines, fans and dynamic compressors
2. Not flexible
3. May have a plain seat or a spherical seat.
What does the spherical seat allow for? (Thick-wall Split bearing)
Allows for alignment during assembly so that point loading does not occur.
What are bushings?
A type of plain bearing that has not been split. May be designed to handle radial or axial loads.
When are bushings usually used?
In poor lubrication conditions, lower load levels or slow speeds.
What is the load and speed capacity of a bushing?
Comparable to the load and speed capacities of split bearings of the same size.
What is the direction of load on a bushing?
Normally radial, but some bushings are flanged or used with thrust washers to accommodate axial loads.
What are the bushing sizes?
Available in commercial sizes, from 1/4" to 5"
What kind of application would you use a bushing?
Same applications as split bearings.
What are the 6 types of bushings?
1. Wrapped bushings
2. Solid bushings
3. Floating bushings
4. Water-lubricated bushings
5. Pre-lubricated bushings
6. Dry lubricated bushings
Features of a wrapped bushing?
Wrapped bushings consist of a steel shell lined with a soft bearing material
What are the advantages of wrapped bushings? (2)
1. Have high fatigue strength
2. Are available in pre-finished sizes that are oversized until installed.
What are the limitations of wrapped bushings?
Available in stock sizes only.
When should pre-finished bushings be used?
In lightly loaded, non-critical applications, as there is variation in the diameter after installation.
Features of a solid bushing?
Bored out of a solid piece of bearing material, such as bronze or Teflon (PTFE)
What are the advantages of solid bushings?
1. Can be bored out to non-standard sizes or can easily be made from suitable round stock for emergency repairs.
2. Can accommodate deep lubricant grooves.
What are the limitations of solid bushings?
Do not have the fatigue strength of wrapped bushings.
Features of a floating bushing?
Consists of an outer bushing with a loosely fitted inner bushing, has oil passages that are force fed.
What are floating bushings intended for?
High speed, high temperature and light load. May also be in heavily loaded slow speed journals.
What does the oil do in a floating bushing?
The oil between the two bushings stabilizes the shaft and dampens vibration
What happens by allowing the heavily loaded inner bushing to revolve freely? (Floating bushing)
The outer bushing is relieved of the forces that make it creep.
What are Water Lubricated Bushings usually made of?
Rubber, Nylon, or Teflon (PTFE)
What are some applications of water lubricated bushings?
Used where water is continually flushing the bearings and makes it impossible for the mechanism to retain any other lubricant.
What are some limitations of water lubricated bushings?
Because of low film strength, these bushings are suitable for only very light loads, such as vertical shafts without side loads.
Because of corrosion, what materials would you use with water lubricated bushings?
What are Pre-lubricated bushings?
Bushings are either made of porous material soaked with lubricant (Sintered Bronze) or have a lining made with pockets filled with lubricant (Oilite bushing).
How is the porous bronze bushing (referred to as sintered bronze or oilite bushing) pre-charged with oil?
By soaking the bushing in oil at 90
How much volume of oil should the porous bronze bushing have?
3% porosity for higher strength up to 30% porosity for higher oil capacity, but lower strength.
Are porous bronze bushings available in round stock?
Yes, these bushings can be made to suit non-standard sized journals or housing bores.
What are the limitations of pre-lubricated bushings? (3)
1. Poor heat distribution means no circulation of the lubricant, best used in start/stop applications.
2. At higher operating temperatures the oil will evaporate
3. Intended for light loads.
What are dry lubricated bushings made of?
They are made from material that is self lubricating (Graphite or Teflon). There is a large range of liner material to suit various environments
What are some features of the dry lubricated bushing?(3)
1. Can run dry or with lubricant (lubricant increases their speed rating)
2. Can be solid or lined
3. Available with a very large range of liner materials to suit various loads, speeds, temperatures and exposure to chemicals
Where are dry lubricated bushings useful?
1. Where lubrication is impossible
2. Start/stop operations that make maintaining a lubricant film difficult
What are the limitations of dry lubricated bushings?
Not suitable for high speeds or heavy loads.
What are some features of a hydrodynamic bearing?(6)
1. Most widely used type of plain bearing
2. Full film of lubricant when journal is up to operating speed
3. Depends on the rotation of the shaft to drag oil into the loaded area
4. Can be a split bearing or bushing
5. The oil inlet is located in the unloaded area
6.Shaft must have a minimum speed in relation to the size to make this work.
Why is there a minimum speed for hydrodynamic bearings?
To create an oil wedge and this speed is dependent on the bearing load. (If the load is increased, the shaft has to rotate faster in order to force the lubricant under the load area)
What causes the most wear on hydrodynamic bearings?
At start-up, there is virtually no oil in the loaded area.
What is the purpose of the chamfer? (Hydrodynamic bearing)
For steady loads that are moderate to heavy. Chamfer assists in the formation of the oil wedge.
How much do loads shift with a bearing supporting Reciprocating or Oscillating Loads?
The load shifts 180* on the bearing
What is the purpose of the groove in the middle of bearings that reciprocate?
Greatly improve the areas to be lubricate, but reduces the load rating to about half.
What do slow-speed bearings require for grease lubrication?
Distribution grooves in the unloaded area.
What happens when a bearing with an unsteady load and high speed develop?
What will an oil whirl do to the shaft?
Cause the shaft to break through the oil film and destroy the surface of the bearing.
What do oil wedges do?
Stabilize oil whirl
What is an added feature widely used on tilting pad bearings?
What advantages does directed lubrication have?(4)
1. Reduces surface temp on the pads by 20
2. Increases load capacity by maintaining a stronger oil film
3. Reduces power consumption 4. Reduces oil flow, allows a smaller lubrication system to be used.
What are some applications of hydrodynamic bearings?
Where are hydrostatic bearings used? (2)
1. Hydrostatic bearings are used on low speed equipment where a hydrodynamic wedge cannot be created or where accurately guided tooling is necessary.
2. Used where shaft stability is essential, stop/start situations or low speeds
What does the hydrostatic bearing depend on?
An external pump to provide a pressurized film of lubricant to support the load shaft.
Can hydrostatic bearings be split or bushing?
Why use a restrictor to regulate oil pressure when using hydrostatic bearings?
So the Shaft stability and its position can be accurately controlled.
What is the minimum speed for hydrostatic bearings?
No minimum speed, a load supporting film can be provided at zero speed.
What does a hydrostatic bearing ensure of at start-up?
Full film of oil with no dry rubbing friction or boundary lubrication. No wear at start-up
What does turbulence do to a hydrostatic bearing?
Reduces a lubricants film strength.
How does oil pressure distribute lubricant with hydrodynamic lubrication?
Occurs where the shaft is turning at the sufficient speed to draw the oil in.
Where are hydrostatic bearings used?
Where the speed is too slow for hydrodynamic lubrication and/or the bearing must be compact, but carry a heavy unsteady load.
What is an oil lift journal bearing?
A special type of hydrostatic bearing with pressurized oil entering under the loaded area. A combination of hydrostatic and hydrodynamic lubrication
What kind of application would the oil lift journal bearing be used?(2)
1. Large turbines and compressors
2. Necessary for bearings supporting heavy rotors that squeeze out the oil film when they have been shut down.
What will happen to the oil lift journal bearing if the jacking oil is not up to pressure before the shaft is turned?
Immediate destruction of the bearing.
What are hybrid bearings?
They depends on both hydrodynamic & hydrostatic lubrication
How is oil delivered to hybrid bearings?
Oil delivered under pressure. They do not have pockets for lubrication.
When can a hybrid bearing hold a load?
Only when both hydrodynamic action and hydrostatic action are operating together.
What are some applications for using hybrid bearings?
Where there is an imbalance, such as an eccentric weight on the rotor that causes the radial load to increase as the speed increases.
How do thrust bearings hold the shaft?
They hold a shaft axially.
What kind of support is required for a thrust bearing?
One or multiple support surfaces perpendicular to the shaft axis. The bearing rides against a thrust collar to control axial movement.
What are 3 various designs of thrust bearings?
1. Flat plate
2. Tapered land
3. Tilting pad.
Do all 3 types of thrust bearings require oil lubrication?
What are some features of the flat plate thrust bearing?(3)
1. Simplest and cheapest
2. Light loads
3. Radial grooves to improve hydrodynamic lubrication (Oil feed should be from the inside out to take advantage of centrifugal forces.)
What are some features of the tapered land thrust bearing?(3)
1. Sometimes referred to as "profiled pad thrust bearing" 2.Greater load capacity than flat plate thrust bearing (due to its superior hydrodynamic lubrication)
3. The flat land prevents excessive contact pressure when the shaft is stationary or on start-up.
What are some features of the tilting pad thrust bearing?
1. Also called a Kingsbury or Michell
2. A stationary base ring with tilting segments of bearing material attached to its face
3. The thrust collar transmits the axial load against the tilting pad assembly
What are some parts and operations of a tilting pad thrust bearing?
The base ring, pivots & bearing material is fitted to the housing.
The thrust collar is ground & fitted to the shaft. Oil is pumped between these two surfaces. The rotating thrust collar forces an oil wedge between the 2 surfaces.
What are the 2 functions of the thrust collar?
1. To transmit the thrust load to the tilting pads and base ring
2. Draw the lubricant into the loaded areas.
What are some extra features of tilting pad thrust bearings? (Also called a Kingsbury or Michell)
They have bearing surfaces on both sides to take a reversing load & a leveling link to ensure these forces are transmitted equally to all thrust bearing surfaces.
When are oil lifts required?
Required on tilting pad thrust bearings on axial inlet centrifugal compressors.
(Used when there is a heavy load at start up & most often this load is in a vertical configuration.)
What are conventional bearings?
The journals are supported on a film of oil.
Where are magnetic bearings used?
On large high-speed centrifugal compressors.
What are some advantages of magnetic bearings?
1. No lubrication
2. No loss of power to frictional forces
3. 2 ½ times rotational speed
4. Work in any environment & any temperature
5. Improved rotor stability & accuracy.
What are the 3 types of air bearings?(Gas lubricated bearings)
3. Air lift
When should pressurized gas support a loaded journal?
Lightly loaded high-speed applications
How do air lifted lubricated bearings work?
Operate as the lubricated beds of machine tools except the lubricant is forced, under pressure, between the two surfaces.
What are some features of aerodynamic bearings?
A layer of pressurized gas that is supplied by the speed of the rotating shaft. They handle light loads but can have speeds in excess of 50,000 rpm.
What are some features of the aerostatic Bearings?
Pressurized by an external source of normally around 100 psi & can also attain speeds past 50,000 rpm.
What are some features of the airlift bearings?
A Flat perforated plate that is used to move heavy objects around.
What are some desirable Characteristics of Plain Bearing Material?(9)
1. Compressive Strengths (support shaft loads)
2. Fatigue Resistance (handle rapid cycling)
3. High Embedability (Handle abrasive partials)
4. Deformability(Yield to misalignment/imperfections)
5. Wetability (retain a film of lubricant)
6. Thermal Conductivity (Removal of heat)
7 Corrosion Resistance (Acid & oxidized oil.)
8. Bondability (Stick to the backing Material)
9. Compatibility (must not wear or weld at startup)
What is BABBIT (White metal alloy) composed of?
Copper, Antimony, Lead and or Tin. (Tin replaces Lead or Lead replaces Tin).
As the % of tin increases the % of lead decreases.
What are the advantages of BABBIT? (8)
1. Adequate compressive strength
2. High embeddability
3. High conformability
4. High wetability
5. Good thermal conductivity
6. Good corrosion resistance
7. Excellent bondability
8. Low cost
What are the limitations of BABBIT? (2)
1. Low fatigue resistance
2. Low load capacity
Where is Teflon used?
For light duty bushings and is used in a bronze matrix for extra strength.
What are some advantages of Teflon?
Self-Lubricating, and Chemically Inert
What are some 3 disadvantages of Teflon?
1. Limited temperature range
2. Poisonous gas when burnt
3. Absorbs water
Why was the Aluminum/Tin & Aluminum/Silicone Alloys developed?
Specifically for bearings.
(The tin provides the bearing material & the aluminum provides the strength. Aluminum /Silicone was developed for extra strength)
What are 5 advantages to Aluminum/Tin & Aluminum/Silicone Alloys?
1. Higher embeddability
2. Higher corrosion resistance
3. Run with softer shafts
4. Higher load rating
5.May be bonded to a steel back.
What are the limitations of aluminum alloys?
They have less embeddability and conformability than BABBIT.
Why is accurate seating in the housing critical to performance?
A small chip or burr can cause premature wear.
What is crush and spread?
Crush is the amount (only a few thousandths of an inch) a bearing protrudes above the split line of the bearing housing.
Spread is the amount the bearing insert is wider than the bearing housing diameter.
What do you do when you find cracking or flaking on the housing paint?
Old paint must be removed completely and the housing re-coated with a coating that is compatible with the lubricant
What is the paint on the inside of the casting for?
To prevent sand from migrating out of the casting and destroying the bearing and prevent corrosion.
When preparing for a bearing installation, what should be checked on the shaft?
Check the shaft or rotor for straightness
When preparing for a bearing installation, what checks should you do with the journal?
Check the journals at 60 degree intervals & in at least 3 places (for out of round, Size & Taper) for a total 9 measurements
How would you check the shaft or rotor for straightness? (preparing for a bearing installation)
V blocks on a surface plate, or between centers and placing a dial indicator against it in several places.
How do you preform a housing fit check?
Lead wire or plastic gage is placed between the top of the bearing shell & under the cap. (Do not place the measuring material in the bottom as shaft weight will distort the readings, note shim size)
If the crushed thickness & the shim thickness are not the same, what are some of the problems that could be happening?(4) (Housing fit check)
1. A burr or chip under the insert
2. A housing bore that is out of round or oversized
3. A housing bore that is not straight
4. Insert dimensions that are not to specifications
What do you do after you determine the housing fit is correct?
Then the bearing to shaft clearance must be checked. (This is done by using 3 strips of Plastigage across a lightly oiled journal then installing the bearing cap & torqueing down.)
What should you use when removing a bushing in order to avoid damage?
Use a driver and an adapter to avoid damage to the housing bore.
What should you do after removing the bushing?
Inspect & measure the bore, (putting a slight chamfer on both the inside & outside of the new bushing will help greatly with installation) Measure the installed bushing I.D.
What does an adapter prevent when installing bushings?
The bushing from collapsing or distorting
What is the thrust bearing installation?(4)
1. Must be clean
2. After installation Thrust bearing runout must be checked
3. Thrust Bearings must have correct Axial position and a specified clearance
4. They need to be pre-lubed before start-up
What is the Oilite Bearing Installation?(2)
1. They can be resized by a single point cutter
2. Do Not resize using a Reamer, Hone or Grinding
(They come in Fractional sizes)
How would you fit a bearing?
1. Bore it out to the required size
2. Install or open all required ports & Grooves
3. Check the contact area & if needed scrape
4. Check the shaft to bearing clearance.
Why would you bore out a bearing?
To bring its bore to the finished diameter
How would you bore out a bearing? (3)
1. Clamp the shells together and center them in a lathe chuck
2. Use a boring tool to bore the bearing out to the specified diameter
3. Face off and chamfer the end of the bearing.
How would you check the contact area on a bearing that the journal load is supported on all parts of the bearing?
1. Clean the bearing and the journal
2. Apply a light coating of Prussian blue to the entire surface of the journal.
3. Set the journal in the bottom half of the bearing and rotate the bearing
4. Remove mandrel and note the contact are, this appears where the blue has transferred from the journal to the bearing surface
How many times should you scrape the high spots after checking the contact area of the bearing?
Until you get a pattern of blue spots about 1/8" in diameter over at least 85% of the surface.
What are 3 methods of measuring bearing clearance?
1. Lift check (not as accurate as Plastigage)
2. Plastigage check.
3. Measure the bearing bore and journal with a Vernier, subtract the journal diameter from the bore diameter to calculate the clearance. (Considered the most accurate method)
How do you adjust the bearing clearance
By adding or removing the thickness of shims as determined from the clearance check.
What are the 2 types of plain bearings?
1. Journal bearings
2. Thrust bearings
What are the 2 main functions of journal bearings?
1. To support a radial load
2. To hold the journal in location accurately.
Is a hydrodynamic bearing suitable for slow speeds? Why or why not?
No, because it would not develop an oil wedge.
What must be done before start-up to ensure there is no damage to an oil lift bearing?
Jacking oil must be brought up to oil pressure.
Why is a circumferential groove required in reciprocating plain bearings?
To distribute the lubricant.
List 4 advantages of BABBIT as a plain bearing material?
1. Adequate compressive strength
2. High embeddability
3. High wettability
How do you check the contact area of a bearing?
By lightly coating the journal with Prussion blue and rotating inside the shell.
What are 2 advantages of measuring bearing clearance with Plastigage?
1. A more accurate measurement of clearance
2. Reveals changes in clearance along the bearing.
How can clearance be adjusted on some connecting rods?
What is the reason for crush and spread on connecting rod inserts?
To ensure the insert is tightly seated to the rod bores.
How many degrees does the load shift on the plain bearing connected to the crankshaft throw on a reciprocating compressor?
How many pieces are there to a bushing?
What do hydrostatic bearings rely on to achieve adequate lubrication?
Pressurized fluid supplied by positive displacement pump.
What tool would you use to re-size an Oilite bearing?
Single point cutting tool
When does the most wear occur on a hydrodynamic bearing?
At start up.
What design characteristic assists in the formation on a oil wedge in hydrodynamic bearings?
What would cause false loading on a plain split pillow block bearing?
A burr between the bearing insert and the housing
What type of bushing has a soft liner and a steel outer shell?
Which type of plain bearing allows for misalignment during assembly?
Thick wall spherical seat bearing
What is used to check the contact area between a shaft and bearing?
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