Plain/friction Bearings Flashcards
What does plain bearings rely on to prevent the journal and bearing from contacting each other and reduce friction?
Film of oil
What is a plain bearing typically made up of? (2)
Journal
Bushing/shell
What is unique about plain bearings?
No rolling elements
What are the two main types of loads plain bearings support?
Radial
Axial
What load does journal bearing a typically support?
Radial load
What load does thrust bearings typically support?
Axial loads
What is the function of plain bearings? (2)
To hold the journal in the required location
Support the intended load
What is the journal?
The part of the shaft in contact with the bearing
What is the function of the journal?
Used to support a radial load on a shaft
What is another name for a shaft collar?
Thrust collar
What is a shaft collar used in conjunction with?
Thrust bearing
If the axial forces are to be controlled in two directions how is the assembly designed with the use of shaft collars?
1 collar located between 2 thrust bearings.
If the axial forces are to be controlled in one direction how is the assembly designed with the use of shaft collars?
The collar is located against one thrust bearing
What is the term bearing or journal bearing used to describe?
The bushing or shell that fits around the outside of the journal
Why is clearance between a journal and bearing required? (2)
Lubrication
Prevent binding
When is a thrust bearing used to locate a shaft?
When axial load is present
When axial load is present what type of friction bearing is commonly used
Thrust bearing
Why is a thrust bearing used when axial load is present?
Locate a shaft in a specific axial position without interfering with the shafts rotation
Where is a thrust bearings location on a shaft?
Near the component requiring accurate axial location control
(Typically the coupling)
How many thrust bearings are required on a shaft?
Only one per direction of axial thrust
What are the two components that make up a linear bearing?
Guide/rail
Bushing/carriage
What is the guide in terms of linear bearings?
The fixed part of the machine that has a smooth uniform shape that the carriage slides along
What is the carriage in terms of linear bearings?
The component that sits on or around the guide and slides along it’s length
What are journals commonly made from?
The same material as the shaft
What material are bearings made from?
Softer material than the shaft
Why are bearings made from softer material than the shaft?
They are the sacrificial components
What are the considerations made when choosing a material for friction bearings? (9)
Compressive strength
Fatigue resistance
High embeddability
Deformability (conformability)
Wettability
Thermal conductivity
Corrosion resistance
Bondability
Compatibility
What does compressive strength refer to in terms of friction bearing material? (2)
Strong enough to support the shaft load at maximum operating
Not so great that the west resistance is inadequate
When wear resistance is high what becomes lower?
Compressive strength
When compressive strength becomes higher what becomes lower? (2)
Wear resistance
Embeddability
What does fatigue resistance refer to in terms of friction bearing material?
The resistance to cracking and flaking by loads the increase and decrease in rapid cycles
How is fatigue resistance in a friction bearing increased?
Keeping liner material as thin as possible to allow the backing to handle the stresses
What does embeddability refer to in terms of friction bearing material?
How well a material handles contaminants without failing
When embeddability becomes higher what becomes lower?
Compressive strength
What does deformability refer to in terms of bearing material?
Allows a material to yield to a misalignment or imperfection in the shaft (such as change in diameter)
What does wettability refer to in term of friction bearings?
The ability to retain a film of lubricant
What does thermal conductivity refer to in terms of friction bearings? (2)
Allows rapid transfer of heat due to fluid friction
prevents local hot spots
What does corrosion resistance refer to in terms of friction bearings?
Resist corrosion from the acids in the oxidized oil
What does bondability refer to in terms of friction bearings?
Bearing material to form a good metallurgical bond to the backing material
What does compatibility refer to in terms of friction bearings?
Materials must not seize or weld to the journal under rubbing conditions
What is another name for Babbitt
White metal alloy
What can Babbitt alloys be composed of?
Tin
Lead
Copper
Antimony
Bronze
What does lead provide to Babbitt?
Bearing properties
What does bronze or copper provide to Babbitt?
Strength
What are the 8 advantages to Babbitt?
Compressive strength
Embeddability
Conformability
Wettibility
Thermal conductivity
Corrosion resistance
Bondability
Low cost
What is the limitation of Babbitt?
Low fatigue resistance
Where is Teflon used in friction bearings? (2)
Light duty bushings
Or
Bronze matrix for strength
What are the 3 advantages to Teflon?
Self lubrication
Chemically inert
Useful with soft journals
What does chemically inert mean?
Withstand chemical attack
What are the 3 disadvantages to Teflon?
Limited by temperature
Emits poisonous gas when burned
Absorbs water
What does aluminum provide in an aluminum/tin alloy?
Strength
What does tin provide in an aluminum/tin alloy?
Bearing material
What was the aluminum/silcone alloy created for?
Extra strength
What are the 6 advantages to aluminum/tin or aluminum/silcone alloys?
More embeddable than copper/lead or lead/bronze
More corrosion resistant than copper/lead or lead/bronze
Less expensive than copper or bronze
Softer than copper or bronze alloys
Higher load rating than Babbitt
Bond able to steel
What are the disadvantages to aluminum/tin or aluminum/silcone alloys? (2)
Less embeddability and conformability than Babbitt
What bearing component is located on a machine shaft and supports a radial load?
Collar
What material property ensures that the journal and bearing do not weld together?
Compatibility
What bearing material has the best load rating?
Aluminum/tin
What determines the type of bearing design? (4)
Load
Machine speed
Installation requirements
Cost
Where are plain bearings typically used?
To support high loads due to their large surface areas
What are the 3 most common radial load types?
Stationary
Oscillating
Reciprocating
What degree does a oscillating load move?
Shifts through an arc less than 180 degrees
Where is the lubricant film on a stationary radial load?
One area of the bearing
How many directions is the load on a stationary radial load?
Typically 1
In terms of the bearing, where is the load supporting film on a oscillating load?
The film must move along the loaded area
What is the difference between the fluid film between a stationary load and an oscillating load?
The fluid is much thinner but stronger on the oscillating load
How does an oscillating load affect the bearing clearance?
Clearances are much smaller
Why is the cleanliness of the oil more critical in bearings with an oscillating load?
The film is thinner
In terms of bearings what degree does the load shift in a reciprocating load?
The load shifts 180 degrees from one side of the bearing to another
What are the two configurations journal bearings can come in?
Split bearing
Bushing
What features does a thin shell split bushing have? (4)
Flexible
Thin overly on steel backing
Must be seated in a strong housing with an accurate bore
What’s the characteristics of a split bushing?
Light weight
With stands high loads
What is a bushing?
A type of plain bearing that has not been split
What are bushing typically designed to handle?
Radial or axial loads
How can you tell if a bushing can support axial loads by its self?
It will be flanged
What does a wrapped bushing consist of?
Thin steel shell lined with a soft bearing material
What are the advantages of a wrapped bushing? (2)
High fatigue strength
Available in pre finished sizes
Where should pre finished bushings be used? (2)
Only in light loaded
Non crucial applications
What are solid bushings?
Bushings bored out of a solid piece of bearing material
What are the advantages to solid bushings? (3)
Can be bored to non standard size
Easily made
Can accommodate deep lubricant grooves
What is important to do after installing solid bushings?
Ream to their correct size
What is the limitations of solid bushings?
Not as much fatigue strength of wrapped bushings
What are floating bushings?
Bushings that consist of an outer bushing with a loosely fitted inner bushing
Where can floating bushings be used? (2)
Lightly loaded / High speeds
Or
Heavily loaded / low speeds
How can you help stabilize a floating bushing?
Film of oil between their inner and outer bushings
What are water lubricated bushings?
Bushings made on non metallic material that are slippery when wet
What materials can be used for water lubricated bushings? (3)
Rubber
Nylon
PTFE
What type of loads are water lubricated bushings suited for?
Only very light
What are pre lubricated bushings?
Either made of porous material soaked in lubricant or have a lining with pockets filled with lubricants
How are porous bronze bushings charged?
Soaking in oil at 90 degrees Celsius for half an hour
How much oil can porous bronze hold up to?
Max 30% of its volume
What is the limitations of pre lubricated bushings? (3)
Poor heat distribution
Not good for high temps
Only good for light loads
What are dry lubricated bushings?
Bushings made of a slippery material
What are the features of dry lubricated bushings? (3)
Can run dry
Solid or lined
Available in a broad range of materials for different needs
What are the limitations of dry lubricated bushings?
Not good for high speeds or heavy loads
What are hydrodynamic bearings?
A widely used plain bearing, Designed to develop full film lubrication when a journal is at operating speeds
What are the characteristics of a hydrodynamic bearing? (3)
Both split or bushing
Oil inlet
Supports full film
How do you check the contact area of a plain bearing?
Look for the polished rub area
