Plain bearings Flashcards
Friction bearings or journal bearings are another name for:
plain bearings
This type of bearing has no rolling elements.
plain bearings
Plain bearings can either be 1._____________ bearings to support radial loads or 2._______________ bearings to support axial loads.
- journal
- thrust
What is the function of plain bearings?
to hold the journal in the required location and support the intended load
What is another name for a thrust load?
an axial load
The _______________ is the part of the shaft in contact with a bearing.
journal
On a plain bearing, a shaft collar is used in conjunction with a ___________ bearing.
thrust
What are shaft collars also known as?
thrust collar and runners
How can axial force be controlled in both directions when using a plain thrust bearing?
by using one collar sandwiched between two thrust bearings
What is the term bearing, or journal bearing used to describe when talking about plain bearings?
the bushing or shell that fits around the outside of the journal
True or false:
The term bearing typically describes a radial type of load.
true
- generally the axial or thrust component would be mentioned
What type of bearing is used to locate a shaft in a specific axial position without interfering with the shafts rotation?
a thrust bearing
What two components make up linear bearings?
- guide or rail
- bushing or carriage
Why are bearings usually made of softer materials than the shaft journals that they support?
to make the bearing the sacrificial component
What material are shaft journals usually made of?
the same material as the shaft, just machined to a tighter tolerance
True or false:
Bearing material must be strong enough to support the shaft loads at minimum operating temperatures.
false;
at maximum operating temperatures
What effect does wear resistance have on the compressive strength of a bearing?
wear resistance ↓ = compressive strength ↑
This type of bearing resistance is resistance to cracking and flaking by loads that increase and decrease in rapid cycles.
fatigue resistance
How is fatigue resistance of a journal bearing increased?
by keeping the liner material as thin as possible to allow the backing to absorb the stresses
When it comes to bearings, what is embeddability?
how well the bearing material handles contaminants without failing
True or false:
A bearing material with high embeddability allows abrasive contaminants to become embedded into the bearing material.
true
What effect does compressive strength have on the embeddability of a bearing material?
compressive strength ↑ = embeddability ↓
This journal bearing material characteristic allows the material to yield to misalignment or imperfections in the shaft.
deformability/conformability
When referring to the conformability of bearing material, this is referring to:
the deformability
conformability = deformability
________________ ensures that all parts of a plain bearing carry a share of the load.
deformability/conformability
When referring to a bearing, what is wettability?
the ability to retain a film of lubricant
Why is it a good for a plain bearing to have good thermal conductivity?
allows for the rapid transfer of heat (due to fluid friction) out of the oil, preventing local hot spots that cause seizure
What can happen if bearing materials are not compatible with the journal?
wear can be adhesive in nature, so if the materials are not compatible, they will wear excessively during times when there is less then full film lubrication
When it comes to bearings, what is white metal alloy?
Babbitt
What 4 metals make up Babbitt?
- tin
- lead
- copper
- antimony
Babbitt has _____________ compressive strength.
adequate
Babbitt has _____________ embeddability.
high
Babbitt has _____________ conformability.
high
Babbitt has _____________ wettability.
high
Babbitt has ____________ thermal conductivity.
good
Babbitt has ____________ corrosion resistance.
good
Babbitt has _____________ bondability.
excellent
When a bearing material is said to have good bondability, what does that mean?
the material forms a good metallurgical bond to the backing material
What is one of the main advantages of Babbitt?
low cost
What is the main limitation of Babbitt?
low fatigue resistance
_____________ is used for light duty bushings and is also used in a bronze matrix for extra strength.
Teflon
What are the 3 advantages of Teflon in plain bearings?
- self-lubricating
- chemically inert (chemical resistant)
- used with soft journals
What are the 3 disadvantages of Teflon in plain bearings?
- temperature limitations (will burn if it gets too hot)
- emits poisonous gas when burned
- absorbs water (not suitable for submerged bearings)
1.What does the aluminum/tin alloy bearing material consist of?
2. What is the purpose of the above components?
- free tin in an aluminum matrix
- tin = bearing material, aluminum = strength
Why was the aluminum/silicone bearing material created?
for extra strength
What is the main advantage of aluminum/tin and aluminum/silicone when compared to Babbitt?
higher load rating
What are the main limitations of aluminum/tin and aluminum/silicone when compared to Babbitt?
less embeddability and conformability
What material property ensures that the journal and bearing to not weld together?
compatibility
What 4 things determine the type and design of a bearing?
- load
- machine speed
- installation requirements
- cost
What are the 3 most common radial loads?
- stationary load
- oscillating load
- reciprocating load
___________ radial loads are present on the bearings supporting a horizontal rotor.
stationary
With this type of radial load, the load supporting lubricant film will stay in one area of the bearing.
stationary load
With a _____________ load, the load generally acts in one direction and on one area of the bearing.
stationary
This type of radial load shifts through an arc less than 180 degrees.
oscillating load
What types of radial loads have a thinner, stronger fluid film than a stationary load?
oscillating and reciprocating
This type of radial load shifts 180 degrees, from one side of the bearing to the other.
reciprocating load
What 2 configurations do journal bearings come in?
- split
- bushing
What is the most common overlay on a split bearing?
Babbitt
This type of radial bearing consists of a flexible thin shell overlay on a steel backing inserted into a very strong housing
split bearing
True or false:
Split bearings can have intermediate layers of material to bond the overlay to the backing.
true
What are the main application of split bearings?
automotive engines
True or false:
Split bearings have low load capacities.
false;
they have very high load capacities
Which type of radial bearing fits into a more compact housing: split or bushings?
split
This type of bearing is available in oversize and undersize, which allows grinding of the journal for reuse.
split bearings
Plain and spherical seats are found on what type of plain bearing?
thick walled split bearings
What are 2 advantages of spherical seat bearings?
- simplified alignment
- minimizes edge loading
The load direction on plain bushings is normally radial. What must happen for a bushing to be able to handle an axial load?
it must be flanged and used with thrust washers
True or false:
The load and speed capacity of split bearings is greater than that of bushings.
false;
they are comparable
What is a wrapped bushing?
a bushing made of a thin steel shell lined with soft bearing material
What are the 2 advantages of wrapped bushings?
- high fatigue strength
- available in pre-finished sizes that are oversized until installed (compressed)
Why must pre-finished bushing be limited to lightly loaded, non-critical applications?
because there will be a variation in the diameter after installation (compression will not be even along the bushing length)
What is the main disadvantage of wrapped bushings?
they are available in stock sizes only
What are the 2 main advantages of solid bushings?
- can be bored to non-standard sizes
- can accommodate deep lubricant grooves
What must be done to solid bushings after installation?
they must be reamed to final diameter
What is the main limitation of solid bushings?
they do not have the fatigue strength of wrapped bushings
What is a floating bushing?
an outer bushing with a loosely fitted inner bushing
What types of applications are floating bushing used for?
- light, high speed loads
- heavy, slow speed loads
How can a floating bushing stabilize a high speed light load?
the film of oil between the inner and outer bushing allows the inner bushing to shift to dampen vibrations and oil whirl
What effect does a floating bushing counteract when installed in a heavy, slow moving journal?
the heavy load tends to force a normal bushing to creep;
by having two journals, the inner journal revolves freely, and the outer bushing is relieved of the forces the make it creep
Why is rubber the most common material used in water lubricated bushings?
its slippery when wet
What is main limitation of water lubricated bushings?
light loads;
water has poor film strength, so it can’t handle heavy loads
These types of bushings are either made of porous material soaked with lubricant or have a lining made with pockets filled with lubricants.
pre-lubricated bushings
A pre-lubricated bushing with porous bronze can hold up to ____% of its volume in oil.
30%
What are the 2 types of porous bronze available for pre-lubricated bushings?
- 3% porosity with higher strength
- 30% porosity with lower strength
What are the 3 main limitations of pre-lubricated bushings?
- poor heat distribution
- not suitable for high temps
- light loads only
This type of bushing can run dry or with lubricant.
dry lubricated bearings
What are dry lubricated bushings?
bushing made of slippery material, such as graphite or PTFE.
True or false:
Dry lubricant bushing can be made of a bronze matrix that oozes graphite or PTFE under load pressure.
true
What is the main advantage of dry lubricant bushings?
they can be placed in equipment locations where lubrication is impossible
What are the main limitations of dry lubricant bushings?
not suitable for high speed or heavy loads
What is a hydrodynamic bearing?
a bearing that develops a full fluid film at operating speed
In a hydrodynamic bearing, where is the oil inlet located?
in the unloaded area
True or false:
Lubrication should commonly enter the unloaded zone.
true
What effect does load have on the minimum speed of a hydrodynamic bearing?
load increases = speed increases
- the shaft must rotate faster to force the oil under the loaded area
For a hydrodynamic split bearing supporting a steady load in one direction, what is added to the area of a split to assist in the formation of an oil wedge?
a chamfer (beveled edge)
What is the most common hydrodynamic bearing design?
split bearing with chamfered edges
With reciprocating and oscillating loads, what physical feature is added to split bearings to ensure lubrication in all areas of the bearing?
a circumferential oil groove
What effect will a circumferential oil groove have on the load rating of a split bearing?
reduces it by half
What auxiliary piece of equipment must a split bearing have when operating with reciprocating or oscillating loads?
an oil pump;
this ensures oil is in all areas of the bearing
What physical feature is used on low speed plain bearings utilizing grease lubrication to ensure proper lubrication distribution?
distribution grooves in the unloaded area
This condition is when an unsteady load at high speed develops a wave of oil the circulates around the journal, potentially causing the shaft to break through the oil film.
oil whirl
What is it called when the pressure wave of an oil whirl whips around the shaft, causing it to break through the oil film and impact the bearing?
oil whip
Which 2 types of bearings are used to combat oil whirl?
- lobed
- pivoting shoe
This type of bearing is used to disrupt oil whirls and is found in light applications, such as turbochargers and gas turbines.
lobed bearing
This type of bearing absorbs oil whirls and is commonly found in heavy turbine compressor applications.
pivoting shoe bearings
True or false:
Even though a pivoting shoe bearing can have an oil lubrication port, it is still considered a hydrodynamic bearing.
true
What can happen if the shoes are unable to pivot (due to fouled lubrication)?
an oil whip can occur at light load conditions
What are the 4 advantage of directed lubrication on a tilting pad bearing?
- reduces surface temp by 20℃
- increases load capacity
- reduces power consumption
- reduces oil flow
These types of bearings are found on equipment that operate too slowly to form hydrodynamic film or operate with heavy shock loads that can break through the fluid film.
hydrostatic bearings
____________ bearings depend on an external pump to provide a pressurized film of lubricant to support the loaded shaft.
hydrostatic
How is oil admitted in a hydrostatic bearing?
through recesses in the loaded and unloaded areas
What can happen if one of the oil ports in a hydrostatic bearing becomes plugged?
pressure imbalances, resulting in shaft deflection and instability
What is the minimum speed of a hydrostatic bearing?
no minimum speed;
the oil film is created by an external pump, not rotation
What advantage does a hydrostatic bearing have over a hydrodynamic bearing at start?
hydrostatic bearings have no wear at start up
Hydrostatic bearings usually have loads and speed limits that are lower when compared against purely hydrodynamic bearings. Why is this?
the oil pockets increase turbulence in the lubricant
True or false:
Hydrostatic bearings can support very high loads, but only if the speed is zero or only a few feet per minute.
true
What effect does turbulence in a bearing have on a lubricant?
it reduces the lubricants film strength
________________ bearings are used where the shaft load may change suddenly, but shaft position must be accurately maintained.
Hydrostatic
_________________ bearings are used where the speed is too slow for hydrodynamic lubrication and/or the bearing must be compact and carry a heavy, unsteady load.
Hydrostatic
This type of journal bearing has pressurized oil entering under the loaded area only, and is used to raise a shaft off the bearing surface before start.
oil lift journal bearing
- this is a hydrostatic bearing
Is an oil lift journal bearing a hydrodynamic or hydrostatic bearing?
hydrostatic
True or false:
The pump in an oil lift journal bearing runs continuously.
false;
the pump shuts off once the shaft reaches speeds that allow for hydrodynamic lubrication (*usually)
What can happen if the jacking oil is not up to pressure in a oil lift journal bearing before starting the machine?
bearing failure due to lack of lubrication
What types of applications are oil lift journal bearings found on?
heavy applications that squeeze out the oil under the loaded area when the the machine has been shut down
This type of bearing depends on both hydrodynamic and hydrostatic lubrication.
hybrid bearings
Which type of plain bearing uses lubrication ports only around the loaded area?
hybrid bearing
______________ bearings support a full load only when both hydrodynamic action and hydrostatic action are operating together.
Hybrid
Which type of plain bearing has the highest capacity of all bearings?
hybrid bearings
What type of plain bearing would be used where there is an load imbalance, such as an eccentric weight on the rotor that causes the radial load to increase as speed increases.
a hybrid bearing
What are 3 common types of plain thrust bearings?
- flat
- tapered land
- tilting pad
This type of plain thrust bearing is the cheapest to manufacture, and usually has radial grooves machined into the thrust washer to improve hydrodynamic lubrication.
flat plate thrust bearings
If a flat plate thrust washer is oil fed, where should the supply oil be fed to?
the inner diameter;
the centrifugal force of rotation will throw the oil to out the outside diameter
This type of plain thrust bearing has tapered lands machined into the surface of the bearing which help to form a strong oil wedge.
tapered land thrust bearings
Rate the following plain thrust bearings in terms of load capacity, from greatest load capacity to least.
A. flat
B. tapered land
C. Kingsbury
- Kingsbury
- tapered land
- flat
What are 2 other names for tilting pad thrust bearings?
Kingsbury or Michell thrust bearings
This type of bearing consists of a base ring with tilting segments of bearing material attached to its face.
Kingsbury/Michell/tilting pad thrust bearing
What are the tilting pads on Kingsbury thrust bearings made of?
a steel backing with a Babbitt face
Why do the tilting pads on Michell bearings tilt?
to force an oil wedge between itself and the collar
What are the 2 functions of a thrust collar on a Michell bearing that does not have direct lubrication?
- transmit thrust to the tilting pads and base ring
- draw lubricant into the loaded areas of the bearing
Which type of plain thrust bearing may require an oil lift system for heavy loads?
Kingsbury/Michell/tilting pad thrust bearing
What does an oil lift do on a Kingsbury thrust bearing?
separates the pads from the thrust collar until the required speeds are achieved
Any type of plain bearing that has the journals supported on a film of oil is classified as a:
conventional bearing
What are the 2 main advantages of magnetic bearings?
- no wear
- no speed limitations
What is the main limitation of magnetic bearings?
they require a constant power source to work;
also, they require secondary bearings to catch the shaft should a unexpected power interruption occur
What are the 2 types of air bearings?
- aerostatic
- aerodynamic
This type of air bearing utilizes a supply of compressed air that is pushing into the bearing.
aerostatic
This type of air bearing relies on the speed of the spinning shaft to create an air cushion.
aerodynamic
What is main limitation of air bearings?
cost;
the tolerances required for these bearings to work make them very expensive to manufacture
What are the 3 common types of slides found in plain linear bearings?
- boxway
- dovetails
- bushings
This type of plain linear bearing consists of a bearing with a rectangular shape with a flat top surface and matching carriage shape.
a boxway linear bearing
With this type of linear plain bearing arrangement, the sides of the guides and the carriage are angled to form a trapezoid.
dovetail plain linear bearings
If a machine has a speed range anywhere between 0 and 3600 RPM, which of type of bearing would be suitable?
hydrostatic or hybrid
If a pendulum was supported by a journal and bearing, what type of load is the bearing supporting?
an oscillating load
Which feature of the tapered land thrust bearing helps to form an oil wedge between the bearing and the collar?
the tapered section
Which type of bearing is more resistant to vibration: a anti-friction roller bearing or a plain bearing?
a plain bearing
What bearing feature allows a hydrodynamic bearing to support a reciprocating or oscillating load?
oil grooves
_______________ lubrication uses velocity to deliver oil to bearings.
Hydrodynamic
What are 2 common methods of supplying oil to the bearing for hydrodynamic lubrication?
- pump
- slinger ring or splash
What are the 2 main limitations of hydrodynamic lubrication?
- film cannot form at start-up
- unsuitable for applications with frequent start/stops
_______________ lubrication uses pressure to force to fluid film into the loaded area.
Hydrostatic
What are the 3 components needed for hydrostatic lubrication?
- bearing with oil pockets
- restrictors on oil ports
- pump
By how much do restrictors fitted to the oil entry ports on hydrostatic bearings reduce the pressure of the oil?
by 1/2
What is the best plain bearing solutions for slow speed, heavily loaded bearings where low friction and accurate shaft position are important?
hydrostatic bearings
Which type of bearing has no wear at start-up: a hydrodynamic or hydrostatic bearing?
hydrostatic
What are the 3 components that make up a hybrid plain bearing?
- oil entry ports around the circumference, without pockets
- pressure restrictors at the oil ports
- pump
Which type of plain bearing has the highest load capacity of any bearing?
hybrid bearings
This type of plain bearing has an oil entry port at the bottom of the journal only.
oil lift bearing
Where is the oil inlet on hydrostatic bearings?
360 degrees around the journal in the oil pockets
- What is a fixed or locating bearing?
- Why is it called this?
- a bearing that supports both thrust and axial loads
- because the shaft location is controlled by this location
Purely radial bearings are sometimes called _________ bearings.
floating
On a motor or pump, the side that is closest to the coupling is called the:
inboard side
What is the advantage of placing a thrust bearing on the same end of a shaft as the coupling (closest to the coupling)?
the reduce movement at the coupling due to thermal expansion
What is the advantage of placing a thrust bearing at the opposite end of a shaft to the coupling?
it will help to maintain internal clearances, and all thermal expansion will be transmitted to the coupling
How many axial bearings are used to located a shaft?
1
If a line shaft with radial bearings is belt driven near the end, where is the fixed bearing located?
at the end closest to the belt drive
What are the 3 most common materials that make up a tri-metal shell split bearing?
- Steel backing
- Copper intermediate layer
- Babbitt overlay
What is edge loading a bearing?
A situation where a shaft with a heavy load will sag when not rotating. This sagging places additional pressure on the bearing races, leading to spalling a flaking in that area.
What are 2 other names for split plain bearings?
- Tri-shell/Tri-metal
- Overlay
What is the minimum “contact percentage” on friction bearings?
85%
*this can be verified with Prussian blue and wiping the bearing against the shaft
