Bearings - Plain

Question 0

What are the main functions of a journal (machine) bearing?

Answer 0

• to hold the journal in it’s required location
• to support the radial load
• reduce friction

Question 1

What is the difference between radial load and axial (thrust) load?

Answer 1

Radial Load that is applied parallel to the shaft.

Thrust load applies pressure parallel to the axis

Question 2

What is the area of greatest wear on a filter thrust bearing?

Answer 2

The trailing edge of the pads

Question 3

What are three types of thrust bearings?

Answer 3

• Flat plate
• Tapered land
• Tilting Pad

Question 4

Flat Plate

Answer 4

-simplest and cheapest, light loads and have radial grooves machined in to improve hydrodynamic lubrication.

Question 4

Tapered Land

Answer 4

-profiled pad with radial and tapered grooves for better hydrodynamic lubrication

Question 5

Tilting Pad

Answer 5

• stationary ring with tilting segments.

Question 7

What is the best thrust bearing?

Answer 7

Kingsberry tilted pad thrust bearing

Question 8

What is the difference between anti-friction and plain bearings?

Answer 8

Plain bearings have no moving parts.

Question 9

Name 3 types of radial loads?

Answer 9

Stationary
Oscillating
Reciprocating

Question 10

Stationary Load

Answer 10

• the load acts in one direction and on one area of the bearing - turbines, fans
• radial load and in one direction

Question 11

Oscillating Load?

Answer 11

Acts on the bearings in the crank end of the connecting rods and wrist pins. The load shifts through an arc less than 180 degrees

Question 12

Reciprocating Load?

Answer 12

Acts on the bearings in the crankshaft end of connecting rod. The load shifts 180 degree from one side of the bearing to the other.

Question 13

Split Bearings

Answer 13

• a thin shell inserted in the housing (.001-.005 thick)
• shell is flexible
• shell must be seated in a strong housing that has a very accurate bore
• most common overlay is babbit
• thin overlay on a steel backing

Question 14

Advantages of split bearings (thin wall)

Answer 14

• very high load capacity
• are more accurate
• inexpensive
• quick to change
• available in over and under size
• consistent quality for dimensions and strength of bonding to backing

Question 15

Split Bearing (thick wall)

Answer 15

• used in gearboxes, turbines, fans and dynamic compressors
• not flexible
• a plain or spherical seat

Question 16

Spherical Seat for split bearings

Answer 16

• allows the bearing to align its bore with the sag in the shaft of a heavy rotor during installation
• externally self aligning
• usually used on large turbines with a long span between bearings

Question 17

Bushings

Answer 17

• plain bearing thats not split

- mostly handle radial loads, but can use thrust washers for axial loads

Question 18

Floating Bushing

Answer 18

• has an outer ring with a loosely fitted inner ring
• support lightly loaded high speed shafts
• unstable
• stabilized by the film of oil between the inner and outer bushing. This film allows inner bushing to shift and dampen vibration or oil whirl

Question 19

Oil Whirl

Answer 19

• A wave that circulates around the journal that can whip the shaft around.
• can occur with an unstable load at high speeds

Question 20

Hybrid Bearings

Answer 20

• depend on both hydrodynamic and hydrostatic lubrication to support a load
• lubrication ports used only around the loaded area
• can not support a full load at start up because there is no hydrodynamic action

Question 21

Oil lift journal bearings

Answer 21

• have pressurized oil entering under the loaded area only
• hydrostatic pressure lifts the shaft off the bearing before it starts to turn and continues to hold it there until hydrodynamic pressure can take over.
• for heavy loads
• Jacking oil must be brought up to pressure before start up

Question 22

Hydrostatic bearing

Answer 22

• found on equipment that operate too slow to form a hydrodynamic film
• depend on external pump to provide the pressurized film
• good for start up, no wear
• high load at very slow speed

Question 23

Hydrodynamic bearing

Answer 23

• develop a full film of lubricant when the journal is up to operating speed
• lubrication is directed to the leading edge of the pad
• trailing edge of the pad has the greatest wear
• depends on the rotation of the shaft to drag oil into the loading area
• any load at high speed
• oil inlets are in the unloaded area
• Fluid adhesion to both journal and bearing surfaces combined with cohesion creates a wedge which when pulled into the load area creates a pressure rise and supports the shaft

Question 24

Fatigue resistance

Answer 24

-the resistance of the bearing material to cracking and flaking by loads that increase and decrease in rapid cycles

Question 25

Why do tilting pads tilt?

Answer 25

- They tilt as the thrust collar rotates and forces an oil wedge between the two surfaces - can have titling pads on both sides to provide thrust control in both directions

Question 26

Oil Whip

Answer 26

- The pressure from the oil whirl can whip the shaft around | - It can cause the shaft to break through the oil film and destroy the surface of the bearing.

Question 27

Circumferential grooves in reciprocating plain bearings

Answer 27

Allow the distribution of lubrication

Question 28

Scaping

Answer 28

- Removes high spots from bearings surface so the entire bearing surface supports the load. - Repeat until pattern of blue spots about 1/8 in diameter over at least 85% of the surface.

Question 29

Locking tangs and dowels

Answer 29

anti friction tabs

Question 30

Crush

Answer 30

- the distance that a new insert protrudes above the split line of the bearing housing - heat transfer - ensures there is positive contact to the housing

Question 31

Spread

Answer 31

- the amount that the insert diameter is wider than the bearing housing bore - ensures there is positive contact to the housing

Question 32

Gas-lubricated Bearings

Answer 32

-Has pressurized gas that supports a loaded journal for lightly loaded high-speed application

Question 33

Advantages of aluminum alloys

Answer 33

- more embeddable than copper/lead or lead/bronze - higher load rating than Babbit - can run with softer shafts than the copper or bronze alloys

Question 34

Disadvantages of Aluminum

Answer 34

- pure aluminum tends to gall or seize | - larger clearances are needed due to its coefficient of thermal expansion

Question 35

Pre-Lubricated Bearings

Answer 35

- either made of porous material soaked with lubricant or have a lining made with pockets filled with lubricants - the porous bronze can hold 3% porosity of high strength and up to 30% for low strength. - poor heat distribution - intended for light loads - good for start/stops

Question 36

Dry Lubricated Bearings

Answer 36

- made of a slippery material like graphite or PTFE - can also be made with a bronze matrix containing graphite or PTFE, which oozes from the bronze under load pressure. - can run dry or with lubricant - can be solid or lined - start/stop, high temperatures

Question 37

Oil groove location?

Answer 37

- area of greatest clearance - unloaded zone for hydrodynamic - loaded zone for oil lift and hybrid

Question 38

Tapered Adapter Sleeve

Answer 38

-can be mounted anywhere on a shaft - think anywhere/adapter

Question 39

Withdrawal Sleeve

Answer 39

- for mounting against a shaft shoulder

Question 40

Fixed/Floating Bearing situation

Answer 40

-fixed bearing always goes to loaded end as that is where the more important tolerances are

Question 41

Magnetic bearing

Answer 41

-large high speed centrifugal compressors

Question 42

Oil lift thrust bearing

Answer 42

- required on titling pad thrust bearings on axial inlet centrifugal compressors - heavy loads on vertical shafts - jacking oil is required for heavy loads until high speeds are reached