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Second Year > Bearings - Plain > Flashcards

Bearings - Plain Flashcards

0
Q

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

A
  • to hold the journal in it’s required location
  • to support the radial load
  • reduce friction
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1
Q

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

A

Radial Load that is applied parallel to the shaft.

Thrust load applies pressure parallel to the axis

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2
Q

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

A

The trailing edge of the pads

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3
Q

What are three types of thrust bearings?

A
  • Flat plate
  • Tapered land
  • Tilting Pad
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4
Q

Flat Plate

A

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

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4
Q

Tapered Land

A

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

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5
Q

Tilting Pad

A
  • stationary ring with tilting segments.
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7
Q

What is the best thrust bearing?

A

Kingsberry tilted pad thrust bearing

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8
Q

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

A

Plain bearings have no moving parts.

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9
Q

Name 3 types of radial loads?

A

Stationary
Oscillating
Reciprocating

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10
Q

Stationary Load

A
  • the load acts in one direction and on one area of the bearing - turbines, fans
  • radial load and in one direction
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11
Q

Oscillating Load?

A

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

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12
Q

Reciprocating Load?

A

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.

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13
Q

Split Bearings

A
  • 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
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14
Q

Advantages of split bearings (thin wall)

A
  • 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
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15
Q

Split Bearing (thick wall)

A
  • used in gearboxes, turbines, fans and dynamic compressors
  • not flexible
  • a plain or spherical seat
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16
Q

Spherical Seat for split bearings

A
  • 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
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17
Q

Bushings

A
  • plain bearing thats not split

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

18
Q

Floating Bushing

A
  • 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
19
Q

Oil Whirl

A
  • A wave that circulates around the journal that can whip the shaft around.
  • can occur with an unstable load at high speeds
20
Q

Hybrid Bearings

A
  • 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
21
Q

Oil lift journal bearings

A
  • 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
22
Q

Hydrostatic bearing

A
  • 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
23
Q

Hydrodynamic bearing

A
  • 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
24
Q

Fatigue resistance

A

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

25
Q

Why do tilting pads tilt?

A
  • 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
26
Q

Oil Whip

A
  • 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.

27
Q

Circumferential grooves in reciprocating plain bearings

A

Allow the distribution of lubrication

28
Q

Scaping

A
  • 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.
29
Q

Locking tangs and dowels

A

anti friction tabs

30
Q

Crush

A
  • 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
31
Q

Spread

A
  • the amount that the insert diameter is wider than the bearing housing bore
  • ensures there is positive contact to the housing
32
Q

Gas-lubricated Bearings

A

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

33
Q

Advantages of aluminum alloys

A
  • more embeddable than copper/lead or lead/bronze
  • higher load rating than Babbit
  • can run with softer shafts than the copper or bronze alloys
34
Q

Disadvantages of Aluminum

A
  • pure aluminum tends to gall or seize

- larger clearances are needed due to its coefficient of thermal expansion

35
Q

Pre-Lubricated Bearings

A
  • 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
36
Q

Dry Lubricated Bearings

A
  • 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
37
Q

Oil groove location?

A
  • area of greatest clearance
  • unloaded zone for hydrodynamic
  • loaded zone for oil lift and hybrid
38
Q

Tapered Adapter Sleeve

A

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

39
Q

Withdrawal Sleeve

A
  • for mounting against a shaft shoulder
40
Q

Fixed/Floating Bearing situation

A

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

41
Q

Magnetic bearing

A

-large high speed centrifugal compressors

42
Q

Oil lift thrust bearing

A
  • 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

Second Year (19 decks)

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