Bearing materials- desired characteristics - PowerPoint PPT Presentation

1 / 16
About This Presentation
Title:

Bearing materials- desired characteristics

Description:

Bearing materials- desired characteristics Load capacity-The allowable compressive strength the material can withstand without any appreciable change in shape is the ... – PowerPoint PPT presentation

Number of Views:87
Avg rating:3.0/5.0
Slides: 17
Provided by: iit6
Category:

less

Transcript and Presenter's Notes

Title: Bearing materials- desired characteristics


1
Bearing materials- desired characteristics
  • Load capacity-
  • The allowable compressive strength the material
    can withstand without any appreciable change in
    shape is the primary deciding factor in deciding
    a bearing material
  • Plain bearings are expected to have the following
    characteristics for the ease of functioning and
    satisfying the design criteria
  • Strength to take care of load-speed combinations
  • Fatigue strength, where bearing materials are
    subjected to stress cycle as in internal
    combustion engines
  • The retention of strength characteristics of
    softer bearing materials at temperature of
    operation which may rise within the design limit
  • The material must easily conform to shape of the
    journal and should be soft enough to allow the
    particulate contaminants to get embedded

2
Bearing materials- desired characteristics
  • Compatibility-
  • The shaft and bearing materials in rubbing
    condition should not produce localized welds
    leading to scoring or seizure.
  • A good bearing-shaft metal combination is
    necessary
  • Corrosion resistance-
  • The oxidised products of oils corrode many
    bearing alloys.
  • Some protection can be provided by forming a thin
    layer of anti-corrosion materials on the bearing
    alloy surface
  • Conformability-
  • It helps to accommodate misalignment and increase
    the pressure bearing area (reduce the localized
    forcse).
  • Relatively softer bearing alloys are better in
    this respect
  • Embeddability-
  • It is the ability of a material to embed dirt and
    foreign particles to prevent scoring and wear
    (decrease 3rd. Body abrasion).
  • Materials with high hardness values have poor
    embeddability characteristics

3
Bearing materials- desired characteristics
(contd.)
  • Low coefficient of friction- the material
    combinations of sliding surfaces, along with the
    lubricant should provide a low friction
    coefficient for reducing damage and lower running
    costs
  • Low thermal expansion- The size should remain
    nearly constant during periods of temperature
    change
  • High thermal conductivity- The ability to
    dissipate heat quickly due to friction
  • Wettability- An affinity for lubricants so that
    they adhere and spread to form a protective film
    over the bearing surface
  • Relative hardness-
  • The bearing material should usually be softer
    than that of the journal to prevent shaft wear
    but hard enough to resist adhesive and abrasive
    wear of its own surface.
  • Bearings are more easy to replace than shafts
    (that require dismantling of the whole engine).
    If one bearing is worn out only that bearing
    needs replacement instead of the whole shaft

4
Bearing materials- desired characteristics
(contd.)
  • Elasticity- should be elastic enough to allow the
    bearing to return to original shape upon relief
    of stresses that may cause temporary distortion,
    such as misalignment and overloading
  • Availability- The material should be readily and
    sufficiently available, not only for initial
    installation but also to facilitate replacement
    in the event of bearing failure
  • Cost- The economic consideration is the ultimate
    deciding factor in selecting a bearing material

5
Aluminium alloys
  • Good fatigue strength, load bearing capacity,
    thermal conductivity, and corrosion resistance
  • Less expensive than babbitt materials
  • Most aluminium allows contain tin as an element
    which remains in the free state to provide a
    better bearing surface
  • The strongest aluminium alloy used is
    aluminium-silicon
  • Thermal expansion is relatively high and this
    restricts their usage at high temperatures
  • Emeddability, conformability, and compatibility
    are not very good and these are improved by
    providing a babbitt overlay

6
Cadmium and silver alloys
  • Cadmium
  • Cadmium alloys offer good fatigue resistance and
    excellent compatibility characteristics
  • Their corrosion resistance is poor and are they
    are expensive
  • Silver
  • Used as deposited material on steel with an
    overlay of lead
  • The addition of lead improves the embeddability,
    anti-weld and anti-scoring properties

7
Multilayered bearings
  • Steel is normally used as a backing material
  • To obtain good embeddability conformability and
    compatibility, a layer or coating of relatively
    softer material is put on a backing material
  • These plain bearings are known as multilayered
    bearings
  • When there is one layer of coating material, it
    is termed bimetal bearing
  • Bearings consisting of a thin layer of soft
    material electroplated or cast on a layer of a
    bimetal bearing are known as trimetal bearings
  • In bimetal bearings the maximum thickness of the
    overlay is kept within 800 mm

8
Multilayered bearings (contd.)
  • The thickness of overlay can be as low as 120 mm
    for babbitts
  • The wall thickness of backing material in bimetal
    bearings is of he order of 0.3 times the bore
    with a minimum value of 1.5 mm
  • In trimetal bearings the surface layer thickness
    could be as low as 25 mm
  • With increase in babbitt thickness, the overall
    fatigue strength decreases

9
Plain bearingsSliding motion takes place
10
Types of plain bearings
  • Porous bearings
  • Made using powder metallurgy techniques by
    sintering powdered bronze, iron, brass, graphite
    etc. and obtaining the requisite bearing housing
    shape by compressing the powder.
  • This yields a porous bearing housing which is
    then impregnated with oil.
  • The quantity of oil depends on load and speed for
    which the bearing is used.
  • The variation of pressure during the operation of
    the bearing along with the circumferential
    direction and the temperature variation causes
    oil to flow through the pores due to capillary
    action into the clearance space between the
    bearing housing and journal.
  • From the loaded portion bearing, the oil flows
    back along the pores into the bearing housing.
  • Applications are mixers, washing machine, garden
    equipment etc.

11
  • Rolled or strip bearings
  • Made by rolling a sheet or strip and due to the
    nature of the manufacturing process, the bearing
    housing is split requiring various joining
    techniques to close this split.
  • Sometimes they are provided with a fiber lined
    cloth of PTFE/Graphite fibers on the inside of
    the housing for friction reduction and improved
    strength.
  • Among non-metallic bushes, rubber and graphite
    have been traditionally used.
  • Nylon is a valuable plastic material for bushing
    because of low friction though it has low
    strength and is suitable for low speeds due to
    heating effect.
  • These are used in grinders and mixers because of
    resistance to corrosion and quiet operation.

12
Non-metallic bearing materials
  • Rubber bearings
  • Used where quiet operation is desired, large
    clearances and misalignment encountered.
  • Found in bearings for propeller and rudder shafts
    of boats and ships.
  • Found where water acts as a lubricant or likely
    to be a contaminant.
  • Even when sand and gravel are present, the
    resilience of water is an added advantage.
  • Wet rubber is very slippery hence its use in such
    situations to reduce friction.

13
  • Carbon graphite
  • Used for food handling equipment and in the
    textile industry.
  • Because of the self lubricating property, no
    additional lubricant is required, hence limiting
    lubricant contamination prospects.
  • Since it is resistant to corrosion, it can be
    used even in water.
  • Teflon (PTFE)
  • Another plastic with self-lubricating properties
    and low friction.
  • However, it has poor strength, low wear
    resistance and a tendency to deform under load.
  • Reinforcing with fibers improves the strength.

14
  • Phenolic plastic bearings
  • Laminated phenolics are formed by treating sheets
    of either paper or cotton fabric with asbestos or
    other filler materials bonded using phenolic
    resin.
  • These are stacked to obtain the desired thickness
    and subjected to heat and presssure to bond the
    sheets firmly and later formed into required
    shapes.
  • Used in aircraft landing gears and in several
    applications where water is a lubricant, such as
    in rolling mills where water is used for cooling
    and to lubricate.
  • Also used in rudder bearings and centrifugal
    pumps .

15
Grooved bearings
Groove
Grooves are provided in bearing surfaces to
enable oil flow into the bearing area and to
spread along the surface
16
Non-circular journal bearings
  • Oil lubricated bearings have serious limitations
    as the surface speeds increase.
  • The limit to operation is due to journal whirling
    in the clearance space within the housing in such
    a manner as to cause danger of surfaces coming in
    contact leading to failure
  • For this various non-circular configurations have
    been devised to accommodate for the whirl and
    provide better stability
Write a Comment
User Comments (0)
About PowerShow.com