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MD2 > SLIDING CONTACT BEARING PPT > Flashcards

SLIDING CONTACT BEARING PPT Flashcards

1
Q

is a machine element which supports another moving
machine element (known as journal).

In a general sense, the function of __________ is to permit constrained relative motion of rigid parts.

It permits a relative motion between the contact surfaces of the members, while carrying the load.

A

bearing

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2
Q

A little consideration will show that due to the relative motion
between the contact surfaces, a certain amount of power is wasted in overcoming frictional resistance and if the rubbing surfaces are in direct contact, there will be ______________

A

rapid wear

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3
Q

In order to reduce frictional resistance and wear and in some cases to carry away the heat generated, a layer of fluid (known as _________) may be provided.

The __________ used to separate the journal and bearing is usually a mineral oil refined from petroleum, but vegetable oils, silicon oils, greases etc., may be used.

A

lubricant

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4
Q

In _______________, the load acts perpendicular to the direction of motion of the moving element

A

radial bearings

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5
Q

In _______________, the load acts along the axis of rotation

A

thrust bearings

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6
Q

In __________________, the sliding takes place along the surfaces of contact between the moving element and the fixed element. The_______________ are also known as plain bearings.

A

sliding contact bearings

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7
Q

In _________________, the steel balls or rollers, are interposed between the moving and fixed elements. The balls offer rolling friction at two points for each ball or roller.

A

rolling contact bearings

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8
Q

sliding contact bearings in which the sliding action is guided in a straight line and carrying radial loads.

Such type of bearings are usually found in cross-head of steam engines.

A

Slipper or guide bearings

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9
Q

sliding contact bearings in which the sliding action is along the circumference of a circle or an arc of a circle and carrying radial loads.

A

Journal or sleeve bearings

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10
Q

When the angle of contact of the bearing with the journal is 360° then the bearing is called a _______________.

This type of bearing is commonly used in industrial machinery to accommodate bearing loads in any radial direction

A

full journal bearing

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11
Q

When the angle of contact of the bearing with the journal is 120°, the bearing is said to be ___________________.

This type of bearing has less friction than full journal bearing, but it can be used only where the load is always in one direction.

The most common application of the ________________ is found in rail road car axles.

A

partial journal bearing

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12
Q

The full and partial journal bearings may be called as _____________ because the diameter of the journal is less than that of bearing

A

clearance bearings

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13
Q

When a partial journal bearing has no clearance i.e. the diameters of the journal and bearing are equal, then it is called a

A

fitted bearing

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14
Q

The ______________ are those in which the working surfaces are completely separated from each other by the lubricant. Such type of bearings are also called as hydrodynamic lubricated bearings.

A

Thick film bearings

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15
Q

The ______________are those in which, although lubricant is present, the working surfaces partially contact each other at least part of the time. Such type of bearings are also called boundary lubricated bearings.

A

Thin film bearings

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16
Q

The ________________ are those which operate without any lubricant present.

A

Zero film bearings

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17
Q

The _________________ are those which can support steady loads without any relative motion between the journal and the bearing.

This is achieved by forcing externally pressurized lubricant between the members.

A

Hydrostatic or externally pressurized lubricated bearings

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18
Q

In ________________________, there is a thick film of lubricant
between the journal and the bearing.

A little consideration will show that when the bearing is supplied with sufficient lubricant, a pressure is build up in the clearance space when the journal is rotating about an axis that is eccentric with the bearing axis.

The load can be supported by this fluid pressure without any actual contact between the journal and bearing.

The load carrying ability of a ________________ arises simply
because a viscous fluid resists being pushed around.

Under the proper conditions, this resistance to motion will develop a pressure distribution in the lubricant film that can support a useful load.

A

hydrodynamic lubricated bearings

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19
Q

the flow of a viscous fluid in a converging channel (known as ____________________)

A

wedge film lubrication

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20
Q

the resistance of a viscous fluid to being squeezed out from between approaching surfaces (known as ______________).

A

squeeze film lubrication

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21
Q

The load carrying ability of a ________________________ results then the journal and/or the bearing rotates relative to the load.

The most common case is that of a steady load, a fixed
(nonrotating) bearing and a rotating journal.

A

Wedge Film Journal Bearings

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22
Q

In a wedge film journal bearing, the bearing carries a steady load and the journal rotates relative to the bearing.

But in certain cases, the bearings oscillate or rotate so slowly that the wedge film cannot provide a satisfactory film thickness.

If the load is uniform or varying in magnitude while acting in a
constant direction, this becomes a thin film or possibly a zero film problem.

But if the load reverses its direction, the ____________ may develop sufficient capacity to carry the dynamic loads without contact between the journal and the bearing.

Such bearings are known as

A

Squeeze Film Journal Bearing

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23
Q

The maximum bearing pressure is considerably greater than the average pressure obtained by dividing the load to the projected area. Therefore the bearing material should have high _______________ to withstand this maximum pressure so as to prevent extrusion or other permanent deformation of the bearing.

A

Compressive strength

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24
Q

The bearing material should have sufficient ____________ so that it can withstand repeated loads without developing surface fatigue cracks.

A

Fatigue strength

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25
Q

It is the ability of the bearing material to accommodate shaft deflections and bearing inaccuracies by plastic deformation (or creep) without excessive wear and heating.

A

Comformability

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26
Q

It is the ability of bearing material to accommodate
(or embed) small particles of dust, grit etc., without scoring the material of the journal.

A

Embeddability

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26
Q

Many high capacity bearings are made by bonding one or more thin layers of a bearing material to a high strength steel shell.

A

Bondability

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27
Q

The bearing material should not corrode away under the action of lubricating oil. This property is of particular importance in internal combustion engines where the same oil is used
to lubricate the cylinder walls and bearings. In the cylinder, the lubricating oil comes into contact with hot cylinder walls and may oxidise and collect carbon deposits from the walls.

A

Corrosion resistance

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28
Q

The bearing material should be of high ______________ so as to permit the rapid removal of the heat generated by friction.

A

Thermal conductivity

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29
Q

The bearing material should be of low coefficient of _____________, so that when the bearing operates over a wide range of temperature, there is no undue change in the clearance.

A

Thermal expansion

30
Q

The tin base and lead base babbits are widely used as a bearing material, because they satisfy most requirements for general applications. The babbits are recommended where the maximum bearing pressure (on projected area) is not over 7 to 14N/mm2

When applied in automobiles, the babbit is generally used as a
thin layer, 0.05 mm to 0.15 mm thick, bonded to an insert or steel shell.

The composition of the babbit metals is as follows :

Tin base babbits : Tin 90% ; Copper 4.5% ; Antimony 5% ;
Lead 0.5%.

Lead base babbits : Lead 84% ; Tin 6% ; Antimony 9.5% ;
Copper 0.5%.

A

Babbit metal

31
Q

The ________ (alloys of copper, tin and zinc) are generally used in the form of machined bushes pressed into the shell. The bush may be in one or two pieces.
The _______s commonly used for bearing material are gun metal and phosphor ______s.

The gun metal (Copper 88% ; Tin 10% ; Zinc 2%) is used for high grade bearings subjected to high pressures (not more than 10 N/mm2 of projected area) and high speeds.

The phosphor bronze (Copper 80% ; Tin 10% ; Lead 9% ;
Phosphorus 1%) is used for bearings subjected to very high pressures (not more than 14 N/mm2 of projected area) and speeds.

A

Bronze

32
Q

The _____________ bearings are usually used with steel journals. Such type of bearings are fairly successful where lubrication is
adequate and the pressure is limited to 3.5 N/mm2 and speed to 40 metres per minute.

A

Cast iron

33
Q

The _______ and _______ lead bearings are mostly used in aircraft engines where the fatigue strength is the most important consideration.

A

Silver

34
Q

The various __________________ are made of carbon-graphite, rubber, wood and plastics.

A

Non-metallic bearings

35
Q

The ___________________ are self lubricating, dimensionally stable over a wide range of operating conditions, chemically inert and can operate at higher temperatures than other bearings.

Such type of bearings are used in food processing and other equipment where contamination by oil or grease must be prohibited. These bearings are also used in applications where the shaft speed is too low to maintain a hydrodynamic oil film.

A

carbon-graphite bearings

36
Q

The _________________ are used with water or other low viscosity lubricants, particularly where sand or other large particles are present.
In addition to the high degree of embeddability and comformability, the rubber bearings are excellent for absorbing shock loads and vibrations.
The rubber bearings are used mainly on marine propeller shafts, hydraulic turbines and pumps.

A

soft rubber bearings

37
Q

The ____________ are used in many applications where low cost, cleanliness, inattention to lubrication and anti-seizing are important.

A

wood bearings

38
Q

The ________ is stronger, harder and more resistant to abrasive wear. It is used for applications in which these properties are important e.g. elevator bearings, cams in telephone dials etc.

A

Nylon

39
Q

The ________ is rapidly replacing Nylon as a wear surface or linear for journal and other sliding bearings because of the following properties:

  1. It has lower coefficient of friction, about 0.04 (dry) as compared to 0.15 for Nylon.
  2. It can be used at higher temperatures up to about 315°C as compared to 120°C for Nylon.
  3. It is dimensionally stable because it does not absorb moisture, and
  4. It is practically chemically inert.
A

Teflon

40
Q

The _____________ used in bearings are mineral oils and
synthetic oils.

The mineral oils are most commonly used because of their cheapness and stability.

A

liquid lubricants

41
Q

A grease is a _____________ having higher viscosity than oils.

The grease are employed where slow speed and heavy pressure exist and where oil drip from the bearing is undesirable.

A

semi-liquid lubricant

42
Q

The ______________ are useful in reducing friction where oil films cannot be maintained because of pressures or temperatures. They should be softer than materials being lubricated.

A graphite is the most common of the ____________ either alone or mixed with oil or grease.

A

solid lubricants

43
Q

It is the measure of degree of fluidity of a liquid. It is a physical property by virtue of which an oil is able to form, retain and offer resistance to shearing a buffer film-under heat and pressure.

The greater the heat and pressure, the greater viscosity is required of a lubricant to prevent thinning and squeezing out of the film.

A

Viscosity

44
Q

The viscosity of the lubricant is measured by ________________.

It determines the time required for a standard volume of oil at a
certain temperature to flow under a certain head through a tube of standard diameter and length.

The time so determined in seconds is the Saybolt universal
viscosity.

A

Saybolt universal viscometer

45
Q

It is a joint property of the lubricant and the bearing surfaces in contact. It is a measure of the lubricating qualities under boundary conditions where base metal to metal is prevented only by absorbed film. There is no absolute measure of __________.

A

Oiliness

46
Q

This property has no relation to lubricating value but is useful in changing the kinematic viscosity to absolute viscosity.

A

Density

47
Q

The term ____________ is used to denote the degree of variation of viscosity with temperature.

A

Viscosity index

48
Q

It is the lowest temperature at which an oil gives off sufficient vapour to support a momentary flash without actually setting fire to the oil when a flame is brought within 6 mm at the surface of the oil.

A

Flash point

49
Q

It is the temperature at which an oil gives off sufficient vapour to burn it continuously when ignited.

A

Fire point

50
Q

It is the temperature at which an oil will cease to flow when cooled.

A

Pour point or freezing point

51
Q

It is the difference between the diameters of the bearing and the journal. Mathematically, ______________, c = D – d

The __________________ (c) in a bearing should be small enough to produce the necessary velocity gradient, so that the pressure built up will support the load.

Also the small clearance has the advantage of decreasing side
leakage.

However, the allowance must be made for manufacturing tolerances in the journal and bushing.

A commonly used clearance in industrial machines is 0.025 mm per cm of journal diameter.

A

Diametral clearance

52
Q

It is the difference between the radii of the bearing and the journal.

A

Radial clearance

53
Q

It is the ratio of the diametral clearance to the diameter of the journal

A

Diametral clearance ratio

54
Q

It is the radial distance between the centre (O) of the bearing and the displaced centre (O′) of the bearing under load. It is
denoted by e.

A

Eccentricity

55
Q

It is the minimum distance between the bearing and the journal, under complete lubrication condition. It is denoted by h0 and occurs at the line of centres. Its value may be assumed as c / 4.

A

Minimum oil film thickness

56
Q

It is the ratio of the eccentricity to the radial clearance

A

Attitude or eccentricity ratio

57
Q

If the ratio of the length to the diameter of the journal (i.e. l / d) is less than 1, then the bearing is said to be __________. On the other hand, if l / d is greater than 1, then the bearing is known as __________.

A

Short and long bearing

58
Q

When the length of the journal (l ) is equal to the diameter of the journal (d ), then the bearing is called __________.

A

square bearing

59
Q

The pressure at which the oil film breaks down so that metal to metal contact begins, is known as ________________ or the minimum operating pressure of the bearing.

A

critical pressure

60
Q

The ___________________ is also a dimensionless parameter used extensively in the design of journal bearings.

A

Sommerfeld number

61
Q

The heat generated in a bearing is due to the fluid friction and
friction of the parts having relative motion.

A

Heat Generated in a Journal Bearing

62
Q

After the thermal equilibrium has been reached, heat will be
dissipated at the outer surface of the bearing at the same rate at which it is generated in the oil film.

A

Heat Dissipated by the Bearing

63
Q

For well designed bearing, the temperature of the oil film should not be more than 60°C, otherwise the viscosity of the oil decreases rapidly and the operation of the bearing is found to suffer. The temperature of the oil film is often called as the ______________ of the bearing.

A

operating temperature

64
Q

is the simplest form of journal bearing. It is simply a block of cast iron with a hole for a shaft providing running fit.

A

solid bearing

65
Q

is an improved solid bearing in which a bush of brass or gun metal is provided.

The outside of the bush is a driving fit in the hole of the casting whereas the inside is a running fit for the shaft.

When the bush gets worn out, it can be easily replaced.

In small bearings, the frictional force itself holds the bush in position, but for shafts transmitting high power, grub screws are used for the prevention of rotation and sliding of the bush

A

bushed bearing

66
Q

A ___________ is used for shafts running at high speeds and
carrying heavy loads.

A ___________ consists of a cast iron base (also called block or pedestal), gunmetal or phosphor bronze brasses, bushes or steps made in two-halves and a cast iron cap.

A

split-bearing

67
Q

A ______________ is used to guide or support the shaft which is subjected to a load along the axis of the shaft.

A

thrust bearing

68
Q

In a _____________________, the loaded shaft is vertical and the end of the shaft rests within the bearing.

A

foot step or pivot bearing

69
Q

In case of ____________, the shaft continues through the bearing. The shaft may be vertical or horizontal with single collar or many collars.

A

collar bearing

70
Q

In a ____________, the shaft continues through the bearing.

The shaft may be vertical or horizontal, with single collar or multicollars.

The collars are either integral parts of the shaft or rigidly fastened to it.

In designing _____________, it is assumed that the pressure is
uniformly distributed over the bearing surface.

A

collar bearing

71
Q

The outer diameter of the collar is usually taken as ___________ the inner diameter of the collar (i.e. diameter of the shaft).

A

1.4 to 1.8 times

72
Q

The thickness of the collar is kept as __________ diameter of the shaft and clearance between collars as _________ diameter of the shaft.

A

one-sixth, one-third

MD2 (22 decks)

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