centrifugal pumps bearings - part 1
TRANSCRIPT
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CENTRIFUGAL PUMPS BEARINGS
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Centrifugal Pumps Bearings
A bearing is a device which supports a shaft to enable it to rotate or slide axially with the least possible frictional
resistance.
Bearing may be broadly grouped into two main categories:
Plain Bearings in which the surface of one component slides over the surface of another. The materials for the bearings are carefully
selected and the surfaces specially prepared in order to minimize friction and wear.
Rolling Element Bearings (anti-friction bearings) in which a series of ball, rollers or needles are interposed between the two
surfaces in order to reduce friction to a low level.
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Plain Bearings
Rolling Element Bearings
Centrifugal Pumps Bearings
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Centrifugal Pumps Bearings
Bearings are also classified according to the type of function they perform:
Journal Bearings which constrain radial movement of the shaft .
Thrust Bearings which constrain axial movement of the shaft
Bearings selection will be largely based on the type of load they have carry.
Radial load Axial load Combined load
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Centrifugal Pumps Bearings
Plain Bearings
Outer surface
Internal surface
A film of lubricant
Surfaces materials
A film of lubricant (if used) and it’s correct distribution.
Temperature control (by sufficient supply of lubricant or by passing water around a jacket which encloses the bearing.
Principles of Operation
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Centrifugal Pumps Bearings
Plain Bearings
A plain bearing material must possess the following important properties:
Materials for Plain Bearings
Compressive Strength – To resist shaft loading without deforming or collapsing.
Fatigue Strength – To resist fluctuating dynamic loads.
Embeddability –To embed foreign matter and so protect the journal from wear and scoring.
Conformability – To tolerate misalignment and deflection under load.
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Centrifugal Pumps Bearings
Plain Bearings
Compatibility – To tolerate momentary metal-to-metal contact without seizure occurring.
Materials for Plain Bearings
Corrosion Resistance – To resist attack by water or oxidized lubrication products.
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Centrifugal Pumps Bearings
Plain Bearings
Some of the more commonly used bearing materials are described below.
Materials for Plain Bearings
Whitemetals
Known also as Babbitt alloys .
They are either tin-based or lead-based alloys and contain a significant percentage of antimony.
They are usually applied as a coating deposited on to a shell made of either steel, cast-iron or bronze.
Whitemetals
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Centrifugal Pumps Bearings
Plain BearingsMaterials for Plain
Bearings Whitemetals
They have excellent properties in all respects, except that their compressive and
fatigue strength declines rapidly as temperature increases.
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Centrifugal Pumps Bearings
Plain BearingsMaterials for Plain
Bearings Copper-lead Alloys:
They provide strength and fatigue resistance up to four times greater than
Whitemetals.
Their conformability and Embeddability are not as good, but may be improved by coating with a thin layer of whitemental.
Whitemetals Copper-lead Alloys
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Centrifugal Pumps Bearings
Plain BearingsMaterials for Plain
Bearings Bronze:
Bronzes can be produced with a wide range of properties and are probably the most
economical of all bearing materials .
four types are normally used:
Lead-bronze – Can be used without a steel backing, but has low conformability and must be accurately aligned. It is easily cast and machined, and is used for moderate speed and
loading applications.
Tin-bronze – Has relatively high hardness and used in applications where loads are high but speeds are low. Tin-bronzes
require reliable lubrication and a hardened journal.
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Centrifugal Pumps Bearings
Plain BearingsMaterials for Plain
Bearings Bronze:
Bronzes can be produced with a wide range of properties and are probably the most
economical of all bearing materials .
four types are normally used:
Aluminum-bronze
Has good resistance to wear and shock loads and is suitable for operating temperature of 260 oC and above.
It is best used for heavy duty, low speed applications.
Phosphor-bronze
Also useful for heavy load, high temperature applications.
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Centrifugal Pumps Bearings
Plain BearingsMaterials for Plain
Bearings For more specific details regarding bearing
materials, manufacturer's information or appropriate standards should be consulted.
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Centrifugal Pumps Bearings
Rolling Element BearingsPrinciples of Operation
Outer race
Cage
Inner race
Ball
Roller
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Centrifugal Pumps Bearings
Rolling Element BearingsPrinciples of Operation
The concept behind a bearing is very simple: Things roll better than they slide. The wheels
on your car are like big bearings. If you had something like skis instead of wheels, your
car would be a lot more difficult to push down the road.
Bearings reduce friction by providing smooth metal balls or rollers, and a smooth inner and outer metal surface for the balls to roll
against. These balls or rollers "bear" the load, allowing the device to spin smoothly.
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Centrifugal Pumps Bearings
Rolling Element BearingsPrinciples of Operation
The material requirements are quite different from plain bearings.
Because the relative motion between the moving elements is one of rolling rather than
sliding.
Why?
Instead of the bearing materials needing to be soft in comparison with the journal, both the rolling elements and their raceways are made of
specially hardened steel. Why?This reduces the rolling frication because of
the smaller indentation produced by the load, but friction can not be entirely eliminated and
so lubrication must still be provided.
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Centrifugal Pumps Bearings
Rolling Element BearingsBall Bearing:
These bearings can handle both radial and thrust loads, and are usually found in applications where
the load is relatively small.
In a ball bearing, the load is transmitted from the outer race to the ball, and from the ball to the inner
race. Since the ball is a sphere, it only contacts the inner and outer race at a very small point, which
helps it spin very smoothly.
Why?
This means that there is not very much contact area holding that load, so if the bearing is overloaded, the balls can deform or squish, ruining
the bearing.
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Centrifugal Pumps Bearings
Rolling Element BearingsThrust Ball Bearing:
It is mostly used for low-speed applications and cannot handle much radial load
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Centrifugal Pumps Bearings
Rolling Element BearingsRoller Bearing:
Can carry heavier loads , provide greater rigidity and less expensive for heavier loads.
In these bearings, the roller is a cylinder, so the contact between the inner and outer race is not a point but a line. This spreads the load out over a larger area, allowing the bearing to handle much
greater loads than a ball bearing. However, this type of bearing is not designed to handle much thrust
loading.
A variation of this type of bearing, called a needle bearing, uses cylinders with a very small diameter. This allows the bearing to fit into
tight places.
Why?
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Centrifugal Pumps Bearings
Rolling Element BearingsThrust Roller Bearing:
It can support large thrust loads They are
often found in and between the housing
and the rotating shafts.
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Centrifugal Pumps Bearings
Rolling Element BearingsTapered Roller Bearing:
It can support large radial and large thrust loads.
In some applications, tapered roller bearings are used where
they are usually mounted in pairs facing opposite directions so that
they can handle thrust in both directions.
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Centrifugal Pumps Bearings
Rolling Element Thrust Bearings
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Centrifugal Pumps Bearings
Rolling Element Journal Bearings