Centrifugal Pumps Bearings - Part 1

CENTRIFUGAL PUMPS BEARINGS

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.

Plain Bearings

Rolling Element 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


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


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.


Plain Bearings

Compatibility – To tolerate momentary metal-to-metal contact without seizure occurring.


Corrosion Resistance – To resist attack by water or oxidized lubrication products.


Plain Bearings

Some of the more commonly used bearing materials are described below.


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


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.



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



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.



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.



Bearings For more specific details regarding bearing

materials, manufacturer's information or appropriate standards should be consulted.


Rolling Element BearingsPrinciples of Operation

Outer race

Cage

Inner race

Ball

Roller



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.



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.


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.


Rolling Element BearingsThrust Ball Bearing:

It is mostly used for low-speed applications and cannot handle much radial load


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?


Rolling Element BearingsThrust Roller Bearing:

It can support large thrust loads They are

often found in and between the housing

and the rotating shafts.


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.


Rolling Element Thrust Bearings


Rolling Element Journal Bearings

Centrifugal Pumps Bearings - Part 1

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