1

Vane type pump

• Vanes are usually in the form of blades, buckets, rollers or slippers.

• The rotor runs eccentric to the casing• Due to this, they force fluid into and out

from the pump chamber• In the fig. the rotor and vanes are enclosed

in a ring.• When the rotor rotates, the vanes shift

outward due to centrifugal action and make contact with the ring.

• On the suction side, the volume between vanes is increasing and fluid enters space between vanes from the suction ports in the end plates.

• On the delivery side, the volume between vanes decreases and fluid is squeezed out through the delivery ports

Ref: http://cache.eb.com/eb/image?id=3657&rendTypeId=4

ring

The volume flow per revolution can be written as

Where d = rotor diameter, e = eccentricity

de2

e2

de

2

dQ

22

t

2

Ref: http://www.roymech.co.uk/images1/lobepump.gif

Ref: http://knowledgepublications.com/doe/images/DOE_Mechanical_Science_Three-Screw-High_Pitch_Screw_Pump.gif

• Similar to gear pumps as the fluid flows around the casing

• As the lobes come out of mesh, they create expanding volume on the inlet side of the pump. Liquid flows into the cavity and is trapped by the lobes as they rotate.

• Liquid travels around the interior of the casing in the pockets between the lobes and the casing it does not pass between the lobes.

• Finally, the meshing of the lobes forces liquid through the outlet port under pressure.

As the screws are turned, the fluid is passed through the screw threads that form a helix

3

Deterioration of lubricating oil• May become contaminated by insoluble matter such

as dirt, dust or metallic wear particles.• May be contaminated by finely dispersed carbon,

resulting from incomplete combustion.• By water due to leakage from cooler or condensation

from combustion products.• Oil soluble impurities due to oxidation of the oil.• Deteriorated oil leads to excessive wear and higher

maintenance costs.• Steps should therefore be taken to remove the

contaminants.

4

Filters

• Contaminants in a fluid system are generally removed with the help of a filter or separator.

• It is a device which captures particles by providing restriction to the passage of contaminants while offering minimum resistance to flow.

• In surface filtration, particles are deposited on the surface of a medium provided they are larger than the pores.

• The filtration action is on a single geometric surface.

• The filtration action in a depth filter is throughout the volume and smaller particles move through the initial barrier.

5

Ref: http://www.lenntech.com/filtratie/english/notion/depth-and-surfacefiltration.htm

Porous mesh Porous material

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Methods of separation

A separator removes contaminants by virtue of physical properties of the contaminant.•Some of the principles exploited are:– Adsorbent- removal of contaminants by molecular

adhesion– Centrifugal- Removal of immiscible fluid and solid

contaminants by action of centrifugal force– Electrostatic- Contaminants are removed by applying an

electric charge– Magnetic- Removal of contaminants by magnetic field– Vacuum- removal of contaminants by applying sub-

atmospheric pressure.

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Choice of separation technique

The selection of separation technique is based on:• Solid or liquid concentration level• Density of solid particles• Fluid flow rate• Particle size range• Power available• Space availability• Separation rate• Operating and maintenance cost

8

Filter medium criteria

The different properties that a successful filter medium should have are:

•It should retain solid particles to be filtered, giving a reasonably clear filtrate.

•It should not plug or blind easily.

•It must be resistant chemically, biologically and strong enough physically to withstand the process conditions.

•It should be cost effective.

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Edge type filters

Spindle

•Consist of a number of discs placed on top of each other, held apart by spacers.

•Oil is forced through the discs from the outside.

•Particles collect at the edge of the discs in between two discs.

•Cleaner blades help in removing the particles from the disc edges by scraping them off while being rotated manually.

Disc

Spacer

Cleaner blade

Particle

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Lolos strainer: used in automotive engines

• Consists of cylindrical drum wound with hard wire spaced at a predetermined distance (0.0005 to 0.05 in.)

• Oil flows from the outside to the inside, leaving solid material at the edges.

Purolator metal element filter:•Filtering element consists of a specially shaped wire coiled in a helix.

•The wire has a tapered cross section with a flat outer edge.

•The coils are kept apart by projections of uniform height.

•The height of these projections determines the degree of filtration that can be achieved.

•Dirty oil is forced through the space between coils and leaves through the central outlet.

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Centrifugal purifier

•Oil is fed to a bowl which is rotating at high speeds.

•Because of the difference in specific gravity, solid particles are flung to the edge of the bowl.

•Water, having higher specific gravity than the oil, gathers adjacent to the solid particles.

•The point of separation can be located and the water and solid particles are separated while the oil is allowed to flow through.

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Maintenance and other features of plain bearings

• Many bearings fail before their designed life is over

• Many of these failures can be traced to improper maintenance

• Maintenance techniques of different types of bearings differ due to differences in their basic design features, nature of application and the surroundings in which they are functioning

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Reasons for failureFailure of bearings happen due to one or a combination of the following

reasons:– Improper lubrication– Incorrect installation– Excessive vibration – Incorrect clearance– Contamination

• Improper clearance or a flaw in installation can be checked in the initial stage of assembly or during shutdown (usually with feeler gauges).

•Problems due to the presence of excessive contamination, high vibration and improper lubrication manifest during operation.

• It is therefore necessary to monitor the supply pressure of lubricant at the bearing inlet, lubricant temperature at bearing outlet and contamination level of oil continuously or at periodic intervals.

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Lubricant flow at bearing inlet•When a journal rotates, oil continuously flows out from the

sides.

• The oil flow is maintained by supply of lubricant through bearing grooves.

• The supply pressure is normally recommended by manufacturers.

• If the operation is continued with inadequate supply of lubricant, the film thickness can become unsafe and contact between shaft and bearing may take place leading to eventual failure.

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Clearance• Manufacturers suggest an optimum clearance taking into

account the load support requirement, speed, permissible power loss, and viscosity drop of oil.

• In order to support the applied load, the journal has to run eccentric to the bearing.

• Due to increased shear rate through the reduced passage, there is heating and subsequent decrease in viscosity of the oil.

• As the oil becomes thinner, the film thickness tends to lower and may eventually become unsafe for operation.

• The diametral clearance depends on type of load, severity of load and application.

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5

25.125.0

d 10

dN6.3C

Where N is the rpm and d is the journal diameter in mm.

The minimum value of Cd should not be less than 25 m.

For more severe applications such as engine bearing, the clearance is rather less and can be calculated from

With a mininum acceptable value of 0.00075d.

For low load applications, the diametral clearance can be estimated from the following relation

)000,00,50

Ndd0009.0(Cd

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AlignmentThe alignment of housing bore and bearing back should be checked during maintenance.

Both surfaces should conform to each other to a high degree of accuracy.

Any deviation will lead to reciprocating motion between surfaces.

The amplitude of vibration may be as low as 1 m but this can cause premature fatigue failure.

An additional requirement to prevent fretting is to maintain surface finish for both bearing back and housing bore within a limit of 0.8 m and 1.6 m respectively.

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Alignment (contd.)• The misalignment in the rotor-bearing system can be as a result of

deflection of shaft or housing or both.

• The alignment causes edge loading of the bearing and creates non-uniform film thickness in the journal surface along the line parallel to its axis.

• The acceptable limit of misalignment is 0.0002 radians.

• If the value exceeds the amount, the load capacity reduces and the new value of load can be estimated by multiplying the design load with the factor {(0.001-m)/0.0008}, where m stands for the misalignment value in radians.

• To keep the misalignment within permissible limits, the bearing axis is located parallel to the local shaft centerline (distances are measured from certain reference positions).

• The required deflection and slope at bearing locations is calculated and aligned the bearings are aligned according to these values.

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Installation of split bearings• The bearing halves are manufactured with a slight increase in the length between

joints compared to that for the housing bore.

• This excess length is called free spread.

• A slight force can assemble the bearing shells in the housing and in the region of joint without slackness.

• The advantage of this is that the bearing shell becomes located tightly in the housing and the friction between the bore and bearing shell allows it to stay in location.

• For reasons such as improper lining thickness, excessive strain imposed during fitting and high temperature rise, there can be loss in free spread.

• In some cases free spread in one half can become positive while the other half can become negative.

• The bearing with the negative free spread should not be refitted

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Bearing shell

Bearing housing

Free spread

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Contamination• Solid particles, commonly known as particulates, in a

lubrication system can be very hazardous for a bearing.

• Even if the fluid is initially free from these particles, they are generated as a result of surface interaction.

• Surface damage due to particles reduce the fatigue life of a bearing by a factor of 15-50 times compared with contamination-free operation.

• The wear debris generated in the solid-fluid impact are of wide range in magnitude.

• The life of the bearing is seriously affected if the particle size is bigger than film thickness.

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Contamination (contd.)• A fraction of the debris, particularly smaller ones, gets

embedded in the lining material.

• The rest of them rub against the shaft and the bearing surface perpetuating more debris.

• Each individual particle may not cause much damage, but if they are in large numbers, due to silting, failure may take place.

• Therefore filtration of oil is necessary.

• Contamination can also be due to ingression of foreign particles and moisture.

• For removal of moisture as well as particles, centrifuges are used.

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Lubrication• Plain bearings are usually lubricated by grease or oil.

• The use of grease is restricted to a speed of 2 m/s for the reason of inadequate heat dissipation.

• The grease provides excellent protection against environmental contamination.

• Mineral oils could be used for a wide range of load and speed conditions.

• They carry away heat and offer lower friction.

• In some situations oil is fed by application of pressure for greater dissipation of heat as well as to ensure complete separation of surfaces.

• High speed and heavily loaded journal and thrust bearings in machine tools, engines and compressors are lubricated by pressure-fed mechanism to restrict the rise of temperature and avoid metal-to-metal contact during start and stop.

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Lubrication (contd.)

• For oil lubricated bearings it is essential to use the lubricant with correct viscosity and having appropriate additives.

• Failure to select the proper lubricant can lead to the bearing operating at dangerously low film thickness or generating high temperature.

• The oil loses its original viscosity value due to usage.

• The drop in viscosity should be checked during maintenance.

• If the drop is 10% or more, the oil should be replaced by fresh oil.

• A bearing with grooves allows oil to enter the clearance space and the location of the groove is of prime importance to enable oil to flow in.