Centrifugal Pumps Bearings - Part 2

Centrifugal Pumps Bearings

Bearing Assembly Design

Shaft and Housing Fits:

As a general rule, a rolling element bearing should be installed so that its inner race is a tight fit on the rotating

shaft and the outer race is an interference fit in the stationary housing. The tolerances for shafts and bearing

housings should be established using the recommendations of bearing manufacturers or ISO standards.



Thermal Expansion:

Shaft



Thermal Expansion:

Shaft

Held BearingFree Bearing



Types of Mounting:Shaft Nut and Locking Washer:

This is one of the most common arrangements and can be used for both single and double row

radial bearings. After tightening the nut, the tab washer is bent against one of the flats on the nut, so preventing it turning.



Types of Mounting:Slotted Nut and Pin :

A castle nut or slotted nut may be used with a split pin or cotter

pin in order to provide adjustment for tapered roller

bearings .



Types of Mounting:End Plate and Locking Screws :

This arrangement may be used as an alternative to the slotted

nut and pin.Each screw has a small hole

drilled through the head, through which a wire is passed and its

ends twisted together. The wire thus prevents the screws from

becoming loose.



Types of Mounting:End Cap and Shims :

This arrangement may be used for tapered roller bearings or for angular contact bearings where

pre-load has to be set with adjusting shims.



Types of Mounting:Snapring :

When it is not suitable for the housing to contain a shoulder,

then a snapring can be used to locate the bearing in the housing .

This requires a special bearing with a recess machined in the

outer race to enable the snapring to be fitted.



Types of Mounting:Adaptor Sleeve :

Some types of bearing are made with a tapered bore and are

mounted as "floating" bearings on an adaptor sleeve .

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Typical Pump Bearing Arrangements:

Ball Radial and Ball Thrust :For shaft diameters up to about 55 mm and speeds up to 3600

rpm, anti-friction double-row angular-contact bearings may

be used for both radial loads and thrust loads in either

direction.



Ball Radial and Ball Thrust :Because of the rolling action of

the balls between the races, the heat generated is not as

great as a plain bearing. Most of the heat which is produced is due to the sliding action of

the balls against their retaining cage which maintains equal

ball spacing.



Ball Radial and Ball Thrust :Lubrication is usually provided

by an oil ring which hangs from the shaft with its lower part

dipped into an oil reservoir in the bearing housing. As the

shaft rotates, the oil ring splashes oil to the bearings.



Sleeve Radial and Ball Thrust :For shaft diameters and speeds which are above the limits for ball radial and ball thrust construction, cylindrical sleeve bearings are used to carry the radial loads.Axial loads in either direction are taken by two ball bearings mounted back-to-back.



Sleeve Radial and Ball Thrust :The sleeve bearings are steel

or bronze lined with Babbitt and are split parallel to the shaft.

Lubrication may be by oil rings and cooling coils may be built in to the oil reservoirs through which either cold water or the

pumped product may be circulated to remove excess

heat from the oil. mounted back-to-back.


Bearing Lubrication:

The choice of lubricant is determined by the following factors:

Load - The load on the bearing will determine the pressure against which the lubricant must work.

Speed - The lubricated surfaces will wear faster at higher speeds.

Temperature - The properties of the lubricant may be affected by the operating temperature.

Environment - The lubricant may have to cope with the presence of water or corrosive substances.



Lubricant selection should normally be in accordance with the recommendations of the equipment manufacturer.

However, as a general rule, the following guide is useful to remember:

For light loads and high speeds, use a lubricant of low viscosity.

For heavy loads and low speeds, use a lubricant of high viscosity.



The decision whether to use oil or grease as the lubricant will depend upon operating conditions. The following

comparative advantages may be useful to consider when making a choice:

Oil:

Assists cooling.

Absorbs less torque.

Feeds more easily and form a central supply.

Washes away dirt.

Can also lubricate other elements such as gears.



Grease:

Permits simpler bearing designs.

Provides better sealing against foreign matter.

Is easier to contain and seal.

Allows longer periods without attention.



The following basic principles govern the application of lubricants:

Cleanliness is vital. Lubricating equipment must be kept free from dirt and other foreign matter.

Lubricants of different brand or grade should never be mixed. If it is necessary to change the type of lubricant, then the

equipment should first be drained and cleaned out.

An excess of lubricant, especially grease, will cause heat to build up. This will eventually break down the lubricant and bearing

failure will result.



Lubricant filters and strainers should always be changed at the recommended time intervals.

The selection of lubricant for a given application should be as recommended by the equipment manufacturer.

Inadequate lubrication can often be identified by the operating condition of the bearing, such as excessive noise or temperature. In general, if a bearing feels too hot to touch with a hand, then its

lubrication may not be correct and should be checked.



Lubricants are potentially hazardous materials and should be properly stored with regard to safely and effect on the environment.


Bearing Lubrication:Oil:

Oil lubrication is often used for light to moderate duty in high-speed applications.

The oil should be changed at regular specified intervals and this should be done sooner if the oil has become dirty or contaminated.

When changing oil, it is good practice to thoroughly clean the bearing housing or sump with flushing oil before adding the new lubricant. This ensures that all foreign matter is removed.


Bearing Lubrication:Grease :Grease lubrication is generally used for heavy loads at low to moderate speeds.

When grease is used in pump bearings, make sure it is of the type specified by the pump manufacturer.

Usually, soda-soap-compounded grease that has a mineral oil base is acceptable.

All greases used for bearings should be free from fillers such as clay, mica or talc. These mineral fillers are highly abrasive, even when finely ground and mixed with oil or grease.


Bearing Lubrication:Grease :Pumps are greased at around 3 monthly intervals and this may be extended to 6 months if the pump is not operated frequently.

When the pump is running, however, the bearings should be checked daily for noise, cleanliness and operating temperature.


Bearing Lubrication:Methods of Lubrication:

Lubrication methods include: Oil reservoirs.

Constant-level oilers .

Drip-feed .

Ring type oilers .

Pressurized systems.

Whatever method is used, daily checks should be made to ensure that the correct amount of lubricant reaches the bearings.


Bearing Lubrication:Methods of Lubrication:The constant-level oiler is a combination drip feed and ring type oiler.

The ring used to distribute the oil is the same as any other ring oiler.

Its purpose is to maintain a constant oil level in the sump by replacing the oil that leaks out.


Bearing Lubrication:Methods of Lubrication:


Bearing Lubrication:Methods of Lubrication:Some pumps are fitted with grease cups . Lubrication is carried out by turning the cap or handle one or two turns at regular intervals.


Bearing Seals:There are two main types of bearing seals:

Contacting type seals (rubbing).

Clearance type seals (no rubbing).


Bearing Seals:

A

D

G

F


Bearing Seals:

B

The simplest type of clearance seal is obtained by providing a relatively long close-clearance fit

between the shaft and housing. One or more grooves may be arranged in the housing to

help prevent the escape lubricant.

Slingers are often fitted to clearance seals when they are used for oil-lubricated bearings. The slingers help to prevent the escape of any oil which may have passed the seal by flinging it into a groove in the seal housing, from where

it can drain back into the bearing housing .


Bearing Seals:

B

Another type of clearance seal is the labyrinth type . In a labyrinth seal, the long and staggered close-clearance path is filled

with lubricant and offers a high resistance to flow in either direction.

Clearance type seals can operate without any restrictions regarding speed or

operating temperature, but they do permit more leakage of lubricant than rubbing

contact seals and also are not as effective in keeping out dirt and other foreign matter.

Centrifugal Pumps Bearings - Part 2

Documents

bearing housing

types of bearing

plain bearing

special bearing

floating bearings

shaft free bearing

rolling element bearing

tapered roller bearings