BEARINGS

1. INTRODUCTION

A rotating shaft in any item of machinery has to be supported and held in position

(located) so that;

the mass (load) of the shaft and any attached components is supported,

axial or radial forces (thrust) generated by rotation are absorbed,

the shaft can rotate freely with minimum wear to the shaft or its supports,

the shaft is located (held) in the correct operating position relative to other

components and the supports and

the minimum of effort (power/ energy) is needed to overcome the frictional forces on

the shaft.

In most machines bearings of some type or other are used to meet these requirements.

The type of bearings selected for a particular application will mainly depend on the

following:

Forces (Dynamic, Static, Axial, and radial loading) acting on the bearing

Speed at which the shaft is revolving

Operating environment

Frictional forces involved

Bearings are subject to a certain degree to rolling or sliding friction. Lubricants are used

to reduce this friction to a minimum. Lubrication however cannot reduce friction

completely because of the friction within the lubricating fluid itself. This fluid friction,

which directly relates to the viscosity, is substantially less than either rolling friction or

sliding friction. However, lubrication is an important factor in bearing operation,

particularly at high speeds and under heavy loads.

2. PURPOSE OF BEARINGS

To effectively locate rotating shafts and machine components in both vertical and

horizontal positions during operation.

© ESKOM 2004

1

3. SAFETY

3.1 Personal safety

It is important to exercise particular caution when working in the vicinity of

rotating equipment and machinery. The following safety rules must be adhered

to at all times:

Do not wear loose hanging clothes

Do not climb over rotating machines

Horse playing is not allowed near rotating machinery

Always beware of hot surfaces which may cause severe burns

Always exercise surgical conditions when working with bearings (lubrication)

NEVER use rags, cotton waste or metal scrapers to remove excess oil or

grease from revolving shafts. This is a dangerous practice and can lead to

serious injuries.

It is also important to make sure that any personal protective equipment that is

required to perform certain duties, are always available when needed.

4. FORCES ACTING ON A ROTATING SHAFT

4.1 Dynamic loading

The dynamic load rating of a bearing is defined as the constant radial (or centric

axial) load which the bearing can theoretically endure for a basic life rating of one

million revolutions. This phenomenon is normally catered for in the

manufacturing process of all the different types of bearings that are available.

In single row angular contact bearings, the radial load relates directly to the radial

component of that load which causes only radial displacement of the bearing

rings.

4.2 Static loading

Static loading is the load acting on the bearing when the speed of rotation of its

rings in relation to each other is zero.

© ESKOM 2004

2

All bearings are thus subjected to load under static conditions because they will

become stationary at some point during operational sequences. The safe static

load for each type of bearing is thus established during the manufacturing

process to ensure that the bearing does not sustain damage to the raceways and

rolling elements during static periods. The damage incurred as a result of

excessive static loading is usually in the form of permanent indentations which

are sufficient to cause unsatisfactory operation (vibrations) of the bearing under

running conditions.

In general terms, the static load rating is the load on a non-rotating bearing

determined by the allowable amount of permanent damage or deformation at the

most heavily loaded point of contact between the raceways and rolling elements

and not by fatigue of the material.

4.3 Axial and Radial loading

Refer to Figure 1. A rotating shaft is subject to two principal forces, those that act

along the axis of the shaft (Axial) and those that act at right angles to the axis of

the shaft (Radial). This means that when the shaft rotates, forces act in a radial

direction away from the shaft at a point around its circumference. A horizontal

shaft always has the mass (weight) of the shaft acting downwards as it rotates

(i.e. radial force). The mass of a vertical shaft also acts downward in which case

this force is considered to be an axial force.

If not contained, radial forces will cause a shaft to ‘wobble’; particularly at low

speed. At higher speeds, centrifugal forces will tend to absorb any minor

imbalance in the radial forces.

FIGURE 1: FORCES ON A ROTATING SHAFT

© ESKOM 2004

3

Axial forces can cause the complete rotating mass (shaft + attachments) to move

and damage other components. Typically, thrust bearings are used to both

contain axial forces (thrust) and locate the shaft in a fixed radial position.

5. BEARING TYPES

Heavy industrial machines like those used in power stations use a wide variety of

bearings. All these however fall into one of two main categories, namely:

Fluid film bearings

Rolling contact bearings

5.1 Fluid Film Bearings

Fluid film bearings are classified according to the direction of the load they are

supporting. Plain journal bearings are cylindrical in shape and carry the mass

(weight) of the rotating shaft. Thrust bearings are typically disc shaped and are

used to prevent the shaft moving axially (lengthwise) by absorbing any force

caused by axial movement of the shaft. Both bearing types rely on the formation

of an oil film between the bearing surface and the shaft to reduce friction and to

support the load.

5.1.1 Pedestal mounted Plain Journal Bearing

Plain journal or sleeve bearing, illustrated in Figure 2 are commonly called

Plummer blocks or bushings. They comprise a metal sleeve that fits

around the shaft and is enclosed and held by a bearing housing. The part

of the shaft that turns within the bearing is called the journal.

In some machines the bearing housing is no more than a hole bored

through the casing with a thin metal sleeve between the casing and shaft.

This arrangement is sometimes called a bushing or bearing bush.

© ESKOM 2004

4

FIGURE 2 : HORIZONTALLY SPLIT JOURNAL BEARING

A basic plain journal bearing, comprising a solid sleeve and has the

disadvantage that it can only be removed from the shaft by sliding it from

an end. With large machines this can be a time consuming, if not an

impossible task. Removal is made easier by splitting the bearing sleeve

horizontally along the axis of the shaft so allowing the shaft to be lifted out

when the upper half of the bearing is removed. A split journal bearing is

also easier to replace because the shaft does not have to be removed

completely. With the upper half removed, the shaft only has to be raised

sufficiently to roll the lower bearing half into position below the shaft.

Large split steel shell journal bearings are normally found on jaw crushers,

large fans, turbines and generators. The bearing shell is a steel casting

that is split into an upper and lower half. Each half is lined with a white

metal (tin/ antimony alloy) such a Babbitt metal. The shell is supported by

© ESKOM 2004

5

a pedestal that stands on the machine bedplate or foundations. The

bearing is oil lubricated using a pressure or ring/chain system.

5.1.2 Tilting-Pad Journal

This type of bearing, illustrated in Figure 3, has a segmented bearing

surface. The segments are supported on pivots so that they can move to

adjust to the best position for proper lubrication. The formation of a

lubricating oil film (wedge) between the pad and shaft journal is essential to

achieve radial stability and to prevent damage caused by metal to metal

contact when running.

FIGURE. 3 : TILTING PAD JOURNAL BEARING

NOTE: Tilting pads are also used for thrust bearings.

Oil Wedge Formation

With tilting pad bearings, two conditions must be satisfied before the film

is established:

- There must be relative motion between the two surfaces;

This is because the oil film breaks down when the shaft stops turning.

In practice the film will breakdown before the shaft stops during

rundown.

© ESKOM 2004

6

- There must be a continuous supply of oil to the bearing

surfaces.

One of the surfaces must be inclined at a small angle relative to the

other so that the oil is shaped like a thin wedge, tapering in the

direction of motion. In cylindrical bearings the wedge must be

imagined as being wrapped around part of the shaft.

Figure 4 illustrates how the oil film is developed as the shaft goes

from standstill up to running speed:

FIGURE 4 : JOURNAL BEARING OIL WEDGE FORMATION

Step 1 - When stopped, the shaft rests in the bottom of the bearing;

the mass of the shaft tends to squeeze the oil out of the bearing until

there is little or no oil adhering to the surfaces.

Step 2 - As rotation commences the shaft will tend to ‘climb’ up the

bearing to a point where it begins to slip. At this stage there is a

sliding of metal on metal and some boundary lubrication from the oil

adhering to the metal.

Step 3 - As the speed of the shaft increases the lubricant is dragged

down until it forms a thin layer under the shaft and film lubrication is

© ESKOM 2004

7

established. The shaft now assumes a central position but is raised

slightly in the bearing by the oil film.

Step 4 – As the speed increases, the bringing oil to the wedge

increase the wedge/ film thickness. When this is completed the shaft

assumes the position illustrated with the ideal wedge-shaped oil film.

The tilting pad bearing is built to allow lubricant to reach the pad

surfaces and is slightly larger than the shaft. The clearance between

the bearing and shaft allows lubricant to circulate around the shaft

between the sliding surfaces. This clearance area varies, depending

on shaft size and speed, bearing load and the type of lubricant used.

Plain journal bearings often have oil grooves or channels cut into

them that help to distribute lubricant evenly over the bearing surface.

Where the load carried by the journal is large, such as in turbines or

large fans, it is possible that the oil film will become broken or

interrupted at low shaft speed or at a standstill. This will result in

metal-to-metal contact that will cause bearing and journal wear.

Where this may occur, the bearing is equipped with a ‘lift’ or jacking

oil system that is used at low shaft speeds, during machine start up

and rundown. With a jacking oil system, the shaft is hydraulically

lifted by high-pressure oil that is pumped into the area between the

bottom of the journal and the shaft. The pressure and quantity of oil

used is sufficient to raise and support the shaft on an oil film

regardless of if the shaft is at standstill or rotating. Jacking oil

systems are normally shutdown when the shaft is rotating at a speed

that is sufficient to maintain a stable oil film.

5.1.3 Thrust Bearings

There are many types of thrust bearings, the most common of which is the

tilting pad or Mitchell type bearing. This bearing holds the rotating shaft in

position in the casing and counteracts any axial forces tending to move the

shaft. The basic thrust bearing, illustrated in Figure 5, comprises a shaft-

mounted thrust collar that is ‘sandwiched’ between segmented thrust and

surge pads. These pads are supported by a cast collar or housing that is

© ESKOM 2004

8

filled with oil. Seals are fitted between the shaft and housing to prevent oil

leakage.

FIGURE 5 : BASIC THRUST BEARING

As with the tilting-pad bearings discussed previously, the bearing pads

comprise a gunmetal body that is faced with a white metal bearing surface.

The pads are also pivoted so that they can tilt to create the necessary

lubricating oil wedge. Surge pads are fitted to absorb any momentary

reversal of thrust (e.g., during start up). The oil wedge also transmits the

axial forces imposed by the shaft journal, via the pads, to the housing. In

most installations the bearing housing is split so that it can be removed and

replaced without removing the shaft.

Tilting Thrust Pad Oil-wedge Formation

As with journal bearings, the tilting action of the pad is essential for the

formation of an oil wedge. As illustrated in Figure 6, the pressure in the

oil film reaches a peak approximately two thirds of the way across the

pad, and it is this pressure that enables the axial thrust forces to be

absorbed. As the pad is of a limited size, oil leaks from all the edges as

© ESKOM 2004

9

illustrated. The effect of this is that the pressure distribution across the

pad varies from a maximum at the centre of the pad to a minimum at the

outside edge.

FIGURE 6: OIL WEDGE FORMATION

Oil Cooling

Because the bearing housing is flooded with oil there is considerable

churning and heat is generated. This effect may be reduced by baffles

inside the housing or by using an external oil cooler. With large Mitchell

bearings the thrust pads are fitted on a flat vertical plane so that they

take the thrust load equally. The pads are each supported by pistons

connected to a closed hydraulic system.

5.1.4 Tapered-land Bearing

The tapered land thrust bearing has two solid bearing elements that are

fitted either side of the shaft thrust collar. The bearing element surface is

segmented by radial grooves that are machined to create tapered ‘lands’.

© ESKOM 2004

10

These lands promote oil wedge formation between the surfaces to provide

lubrication and thrust transmission to the stationary housing.

5.1.5 Bearing Lubrication

The choice of oil characteristics for the fluid film type bearings has to take

account of the operating conditions. Key among these is the oil viscosity at

start and normal operating temperatures.

Oil with a low viscosity would be squeezed out between the surfaces by the

load, causing the film to rupture and result in metal-to-metal contact. Oil

with an excessively high viscosity would cause additional fluid frictional

losses and consequent heating of the oil and bearing. Similarly the ability

of the oil to ‘wet’ the surfaces is important so that there is always a thin film

of oil, even at standstill.

In practice a compromise is made; the viscosity must be high enough to

maintain the oil film, with a margin of safety, even at the highest working

temperature, but must not be so high as to overheat the bearing. In many

forced lubrication installations the oil in the tank is maintained above a set

temperature to ensure that the viscosity remains above an acceptable level

so that it flows and forms a wedge easily.

5.2 Rolling Contact Bearings

Rolling contact bearings are primarily designed to support and locate rotating

shafts or parts so that load can be transferred from the rotating to the stationary

members. They also are required to allow free rotation with the minimum of

friction. This type of bearing comprises several rolling ‘elements’ (Figure 7) that

are held between an inner and outer ring. The rolling elements are either balls or

rollers (cylindrical, tapered or spherical). The essential differences of application

between ball bearings, roller bearings and needle bearings are:

Ball bearings : low load carrying capacity; higher speeds

Roller bearings: higher load carrying capacity, lower speeds

Needle bearings: low load carrying capacity, moderate speeds, high

accuracy

© ESKOM 2004

11

FIGURE 7 : BEARING ROLLING ELEMENTS

FIGURE 8 : BALL AND ROLLER BEARINGS

Both types of rolling element are normally held in position, between the bearing

rings, by a cage or retainer. The cage also damps any vibrations of the rolling

elements. Rolling bearings offer less friction compared to plain bearings,

especially on starting (Figure 8).

© ESKOM 2004

12

Bearing friction is minimised by using as many rolling elements as is practical to

support the load. Lubrication is also used to reduce friction further by introducing

lubricant to the contact zones between the rolling elements, rings and cage.

Lubrication also:

Protects the bearing surfaces from dust, water and foreign matter

Protects the surfaces from corrosion

Absorbs and dissipates heat caused by friction between the contact surfaces.

5.2.1 Ball Bearings

Ball bearings work on a point contact so they are suited for high speed and

light loads applications; they are also less expensive than roller bearings.

A ball bearing assembly comprises of a number of steel balls that turn

within two rings or races (Figure 9).

The inner race has a groove on its outside surface in which the balls roll.

There is usually a similar groove on the inside surface of the outer race.

The balls are enclosed by a cage or retainer. The cage separates the balls

and prevents them from being forced out of the bearing. The balls must be

separated because friction would increase greatly if they came in contact

with each other while rolling.

Bearing lubrication is essential and seals may be used to retain lubricant

and prevent dirt or moisture entering the bearing races (Figure 10).

FIGURE 9 : BEARING AND CAGE

© ESKOM 2004

13

FIGURE 10 : BEARING SEALS

Non-rubbing gap and labyrinth seals use lubricant (oil/ grease) to seal or

‘flush’ the gap between the bearing outer and inner rings. Rubbing felt,

radial or V-ring seals close the contact points between the moving parts.

There are several variations of the basic ball bearing design that are

intended for different applications. The most common types are:

Radial Ball Bearings

- Deep Groove

- Single and Double Row Angular Contact

- Four point

- Self-Aligning

Thrust Ball Bearings

- Straight Thrust

- Angular Contact

Radial Roller Bearings

- Cylindrical Roller

- Needle Roller

- Tapered Roller

© ESKOM 2004

14

- Barrel Roller

- Spherical Roller

Thrust Roller

- Cylindrical Thrust

- Spherical Thrust

5.2.2 Roller Bearings

Roller bearings are constructed in much the same way as ball bearings.

They comprise a number of rollers that turn between two races or rings.

Roller bearings also have a cage or retainer that keeps the rollers equally

spaced around the bearing. This is to ensure that forces on the bearing are

equally distributed and that the proper shaft position is maintained.

As described previously, roller bearings are classified according to the

shape of the roller. The cheapest and most common is the cylindrical

roller; the races do not need a special shape to match the rollers.

Tapered Roller

The tapered roller bearing has rollers that are smaller at one end than at

the other. This design requires the race surface to be angled for it to

make full contact with the rollers (Figure 11). A tapered roller bearing

will carry some thrust force as well as radial force due to the tapered

shape of the race and cage.

Spherical Roller

Spherical roller bearings have rollers shaped like a barrel that is thick in

the middle and thin at the ends. The outer race is a segment of a

sphere, while the inner race is dished out so that it comes into contact

with the rollers. This construction allows for good force distribution even

when the bearing is slightly misaligned. Full contact can be maintained

even if the inner race is slightly twisted.

© ESKOM 2004

15

FIGURE 11 : TYPICALTHRUST BEARINGS

5.2.3 Needle Roller Bearings

Again, Needle Roller bearings are constructed in much the same way as

roller bearings. They comprise a number of rollers that turn between two

races or rings (Figure 12). Needle Roller bearings also have a cage or

retainer that keeps the rollers equally spaced around the bearing. This is to

ensure that forces on the bearing are equally distributed and that the proper

shaft position is maintained.

A needle roller is one that has a length several times its diameter and is of

such a small size that roller guidance through rib faces is not achievable.

They therefore have a smaller radial section than conventional ball and

roller bearings thereby offering space saving, initial cost saving and weight

reduction.

© ESKOM 2004

16

Needle roller bearings are designed only for radial load and can not be

applied for axial loads, consequently they are not used to provide axial

location.

Full complement types are suitable for low speeds whereas caged types

will operate at moderate speeds.

Misalignment of needle roller bearings has a greater effect on needle roller

skewing and bearing life than for conventional roller bearings.

FIGURE 12 : MACHINED TYPE NEEDLE ROLLER BEARING

Needle Roller bearings are lubricated with a newly developed thermosetting

solid-type lubricant. A large amount of lubricating oil and fine particles of

ultra high molecular weight polyolefin resin are solidified by heat treatment

to fill the inner space of the bearing. As the bearing rotates, the lubricating

oil is squeezed out onto the raceway in proper quantity, keeping the

lubrication performance for a long period of time. These bearings are

referred to as Capilube Bearings and are available in all Needle Bearing

series with outer diameter not exceeding 80 mm.

© ESKOM 2004

17

5.3 Bearing Installation

Bearings are installed in a variety of ways depending on their duty, location within

the machine and any lubrication or cooling requirements. Typically, where a

shaft is to be supported, the outer bearing race is located within a bore in the

machine casing or held in a pedestal housing (Figure 13). The shaft passes

through the bearing’s inner race and may be a sliding or interference fit. If a shaft

is supported by two in-line bearings, such as a fan shaft, the shaft is only fixed

(located) to one bearing leaving the other ‘floating’. This is done to accommodate

thermal expansion of the shaft. In many machines the fixed bearing is at the

drive-end of the shaft (Figure 13 a).

FIGURE 13a : SHAFT / BEARING ARRANGEMENT

5.4 Bearing Lubrication

The bearing may be oil or grease lubricated. Pedestal bearing housings lend

them to oil filling, oil circulation (external cooling) or grease packing.

5.4.1 Greasing

Greased packed housings normally have a grease nipple and a grease

vent hole. Grease is then expelled from the vent during greasing. Where

the bearings are exposed to moisture, dirt or dust (e.g., conveyor tail shaft)

the housing/ shaft seal is arranged so that grease is expelled via the seal

such that greasing ‘flushes’ the bearing housing.

© ESKOM 2004

18

FIGURE 13 : PEDESTAL BEARING (Fixed Type)

5.4.2 Oil Filling

Oil filled housings are normally equipped with a sight level glass that shows

the level of oil in the housing (Figure 14). Oil filling and drain plugs may be

incorporated with the level indicator located on the housing.

FIGURE 14 : TYPICAL BEARING OIL LEVEL SIGHT GLASS

© ESKOM 2004

19

6. TYPICAL APPLICATION OF BEARINGS

Bearings have to be carefully selected for different applications. As mentioned in

paragraph 1 the type of bearings selected for a particular application will mainly depend

on the following:

Forces (Dynamic, Static, Axial, and radial loading) acting on the bearing

Speed at which the shaft is revolving

Operating environment

Frictional forces involved

The following are typical examples of machines on which the various types of bearings

are used:

6.1 Journal bearings

Horizontally split journal bearings are widely used on machines which operate

under heavy loads such as Turbines, Induced Draught fans. On Circulating

Water (CW) pumps where the bearings are mounted vertically, it is usually

combined with a thrust bearing to accommodate the vertical thrust exerted by the

weight of the shaft.

6.2 Thrust Bearings

During normal operation, Steam Turbine shafts are subjected to continuous axial

thrust due to the effect of the high pressure steam on the turbine blades. This

thrust must be limited to prevent contact between the moving Turbine blades and

the stationary diaphragms.

FIGURE 14a : POSITION OF THRUST BEARING ON A TYPICAL

TURBINE

© ESKOM 2004

20

All modern Turbines are therefore designed with their blading arranged in such a

way to largely counter-act the thrust (Figure 14a). This is however not sufficient

and it is thus also necessary to fit a thrust bearings to reduce axial movement to

an absolute minimum.

6.3 Rolling Contact bearings

Rolling contact bearings are suited for operation on machines where dust and

sludge is handled. The reason for this is that these bearings are pre-lubricated

and sealed.

Typical applications are:

Gearboxes

Slurry pumps

Electric motors

Conveyor idlers

7. CARE OF BEARINGS

There are very few machines on a power plant that do not have bearings in one form or

another. The failure of only one bearing can cause extensive damage and put a

complete generating unit out of service for several weeks. Depending on the

circumstances this can cost the station millions of Rand for repair and loss revenue.

Bearing failure can often be prevented by ensuring that proper checks, inspections and

maintenance is done before a plant is started and while it is in operation. Depending

on the system typical running checks that MUST be done at least once a shift are:

Check the bearing oil pressures (pressure oil system). Any observed pressure that

is outside the specified limits must be reported immediately. A high pressure is also

cause for concern as it may be the result of a blocked filter or oil way.

Check bearing temperatures; they should be within the specified limits; particularly

below the maximum specified. Bearing temperature will rise quickly after the plant

has been started, and then reach its steady normal running value. Where the

temperature continues to rise rapidly, report it immediately and have the plant shut

down.

© ESKOM 2004

21

Check that the oil levels, both 'standing' and 'running' (where this is indicated) are

correct. The level may be visible on a dipstick, or in sight-glass type level gauge. It

must be remembered that an excessively oil level may overheat the machine and

bearing and will slow down or stop an oil ring lubricator.

Check for abnormal oil level rise or fall particularly where oil-cooling systems are

used. A rise in level could be caused by cooling water entering the system or

reservoir. Such water will cause the oil to emulsify (creamy appearance) and

reduce its lubricating properties. A fall in level may indicate oil entering the cooling

water

With oil ring lubricated bearings, check the ring to see that it is in its correct position

and free to move or turn. When the plant is running, the ring must turn with the

shaft. If the ring has stopped turning contact your supervisor or the Maintenance

department. DO NOT attempt to get it moving using metal rods or fingers.

Oil flow must also be monitored on a continuous basis. If the oil flow to a bearing is

insufficient, make sure that the in line filters and strainers are clean. Where duplex

strainers are provided, change them over and defect the clogged strainer end follow

up to ensure that it gets cleaned as soon as possible.

Always be on the look out for emulsified oil. Emulsification will cause the oil to

become a milky white colour and foaming will occur. This usually occurs when

water is leaking into the oil in oil coolers. Inspect oil coolers for leaks.

Keep level gauge glass and breather clean. See that the small hole on the level

gauge glass is not blocked as it may give a false high oil level indication.

Check for and report any oil leaks immediately. If necessary, maintain the oil level

(top up) until the plant can be shut down. If the oil leak is severe shut down the

equipment immediately.

Check automatic bearing lubricating devices for correct operation; see that the

discharge rate of lubricant is adequate.

© ESKOM 2004

22

DO NOT over grease bearings; too much grease will increase fluid friction and

generate excessive heat causing bearing overheating and subsequent failure.

Check that oil filler caps are fitted and secure, this is essential to prevent dirt

entering.

The Four Rs (Always keep them in mind!)

Always exercise caution when oiling a bearing. Always use the Right amount of the

Right oil in the Right place at the Right time.

8. POTENTIAL OUT-OF-NORMAL BEARING CONDITIONS

During plant inspections it is possible to detect possible faults and other abnormal

operating conditions on bearings. This can be achieved by using out natural senses of

smelling, hearing, feeling and seeing. The following abnormal conditions can be

observed in these ways:

8.1 Noise

Damage to bearings normally manifests in a humming noise. Thus, by simply

listening to the sound of a running bearing, its physical condition can be

diagnosed. Should any abnormal sound be emitted from the bearing housing the

following must be checked:

Sufficient lubricating oil available

If it is ring lubricated, check that the ring is rotating

If the lubrication is in order, the bearing must be reported as defective without

delay.

8.2 Heat

A damaged bearing will run hot. By simply placing the palm of your hand on the

bearing housing can tell whether it is running abnormally hot. If the temperature

is such that a burning sensation is experienced within 4 to 5 seconds, it can be

accepted that it is running hot. Once again, the lubrication of the bearing can be

checked for satisfactory operation. If the lubrication is in order, and the bearing is

still running hot, report it immediately.

© ESKOM 2004

23

NOTE: Special care must be taken not to sustain burns when inspecting

bearings that are suspected of running hot.

8.3 Vibration

Although varying levels of slight vibration is very difficult to recognise it is a skill

that can be practiced and developed. Simply by placing a hand on the bearing

housing which is vibrating, a continuous shivering feeling will be felt. To develop

this skill, it is good practice to compare different bearings on the same machine.

Should any bearing be diagnosed as vibrating excessively, it must be reported

immediately.

8.4 Leaks

Lubricating oil leaks on bearings is not only a threat to the reliable operation of

the machine but it is also a safety threat and fire hazard. Leaking bearings must

be reported and repaired immediately. The oil that has leaked out of the bearing

must be cleaned immediately.

The bearing lubrication must be checked and maintained at its normal working

level until the machine can be taken off load for repairs.

9. BEARING PROTECTION SYSTEMS

To ensure continuous and reliable operation of bearings, they are in many cases

equipped with condition monitoring and protection devices. These devices include the

following:

9.1 Bearing vibration monitoring and protection

Bearing vibration pick-ups are installed on large machine bearings such as those

on Steam Turbines which are rotating at high speeds. The reason for providing

protection against vibration is that it could result in disaster if it is not noticed and

the necessary corrective actions taken in time. Severe vibrations may result in

serious damage to bearings, machine foundations and in the case of precision

machines such as steam turbines, cause internal rotating components to ‘rub’

against each other.

© ESKOM 2004

24

Bearing vibrations are measured in Micro-Siemens (µs). In many cases,

recording instruments are provided in the control room where the vibrations of

each bearing on the machine are continuously recorded. Any abnormal

increases are thus immediately visible.

For large turbines the normal vibration level for any bearing is typically in the

region of 0, 02 to 0, 04µs. Should the vibration severity increase and reach a

typical value of 0.1 to 0, 12µs an audible and visual alarm will be raised to warn

the operator about deteriorating conditions. Should the vibration levels increase

in excess of 0, 15 to 0, 2µs an automatic tripping device will trip the machine to

avoid any further damage to the bearings, machine or other components.

Normally, a simultaneous rise in bearing metal temperature and lubricating oil

return flow together with abnormal bearing vibrations is a sure indication that

bearing damage has occurred.

9.2 Metal temperature monitoring and protection

Metal temperature monitoring and protection devices are also provided for each

bearing on large machines. The purpose of these instruments is to monitor the

operational condition of each bearing individually. Higher than normal bearing

metal temperatures is a sure indication that the bearing is not in a healthy state.

In cases where a bearing is damaged to such a degree that the metal

temperatures are increasing to values above its normal operating temperature,

the rate of deterioration usually becomes worse. For this reason, tripping devices

are also normally provided to trip the machine if the bearing metal temperatures

reach temperatures of typically 80°C to 85°C.

Monitoring of bearing metal temperatures is accomplished by thermocouples

which are fitted to the bearing housing. The electrodes of the thermocouple are

usually positioned as close as possible to the inner core of the bearing housing to

minimise the delay of heat conduction through the bearing metals.

© ESKOM 2004

25

9.3 Lubricating oil pressure monitoring and protection

The single most important factor upon which efficient bearing operation is

dependant is adequate lubricating oil supply. The only reliable assurance that a

bearing is adequately lubricated is indicated by the lubricating oil pressure and

associated bearing oil flow.

To accomplish this, oil pressure measuring devices and local pressure gauges

are provided to enable the operator to monitor lubrication parameters.

A pressure measuring device comprises of tapping points and impulse lines on

the lubricating oil supply line to the bearing. The impulse lines are connected to a

pressure transmitter which transmits an electronic pressure signal to the remote

control panel.

The value of the lubricating oil supply pressure differs from application to

application.

9.4 Lubricating oil flow measuring devices

The most common method to measure lubricating oil flow is with a differential

flow meter. This technique requires the oil pressure to be measured on both

sides of an imposed restriction (orifice plate) in the path of normal flow. The flow

rate is then calculated based on the change in pressure across the restriction.

A pressure transmitter is used to transmit the differential pressure signal to

electronic devices where the signal is processed to provide a signal

corresponding to the actual pressure in the lubricating oil supply pipe.

© ESKOM 2004

26

10. SELF ASSESSMENT

After having studied all the material, you can now request the Self Assessment from

your Facilitator. Make sure that you have mastered all the information contained in this

manual and attempt not to look for the correct answers in the text.

If you do not master the Self Assessment at the first attempt, re-study the material,

make notes of the topics that are not absolutely clear to you and approach your

Facilitator to explain these issues again.

When you feel confident that you have mastered this manual, you can then request to

write the Criterion Test.

GOOD LUCK!

© ESKOM 2004

27