Thermal Power Plants INDIA: All About Bearings

KNOW ALL ABOUT BEARINGS

We provide technical consultancy for all areas of thermal power plants.

Objectives of this article :

To achieve maximum bearing performance.

To reduce costly down time.

To minimize labor requirements.

To ensure that the equipment, in which the bearings are installed, runs smoothly / effectively.

Reasons for bearing failure :

	*Reasons*	*Counter measures*
1.	Bearing Quality	- Avoid installing spurious bearings
2.	Operating Environment	- Proper alignment. - Protection from temperature, moisture & contamination.
3.	Proper installation	- Knowledge of proper installation. - Knowledge of use of proper tools.
4.	Proper Maintenance	- Lubrication. - Maintenance schedules. - Monitoring operating conditions

Selection of bearing :

Each bearing type is designed to meet the requirement of specific bearing applications.

Whenever a bearing fails prematurely or repeatedly, it is very important to establish the cause of the damage, so that preventive action can be taken and proper replacement bearing can be selected.

Rolling bearings include radial and thrust bearings for radial and axial loads and some bearings are designed for continued radial and axial loads.

Generally ball bearings are recommended for light and moderate board roller bearings are recommended for heavy loads.

Ideal condition :

1) The actual load is less than designed load.

2) When the rolling surfaces are effectively separated by a lubricant film.

3) When surface damage caused by contamination is limited.

Under ideal condition bearing can attain a much longer life / infinite life.

Failure is initiated by fatigue, wear, corrosion contamination, seal failure, etc.

Bearing life is defined as the number of revolutions or the number of operating hours at a given constant speed which a bearing is capable of enduring before the first sign of fatigue occurs on one of its rings or rolling elements.

Of the bearings which do fail, approximately one third “die” from old age, i.e. fatigue of the bearings surfaces.Another third fail because of poor lubrication and rest fail mainly as a result of contaminations entering the bearings or because of handling damage or faulty mounting.

However, pattern varies between different industrial segments.

How bearing failure starts

Fatigue is the result of shear stresses cyclically appearing immediately below the load carrying surface. After a time these stresses cause cracks which gradually extend up to the surface. As the rolling elements pass over the cracks fragments of material break away and this is known as sapling or flaking. The flaking progressively increases in extent and eventually makes the bearing un-serviceable.

This type of bearing damage is a relatively long drawn out process, making its presence known by increasing noise and vibration. Therefore, it gives user adequate time to replace the bearing before it fails completely.

When cracks forms on the surface and grow into the material, surface distress is at hand. All surface contains roughness. If the oil film has a proper thickness in relation to the surface roughness, the probability for surface distress is very low.

If however the load is above the fatigue load limit, normal material fatigue may sooner or later occur.

Replace with care :

Bearings are calculated to run under very specific operating conditions. Too often, changes involving the use of a different lubricant, higher machine speeds, higher loads, and change in lubrication system,are made without anticipating possible negative effects.

Therefore, when replacing a bearing no other changes should be made, which would adverse affect bearing operation.

What to look for during operation ?

Checking the condition of a machine in operation and planning for its necessary overload is becoming an issue of increasing importance.

Bearings constitute a vital factor in all machines with rotating parts. Condition monitoring represents a rapidly growing activity in the field of preventive maintenance. Early indications of bearing damage allow bearings to be replaced during regularly scheduled maintenance and thus avoid unscheduled equipment downtime due to bearing failure.

Bearings in critical machinery or in severe environment should be checked frequently. Many systems and instruments are available for monitoring bearings.

However, for practical reasons not all machines or machine functions are monitored using advanced instrument. In these instances the machine operator or the maintenance engineer must remain alert for bearing trouble signs, such as noise, increase in temperature and vibration.

Listen, Feel and Look are three important factors.

Listen :

ü A common way of identifying an irregularity in running is by listening. With the help of an electronic stethoscope, abnormal noise can be detected and even traced to an individual machine component by an experienced operator.

ü Bearings in good operating condition produce a soft purring noise.

ü Grinding squeaking and other irregular sounds usually point to bearings in poor operating conditions. They are:

A squeaking noise may be caused by inadequate lubrication.

Insufficient bearing clearance can produce a metallic tone.

Indentations in the rear ring raceway of a bearing can cause vibration resulting in a smooth, clear tone.

Ring damage caused by blows during mounting or scoring leads to sounds, which vary according to the operating speed of the bearings.
Intermittent noises may indicate damage to a rolling element. The sound is generated when the damage surface is rolled over.

Dirt in a bearing often causes a grinding sound.

Severally damaged bearing produce irregular and loud noises.

Bearing damage can certainly be detected by listening, but the damage will often reach such an extent that an immediate bearing change has to be carried out.

Using of Electronic Condition Monitoring devices are much safer and more accurate prediction than the early method of placing one end of a wooden strake or a screwdriver against the bearing housing and putting one ear to the other end.

Feel :

ü High temperature often indicates that the bearing is acting abnormally. Overheating is attributed to the bearing lubricant. Running for extended period at temperatures in excess of 125°C can reduce bearing life.

ü Causes of high bearing temperature include insufficient or excessive lubrication, impurities in the lubricant, overloading, bearing damage, insufficient clearance, pinching and high friction in the seals.

Any change in temperature can be an indication of malfunction, if the running conditions have not been altered.

Critical bearings which cause equipment downtime when they fail should ideally be equipped with a heat probe.

Look :

ü Properly lubricated bearings that are adequately protected from dirt and moisture should not exhibit wear.

ü However, it is advisable to visually check a bearing when it is uncovered and to periodically check the seals.

ü Check the condition of seals near the bearings to ensure that they do not permit hot or corrosive liquids and gases to penetrate along the shaft.

ü Keep protective collars and labyrinth seals filled with grease to ensure maximum protection. Replace worn felt and rubber seals as soon as possible.

ü In addition to preventing the entry of dirt seals, it is also important to keep lubricant in the bearing housing. Lubricant leaks at seal locations should be inspected immediately for worn seals, seals defects and loose plugs. Leaks can also result from slacking of the joint between the matching surfaces of the bearing housing or from the grease breaking down by churning and releasing free oil.

ü In order that the automatic lubricating systems perform properly, fill them with oil or grease and ensure that the right amount of lubricant is released.

ü Also take a good look at the lubricants’ discoloration or darkening because these are usually signs that the lubricant contains impurities.

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