Fan bearings

Many types of bearings can be found on fans, of which rolling element and plain bearings are by far the most numerous and form the main part of this Chapter. More exotic bearings, for example air bearings and magnetic bearings, may be used for some very special applications and are briefly discussed.

Other factors which play an important part in the choice of bearings include thermal expansion and heat losses. Any fan when it operates will experience a temperature rise and this can give different amounts of expansion between the stator and rotor which in turn may impose additional forces on the bearings or a requirement to design the overall bearing system to compensate for such events. The load may in some cases contribute to the problem by its own shaft expansion. All bearings have some frictional losses which appear as heat and may require some bearing cooling. Lubrication plays an important part in maintaining bearing temperatures at an acceptable level and in some cases cooling of the lubricant may be essential.

Wherever there is rotating machinery there will be a need for bearings i. e. those components whereby forces are transmitted between solids which are moving relative to each other. It is at such interfaces that friction takes place, accounting in its turn for significant amounts of energy to be added to that required for the air power provided by a fan impeller.

It is also at these interfaces that wear occurs, with a conse­quential risk of malfunctioning and/or overcoming the effects of wear, not only on the impeller and stationary parts, but often more importantly on the fan bearings and shaft.

The change of lubrication from an empirical art to an exact sci­ence, now dignified with the title “Tribology” grew out of the studies of Beauchamp Tower. He reported to an Institution of Mechanical Engineers committee set up in 1879. Osborne Reynolds, that giant of Victorian engineers, analysed these re­sults and in 1886 showed that in certain circumstances, the rel­ative motion and convergent geometry could generate suffi­cient pressure to overcome the loads applied to a bearing and prevent the two surfaces from making physical contact.

General comments

There is a wide variety of bearing types used for fans of which plain and rolling element bearings are by far the most numer­ous and form the main part of this Chapter. More exotic bear­ings, for example air bearings and magnetic bearings, may be used for some very special applications and are briefly dis­cussed.

Although the bearings essentially support and position the im­peller, they may be called upon to withstand some of the other forces imposed by the driven load. The rotor weight will always act downwards whatever the motor attitude but the forces aris­ing from the load, where applicable, may be in any direction and even vary according to the load conditions. The type of bearing selected will depend upon these conditions in addition to any limitations imposed by the environment. There is clearly a dif­ference in the type of bearing used for impellers running hori­zontally or vertically. Except for some very small fans and fans intended to run with the shaft in any direction, particular atten­tion may need to be paid to the choice of bearings.

Other factors which play an important part in the choice of bear­ings include thermal expansion and heat losses. Any fan, when it operates, will experience a temperature rise, or indeed may handle hot gases. This can give different amounts of expansion between the fan casing and bearing support structure, which in turn may impose additional forces on the bearings or a require­ment to design the overall bearing system to compensate for such events. The fan may in some cases contribute to the prob­lem by its own shaft expansion. All bearings have some fric­tional losses which appear as heat and may require some bear­ing cooling. Lubrication plays an important part in maintaining bearing temperatures at an acceptable level and in some cases cooling of the lubricant maybe essential.

The fan attitude, forces from the driven load, air or gas temper­atures and site ambient conditions all affect the bearing reliabil­ity and life. In turn the maintenance requirements are deter­mined by these factors and the type of bearing selected. Generally the manufacturer will fit bearings suitable for the specified requirements but customers may have a preference fora particular bearing type. For example, sometimes rolling el­ement or plain bearings may be suitable and the customer has a preference based on his experiences.

This Chapter covers various aspects of bearing selection, bear­ing housings, operation, lubrication, life and maintenance. Monitoring bearing performance by means of auxiliary equip­ment to protect against failure is also discussed in Chapters 15 and 18.

Kinematic pairs

A machine has been defined as “an apparatus for applying me­chanical power, consisting of a number of interrelated parts, each having a definite function”. The parts in contact, and be­tween which there is a relative motion, form a “kinematic" pair consisting of two solid bodies in contact. Lubrication is inevita­bly necessary for good operation. Often additional elements are included, for example, the balls or rollers and cage of a typi­cal bearing race.

Kinematic pairs fall into two categories:

Lower, in which surfaces touch over a fairly large area whilst sliding, one relative to the other. These would include pistons, sleeve bearings and screws used for converting rotary to linear motion or vice versa.

Higher, in which there is only line or point contact between the surfaces and relative motion may be partly turning and sliding. Examples include wheels on rails, anti-friction (ball and roller) bearings, or gears and pinions.

The majority of modern fans are fitted with rolling element bear­ings. As design has become more advanced, parts have been expected to rotate at higher speeds leading to higher stress lev­els. It has become the norm to get “a quart out of a pint pot”. In general this has favoured the increasing adoption of ball/roller, or anti-friction, bearings.

Condition monitoring

It is inevitable that in every decade there will be a theme to fasci­nate our political masters. Having survived the “white heat of the technological revolution” what now? Undoubtedly one of the contenders is our “business efficiency” and this is recog­nised as vital if we are to expand, or indeed survive, in an in­creasingly competitive world.

The use of CNC machinery for production; of computer sys­tems in the design and accounts departments; and even of so­phisticated marketing techniques in the sales office, all con­tinue apace. Only recently has the efficient maintenance of machinery been recognised as a potential field for extra profit.

Condition monitoring techniques have frequently been intro­duced but have themselves been monitored for cost effective­ness. Companies have often wasted money on such systems but the losses have been ignored. Perhaps maintenance itself should be more closely investigated instead of being accepted as an inevitable overhead.

Mechanical methods of condition monitoring are of most inter­est where the fan has ball/roller bearings (higher pairs), al­though some can be of use in analysing the special problems of sleeve bearings. Chemical methods can be of value in all cases.

The cost of preventative maintenance programmes, involving periodic stopping, stripping down and re-starting of an installa­tion, is becoming prohibitive. This is particularly so with capital intensive or even automatic plant. Various techniques have therefore been developed to determine the condition of fans whilst they are running, with the intention that only when there is an indication of impending damage or malfunctioning due to ex­cessive wear, will they be stopped. These techniques may be conveniently grouped under two headings and some examples are given for each:

Mechanical

• Vibration analysis — For general monitoring of plant condi­tion.

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• Spike energy detection — Methods for early warning of bearing failure.

• Shock pulse measurements — Methods for early warning of bearing failure

• Kurtosis monitoring — Methods for early warning of bearing failure

Further information on these techniques as applied to fans is given in Chapter 15.

Chemical

• Spectrographic oil analysis programmes (SOAP)

• Heat detection and thermography

• Ferrography or particle analysis

Further information on these techniques as applied to fans is given in Chapter 18.

Posted in Fans Ventilation A Practical Guide


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