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.
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.
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.
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:
• Vibration analysis — For general monitoring of plant condition.
• 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.
• 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