spacing <strong>of</strong> <strong>the</strong> fan from <strong>the</strong> measuring place. The air velocity fields<strong>at</strong> a serial fan connection are introduced in graph 4, where <strong>the</strong>influence <strong>of</strong> fan spacing on <strong>the</strong> measured place can be seen.From <strong>the</strong> measured air velocity fields were calcul<strong>at</strong>ed averagespeeds in <strong>the</strong> particular places <strong>of</strong> measuring. For each variant <strong>of</strong>fan combin<strong>at</strong>ion on <strong>the</strong> physical model and for each particularplace we got <strong>the</strong> following average values <strong>of</strong> speed (vstr):Variant a) measuring place A: v str 1.80 ms 1 , measuring placeB: v str 1.43 ms 1 , measuring place C: v str 1.44 ms 1 ,Variant b) measuring place A: v str 1.55 ms 1 , measuring placeB: v str 1.23 ms 1 , measuring place C: v str 1.64 ms 1 ,Variant c) measuring place A: v str 1.42 ms 1 , measuring placeB: v str 1.16 ms 1 , measuring place C: v str 1.10 ms 1 ,Variant d) measuring place A: v str 1.62 ms 1 , measuring placeB: v str 1.40 ms 1 , measuring place C: v str 1.36 ms 1 ,Graph 1 Measured speeds <strong>at</strong> parallel fan connectionGraph 2 Measured speeds <strong>at</strong> 2 4 parallel fan connectionGraph 3 Measured speeds <strong>at</strong> 4 2 parallel fan connection42 ● COMMUNICATIONS 3/2008
Graph 4 Measured speeds <strong>at</strong> serial fan connection4. Vibr<strong>at</strong>ion measurement <strong>of</strong> jet fansand <strong>the</strong>ir analysis.An important requirement placed on a jet fan is also its abilityto ensure <strong>the</strong> required immedi<strong>at</strong>e ventil<strong>at</strong>ion <strong>of</strong> <strong>the</strong> road tunnel.When this requirement is not s<strong>at</strong>isfied (because <strong>of</strong> <strong>the</strong> fan breakdown),<strong>the</strong> consequences can be f<strong>at</strong>al. The reliability and availabilityare <strong>the</strong>n essential properties <strong>of</strong> this object [5].The preventive maintenance can be seen as a rel<strong>at</strong>ively simpletool for increasing trouble-free oper<strong>at</strong>ion and availability, using <strong>the</strong>inform<strong>at</strong>ion about <strong>the</strong> object condition, for example, by <strong>the</strong> measurement<strong>of</strong> its vibr<strong>at</strong>ions.This maintenance str<strong>at</strong>egy is an acceptable solution till <strong>the</strong>total cost <strong>of</strong> maintenance is not very high for <strong>the</strong> oper<strong>at</strong>ing organiz<strong>at</strong>ionand <strong>the</strong>y do not need to predic<strong>at</strong>e a fault condition <strong>of</strong> <strong>the</strong>mechanism. However, <strong>the</strong> procedures <strong>of</strong> vibr<strong>at</strong>ion diagnostics areable to provide d<strong>at</strong>a for prediction as well as inform<strong>at</strong>ion for proactivemaintenance (on <strong>the</strong> base <strong>of</strong> which <strong>the</strong> reasons <strong>of</strong> failuresare detected). Because <strong>of</strong> this reason, it is essential to deal withvibr<strong>at</strong>ion diagnostics <strong>of</strong> fans. The sources <strong>of</strong> vibr<strong>at</strong>ion for fans canbe a rotor unbalance (it is indic<strong>at</strong>ed by a high vibr<strong>at</strong>ion <strong>at</strong> 1RPM(rot<strong>at</strong>ion per minute) in <strong>the</strong> frequency spectrum), misalignment(angularity or <strong>of</strong>f<strong>set</strong>) on <strong>the</strong> interconnection between <strong>the</strong> driverand <strong>the</strong> fan (with a high axial or radial vibr<strong>at</strong>ion in spectrum onboth 1RPM and 2RPM and 180° out-<strong>of</strong>-phase across <strong>the</strong> coupling),failure <strong>of</strong> bearings (detectable by significant peaks in spectrums<strong>at</strong> <strong>the</strong> bearing defect frequencies), or hydrodynamic processescaused by an unequal gap between <strong>the</strong> impeller and <strong>the</strong> st<strong>at</strong>or <strong>of</strong><strong>the</strong> fan (identifiable on <strong>the</strong> blade frequency <strong>of</strong> <strong>the</strong> machine) [6].An electro motor is a source <strong>of</strong> vibr<strong>at</strong>ion, too – st<strong>at</strong>or eccentricity,short-circuited insul<strong>at</strong>ion and loose iron (st<strong>at</strong>or problemsbecome evident from high vibr<strong>at</strong>ion <strong>at</strong> 2 line frequency in spectrum),eccentricity <strong>of</strong> rotor (with side-bands with interval <strong>of</strong> polefrequency), broken or cracked rotor bars or shorting rings (presence<strong>of</strong> rotor bar frequency), phase problems – loosening <strong>of</strong> connectors(side-bands with interval <strong>of</strong> 1/3 <strong>of</strong> <strong>the</strong> network frequency),etc. The properties <strong>of</strong> <strong>the</strong> framework are important, too, for exampleits mechanical looseness (multiples <strong>of</strong> <strong>the</strong> speed frequency inspectrum) or excit<strong>at</strong>ion <strong>of</strong> its n<strong>at</strong>ural frequency.In <strong>the</strong> case <strong>of</strong> fans with a gearbox to <strong>the</strong> source <strong>of</strong> vibr<strong>at</strong>ionsbelong also meshing spur gears with considerable amplitudes ongear frequencies, <strong>the</strong>ir multiples and with <strong>the</strong>ir side-bands in <strong>the</strong>spectrums.In Fig. 8 is depicted <strong>the</strong> frequency spectrum <strong>of</strong> oscill<strong>at</strong>ingspeed with one dominant amplitude on <strong>the</strong> speed frequency <strong>of</strong> <strong>the</strong>fan which is typical for sign<strong>at</strong>ure <strong>of</strong> <strong>the</strong> rotor unbalance <strong>of</strong> <strong>the</strong> fan.The Faculty <strong>of</strong> Mechanical Engineering <strong>of</strong> <strong>the</strong> University <strong>of</strong>Zilina deals also with <strong>the</strong>ses problems, and we are able to <strong>of</strong>fer ourpartners <strong>the</strong> solution <strong>of</strong> vibr<strong>at</strong>ion measurements on fans. It doesnot have to be only <strong>the</strong> safety system switching <strong>of</strong>f <strong>the</strong> fan afterreaching a certain level <strong>of</strong> vibr<strong>at</strong>ions, but also <strong>the</strong> on-line, respectivelyon-line monitoring and analysis <strong>of</strong> <strong>the</strong> measured d<strong>at</strong>a from<strong>the</strong> point <strong>of</strong> view <strong>of</strong> predictive and pro – active maintenance.Fig. 8 Spectrum <strong>of</strong> oscill<strong>at</strong>ing speed with one dominant peak <strong>at</strong> 38 Hz,(speed frequency), demonstr<strong>at</strong>ion <strong>of</strong> <strong>the</strong> rotor unbalance.COMMUNICATIONS 3/2008 ●43
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- Page 46 and 47: REVIEWmeters from the point of view
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- Page 55 and 56: REVIEWRadomir Brkic - Zivoslav Adam
- Page 57 and 58: REVIEWfailures. For example, for an
- Page 59 and 60: REVIEWJan Krmela *COMPUTATIONAL MOD
- Page 61 and 62: REVIEWFor this reason tyres and whe
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- Page 67 and 68: REVIEWIvana Olivkova *PASSENGER INF
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- Page 75 and 76: BOOK REVIEWKavicka, A., Klima, V.,
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