FUN FAIR RIDE shows its se­ri­ous side

Mel­bourne Star Ob­ser­va­tion Wheel, by Mark Ciechanow­icz, Man­ager – In­dus­trial Ser­vices, Scha­ef­fler Aus­tralia, first pre­sented at the In­ter­na­tional Ma­chine Vi­bra­tion Anal­y­sis Con­fer­ence ( IMVAC), Gold Coast, Septem­ber 2017.

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Typ­i­cal bear­ing vi­bra­tion anal­y­sis is done us­ing ac­celerom­e­ters, which are highly ac­cu­rate and ef­fec­tive mea­sur­ing tools at high speeds and when the vi­bra­tion fre­quency falls be­tween 10Hz and 10kHz.

How­ever, there are lim­i­ta­tions of us­ing this method of vi­bra­tion anal­y­sis, such as when the speed is slower (es­pe­cially un­der 50 RPM), when there are ex­treme fluc­tu­a­tions in speed, when the ro­ta­tion process has a short du­ra­tion or when os­cil­lat­ing pro­cesses are present.

For very slow speed ap­pli­ca­tions, Acous­tic Emis­sion is a far more ef­fec­tive mea­sure­ment tech­nique, and cov­ers vi­bra­tion fre­quen­cies from 50kHz right through to 1MHz. Sounds that fall in this range in­clude plas­tic de­for­ma­tion of ma­te­ri­als, crack for­ma­tion and prop­a­ga­tion, ma­te­rial frac­tures and fric­tion.

These can all help pre­dict and pre­vent se­ri­ous prob­lems and down­time aris­ing in a mul­ti­tude of slow speed in­dus­trial, man­u­fac­tur­ing, pro­cess­ing and ma­te­ri­als han­dling ap­pli­ca­tions, in­clud­ing slew­ing ring bear­ings on stacker/re­claimers, air pre­heater bear­ings in power gen­er­a­tion plants and con­verter bear­ings in steel plants.

MEL­BOURNE STAR OB­SER­VA­TION WHEEL

Scha­ef­fler’s in­stal­la­tion of its spher­i­cal roller bear­ings on the Mel­bourne Star Ob­ser­va­tion Wheel, the largest fer­ris wheel in the South­ern Hemi­sphere, pro­vided an ideal op­por­tu­nity to test the ef­fec­tive­ness of Acous­tic Emis­sions as a con­di­tion mon­i­tor­ing tech­nique for very slow speeds. The fer­ris wheel has a ro­ta­tional speed of 0.029min-1.

The bear­ings in­stalled on the ob­ser­va­tion wheel were Scha­ef­fler’s dou­ble row spher­i­cal roller bear­ings, de­signed for long life, high re­li­a­bil­ity and high safety re­quire­ments. They fea­ture a ro­tat­ing outer ring and have a qua­sistatic load con­di­tion.

CHAL­LENGES AND SO­LU­TIONS

The project was par­tic­u­larly chal­leng­ing, be­cause the main ro­tat­ing hub bear­ings are lo­cated 63m above ground with lim­ited ac­cess that can only be done out­side of open­ing hours. Ad­di­tion­ally, hub space is lim­ited and tem­per­a­tures inside fluc­tu­ate con­sid­er­ably.

Scha­ef­fler suc­cess­fully in­stall four Acous­tic Emis­sion sensors with am­pli­fiers for each bear­ing. They were in­stalled ax­i­ally within the hub, spaced 90 ˚ apart. The sen­sor am­pli­fiers out­put the raw sen­sor mea­sure­ment in dB and the cor­re­spond­ing com­puted dis­tress level for the con­nected sen­sor.

Two ro­bust eight- chan­nel Scha­ef­fler

FAG ProCheck de­vices were in­stalled for all data pro­cess­ing from the Acous­tic Emis­sion sig­nal in­puts. The FAG ProCheck de­vices are de­signed to op­er­ate ef­fi­ciently un­der harsh en­vi­ron­men­tal, ro­ta­tional and tem­per­a­ture con­di­tions.

Scha­ef­fler then utilised their On­line Mon­i­tor­ing Cen­tre (OMC) to con­duct re­mote mon­i­tor­ing of all di­ag­nos­tics from the Acous­tic Emis­sion sig­nals. An in­dus­trial telecom­mu­ni­ca­tions kit was set up to trans­mit the mea­sured data over a 3G net­work to the OMC.

Two types of mea­sure­ment were taken to en­sure an ac­cu­rate over­all pic­ture of the ma­chine’s vi­bra­tion was formed. Firstly, snap­shots were taken of am­pli­fier out­puts over a small arc of ro­ta­tion (16 sec­onds, or ap­prox­i­mately 3 ˚ ), and sec­ondly av­er­ages of am­pli­fier out­puts over one com­plete ro­ta­tion (43 min­utes or ap­prox­i­mately 1.4 rev­o­lu­tions) was recorded.

Mea­sure­ments are only taken when the wheel was in mo­tion, us­ing the mo­tor speed and cur­rent as val­i­da­tion, and there is no dis­crim­i­na­tion be­tween the di­rec­tion of ro­ta­tion. Data is au­to­mat­i­cally trans­mit­ted back to the OMC ev­ery 24 hours.

RE­SULTS

Two main mea­sure­ments are used to an­a­lyse the bear­ing’s con­di­tion, Dis­tress and dB level. The com­puted Dis­tress is trade­marked by the sen­sor man­u­fac­turer, but in gen­eral terms is a sum­ma­tion of the clicks, crunches, groans and whis­tles gen­er­ated by de­fec­tive ro­tat­ing ma­chin­ery. If Dis­tress is 0- 5 units, the bear­ing is in ex­cel­lent con­di­tion, 5-10 units means it is in sat­is­fac­tory con­di­tion and greater than 10 units means the bear­ing is in sus­pect con­di­tion and may need main­te­nance or re­plac­ing.

For dB read­ings, when the over­all trend is start­ing to in­crease, rather than re­main at a sta­ble and con­sis­tent level, that is when it is time to check the bear­ings.

Scha­ef­fler also takes grease sam­ples from the bear­ings inside the hub ev­ery four months, which are sent to the lab­o­ra­tory for de­tailed anal­y­sis. A Par­ti­cle Quan­tifi­ca­tion (PQ) in­dex is es­tab­lished to mea­sure the to­tal fer­rous (Iron) par­ti­cles present in the sam­ple.

In analysing re­sults of con­di­tion mon­i­tor­ing, it’s im­por­tant to take note of any trends.

It’s also im­por­tant to cor­re­late and con­firm the pres­ence of a prob­lem us­ing mul­ti­ple tech­niques. Us­ing more data, from var­ied sources, re­duces the chance that it was just an anom­aly, and will strongly con­firm that there’s a prob­lem.

Since im­ple­men­ta­tion in 2014, over­all lev­els have been rel­a­tively sta­ble. Some in­stances of el­e­vated am­pli­tudes of Acous­tic Emis­sion as well as spec­tro­gram wear par­ti­cles in the grease sam­ples have oc­curred over this time, but not to con­cern­ing lev­els.

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