TRIBOLOGY AP­PLI­CA­TIONS

Lubricant World - - CONTENTS - Prof. Dr. Er­tuğrul Du­rak Sü­ley­man Demirel Uni­ver­sity Me­chan­i­cal En­gi­neer­ing De­part­ment Aca­demic Mem­ber er­tu­grul­du­[email protected]

El­e­ments that carry power trans­mis­sion el­e­ments such as gear wheel, belt and pulley or chain wheel mech­a­nisms are called shafts. Shafts that as­sume this car­ry­ing task are sup­ported with jour­nal bear­ings and ball bear­ings ( rollers). Part of shafts that is en­closed by a bear­ing is called the jour­nal. That's why these are called jour­nal bear­ings.

A jour­nal bear­ing sys­tem con­sists of two main parts: a rolling jour­nal and a fixed bear­ing block. The bear­ing may cover the jour­nal par­tially or com­pletely. The bear­ing can be called full bear­ing or par­tial bear­ing ac­cord­ingly. There is a gap be­tween bear­ing and jour­nal di­am­e­ters. This is called bear­ing clear­ance ( c). Bear­ing clear­ance is used for as­sem­bling the shaft and bear­ing, cre­ates the gap needed for the lu­bri­cant, helps de­crease ther­mal ex­pan­sion, and helps min­i­mize the ax­ial mis­align­ment and/ or de­flec­tion prob­lems.

Nom­i­nal di­men­sions of bear­ing and jour­nal are the same; but the nec­es­sary cav­i­ta­tion for oil film is cre­ated be­tween bear­ing sur­face and jour­nal sur­face by se­lect­ing ap­pro­pri­ate tol­er­ances in de­sign and pro­duc­tion. If we take bear­ing di­am­e­ter as D and jour­nal di­am­e­ter as d, the for­mula to cal­cu­late bear­ing clear­ance is c = D- d. The bear­ing clear­ance to jour­nal ra­dius ra­tio is called rel­a­tive bear­ing clear­ance. It is usu­ally shown as in bear­ing de­sign, and it is cal­cu­lated as = ( D- d)/ d. Rel­a­tive bear­ing clear­ance ( ) depends on var­i­ous fac­tors such as the load on the bear­ing, speed of the shaft, di­men­sions and tol­er­ances. It should be un­der­lined that per­for­mance of the bear­ing to be de­signed will be greatly af­fected by any change in this num­ber.

As the bear­ing clear­ance will vary be­tween the high­est and low­est limit val­ues ac­cord­ing to the di­men­sion tol­er­ances of the bear­ing and the shaft, av­er­age bear­ing clear­ance and av­er­age rel­a­tive bear­ing clear­ance val­ues are con­sid­ered in bear­ing de­sign cal­cu­la­tions. How­ever, for safety, min­i­mum and/ or max­i­mum bear­ing clear­ances should also be checked by tak­ing into con­sid­er­a­tion the sur­face rough­ness val­ues.

As a gen­eral de­sign cri­te­rion, lower value is se­lected at low shaft speeds and high bear­ing loads, while higher value is used at high shaft speeds and low bear­ing loads. When bear­ing clear­ance is high, the pres­sure in bear­ing in­creases and the pres­sure dis­tri­bu­tion curve ta­pers at the cen­ter of the bear­ing. In prac­tice, it is gen­er­ally ac­cepted that the bear­ing clear­ance be­tween shaft ( jour­nal) and bear­ing should be around 1‰ of the shaft di­am­e­ter. If we take ≥ 0.002 un­der op­er­at­ing con­di­tions with high shaft speed and low bear­ing load, fric­tion is de­creased and there is less tem­per­a­ture in­crease in bear­ing. Ac­cord­ing to the shaft speed, the = 0.8 10- ( U) for­mula is

3 1/ 4 rec­om­mended for av­er­age rel­a­tive bear­ing clear­ance. In the for­mula, speed is pe­riph­eral speed in terms of [ m/ s]. It should be un­der­lined that the bear­ing tem­per­a­ture in real op­er­at­ing con­di­tions will be much higher than in pro­duc­tion con­di­tions, and the bear­ing clear­ance may change due to the wear on jour­nal and bear­ing sur­faces as a re­sult of op­er­a­tion over time. ( to be con­tin­ued)

Ref­er­ences: Ba­balık, F.C., Çav­dar K., Mak­ina Ele­man­ları ve Kon­strük­siyon Örnek­leri, 8th Edi­tion, Dora Print­ing-Pub­lish­ing and Dis­tri­bu­tion Ltd. Co., Tevrüz, T., Makine Ele­man­ları ve Kon­strük­siyon Örnek­leri, Vol. 2., Çağlayan Book­store, Ge­mal­mayan, N., https://web­sitem.gazi.edu.tr/site/ni­hat­gem/ files, Kur­banoğlu, C., Mak­ina Ele­man­ları, Te­ori, Kon­strük­siyon ve Prob­lem­ler, No­bel Print­ing House, Du­rak, E., In­dus­trial Lu­bri­ca­tion Tech­niques Lec­ture Notes)

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