Figure 18-6. Metal diaphragm coupling (one end shown). (Courtesy ofKoppers Companyy Inc.)
Figure 18-7. Schematic ofa typical diaphragm coupling. (Courtesy ofKoppersCompanyy Inc.)
are caused by the torqueeffects，centrifuga1forces，and axia1 def1ection. Standard methods for ca1cu1ating centrifuga1 forces in a rotating disc show that both tangentia1 and radia1 stresses increase rapid1y with a decrease in the radius.
The stresses imposed by axia1 def1ection are much greater at the hub thanat the rim as seen in Figure 18-9. Therefore，to maintain uniform stresses in the diaphragm when a11 the various forces acting on the diaphragm are attheir maximum，the diaphragm must be used to connect the contoured profi1e at both the hub and the rim to reduce stresses.
.iaphragm coup1ings are more susceptib1e to axia1 movement prob1emsthan gear coup1ings，since the diaphragm has a maximum def1ection that cannot be exceeded.
Theoretica11y，a diaphragm coup1ing wi11 have no prob1ems or fai1ures as 1ong as it is operated within ""design 1imits.'' The diaphragm fai1s from excessive torque. Two distinct modes of fai1ure can be found.one at zero axia1 dis-p1acement and the other at 1arge axia1 disp1acement. .ero axia1 disp1acement is characterized by a circu1ar crack1ine that goes through the thinnest portionof the diaphragm. The crack is re1ative1y smooth，and there is no buck1ing ofthe disc. The 1arge axia1 movement and angu1ar misa1ignment，which 1ead todisc fai1ure，is characterized by a crack1ine that fo11ows a random path from thethinnest to the thickest portion of the disc. The crack1ine is very irregu1ar，and there is severe buck1ing of the unfai1ed part of the disc. Fai1ure in this mode shows that the crack1ine propogates some 270o before disc buck1ing takesp1ace，indicating that the torque 1oad makes on1y a sma11 contribution to the tota1 stresses in the disc. Meta1 diaphragm coup1ings can a1so have prob1emsdue to corrosive action on the diaphragms. Thus，care must be taken to app1y coating to protect against damage from a harsh environment.
.etal Disc Couplings
The main difference between the meta1 diaphragm coup1ing and the typica1 meta1-f1exing disc coup1ing is that a number of discs rep1ace the sing1e diaphragm between the hubs and the spacer. Figure 18-10 shows a schematic of this type of coup1ing. A typica1 meta1-f1exing disc coup1ing consists of two hubs rigid1y attached by interference fit or f1ange bo1ting to the driving and driven shaft of the connected equipment. Laminated disc sets are attached to each hub to compensate for the misa1ignment. A spacer spans the gap between the shafts and is attached to the f1exing e1ements at each end.
The functiona1 requirements and characteristics of the f1exing e1ements are to transmit rated torque as we11 as any system over1oads without buck1ing or permanent deformation. In other words，they must possess torsiona1 rigidity.However，under conditions of para11e1，angu1ar，and axia1 misa1ignment，the f1exing e1ement must have sufficient f1exibi1ity to accommodate these condi-tions without imposing excessive forces and moments on equipment shafts and bearings. Both of the previous requirements must be met whi1e main-taining stress 1eve1s that are safe1y within the fatigue 1imit of the f1exing materia1. Meta1-f1exing coup1ings have been known to exhibit occasiona1 1arge-amp1itude vibrations in the axia1 direction when excited at the natura1 frequency of the coup1ing.
The amount of damping present in a meta1-f1exing coup1ing is thought tobe re1ative1ysma11，a1though it is known to be greater for the 1aminated disc-type construction than for a coup1ing consisting of a sing1e-piece membrane. The reason for the greater damping in the 1aminate disc configuration is thatunder conditions of axia1 movement，a microscopic amount of motion takesp1ace between adjacent 1amina，as shown in Figure 18-11. Since the e1ementis c1amped together under a bo1tpre1oad，there is a frictiona1force，which resists s1iding.
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