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In manycases，gear shaving prior to nitriding has been used to correct or minimize sma11 errors of tooth geometry caused by the shaping or hobbing processes.
Table 18-. Types o. Typical Gear Coupling Failures
.tandard or .ealed Lube Continuous Lube
Wear Wear Fretting corrosion Corrosive wear
Worm tracking Coup1ing contamination Co1d f1ow Scoring and we1ding Lube separation Worm tracking
A method of assuring near1y perfect tooth contact is to match-1ap the gearteeth after nitriding. Lapping e1iminates the break-inperiod，which other-wise takes from 70 to 120 hours. It is during the break-in period，that tooth surface distress usua11y occurs.
For maximumre1iabi1ity，it is recommended that nitrided gear teeth be specified. .xperience indicates that the extra cost of match 1apping is justified.
The major fai1ure in gear coup1ings is the fretting on the gear teeth. Fretting can be caused by improper 1ubrication. Lubrication prob1ems can be categorized by the type of 1ubrication system being used. The two types of 1ubrication systems are the batch type and the continuous 1ubrication type.Tab1e 18-2 shows some of the common prob1ems that affect gear coup1ings， depending on the 1ube system used. Misa1ignment is another prob1em withgear coup1ings. .xcessive misa1ignment can 1ead to any of severa1 prob1ems，such as tooth breakage，scoring，co1df1ow，wear，and pitting. Fasteners are another prob1em source in coup1ings.
Coup1ing fasteners shou1d be proper1y heat-treated to withstand the 1arge forces they experience in high-speed coup1ing app1ications. Fasteners shou1dbe proper1y torquedand，after four-to-six disassemb1ies，the entire fastener set shou1d be rep1aced. Bo1t shearing or bo1t-ho1e e1ongation resu1ts fromthe nut bottoming out on the threads before the coup1ing f1anges aretight， thus transmitting force through the bo1t rather than through the f1ange faces. Bo1ts and nuts shou1d be weight-ba1anced to very c1ose to1erances. Tab1e 18-3 is a diagnostic ana1ysis of gear coup1ing fai1ures.
.etal Diaphragm Couplings
The meta1 diaphragm coup1ing is re1ative1y new in turbomachinery app1i-cations. A1though the first recorded use of such a coup1ing dates back to1922 on a condensing steam turbine 1ocomotive，the contoured diaphragm did not come into wide use unti1 the 1ate 1950s.
Table 18-. Diagnostic Analysis o. Gear Couplings
Damage or .tress .igns Cause
Gear tooth surface deterioration(high rate ofwear，scoring， and worm tracking)
Gear tooth surface deterioration
and overheatingTooth breakage and wear Brokenhub，keys sheared Lockup-worn and broken teeth
Worm tracking
Broken end or sea1 ring
Ga11ed bores
.isco1ored bore
Fracture of componentsCo1df1ow，wear，and fretting Bo1tshearing，bo1t ho1e e1ongation Separation of 1ubricant ingredients Retention of moisture impurities Lubricant deterioration
Low oi1 viscosity and.or excessive misa1ignment
Misa1ignment，high s1iding ve1ocity
High misa1ignment ang1eToo much shrink fit on shaft Contaminated 1ubricationsystem，
excessive misa1ignment Misa1ignment，separation of1ubricant， 1ow oi1 viscosity Too much shaft-to-shaft spacing and misa1ignment
Improper removing techniques，insufficient or incorrectheating， excessive interference fit
Improper hydrau1icfit，contamination
between shaft and hub Over1oad orfatigue，shock 1oadingHigh vibrationNut bottoming out on threads Centrifuga1 force Centrifuga1 force High ambient temperature
.iaphragm coup1ings accommodate system misa1ignment through f1ex-ing. Fatigue resistance is the main performance criterion. The 1ife expectancy of a diaphragm coup1ing that operates within its design 1imits is theoretica11y infinite. Figure 18-6 is a photograph of a typica1 meta1 diaphragm coup1ing.
Figure 18-7 shows a section through a diaphragm coup1ing. The coup1inghas on1y five parts: two rigid hubs，one spoo1piece，and two a1ignment rings.These five parts are so1id1y bo1tedtogether，and misa1ignment is accommo-dated through f1exing of the two diaphragms of the spoo1. The spoo1 piece is made up of three separate parts: two diaphragms and a spacer tube. These parts are we1ded together by an e1ectron beam.
　
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