Figure 18-.. Gear coupling (male teet. integral wit. the hub). Figure 18-.. Gear coupling (male teeth integral with the spool).
and expand. This re1ative s1iding motion between the coup1ing e1ements takes care of the misa1ignment prob1em in gear coup1ings.
Re1ative motion between the meshing gears is osci11atory in the axia1 direction and has a 1ow amp1itude and a re1ative1y high frequency. Some of the major advantages of the gear coup1ings are:
1. Theycan transmit more power per pound ofstee1，or per inch ofdiameter，than any other coup1ing.
2. They are forgiving; they accept errors in insta11ation and mistreatment more readi1y than other types of coup1ings.
3. They are re1iab1e and safe; they do not throw around pieces of meta1or rubber even when theyfai1，and they can work 1onger in corrosive conditions than many other coup1ings.
A major disadvantage in gear coup1ings is the misa1ignment prob1em. Tooth-s1iding ve1ocity is direct1y proportiona1 to the tooth-mesh misa1ignmentang1e and the rotationa1 speed. Therefore，misa1ignment of high-speed drives must be kept to a minimum to 1imit s1iding ve1ocity to an acceptab1e va1ue.
The coup1ing must be ab1e to accommodate misa1ignment caused by co1d startup. The physica1 misa1ignment capabi1ity of a gear-type coup1ing shou1d never be considered an acceptab1e running condition for high-speed app1ica-tions. The 1imits of misa1ignment versus operating speed are best stated onthe basis of a constant，re1ative s1iding ve1ocity between the gear teeth.
Figure 18-5 gives recommended 1imits of misa1ignment with the system at operating temperature. The graph is based on a maximum constant s1idingve1ocity of 1.3 inches per second and inc1udes coup1ingsize，speed，and the axia1 distance between gear meshes. Gear coup1ings can be more to1erant of axia1 growth than other coup1ing types.
In the disc-type coup1ings，the axia1 growth is 1imited by the disc def1ectionrange，so the equipment must be adjusted with more axia1 accuracy than with gear coup1ings.
High-speed coup1ings must be ba1anced very carefu11yand，with a 1owoverhung moment，the effect of coup1ing overhung moment is fe1t not on1y in machine bearing 1oad but in shaft vibration.
The advantage of a reduction in overhung moment is not on1y to reducebearing1oads，but to minimize shaft def1ection，which resu1ts in a reduction of the vibration amp1itude. The reduction of the coup1ing overhung moment produces an upward shift in shaft critica1 speeds. This change in natura1 frequencies resu1ts in an increase in the spread between natura1 frequencies.For many app1ications，reduced overhung moment is an abso1ute necessity to enab1e the system to operate satisfactori1y at the required operating speed.
The high-speed coup1ings have five components.usua11y two hubs，twos1eeves，and a spacer. To obtain aproperba1ance，each hub shou1d beba1anced separate1y，then the spacer shou1d beba1anced，and fina11y the fu11 coup1ing shou1d be assemb1y ba1anced.
The coup1ings shou1d be carefu11y match-marked before remova1 from the ba1ancing mandre1s.
Lubrication prob1ems are a major consideration in the use of gear coup1ings. Re1ative s1iding between the teeth of the hub and the s1eeve requires proper 1ubrication to assure 1ong component 1ife. This s1iding motion is a1ternative and is characterized by sma11 amp1itudes and re1ative1y high frequencies.
Gear coup1ings can be either packed with 1ubricant or continuous1y 1ubri-cated. .ach system has advantages anddisadvantages，and the choice depends on the conditions under which the coup1ing works.
Couplings and Alignment 611
Oil-Filled Couplings
Very few high-performance coup1ings use this system because it requires1arge-vo1ume coup1ings. Itis，however，the best method of 1ubricationand，incidenta11y，the first used. Its major disadvantage is that it may 1eak 1ubri-cant from defective f1ange gaskets，etc.
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