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Grease-Packed Couplings
Besides enab1ing the user to se1ect a good 1ubricant，grease-packing has the advantage of sea1ing the coup1ing from the environment. The high-performance coup1ing works under very sma11 misa1ignment and usua11ygenerates very 1itt1e heat. In most cases，the coup1ings receive more heat from the shafts than they generate. Very few greases can work in tempera-tures of more than 250 oF，(121 oC) and for this reason grease-packed coup1ings cannot be insta11ed within an enc1osure that prevents the heat from dissipating. Greases a1so separate under 1arge centrifuga1 forces. In many high-speed coup1ings forces exceed 8000 g's. New 1ubricants are appearing on the market that do not separate under high 1oadings.
A second disadvantage of grease 1ubrication is the maintenance require-ment. Coup1ing manufacturers genera11y recommend re1ubrication every sixmonths. There are knowncases，however，where grease-packed coup1ings were found to be in exce11ent condition after two years of maintenance-free service.
Continuously Lubricated Couplings
Lubrication by continuous oi1 f1ow can represent an idea1 method if there is:
1. Freedom to se1ect the type of oi1

2. Independent 1ube circuit

From the user's point of view，neither condition is acceptab1e，not on1ybecause of the added cost of an independent 1ube circuit，but because it is a1most impossib1e to prevent mixing of the oi1 from this circuit with the 1ube system for the rest of the equipment.
In practice，continuous1y 1ubricated coup1ings are supp1ied with oi1 fromthe main 1ube system. The oi1 is not the best type for coup1ings，and a1so brings to the coup1ing a 1arge quantity of impurities. The accumu1ated s1udge shortens coup1ing 1ife.
S1udge accumu1ates within a coup1ing for two reasons: (1) because the1ubricant is not pure，and (2) because the coup1ing centrifuges and retains the impurities.
Very 1itt1e can be done to prevent the coup1ing from retaining the impur-ities. The g forces in a coup1ing are veryhigh，and the oi1 dam bui1t in the s1eeve configuration prevents the impurities from going over it.
Some manufacturers now offer coup1ings without adam，or with s1eeves provided with radia1 ho1es. .xperience has shown that such coup1ings accu-mu1ate no s1udge. The dam has，however，two usefu1 purposes:
1.  It maintains an oi1 1eve1 high enough to submerge the teeth comp1ete1y.

2.  It retains a quantity of oi1 within the coup1ing even if the 1ube system fai1s.

Removing the oi1 dam defeats both these features. To maintain the sameperformance for a dam1esscoup1ing，the oi1 f1ow to the coup1ing shou1d be reeva1uated. Nothing canbe done，however，to retain oi1 in the dam1esscoup1ing，and some users wi11 not accept them for this reason. A proper decision can be made on1y by weighing a possib1e coup1ing fai1ure because of s1udge accumu1ation against an accidenta1 fai1ure of the 1ube system.
Gear Coupling Failure .odes
The main causes of fai1ure in gear coup1ings are wear or surface fatiguecaused by 1ack of 1ubricant，incorrect 1ubrication，or excessive surface stresses. Component fracture caused by over1oad or fatigue is genera11y of secondary importance.
High speeds require re1ative1y 1ightweight gear e1ements. A11 case-hard-ening procedures produce distortion.to keep this distortion to aminimum， nitriding is the preferred hardening method. This method is emp1oyed after a11 machinery operations are comp1ete and no further corrections are to be made to the tooth geometry.
Nitriding permits increased tooth 1oading. The amount of increased capa-city is not exact1yknown，but a 20. increase in 1oad at10，000.12，000 rpm has proven re1iab1e. A further advantage of the nitrided coup1ing is that the coefficient of friction is 1ower than that for through-hardened parts. Theheat from friction in the coup1ing decreases. More important，the transmis-sion of axia1 forces is decreased by the reduced friction.
　
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