Limited End Fioat Couplings

The flexible element coupling is by the nature of its design a limited end-float coupling. In gear applications where flexible element couplings are used on both the low and high speed sides, the gear, if double-helical, may be operated without need of a thrust bearing. The gear elements are allowed to float between the compressor thrust bearing and the motor magnetic center.

For gear couplings, to guard against transmitting thrust continuously from the motor to driven equipment bearings, use gear-type flexible couplings with the limited end-float provision. The following requirements are specified for all large motors:

1. The coupling is required to have a no more than lA-inch end-float ( if a gear type).

2. The motor end-float must exceed the coupling end-float by lA inch, making the minimum permissible motor end-float A inch.

3. The motor magnetic center must be within 3/« inch of the motor's geometric center.

4. The motor centering force must be sufficient to return the rotor to magnetic center against friction of a new, properly lubricated gear coupling.

5. The momentary end thrust transmitted by the motor to the coupling and thus to the driven equipment must not exceed 100 pounds per thousand horsepower. (This provision covers thrust transmitted when breaking away the engaged coupling gear teeth from their rest position or when the coupling separators touch when starting.)

The gear coupling must be modified to provide a limited end-float feature. This feature is desirable when sleeve bearing machines without thrust carrying capability are used. On shutdown, the units can float to their limit stops, for example, on a motor. On shutdown, the motor will float to a bearing face. On restart, the friction in the teeth keeps the rotor on a thrust face, which is not capable of carrying load and causes a bearing failure. To prevent this, a feature is added to a gear coupling limiting this end-float. Happily, the flexible disc coupling is an inherently limited end-float design. On a motor gear compressor arrangement, the parasitic power used by the gear thrust bearings can be eliminated by omitting the thrust bearing entirely and using both a high-speed and low-speed limited end-float coupling.

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