Gear Design and Application

The special purpose gear unit should have a specified API 613 service factor of 1.4 as a minimum. The service factor used in the gear design should he selected to meet the requirements of API for the application.

The gear unit must be tuned to the system in which it operates. The vendor with the train responsibility must analyze the system to be certain the gear unit will not transmit torsional or lateral vibrations to the rest of the system or be damaged by system excitations.

If the gear unit is retrofit, for example when replacing a turbine with a motor and gear unit, the system, in like manner, must be thoroughly analyzed. In either case, the gear supplier must furnish the user with adequate data on the mass elastics of the unit, inertias, bearing characteristics, and the like, to permit the user's own or a consultant's analysis.

Resonant responses must not coincide with excitation frequencies of rotational shaft speed, especially gear meshing frequency (the speed of a shaft times the number of teeth of the gear on that shaft), or other identi fied system frequencies; otherwise, a self-excited system will exist. Lateral response criteria should conform to API 613.

An important consideration of a gear unit is the pitch line velocity (PLY), which is the product of the gear or pinion pitch diameter and the

A general classification of pitch line velocity and corresponding American Gear Manufacturers Association (AGMA) quality values are:

• 20,000 fpm or less—moderate service—AGMA 12

• 20,000 to 30,000 fpm—medium service—AGMA 13

• 30,000 fpm or higher—severe service—AGMA 14

To promote even wear, a hunting tooth design is desirable. In this design, a pinion tooth does not contact a given gear tooth more than once until it has meshed with all the other gear teeth.

For maximum reliability, a gear-unit ratio must be within the capability of a single-step application. Practically, ratios may approach 6:1, but

A double-helical design should be considered, particularly for critical service. Rotation of the helices should be such that they will be apex leading, that is, teeth engage at the centers with mesh progressing to the tooth outer ends. Additionally, rotation should permit the force from the gear mesh to cause the gear to load the lower half of its bearings. The double-helical gear generates no thrust load of its own and can be operated with no thrust bearing if flexible disc or limited end-float type couplings are used on each shaft. The advantage of this approach is the elimination of the horsepower loss associated with a thrust bearing.

The low speed gear shaft and the housing must be designed to permit installation of a stub shaft for a torsiograph unit if an operational problem occurs. API 613 gives the details of the shaft end requirements for attaching a torsiograph. This should be done on all synchronous motor compressors and on multiple driver or multiple compressor case trains.

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