Page 227 - Centrifugal Pumps Design and Application

High Speed Pumps 201

Figure 11-13. Multi-stage high and low speed rotor comparison (courtesy
Worthington Division, McGraw Edison Company).

precision class 10 to 12 are commonly used in moderate- to high-pitch
line-velocity gearing. The American Petroleum Institute (API) also
publishes gearing specifications that are derived from AGMA, but de-
mand more design conservatism. AGMA ratings compare to API ratings
roughly in a ratio of 5:3.
Gear design considers ratings from two standpoints: strength and en-
durance. Strength rating is based upon evaluation of the gear tooth as a
cantilever beam, and dominates in lower-speed, high-torque situations.
Control of the case depth is important in hardened gear design to avoid
through-hardening, or brittle teeth. Endurance rating evaluates gear de-
sign from the standpoint of wear resistance and becomes increasingly
dominant with increasing pitch line-velocities. The lesser of the strength
and endurance ratings at a given operating condition establishes the gear-
ing rating. Best balance between strength and endurance results from
coarse tooth selections for the lower operating speeds to fine-tooth selec-
tion in the high-speed ranges.
A tendency has existed to select spur gears for moderate power trans-
mission and helicals for the higher power ranges. Spur gear geometry
forces design with a contact ratio between 1 and 2, that is to say that the
load is alternately carried by a single tooth or shared by a pair of teeth.
Rating, then, is based upon single-tooth contact. Helical gears provide
smooth meshing and continuous multi-tooth contact, and so in theory
provide substantial increased capacity within a given envelope. This heli-
cal advantage, however, is highly dependent on gear precision, and is
usually assumed to provide added design margin rather than increased
capacity rating.