Page 227 - Improving Machinery Reliability

198 Improving Machinery Reliability

Tooth loading with maximum possible thrust:

(100)(77900) = 128% of rated tangential load
6.150

This analysis demonstrated that a gear-type coupling with an anticipated maximum
coefficient of friction of 0.3 may allow the transmission of excessive axial thrust into
the gear mesh. In other words, if a gear required all of its designed-in service factor
initially, the additional thrust transmitted through gear-type couplings may further
reduce the service factor to a serious degree. While not usually causing immediate
tooth failures, the increased loading may result in gear failure after a relatively short
time of operation should the axial thrust be continuously present.
If the failure analysis concludes a gear mesh failure due to axial thrust, there are a
number of options, some of which are included here.
The coupling and coupling lubrication system can be examined to determine the
cause of excessive frictional forces.
A thrust bearing with lower load-carrying capacity can be installed purposely.
This, in effect, makes the thrust bearing the weak link, or sacrificial component.
Using adequate safeguards, such as temperature thermocouples embedded in the
thrust-bearing material and connected to a shutdown system, potential failures thus
confined to the thrust-bearing replacements are certainly less costly and less time
consuming than gear-tooth failures.
Replace couplings with large tooth pitch diameter gear couplings.
The coupling can be replaced with an axially soft diaphragm coupling which trans-
mits little or no axial thrust. Diaphragm couplings accommodate misalignment
while transmitting power by material flexure. However, the purchaser should rec-
ognize that high axial movement cannot be accommodated by diaphragm cou-
plings. This requires that the initial system axial alignment take into account the
thermal expansion of both coupled shafts.
Replacement gear elements can be purchased with a larger service factor than list-
ed in AGMA 42 1.06.

The calculations illustrated in this section can actually be performed to define the
service factor necessary to prevent gear mesh failures resulting from axial-thrust
transmission.

Determination of Effect of High-speed Coupling Lockup Transmitted
Through Mesh. See Figure 3-80 (A - B + TA = 0).