Page 223 - Improving Machinery Reliability

194 Iwproving Machinery Reliabilitj

However, gear-tooth failures still occur. One basic but frequently overlooked
cause of gear failures is tooth overload due to axial-thrust transmission to the mesh.
The most common source of externally imposed axial thrust arises from gear cou-
plings used to connect the gear box to the driver and driven equipment. Gear-unit
thrust bearings are usually rated to absorb the maximum anticipated axial thrust.
However, due to the transient nature of coupling-transmitted axial thrust, it is rarely
included in the tooth strength and durability ratings.
This section shows how to assess the overload effects of axial-thrust transmission
on double-helical gears as an aid to failure investigation. It also outlines a number of
options to keep coupling-transmitted excessive axial thrust from contributing to a
gearing problem.

Thrust Transmitted Through Gear-Tooth Couplings. Friction between the teeth
of gear-type couplings is responsible for the transmission of axial thrust between two
coupled machines. This friction resists the normal relative movement between coil-
pled shafts that occurs due to thermal expansion or hydraulic forces and results in
transmission of axial forces from one rotor to another through the coupling. The
maximum axial thrust transmitted is F = 2 Tp/D, cos 8, where T is the torque at the
coupling (in pound-inches), p is the coefficient of friction between the coupling
teeth, D, is the coupling tooth pitch diameter, and 0 the coupling tooth pressure
angle. The coefficient of friction, p, is assumed to be 0.15 (API 613), although some
petrochemical industries report values as high as 0.30. Higher values are simply a
reflection of conditions frequently encountered in petrochemical plants with less
than optimum coupling design and/or lubrication.
The coupling friction forces are generally transient in nature. However, relatively
high friction factors have actually been observed to persist for long periods of time
on poorly lubricated gear couplings. The actual value depends on such variablcs as
tooth interference due to thermal and sometimes centrifugal growth, viscosity and
adhesion of lubricant, or lubricant film interruption because of either severe coupling
misalignment or long-term operation with virtually perfect alignment.

Location of Thrust Bearing Must Be Considered. The location of the gear-box
thrust bearing will determine whether thrust transmission will occur for a given
machinery train arrangement. For the train configuration shown in Figure 3-77, the
thrust bearing is generally part of the high-speed pinion shaft assembly. Using a cou-
pling friction coefficient of 0.3, the pinion thrust bearing could be exposed to a max-
imum axial thrust of

(where THSs = torque acting on high-speed shaft, and DpHs = tooth pitch diameter of
high speed coupling) plus MA, the magnetic centering force of the motor.
However, the maximum possible axial force imposed on the gear mesh on Figure
3-77 is not related to FI, but rather, as will be seen later, to the tangential driving
load FT and the magnetic centering force MA of the motor.