Page 196 - Root Cause Failure Analysis

184 Root Cause Failure Analysis

Brake Horsepower
All gear sets have a recommended and maximum horsepower rating. The rating varies
with the type of gear set but must be carefully considered when evaluating a gearbox
problem. The maximum installed motor horsepower should never exceed the maxi-
mum recommended horsepower of the gearbox. This is especially true of worm gear
sets. The soft material used for these gears is damaged easily when excess torsional
load is applied.

The procurement specifications or the vendor’s engineering catalog will provide all
the recommended horsepower ratings needed for an analysis. These recommendations
assume normal operation and must be adjusted for the actual operating conditions in a
specific application.

Speed Transients
Applications that require frequent speed changes can have a severe, negative impact
on gearbox reliability. The change in torsional load caused by acceleration and decel-
eration of a gearbox may exceed its maximum allowable horsepower rating. This
problem can be minimized by decreasing the ramp speed and amount of braking
applied to the gear set. The vendor’s O&M manual or technical specifications should
provide detailed recommendations that define the limits to use in speed-change appli-
cations.

Startup
Start-stop operation of a gearbox can accelerate both gear and bearing wear and may
cause reliability problems. In applications like the bottom discharge of storage silos,
where a gear set drives a chain or screw conveyor system and startup torque is exces-
sive, care must be taken to prevent overloading the gear set.

Backlash

Gear backlash is the play between teeth measured at the pitch circle. It is the distance
between the involutes of the mating gear teeth as illustrated in Figure 14-17.

Backlash is necessary to provide the running clearance needed to prevent binding of
the mating gears, which can result in heat generation, noise, abnormal wear, overload,
andlor failure of the drive. In addition to the need to prevent binding, some backlash
occurs in gear systems because of the dimensional tolerances needed for cost-effec-
tive manufacturing.

During the gear-manufacturing process, backlash is achieved by cutting each gear
tooth thinner by an amount equal to one half the backlash dimension required for the
application. When two gears made in this manner are run together (i.e., mate), their
allowances combine to provide the full amount of backlash.