Page 197 - Root Cause Failure Analysis

GearboxedReducers 185

BACKLASH

Figure 14-17 Backlash (Neale 1993).

The increase in backlash that results from tooth wear does not adversely affect opera-
tion with nonreversing drives or drives with a continuous load in one direction. How-
ever, for reversing drives and drives where timing is critical, excessive backlash that
results from wear usually cannot be tolerated.

Ratios
Gears are defined and specified using the gear-tooth ratio, contact ratio, and hunting
ratio. The gear-tooth ratio is the ratio of the larger to the smaller number of teeth in a
pair of gears. The contact ratio is a measure of overlapping tooth action, which is nec-
essary to assure smooth, continuous action. For example, as one pair of teeth passes
out of action, a succeeding pair of teeth already must have started action. The hunting
ratio is the ratio of the number of gear and pinion teeth. It is a means of ensuring that
every tooth in the pinion contacts every tooth in the gear before it contacts any gear
tooth a second time.

INSTALLATION
Installation guidelines provided in the vendor’s O&M manual should be followed for
proper installation of the gearbox housing and alignment to its mating machine-train
components.

Gearboxes must be installed on a rigid base that prevents flexing of its housing and
the input and output shafts. Both the input and output shaft must be properly aligned.
within 0.002 in., to their respective mating shafts. Both shafts should be free of any
induced axial forces that may be generated by the driver or driven units.

Internal alignment also is important. Internal alignment and clearance of new gear-
boxes should be within the vendor’s acceptable limits, but there is no guarantee that
this will be true. All internal clearance (e.g., backlash and center-to-center distances)