Page 271 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Power Transmission Gears 253

Tooth Contact Check

In gearing, a gear tooth must have an even load across the entire face
width if the stress on that tooth is to be minimized. The type of contact
between gear teeth is instantaneous line contact; therefore, the alignment
between the rotating elements (pinion and gear) is critical. The alignment
is controlled by the accuracy of the rotating elements, the housing, and
the bearings. misting the housing either during shipment or because of
poor foundation conditions will cause poor tooth contact; incorrectly in-
stalled rotating elements or bearings will cause poor contact, and of
course poorly manufactured parts will also cause poor contact. There-
fore, tooth contact should be checked on all new installations, after any
disassembly of the gear unit, and after any housing-to-foundation
change.

How to Check Tooth Contact

Gear tooth contact can be checked two ways. Soft machinist’s blue or
transfer blue can be applied to the teeth of one gear and that gear rolled
by hand through mesh with its mating gear. The transfer of the blue from
one gear to the other is read as the contact. Another method is to paint the
gear teeth with hard blue or layout blue and run the gear unit for a short
while. Then stop the unit, and observe the pattern of “wear-off” of the
bluing. The term “bluing” is used for convenience. Some of the layout
dye or layout blue used is red in color. Some people claim that using this
color makes it much easier to see the contact pattern.
The soft blue method of checking gear tooth contact is usually done
first. Since the unit is not running, this check does not give true contact.
It does, however, give a good indication of what contact will be. If it
indicates poor contact you may choose not to start the unit until the con-
tact is corrected. If the unit has been disassembled, then a soft blue check
before the housing cover is installed may save you a tear-down later to
correct contact. This is especially important if a new set of rotating ele-
ments is installed.
Soft blue is usually applied to three or four teeth on the pinion in two
places 180” apart. The contact should be checked at three or four places
around the gear; however, you must reapply and resmooth the blue on the
pinion after each meshing. If time is very critical, two checks at 90”
apart on the gear will suffice.
First of all, clean the teeth thoroughly with solvent, and spread the blue
on thinly and evenly. A one in. wide good quality paint brush with the
bristles cut off to a length of about one in. makes a good blue application
brush. The blue still will not be even enough, so with a shop rag smooth
it to a very thin and even layer. If the gear set is double helical, center the