Page 272 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

254 Major Process Equipment Maintenance and Repair

pinion and gear meshes. If the gear set is single helical, position the pin-
ion and gear against their proper thrust faces. Now hold a drag on the
gear and roll the pinion blue area through mesh with the gear. Rotation
direction is not important in itself but should be adhered to, since it is
important to check the loaded tooth flank and not the unloaded tooth
flank. Now observe the blue that transferred from the pinion to the gear.
This transfer is the contact pattern. An acceptable contact pattern for hel-
ical gears without lead modification is blue transfer for approximately 70
to 80 percent of the length of the tooth on each helix.
A piece of cellophane tape can be used to remove thh blue pattern from
the gear and save it for maintenance records. After the check, place a
piece of tape on the gear tooth flank and press it firmly on the tooth.
Remove the tape, and then place the tape on a clean sheet of white paper.
Be sure to record where the check was made. The exact position should
be marked on the gear using a light punch mark on a part of the tooth
checked that will not be contacting another tooth surface.
Hard blue or layout blue, the second method of checking tooth contact,
is sprayed or brushed on both the gear and the pinion. First clean the
areas to be blued thoroughly. Just cleaning with a solvent such as naphtha
is not sufficient, since this procedure will not completely remove the lu-
bricant. Additional cleaning with a volatile solvent such as electrical con-
tact cleaner, or lacquer thinner is necessary. If the teeth are not absolutely
free of oil, the blue will not adhere properly, and large flakes will chip off
making the contact check difficult or impossible.
The layout blue is applied to a three or four-tooth-wide area at three or
four places on the gear and at two on the pinion. The unit is then started
and run at full speed. Running conditions may vary from no load to full
load. The best procedure is to run the unit at a very light load (approxi-
mately 20 percent of full load if possible) for twenty minutes or so and
then shut down and check the contact. With higher loads you should run
the unit a shorter time before checking contact. The trick is to run the
unit just long enough to wear the blue off the areas of lower contact
stress. High loads can mask poor contact and give false readings by de-
flecting the gear teeth enough to indicate better contact than that actually
present.
If the soft and hard blue checks show satisfactory contact, operate the
unit at full load until temperatures have stabilized in the system. Shut the
unit down, and re-check the blue wear-off to be sure the contact is still
acceptable.
If poor contact is indicated by the soft or hard blue checks or both,
housing distortion and coupling alignment should be checked. In addi-
tion, any corrective measures recommended by the manufacturer should
be taken before proceeding with installation.