Page 273 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Power Transmission Gears 255

Helix Angle (Lead) Modification

Many wide face width single and double helical gears have helix angle
modifications to correct for torsional twist and bending deflections of the
teeth due to operating loads. When these modifications are made, the he-
lix angles on two mating gears are purposely cut differently so that they
will be the same when the gear teeth deflect as the design load is applied.
In some cases, only the ends of the teeth are relieved (end ease-off), or a
gentle curve is cut into the tooth flank (crowning).
In cases where the helix angles have been altered, the face contact will
not approach the 70 to 80 percent usually recommended on nonmodified
leads. Therefore, when modified leads are furnished, it is necessary to
determine from the manufacturer what blue transfer or wipe off is ex-
pected under different load conditions. Figure 6-28 illustrates possible con-
tact patterns for double helical gears with and without modified leads under
no load conditions. Several possible contact patterns for single helical gears
under no load conditions are depicted in Figure 6-29.

Spiral Bevel Tooth Contact
The tooth contact checking procedure for spiral bevel gears is the same
as that described for helical gears except the expected contact pattern is
different. Spiral bevel gears are generally manufactured in matched sets,
and the contact will vary with the mounting position. These gears are
usually mounted on tapered roller bearings with shims to make it possible
to position the gear and pinion axially and thereby to obtain proper tooth
contact. Figure 6-30 illustrates one contact pattern to be expected OR spiral
bevel gears.
Doweling

After completing the alignment procedures and before starting the
drive, install dowel pins as specified by the manufacturer. To start a unit
and allow the temperature to change without the dowel pins in place will
cause misalignment; alignment cannot be maintained without pinning.
Each drive train should be evaluated, and the best dowel pin locations
should be determined. Most manufacturers furnish starter holes for the
dowel pins, and they have selected what they feel is the best location for
the zero movement point on the gear unit. A good rule to follow is that on
high speed units, the dowel pins should be located as near as possible to
the high speed pinion since this shaft has the most critical coupling align-
ment. Many large, heavy duty, low speed high torque units will require
the dowel pins be placed under the low speed output shaft since this shaft
has the most critical alignment point. On a unit which has rigid couplings