Page 269 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Power Transmission Gears 251

should be determined and the clearance split on either side of that run-
ning position. On single helical gears, the gear and the pinion both are
equipped with thrust bearings; consequently, this procedure applies to
each.
For double helical gearing with a thrust bearing on the low speed shaft,
care must be taken when axially locating the high speed pinion. This is
accomplished by moving the pinion as far as possible in both directions
axially and measuring this movement. The pinion is then centered, and
this is the resulting alignment position. For units running at elevated tem-
peratures, the axial growth of the pinion must also be taken into account.

Thermal and Mechanical Movement

Thermal movements of gear shafts are caused by both environmental
and operational temperature changes, whereas mechanical movements
are caused only by internal gear loads which force the shafts to move
within the bearing clearances. Figure 6-25 shows the positions of the
journal centers for a single stage gear after the gear unit reaches operat-
ing temperature and load. To properly determine the operating positions,
it is necessary to calculate the operating loads and the gear weights, com-
pute the movements due to these mechanical forces, and then add these
movements to those due to the expansion of the gear housing caused by
the elevated operating temperature. When the alignment procedure is
started, these mechanical and thermal movement values should be calcu-
lated and recorded on a worksheet and kept for future reference.
Thermal movement can be calculated by multiplying the coefficient of
thermal expansion (.0000065 in. per in. per "F for ferrous materials) by
the distance involved (either vertical height or horizontal offset) and by
the anticipated temperature rise above ambient. Normally, the tempera-
ture rise will range from 30 to 70°F when an ambient temperature of 60
to 70°F is present. Before making an assumption on the direction of ther-
mal expansion, the location of the dowel pins which hold the gear mount-
ing feet on the base or soleplate must be established. All movement is
calculated assuming a properly fitted dowel pin and is away from the pin.
Mechanical movement must be estimated from bearing clearances and
directions of rotation. A rough rule of thumb for estimating mechanical
movement is to assume zt minimum running clearance of .003 in. or .001
in. per in. of shaft diameter, whichever is greater, at the bearing. This
should only be used for journal, straight roller, spherical roller, and dou-
ble row tapered roller bearings. For single row tapered roller bearings, a
clearance of .003 in. can be assumed for all shaft sizes. If possible, the
mechanical movements should be obtained from the manufacturer of the
gear unit.