ALLOWANCE FOR BEARING CLEARANCE
Where a AGNEE Worm Speed Reducer with shafts mounted on ball or roller bearings, is coupled to a prime mover or a driven
machine with plain bearings, an allowance for "lift" should be made
to accommodate the probable change in centre height when running, due to thickness
of the oil film and upward reaction (if any) under load.
No hard and fast rule can be given as to the exact amount of correction to allow
and this, dependent upon so many factors peculiar to individual cases, is
largely a matter of experience.
EFFECT OF TEMPERATURE
Any change in temperature will alter the height of a shaft above its foundations.
If there is much difference between the temperature of a gear casing and that
of the driven machine, or prime mover, the resulting difference in level may be
appreciable. The coefficient of linear expansion of cast iron is 0.000011mm per
mm per 0C; hence with a centre height of 500mm. above the bedplate
and difference in temperature of 300C between the gear casing and
the machine, the error of alignment, if correct when cold, will be 0.165mm.
Such conditions may, particularly in the case of large units and those which are
close-coupled, impose very severe loads on the bearings. If this is so, it is
recommended that an allowance be made so that the alignment will be more nearly
correct under normal working conditions.
Lining Up
Errors of alignment fall into the categories of angularity (Fig. 1) and eccentricity
(Fig. 2), or combinations of both. Angularity error should be corrected first
by using feeler gauges (Fig. 3) and arriving at a constant gap measured every
900 of rotation, rotating both halves simultaneously.
Eccentricity error can be corrected by using a straight edge (Fig. 4) if both couplings have
the same outside diameter. If not a straight edge should be used in conjunction
with a feeler gauge equal to half the difference in diameters. Here also
checking should be done every 900 while rotating both halves
simultaneously.
