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Factors Affecting the Training of a Belt
Pulleys and Snubs
Carrying Idlers
Relatively little steering effect is obtained
from the crown of conveyor pulleys.
Crowning is most effective when there
is a long unsupported span of belting
(approximately four times belt width)
approaching the pulley. Since this is
not possible on the conveyor carrying
side, head pulley crowning is relatively
ineffective and is not worth the lateral
mal-distribution of tension it produces in
the belt.
Tail pulleys may have such an
unsupported span of belt approaching
them and may be crowned with benefit,
except when they are at points of high
belt tension. The greatest advantage is
that the crown, to some degree, assists
in centering the belt as it passes beneath
the loading point; this is necessary for
good loading.
Take-up pulleys are sometimes crowned
to take care of any slight misalignment
that occurs in the take-up carriage as
it shifts position. In general, crowned
pulleys should be used sparingly, if at all,
on fabric belts. With steel cord belts, all
pulleys must be flat.
All pulleys should be level and should
have their axis at 90 degrees to the
intended path of the belt. They should
be kept that way and not shifted as
a means of training, except that snub
pulleys can have their axis shifted when
other means of training have provided
insufficient correction. Pulleys with their
axis at other than 90 degrees to the belt
path will lead the belt in the direction of
the edge of the belt that first contacts the
misaligned pulley. When pulleys are not
level, the belt tends to run to the low side.
This is contrary to the old rule-of-thumb
statement that a belt runs to the high
side of the pulley. When combinations
of these two occur, the one having the
stronger influence will become evident in
the belt performance.
The belt can be trained with the
troughing idlers in two ways. Shifting
the idler axis with respect to the path of
the belt, commonly known as knocking
idlers, is effective where the entire belt
runs to one side along some portion of
the conveyor.
The belt can be centered
by knocking ahead (in the direction
of belt travel) the end of the idler to
which the belt runs (Fig. 7).
Shifting
idlers in this way should be spread over
some length of the conveyor preceding
the region of the trouble. It will be
recognized that a belt might be made to
run straight with half the idlers knocked
one way and half the other, but this
would be at the expense of increased
rolling friction between belt and idlers.
For this reason, all idlers initially should
be squared with the path of the belt
and only the minimum shifting of idlers
used as a training means. If the belt is
overcorrected by shifting idlers, it should
be restored by moving back the same
idlers, not by shifting additional idlers in
the other direction.
Such idler shifting is effective for only
one direction of belt travel. If the belt
is reversed, a shifted idler, corrective in
one direction, is misdirective in the other.
Hence, reversing belts should have all
idlers squared up and left that way. Any
correction required can be provided with
self-aligning idlers designed for reversing
operation. Not all self-aligners are of this
type, for some work in one direction only.
8
Installation, Maintenance & Troubleshooting Guide
Installation