Page 10 - ConveyorBeltMaintenanceManual2010

8
INSTALLATION, MAINTENANCE & TROUBLESHOOTING GUIDE
i n s t a l l a t i o n
FACTORS AFFECTING THE TRAINING OF A BELT
Pulleys and Snubs
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.
Carrying Idlers
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.