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"Tracking" or training
is defined as the procedure required to make the conveyor belt run
"true" when empty and also when fully loaded.
Tracking the belt is a process of adjusting idlers, pulleys, and loading
conditions in a manner that will correct any tendencies of the belt to run
other than true.
A normal sequence of training is to start with the return run working toward
the tail pulley and then follow with the top run in the direction of belt
travel. Start with the belt empty. After tracking is completed, run the belt
with a full load and recheck the tracking.
Tracking adjustment is done while the belt is running and should be spread
over some length of the conveyor preceding the region of trouble. The
adjustment may not be immediately apparent, so permit the belt to run for
several minutes and at least three full belt revolutions after each idler
adjustment to determine if additional "tracking" is required.
After adjustment, if the belt has overcorrected, it should be restored by
moving back the same idler, and not by shifting additional idlers or
rollers.
If the belt runs to one side at a particular point or points on the conveyor
structure, the cause will probably be due to the alignment, or leveling of
the structure, or to the idlers and pulleys immediately preceding that
particular area, or a combination of these factors.
If a section or sections of the belt run off at all points along the
conveyor, the cause is possibly in the belt itself, in the belt not being
joined squarely, or in the loading of the belt. With regard to the belt,
this will be due to camber. Its condition should improve after it is
operated under full load tension. It is a rare occasion
when a cambered belt (less than 1/2%) needs to be
replaced.
If the belt is not joined squarely, it is necessary to cut away the faulty
joint and make a new one-properly squared.
These basic rules can be used to diagnose a belt running badly. Combinations
of these rules sometimes produce cases which do not appear clear-cut as to
the cause, but if there are a sufficient number of belt revolutions, the
running pattern will become clear and the cause disclosed. In those unusual
cases where a running pattern does not emerge, it is quite likely that at
some point the belt is running so far off that it is fouling super-structure
or mounting brackets, bolts, etc. This results in highly erratic performance
and can be a real problem. We would suggest that in this event the full
tracking procedure be employed. It is quite likely that the erratic
performance will be resolved in the process.
When replacing a used belt, go through the system and square and level all
rollers, idlers, pulleys and bed before training a new belt.
BASIC RULES OF CONVEYOR BELT
TRACKING
- The basic rule which must be kept in mind
when tracking a conveyor belt is simply "THE BELT MOVES TOWARD
THAT END OF THE ROLL/IDLER IT CONTACTS FIRST".
The reader can demonstrate
this for himself very simply by laying a small dowel rod or round pencil
on a flat surface in a skewed orientation. If a book is now laid across
the dowel rod and gently pushed by one's finger in a line directly away
from the experimenter, the book will tend to shift to the left or right
depending upon which end of that dowel rod the moving book contacts first.
Figure 1.
In a straight warp or solid
woven carcass design, however, skew is of little significance.
It is a cosmetic defect and is not indicative of a cambered belt.
- The conveyor structure must be
"TRUE" (relative to the centerline) and "LEVEL"
(side to side).
- ALL pulleys, snub rollers, carrying idlers
and return idlers must be SQUARE with the frame (perpendicular to the
belt centerline) and parallel to each other. Check this by measuring
diagonals, which should be equal.
- Cleanliness is essential to good belt
tracking. Foreign matter creates a new roll on the pulley face,
adversely affecting tracking.
- The conveyor belt must be straight (less
than 1/2 percent camber) and the ends must be
squared and laced properly.
MEASURING BELT CAMBER
If unbalanced warp tensions exist
in a conveyor belt, that belt will usually assume a "crescent" or
"banana" shape when laid flat upon a horizontal surface. This
deviation from a straight line is defined as "camber."
Unroll the belt on a flat surface such as the floor, a flat horizontal
driveway, etc. Grasp one end of the belt and drag it in a perfectly straight
line for 10-20 ft., (3-6 m). If the belt is too heavy for one person to
move, clamp one end of the belt to a forklift and follow the same procedure.
After pulling, the belt should be flat. (If the belt assumes a
"crescent" or "banana" shape, unequal and unresolved
warp tensions exist in the belt).
Note: It is extremely important that the procedure described above be
followed closely. It is difficult to have both edges of the belt at the same
thickness – particularly wide belts. According, if the belt is unrolled on
a flat surface, that belt will always unroll in a banana shape – due to
geometry, not unbalanced warp tensions. "Dragging" one edge of the
belt for 10-20 ft., (3-6 m) eliminates this geometrical consideration and
shows whether the belt is cambered.
Camber is measured by drawing a taut line along one edge of the belt (inward
camber edge) and measuring the maximum deviation from the center point of
the line to the belt. The percentage of camber is computed using the
following formula:
% camber = Maximum Deviation (inches) x 100
Length of Taut Line (inches)
It is recommended that if the percent camber exceeds ½% on a polyester warp
belt construction, or 1% on a nylon warp belt construction, the belt
manufacturer should be contacted.
BELT TENSION
Belt tension must be great
enough:
A) To prevent slippage between the drive pulley and the belt, and
B) To force the belt to conform to the crown on the crowned pulleys.
Slippage will cause excessive wear to both drive pulley lagging and the
belt. Further, an excessive heat buildup on the drive pulley lagging can
result in rubber reversion. (Reversion is the softening of vulcanized rubber
when it is heated too long or exposed to elevated temperatures. It is a
deterioration in physical properties, and frequently results in tackiness.)
Once the pulley lagging has reverted, it frequently will offset onto the
bottom side of the conveyor belt, which will then distribute the reverted
rubber throughout the slider or roller bed of the system. The resulting
tackiness between the bed and the belt will certainly drive current
consumption up; can actually result in a stalled system, and can cause
severe tracking problems.
If belt tension is not high enough to force the belt to conform to the crown
on the pulley, then poor tracking is an almost certain result. A belt, which
does not conform to the crown cannot be guided by that crown.
This suggests a convenient rule of thumb for determining "proper
tension" Simply stop the conveyor and depress the belt at the end of
the crowned pulley. There should be no play between the belt and the crowned
pulley. If there is play-increase the tension. Once proper contact is made
between the edge of the belt and the end of the crowned pulley, that crown
will be guiding the belt. Tracking will be improved, and in all likelihood
sufficient tension will be on the system to avoid slippage at the drive
pulley. Obviously, if slippage still occurs, increase the tension further.
LENGTH
Belt length is normally
determined after the take-up pulley has been brought to within one inch of
the position of minimum travel. This is done by turning the take-up screws,
lifting the automatic counter weight, or counteracting the spring loading.
The extra one inch of take-up allows for enough play to install the belt and
lace pin. Once the belt has been tensioned, initial stretch will be
sufficient to allow the take-up to assume its desired position. If your belt
is to be made endless, add sufficient belt length to make the splice.
TROUGHED IDLER SETS
Troughed idler sets behave just
like flat idlers. The belt will move toward that end of the set which, it
contacts first.
In addition, troughed idler sets exert a powerful gravitational tracking
force. By virtue of the troughed configuration, a portion of each edge of
the belt is held "aloft" at some vertical angle to the horizontal.
Gravitational force is exerted on that raised portion of the belt. If the
belt is not centered in the set, the gravitational force on the one edge
will be greater than the other, tending to "center" the belt in
the troughed
set.
This "gravitational" tracking force is so pronounced that bulk
conveyors usually depend upon it as their major tracking force ... choosing
to use uncrowned pulleys in order to minimize the unequal tensions across
the belt which are the result of pulley crowns.
PULLEY CROWN
A crowned pulley can be regarded as a
special case of our primary rule of tracking as stated above. The right half
of the belt is contacting the center of the pulley sooner than it contacts
the right edge of that pulley and therefore will tend to move toward the
center. The reverse is true of the left half of the belt. The two forces
tend to balance one another by centering the belt.
in addition to this
surface effect, however, there is a strong internal "balancing
of warp tensions occurring". Consider any warp yarn not directly on the
centerline. If the belt is forced off-center and this warp yarn is drawn
toward the mid-point of the crown, tension will be increased on that yarn.
As the belt revolves and that yarn seeks to move back to its normal
position, this tension will diminish. Yarns on both sides of the belt seek
that position which results in the least stress to them, consistent with the
physical structure across which they are stressed and consistent with their
individual position within the matrix of the belt carcass. Accordingly, the
belt will shift on the crowned pulley until these stressing forces are
balanced and minimized-centering the belt.
Experiment has shown that a crown is most effective when it has a long
unsupported span of belt approaching the pulley. The lateral position of
this span can be influenced by the location of the belt on the crown most
easily when there is a minimum of resistance being offered by a supporting
slider bed or by supporting idlers.
In most non-unit-handling conveyors this optimum condition does not exist on
the top run and consequently, crown on the head pulley is of little value in
training the belt. Further, it is a distinct detriment as far as lateral
distribution of tension in the belt is concerned. Head pulleys therefore,
should be uncrowned in normal circumstances. Tail pulleys and take-up
pulleys, which may have a fairly long approaching span without support, can
be crowned with beneficial results.
The effectiveness of the crown is increased as the length of the unsupported
span of belt approaching the pulley is increased up to a length of
approximately 10 feet. Lengthening the unsupported span beyond 10 feet does
not seem to increase the effectiveness of the crown. Diminishing the length
of the unsupported span on the other hand, does diminish the effectiveness
of the crown. The shorter the unsupported span, the less effective the crown
will be.
We recommend a standard pulley crown of 1/16" per foot of pulley face.
This results in an increase in pulley diameter at a point 12" from the
edge of the pulley of 1/8" above the edge diameter. A crown of
1/8" per foot should be considered maximum.
It is further recommended that the crown not be carried beyond a point
18" in from the edge of the pulley. If the pulley width is greater than
36" it is recommended that a trapezoidal pulley be used. In other
words, that pulley will have a flat face in its center equivalent to the
amount that the pulley width exceeds 36".
A double pulley crown can also be used with great effectiveness depending
upon the individual conveyor design.
EQUIPMENT INDUCED CAMBER
Camber can be induced into a
perfectly straight belt by the roll or rolls preceding the camber. If the
roll is cocked, the belt will react and will move toward that end of the
roll, which, it contacts first. This, of course, throws the belt off-center.
If now, subsequent structural adjustments center that roll, the belt
installation will be left with a cambered appearance. This camber may be
removed by simply aligning the roll or rolls, which, are cocked.
SAFETY BULLETIN
This notice is issued to advise you that some previously
accepted shop practices may not be keeping up with
changing Federal and State Safety and Health Standards. Your current shop
practices may not emphasize the need for
proper precautions to insure safe operation and use of machines, tools,
automatic loaders and allied equipment
and/or warn against the use of certain solvents or other cleaning substances
that are now considered unsafe or prohibited by
law. Since many shop practices may not reflect current safety
practice and procedures, particularly with regard to the safe operation of
equipment, it is important that you
review your practices to ensure compliance with Federal and State Safety and
Health Standards.
IMPORTANT
The operation of any machine or power-operated device can
be extremely hazardous unless proper safety precautions
are strictly observed. Observe the following safety precautions:
Always be sure proper
guarding is in place for all pinch, catch, shear, crush and nip points.
Always be sure that
all personnel are clear of the equipment before starting it.
Always be sure the
equipment is properly grounded.
Always turn the main
electrical panel off and lock it out in accordance with published lockout/
tagout procedures prior to making adjustments,
repairs, and maintenance.
Always wear
appropriate protective equipment such as safety glasses, safety shoes,
hearing protection and hard hats.
Always keep chemical
and flammable material away from electrical or operating equipment.
Always maintain a
safe work area that is free from slipping and tripping hazards.
Always be sure
appropriate safety devices are used when providing maintenance and repairs
to all equipment.
Never exceed the
rated capacity of a machine or tool.
Never modify
machinery in any way without prior written approval of the Besser
Engineering Department.
Never operate
equipment unless proper maintenance has been regularly performed.
Never operate any
equipment if unusual or excessive noise or vibration occurs.
Never operate any
equipment while any part of the body is in the proximity of potentially
hazardous areas.
Never use any toxic
flammable substance as a solvent cleaner.
Never allow the
operation or repair of equipment by untrained personnel.
Never climb or stand
on equipment when it is in operation.
It is important that you review
Federal and State Safety and Health Standards on a continual basis. All
shop supervisors, maintenance personnel, machine
operators, tool operators, and any other person involved
in the setup, operation, maintenance, repair or adjustment of Besser-built
equipment should read and understand this
bulletin and Federal and State Safety and Health Standards on which this
bulletin is based. |
