TM 5-810-15
3 ply belt with a maximum calculated operating
not use counter-weighted type cleaning devices on
tension of 5000 pounds, will have a unit tension of:
conveyors faster than 350 fpm or larger than 36-
5000 = 167 piw or 56 ppi
inch as they become ineffective very quickly. V
30
type belt cleaners will be provided on the clean side
(4) Conveyor belt sag between carrying
of the belt before belt take ups and tail pulleys.
idlers should be limited to two percent, except at
load zones limited to 1 percent.
5-9. Other conveying methods.
(5) Belt tension will not exceed 70 percent
a. Bucket elevators. These are used to elevate
of the Rubber Manufacturers Association (RMA)
coal to overhead storage or conveyors, where there
tension ratings under normal operating conditions
is little available space for a belt conveyor. Mallea-
with a vulcanized splice.
ble iron, steel, stainless steel, aluminum or plastic
(6) Conveyor belt will be fire and oil
buckets can be selected depending on the material
resistant conforming to United States Bureau of
conveyed. Care should be taken when selecting
Mines Standards.
nonmetallic type buckets for use in a combustible
(7) Consideration should be given in applica-
environment, due to their ability to retain a static
tions of limited takeup or long conveyors to pro-
electrical charge. Capacity of the bucket elevator
vide a mechanical splice when the belt is first
will be based upon buckets filled to 75 percent
installed so initial stretch can be taken out before
theoretical capacity for loading. Drive horsepower
doing the vulcanized splice. In these cases run-in
will be based upon 100 percent full buckets. There
time will be long enough to eliminate manufactu-
are two common types of elevator used for coal
rer*s stretch. Vulcanized belt splices generally last
handling: centrifugal and continuous.
close to the life of the belt. Mechanical splices last
(1) Centrifugal type. Centrifugal discharge
two to three years.
elevators are the most frequently used type for free
i. Skirtboards. Skirtboards will be provided at all
flowing, fine or small lump materials. Buckets
conveyor loading points. Coal handling system
should be type A or AA as designated by CEMA,
design, especially when handling lignite or western
spaced at intervals to chain or belt. Buckets are
coal requires examination of flow velocity
loaded by a combination of material flowing into
differences between conveyors, vertical drop at
the buckets and material that is scraped up by the
transfer points, and angular relationship of con-
digging action as the buckets pass around the tail
veyors. The width of the skirtboards will be maxi-
pulley. Speeds are relatively high and the centrifu-
mum 3/4 belt width.
gal action controls the discharge from the buckets.
(1) Skirtboard length will be at least 2 feet
Capacities range from 5 to 80 tons per hour (tph).
for every 100 fpm of belt speed plus 3 feet at tail
Elevator will be completely self supporting. Cen-
end. Minimum skirt length will be 8 feet. The skirt-
trifugal type elevators are used extensively in grain
board will terminate above an idler roll, not
service and other free flowing materials.
between.
Centrifugal elevators tend to create more dust and
(2) Skirtboard rubber strips with easy adjust-
cause breaking of friable material, which creates
able clamps will be provided on the lower edge of
problems with boiler requiring a particular size
the skirtboards to prevent the escape of fines.
distribution. Centrifugal elevators if used should be
Wearable liners inside the chute will be installed as
vented, and include a filter to relieve the "air
a dam to keep the material load off the rubber, so
pumping" phenomena at discharge.
it can effectively serve as a fines barrier. Provide
(2) Continuous type. Continuous bucket
continuous skirtboards on feeder conveyors.
elevators are recommended for high capacity heavy
j. Belt cleaners. Belt cleaner units on a troughed
duty service. The buckets are steel, continuously
conveyor belt will consist of a primary scraper on
space on single or double strand chain or on a belt.
the face of the head pulley and one or more
At the head, the discharge from each bucket is over
secondary arm and blade type multiple blade
the back of the preceding bucket which forms a
cleaner to scrape and remove the material that
chute to lower the material to the fixed discharge
bypasses the primary cleaner. Each belt cleaner will
spout. This method of discharge, plus the slow
be held in an easily serviceable mounting system
speed, minimizes breakage of fragile material.
allowing fast and easy blade replacement. The
These types of elevator are not the self digging
cleaners will be held in position against the belt by
type, so a loading leg must be used, requiring a
means of a tensioner which rotates the blades
deeper pit than that needed for a centrifugal
discharge elevator. Capacities from 15 to 300 tph
against the belt yet which allows for relief when
are available. Elevator will be completely self
mechanical splices or other obstructions pass. Do
5-7
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