Operating Considerations for Belt
Conveyors
Guest article by Dr Harold Wright, Alan Jackson and Ray Hodgkinson Similar
problems occur within both the quarrying and the mining industry
when the correct selection of belt conveyors is being considered,
namely the environmental and physical constraints under which the
unit is expected to operate. Both of these have an impact upon the
final selection of appropriate equipment - some of the constraints
that these cause are looked at below:
Motor Power Selection
Within the basic conveyor calculation
correct selection of friction factor is crucial to ensure that the
conveyor is adequately powered - in selecting the friction factor
full regard must be taken of the anticipated operating condition of
the conveyor taking into account such factors as:
- Expected accuracy of alignment
(IE, is the conveyor a permanent installation or will it be
periodically re-sited?)
- are the idler rolls selected of
highest industry standard? - is there internal rotation friction
as low as practical? - is there eccentricity at minimum values?
- are they forward tilted?
- is the conveyor to run in a dusty
atmosphere?
- will the unit operate at low
temperature? (note that special grease may be required in very
low or high temperature conditions)
- does the material to be conveyed
have a high internal friction? (highly angular lumps can create
this as well as the material itself)
- will the conveyor always be
operating at low belt-tension? (low tension creates more belt
sag and higher pulling effort); and
- full account should be taken of
all special and secondary resistances - these include items such
as: acceleration of the load at the transfer point (this is
usually allowed for within the length correction factor for
longer center conveyors); effect of belt ploughs and scraper;
effect of belt tripper units and bunker drag out effect.
It is generally thought that these
last elements are those that caused the discrepancies noted in one
of Dr H Wright's paper "Current UK Drive Formulae for Belt
Conveyors - Fact or Friction"
Drive Format Selection
Differing conditions call for
differing drive formats - such cases can be:
- Will the selection of belt grade
reduce if a dual drive pulley arrangement is used? - will this
reduction pay for the extra complication of the drive?
- can the discharge pulley also be
the drive pulley? (this is usually the most cost effective but
consideration has to be given to the ease of support of the
transmission gear and the potential that it is operating within
the dust generated by the material discharge).
Care has to be used in the selection
of transmission equipment - will the conveyor be subject to run-back
forces?, will it need braking to prevent overrun or reduce stopping
time to acceptable limits? If the required drive power is high
(particularly in relation to the power required to turn the empty
belt) then a gradual start-up will benefit the life of the belting
and the conveyor pulleys - this can be achieved by either use of
fluid or powder coupling or by electronic control of the motor run
up time.
Tensioning Arrangements
 Arguably
the easiest and the one that in the majority of cases produces the
least stress impact into the conveyor belt is the gravity operated
system - either directly formed by having a vertically-moving
pulley, to which is attached the gravity weight or indirectly by
means of a horizontally-moving pulley with the effort of the gravity
weight being applied through a rope system. Where the conveyor
length is relatively short (say less than 200m ) and the conveyor
configuration does not lend itself to a direct gravity system, then
a tensioning device may be used at the tail pulley (usually by
adjustment to the tail pulley position by a screw or similar
method).
More complex arrangements are
possible, utilizing moving pulleys tensioned by rope systems with
tension being applied by electric winch or by hydraulic system or
similar - these being more usually applied to long centered or
complex conveyors.
Conveyor Structure Selection
Selection of the general structure
(and indeed the design of the other major elements of the conveyor)
is affected by the ground upon which the unit is to operate. Ground
with low bearing capability could require pontoon mounted drive etc;
with the structure being carried on sleepers.
Conveyors that are to be repositioned
periodically can also have the same design to facilitate ease of
movement. Idler rolls need to be selected, taking into account the
speed of the conveyor and the nature of the material and tonnage
rate to be conveyed, whilst producing an acceptable life rating of
the bearings (usually no less than 50,000 hours B10 rating).
Special Conveyors
Engineers today have the benefit of
being able to select proven conveyor types that offer solutions to
previously difficult problems. For example:
- Steep angle conveyors can now can
provide high angles of lift, giving potential for conveying out
of pit/quarry without having to follow a shallow angle grade
route
- where required, conveyors can
totally enclose the material (by the use of pipe or the new
Kawasaki Flow Dynamic Conveyor* (FDC), similar conveyor
construction); and
- conventional conveyors can now be
engineered to run around horizontal curves (indeed pipe type
systems can operate around very sharp curves) enabling the most
effective conveyor route to be chosen whilst minimizing conveyor
transfer points.
Conclusion
Conveyors operating within the mine
and quarry industry have their own particular problems but if both
the basic conveyor calculations and equipment selection are carried
out, taking into account all the known factors, then these can prove
to be reliable cost-effective solutions to material transportation
problems.
Editor's Note: This
article by Dr. Wright's was first published in the Mine &
Quarries 2002 Yearbook, by McMillan-Scott plc UK
About our Authors:
The authors of this article can be
contacted on the following e-mail addresses:
Dr Harold Wright
Dr H Wright & Associates
http://www.drhwright.com/
Email: thedoctor@drhwright.com
Telephone: 01642 581677, Fax: 01642 570177
Alan Jackson - alan.jackson@alstec.com
Ray Hodgkinson - raymond@hodg38.worldonline.co.uk
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