
Understanding the relevant factors
System productivity and is directly related to the proper material handling
equipment selection and in-depth understanding of the application. Before hardware can be
selected properly, the following criteria must be studied and defined
carefully:
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Load data (physical properties of the product to be handled)
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Objectives of the application (what is to be accomplished with
the equipment?)
Probably the most common error in selecting transfer mechanisms
like conveyors is the fact that load data and application objectives are not
understood in enough detail. In many cases, they are ignored completely and
hardware is selected on an arbitrary basis or, worse yet, on a "lowest cost"
basis. This is the point where rework happens and systems fail to meet the needs
of the operation.
Breaking down load characteristics is critical
To specify the right equipment, you must understand the load and its
characteristics. You must understand how it interacts with the conveyor,
integrated machinery, other load types, and the inputs/outputs of the system.
What do you need to know first?
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Shape or form. The load must be defined as pallet, box, drum,
wire container, engine block, beer bottle, coal, automatic body, or
other item.
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Dimensions. If the load is a container such as a pallet, box, or
tote, its length, width, and height must be known.
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If it is a unit item,
the dimensions of the interface between product and conveyor -- such as
the load bearing surface -- are critical.
And, in the case of a product
on a container -- such as a pallet load of beer cases -- the dimensions of
both the carrier and the load on the carrier must be known in order to
provide for such factors as overhung clearance.
Load characteristics
Finally, if the load consists
of bulk materials, such factors as density and flow rate must be identified.
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Weight. Maximum and minimum, filled and empty, weights of the
load must be known if it is a container.
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Flow rate. The rate of flow or capacity of a system is usually
stated in units per minute if a unit handling conveyor is used. Normal
measurements of rate or capacity in bulk handling applications are cubic
feet per minute or tons per hour.
Be specific with terminology
Ambiguous terms such as average
rate or throughput capacity should be avoided. The rate usually
varies in different areas of a material handling system. However, there is
always one point -- usually an intersection or junction -- that acts as a
bottleneck, and controls the rate of the entire system. This point
must be identified, because it is a controlling factor in overall system
design and hardware selection. The speed of an individual conveyor may have
nothing to do with the rate or capacity of a system, except that the
conveyor must be capable of handling the maximum required total load-feet
per minute.
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Orientation. The position of the load on the conveyor must be
established. A load length may actually become a height when the item is
placed on a conveyor, tow line, or monorail carrier.
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Footprint. The bottom configuration, or footprint, of a load can
have a strong bearing on the design and cost of a conveyor system. The
following questions should be asked about footprints of different types
of loads:
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Pallet -- Are there block feet or runners? In which
direction? Are there any broken boards, protruding nail heads, or
straps?
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Drum -- Are there chines? Does the bottom bulge outward?
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Carton -- Is there a soggy bottom? Are there protruding
staples?
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Casting -- Does it have irregular shapes? Is the surface
machined? Does it have sharp edges or burrs?
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Steel shape -- Is the bottom surface flat, or is it warped?
Is weld splatter present?
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Container -- Does it have feet? Is there a full-faced bottom?
Is it flat or does it have sharp edges? Are ribs or runners used? Is
weld splatter present?
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Bag -- Is the bottom flat, or is the shape like that of a bag
of water?
Objectives of the system should
be evaluated on a step-by-step, component-by-component basis. The purpose of
each item of hardware should be questioned.
For example, is transportation
the primary purpose of a certain conveyor, or is it accumulation and
surging? Is the conveyor to be used as a moving assembly work table? Can it
be combined with an adjacent conveyor to eliminate a separate drive?
Other typical questions are as follows:
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If a turntable is being considered, can a right-angle transfer be
used instead, to eliminate one drive mechanism? Can a roller gravity bed
be used instead of a power conveyor? Why is the load to be turned or
rotated in the first place?
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In a towline application, is the system strictly for
transportation, or must accumulation take place at some point? Are
accumulation areas absolutely necessary, or are they merely "nice to
have"? Can switches and spurs be combined in some cases to reduce the
number of separate elements?
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The same questions can be asked for a power-and-free system. An
additional question to be asked is, "Is a power-and-free system really
necessary?" Perhaps a combination of several trolley conveyors with
interfacing transfers could do the same job.
The overall system should be
reviewed after individual hardware components have been evaluated. In most
cases, the scope of the system will be changed somewhat after the hardware
analysis.
Frequently, the layout will be simplified, the number of
transferring mechanisms reduced, and nonessential operations eliminated.
This results in cost savings and improved conveyor performance.
