Page 133 - Root Cause Failure Analysis

Conveyors 121

Ciearance Improper clearance is the source of many volumetric-efficiency prob-
lems. It is important to maintain proper clearance between the outer ring, or diameter,
of the screw and the conveyor’s barrel, or housing, throughout the operating life of the
conveyor. Periodic adjustments to compensate for wear, variations in product, and
changes in temperature are essential. While the recommended clearance varies with
specific conveyor design and the product to be conveyed, excessive clearance has a
severe impact on conveyor performance as well.

lnstalletion

Installation requirements vary greatly with screw-conveyor design. The vendor’s
operating and maintenance (O&M) manuals should be consulted and followed to
ensure proper installation. However, as with practically all mechanical equipment,
some basic installation requirements are common to all screw conveyors. Installation
requirements presented here should be evaluated in conjunction with the vendor’s
O&M manual. If the information provided here conflicts with the vendor-supplied
information, the O&M manual’s recommendations always should be followed.

Foundation 0
The conveyor and its support structure must be installed on a rigid foundation that
absorbs the torsional energy generated by the rotating screws. Because of the total
overall length of most screw conveyors, a single foundation that supports the entice
length and width should be used. There must be enough lateral (Le., width) stiffness
to prevent flexing during normal operation. Mounting conveyor systems on decking
or suspended-concrete flooring should provide adequate support.

Support Structure Most screw conveyors are mounted above the foundation level
on a support structure that generally has a slight downward slope from the feed end to
the discharge end. While this improves the operating efficiency of the conveyor, it also
may cause premature wear of the conveyor and its components.

The support’s structural members (Le., I-beams and channels) must be adequately
rigid to prevent conveyor flexing or distortion during normal operation. Design, siz-
ing, and installation of the support structure must guarantee rigid support over the
full operating range of the conveyor. When evaluating the structural requirements.
variations in product type, density, and operating temperature also must be consid-
ered. Since these variables directly affect the torsional energy generated by the con-
veyor, the worst-case scenario should be used to design the conveyor’s support
structure.

Product-Feed System A major limiting factor of screw conveyors is their ability to
provide a continuous supply of incoming product. While some conveyor designs, such
as those having a variable-pitch screw, provide the ability to self-feed, their installa-
tion should include a means of ensuring a constant, consistent incoming supply of
product.