Page 129 - Root Cause Failure Analysis

Conveyors 117

In applications where it is impossible to completely evacuate the conveyor prior to
shutdown, the only viable option is to jog, or step start, the conveyor. Step starting
reduces the amp load on the motor and should control the torque to prevent the shear
pin from failing.

If, instead of step starting, the operator applies full motor load to a stationary, fully
loaded conveyor, one of two things will occur: (1) the drive motor's circuit breaker
will trip as a result of excessive amp load or (2) the shear pin installed to protect the
conveyor will fail. Either of these failures adversely affects production.

Screw
The screw, or spiral, conveyor is widely used for pulverized or granular, noncorrosive,
nonabrasive materials in systems requiring moderate capacities, distances no more
than about 200 feet, and moderate inclines (535"). It usually costs substantially less
than any other type of conveyor and can be made dust tight by installing a simple
cover plate.

Abrasive or corrosive materials can be handled with suitable construction of the helix
and trough. Conveyors using special materials, such as hard-faced cast iron and lin-
ings or coatings, on the components that come into contact with the materials can be
specified in these applications. The screw conveyor will handle lumpy material if the
lumps are not large in proportion to the diameter of the screw's helix.

Screw conveyors may be inclined. A standard-pitch helix will handle material on
inclines up to 35". Capacity is reduced in inclined applications, and Table 9-3 pro-
vides the approximate reduction in capacity for various inclines.

Configuration
Screw conveyors have a variety of configurations. Each is designed for specific appli-
cations or materials. Standard conveyors have a galvanized-steel rotor, or helix, and
trough. For abrasive and corrosive materials (e.g.. wet ash), both the helix and trough
may be hard-faced cast iron. For abrasives, the outer edge of the helix may be faced
with a renewable strip of Stellite(tm) (a cobalt alloy produced by Haynes Stellite Co.)
or other similarly hard material. Aluminum, bronze, Monel, or stainless steel also may
be used to construct the rotor and trough.

Table 9-3 Screw Conveyor Capacity Reductions for Znclined Applications
Inclination, degrees LO 15 20 25 30 35
Reductionincapacity, 8 10 26 45 58 70 78

Source: Theodore Baumeister. ed., Marks' Standard Handbook for Mechanical Engineers, 8th ed. (New
York: McGraw-Hill, 1978).