Page 128 - Root Cause Failure Analysis

116 Root Cause Failure Analysis

clearance but should not be large enough to have areas where the chain-drive bypasses
the product.
A long horizontal run followed by an upturn is inadvisable because of radial thrust.
All bends should have a large radius to permit smooth transition and prevent material
buildup. As with pneumatic conveyors, the ductwork should include cleanout ports at
regular intervals for ease of maintenance.

Primary Drive System Most mechanical conveyors use a primary-drive system that
consists of an electric motor and a speed-increaser gearbox. See Chapter 14 for more
information on gear-drive performance and operation criteria.

The drive-system configuration may vary, depending on the specific application or
vendor. However, all configurations should include a single point-of-failure device,
such as a shear pin, to protect the conveyor. The shear pin is critical in this type of
conveyor because it is prone to catastrophic failure caused by blockage or obstruc-
tions that may lock the chain. Use of the proper shear pin prevents major damage to
the conveyor system.
For continuous applications, the primary-drive system must have adequate horsepower
to handle a fully loaded conveyor. Horsepower requirements should be determined
based on the specific product’s density and the conveyor’s maximum-capacity rating.
For intermittent applications, the initial startup torque is substantially greater than for
continuous operation. Therefore, selection of the drive system and the designed fail-
ure point of the shear device must be based on the maximum startup torque of a fully
loaded system.

If either the drive system or designed failure point is not properly sized, this type of
conveyor is prone to chronic failure. The predominant types of failure are frequent
breakage of the shear device and trips of the motor’s circuit breaker caused by exces-
sive startup amp loads.

Operating Methods
Most mechanical conveyors are designed for continuous operation and may exhibit
problems in intermittent-service applications. The primary problem is the startup
torque for a fully loaded conveyor. This is especially true for conveyor systems han-
dling material that tends to compact or compress on settling in a vessel, such as the
conveyor trough.
The only positive method of preventing excessive startup torque is to ensure that the
conveyor is completely empty before shutdown. In most cases, this can be accom-
plished by isolating the conveyor from its supply for a few minutes prior to shutdown.
This time delay permits the conveyor to deliver its entire load of product before it is
shut off.