Page 130 - Root Cause Failure Analysis

11s Root Cause Failure Analysis

Short-Pitch Screw
The standard helix used for screw conveyors has a pitch approximately equal to its
outside diameter. The short-pitch screw is designed for applications with inclines
greater than 29".

Variable-Pitch Screw
Variable-pitch screws having the short pitch at the feed-end automatically control the
flow to the conveyor and correctly proportion the load down the screw's length.
Screws having what is referred to as a short section, which has either a shorter pitch
or smaller diameter, are self-loading and do not require a feeder.

Cut Flight
Cut-flight conveyors are used for conveying and mixing cereals, grains, and other
light material. They are similar to normal flight or screw conveyors, and the only dif-
ference is the configuration of the paddles or screw. Notches are cut in the flights to
improve the mixing and conveying efficiency when handling light, dry materials.

Ribbon Screw
Ribbon screws are used for wet and sticky materials, such as molasses, hot tar, and
asphalt. This type of screw prevents the materials from building up and altering the
natural frequency of the screw. A buildup can cause resonance problems and possibly
catastrophic failure of the unit.

Paddle Screw

The paddle-screw conveyor is used primarily for mixing materials like mortar and
paving mixtures. An example of a typical application is churning ashes and water to
eliminate dust.

Performance

Process parameters, such as density, viscosity, and temperature, must be constantly
maintained within the conveyor's design operating envelope. Slight variations can
affect performance and reliability. In intermittent applications, extreme care should be
taken to fully evacuate the conveyor prior to shutdown. In addition, caution must be
exercised when restarting a conveyor in case an improper shutdown was performed
and material was allowed to settle.

Power Requirements
The horsepower requirement for the conveyor-head shaft, H, for horizontal screw
conveyors can be determined from the following equation:

H= (Am+ CWLF) X 10-6