By R. Charles Ireland, PhD, PE
Washington, DC Office
Web site: www.CTLGroup.com
Manufacturing facilities depend on hanging conveyor systems. Structural elements supporting these systems are rarely designed for a rational consideration of metal fatigue. Consequently, conveyor support systems may provide adequate performance for years and then experience chronic failures resulting in costly downtime. However, with proper attention to fabrication and connection details, conveyor supporting elements can be designed or upgraded to durably resist cyclic loading problems that can lead to costly interruptions in production.
Past structural investigations have found inadequacies in original hanger designs and cases where well-meaning repair efforts have contributed to subsequent failures. An investigation to address conveyor support failures may include:
- Meetings with conveyor engineering and plant personnel to understand the operating environment and performance expectations
- Field observations, photographs, and measurements
- Metallurgical evaluation of failed materials
- Structural evaluations
- Development of improved hanger designs and repair recommendations
As an example, power and free-type conveyors are used extensively due to handling advantages and are well suited to modern automotive painting operations. Often, the entire conveyor and booth structures are hung from roof trusses using header and hanger steel. Ideally, hangers are connected to the regularly spaced yolk-shaped element that ties the conveyor system together.
Many conveyor hanger failures are not a result of weakness in the hanger due to a lack of connection material. Rather, they are a direct result of metal fatigue triggered by undesirable connection features. Metal fatigue can be viewed as a progressive, localized permanent damage culminating as cracks or complete fracture resulting from fluctuating stresses due to a sufficient number of repeated load applications. There are two challenging aspects of metal fatigue:
It occurs at relatively low load levels, often right in the range of typical operating loads.
Fatigue cracks grow with little or no external indication. Hence, it is very difficult to inspect for and anticipate fatigue failures. They typically occur in a very sudden and catastrophic manner.
For these reasons, the best defense against metal fatigue is a well-designed system that is not susceptible to fatigue in the first place.
Rational fatigue analyses can be performed using guidelines such as those presented in the American Institute of Steel Construction (AISC) Specifications. Such calculations consider member capacities, connection details, load magnitude, and required resistance to cyclic stresses. In addition, the following guidelines should be considered for the development of fatigue-resistant hanger details:
- Avoid details that produce severe stress concentrations or poor stress distribution.
- Provide gradual changes in the section and avoid reentrant notch-like corners.
- Avoid abrupt changes of section or stiffness in members or components.
- Align parts so as to eliminate or minimize eccentricities.
- Avoid attachments on parts subjected to high cycles of repeated loadings.
- Use continuous welds rather than intermittent welds.
- Avoid details that introduce high localized constraint.
Provide suitable inspection of construction details, including verification of high-strength bolted connections and placement of sound welds. Sound welds avoid inclusions, undercut, incomplete fusion, etc.
Establish an appropriate inspection and maintenance program for members that are expected to be fatigue critical.
If fatigue cracks are discovered, take immediate steps to prevent their propagation. ME
This article was first published in the April 2012 edition of Manufacturing Engineering magazine. Click here for PDF.