Despite
the established physical and dynamic properties of specific materials (rubber,
nylon and polyurethane), there are a number of design application issues to
consider in generating optimum wheel performance for demanding applications. Many
changes can be made to improve performance and extend the usable life of
industrial wheels that are experiencing frequent failures (such as dynamic
failures known as blowouts, delaminations or fatigue life). Some of these
changes include the use of alternative polyurethane chemistries, wheel design
changes or even altering manufacturing parameters.
There
are many aspects of an industrial application to consider to determine the
appropriate wheel design, material and manufacturing parameters to optimize
wheel performance. Some of these considerations include:
- General operating conditions of the
application/system and the running surface condition (including joints,
paint, lubricants, proper alignment, etc.);
- Load and speed profile of the
application including the duration of cycles and idle time between cycles;
- Wheel type (i.e., drive/brake wheels,
guide wheels or free running load wheels);
- Dimensional constraints of the
application;
- Vibrations or abrasions that occur
during use;
- Environmental conditions including
exposure to water, chemicals or other contaminants, ambient weather
conditions and ultraviolet light exposure; and
- Application serviceability and the wheel's
targeted life span and preferred mode of failure. For existing
applications, evaluating the design and performance of existing wheels and
common modes of failure is critically important--how a wheel fails can be
as important as when it fails.
The
following case study describes a solution provided for a major retailer that improved
the operating performance of the drive wheels for a large automated materials
handling crane located in the company's largest warehouse/distribution facility
in the U.S. In this case, the existing
solid wheels were constructed from another polyurethane material.
Onsite
Evaluation and Testing
Following
the initial project review with engineers at the distribution facility, Uremet (a
supplier of high-performance polyurethane wheels) conducted an onsite
evaluation of the automated materials handling crane system and its drive
wheels. In addition to general
observations relating to the operating environment and conditions, multiple
tests were conducted and data gathered during the initial evaluation, including
measuring the operating temperature of the existing wheel treads and the
ambient temperature of the facility. Details regarding the load, acceleration,
maximum speed, operating cycle, crane weight and wear requirements were noted.
The existing wheels were examined for wear, fatigue and delamination.
A
general review of the historical operating performance, life span and observed
failure modes for the existing wheel tread was also discussed with the facility's
engineers and maintenance technicians. The average tread life of the existing
wheels was estimated to be less than three months, which caused significant
downtime issues for the system. The existing wheel treads typically failed due
to either a form of delamination known as bondline fatigue or severe fatigue
cracking of the tread material. Bondline fatigue is a failure generally caused
by overloading, fatigue or overheating of the bondline resulting in areas of
separation between the hub and the tread. Fatigue cracking is a failure
typically caused by high-stress concentrations during cyclic loading. In both
instances, the vibration induced by these failures was believed to be causing
fatigue to the crane frame structure and premature failure of the drivetrain
gearboxes. In addition, the existing wheels were only available as new complete
wheel assemblies sourced from Europe, resulting in long lead times and high
replacement costs.
Analysis/Recommendation
Additional
analysis was performed including the examination of the customer supplied wheel
samples. The wheel tread material was confirmed to be an NDI/polyester-based
polyurethane with a hardness of 96 Shore A. Using the data gathered during the onsite
evaluation, the application was analyzed using computer models for targeted
deflections, traction, bondline stresses and load capacity based on historical
experience and previous testing results from Uremet's application simulation
tester. It was determined that the configuration of the existing wheel design
was satisfactory and that an adjustment to the formulation of the polyurethane
tread would provide enhanced performance.
Uremet's
U3000 polyurethane, with a hardness of 78 Shore B, was selected as the optimum
tread for this application. Compared to the original material, the U3000
material has a lower compressive modulus, lower hysteresis, higher tensile and
tear strengths and a slightly higher coefficient of friction.
The
lower compressive modulus of the material results in greater deflection under
the operating preload of the system. The increased deflection greatly reduces
the bondline fatigue of the wheel system by distributing the stress from the
operating loads over a larger area of the bondline. However, the key to the success of the U3000
material is its outstanding dynamic properties, specifically its hysteretic properties
which, as measured by tan delta, are approximately 35 percent lower than the
original NDI-based polyurethane. Hysteretic properties are effectively a measure
of the efficiency of a material in handling high loads and strains. The lower
the tan delta of a material, the less heat is generated within the polyurethane
during use, and therefore, the higher the load bearing capability of the
material. Due to the increased deflection of the material resulting from its
lower compressive modulus, the improved dynamic properties are required to
prevent dynamic failures within the wheel tread. Another enhanced physical
property of the U3000 material is its higher tear strength (in excess of 625pli
in ASTM D-1938 testing), which effectively resists the severe fatigue cracking
that typically plagues solid elastomeric drive wheels.
Results
Sample
wheels with the alternative polyurethane material were manufactured using the
customer supplied wheels stripped of their NDI tread and recoated. With the
U3000 polyurethane chemistry, the retreaded wheels were installed on the system
and have generated significant tangible results, such as:
-
Extended wheel life: The average wheel life increased to approximately 8 to 12 months of run time,
resulting in significant cost savings from reductions in wheel purchases,
maintenance costs and downtime issues. Parameters are still being adjusted to
optimize wear rates with the goal of extending lifespan to upwards of 18
months.
-
Reduce system stress/fatigue: The greater fatigue life and higher tear
strength of the material have completely eliminated all fatigue cracking on the
drive wheels and significantly reduced the tread induced vibrations which,
coupled with the material's lower compression modulus, have resulted in less
stress and fatigue to the crane structure and drive system thereby yielding a
longer life for various components.
Greg Stevens, P.E., is head of research & development
at Uremet Corporation.For more information, visit:
www.uremet.com
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