Adjustable Dumbbell Design [2024]
Summary and Key Details
Developed the design and manufacturing process for an adjustable dumbbell, covering material selection, manufacturing, and durability
Utilized cold-rolled 1018 Steel for the core of the dumbbell, and an insert-injection molded EPDM handle to reduce material costs by 47%
Ensured structural integrity and corrosion resistance using Nickel electroplating and EPDM Grip
Validated loading conditions using SOLIDWORKS FEA and ANSYS FEA
Motivation
This project began as an exploration into consumer product optimization for my 2nd-year materials engineering course, where the goal was to analyze a product's material selection, durability, and design. The objective was to identify flaws in the manufacturing processes or materials used and propose alternative solutions that improved durability, while maintaining performance. Although it started as an assignment, I expanded it into a comprehensive design challenge.
Preliminary Dumbbell Analysis
The adjustable dumbbell set selected for comparison was the CAP adjustable dumbbell set. It was chosen because I owned a set, which allowed for a detailed analysis. Upon initial inspection, the main concerns of the current dumbbell design was corrosion present around the interface between the knurled handle and the shoulders around the handle.
It was crucial to determine whether the corrosion issue stemmed from the dumbbell's design or was unique to the specific product I owned. To investigate, I researched this model online and found an eBay listing featuring the same discoloration, indicative of rust, in the exact same location on an identical dumbbell. This discovery confirmed my initial concern that the design itself was prone to corrosion.
Fabrication and Material Analysis
To identify the factors contributing to corrosion, it was essential to analyze the materials and fabrication methods behind the dumbbell's design.
The handle comprises a steel bar and star-lock threaded collars for securing weight plates. According to the manufacturer, the material is chrome-plated steel, though the specific alloy is not disclosed. Given common industry practices, the handle is likely made of cold-drawn 1018 or 1045 steel, which enhances yield strength. Heat treatment is unlikely, as the associated cost would outweigh its benefits for this application.
The visible lathing patterns and grooves suggest the handle was machined on a lathe, starting from a solid steel rod. The final step in its production is likely chrome electroplating, which deposits the protective coating onto the steel. The fabrication process is illustrated in the image below.
Corrosion Analysis
1018 steel is prone to corrosion which is likely why the manufacturer applied a chromium coating. The chromium layer forms a protective oxide that passivates the surface, creating a barrier between the steel and its environment.
However, this coating's effectiveness can fail due to the conditions in which adjustable dumbbells are typically used. These dumbbells are often stored in humid spaces like basements or garages, where moisture accelerates corrosion.
Sweat from the human body further increases the issue by introducing chloride ions, which can break down the chromium oxide layer. This breakdown removes the passive barrier, exposing the steel to moisture and leading to the formation of iron oxide. Chlorides in sweat, such as NaCl, can react with chromium to form soluble chromium trichloride, depleting the protective layer and leaving the steel vulnerable.
Additionally, the lathing process creates a crevice between the knurled handle and the dumbbell's shoulder. This crevice traps electrolytes, causing stagnation and ion concentration differences, which promote localized, concentrated corrosion (crevice corrosion)
Design Requirements for Improved Dumbbell
Based on the analysis of the design and its prevalent issues, the following criteria were established to guide the improved design:
2.5 - 2.85 cm diameter ends
Research showed that most off-the-shelf weight plates are compatible with this diameter range. To ensure compatibility, the ends of the adjustable dumbbell must adhere to this specification.
Groove Pattern
To maintain adjustability, the design must accommodate the same groove pattern required for star-lock threaded collars, regardless of the material or fabrication method.
Supports 50-lb on Both Ends
Most comparable dumbbell kits support a maximum of 30 lbs (15 lbs per side). Supporting 50 lbs per side was the decided metric to provide a 3.33 Factor of Safety (FOS).
Corrosion Prevention against Chloride
To enhance durability, the new design must incorporate improved corrosion prevention, particularly against chloride exposure.
Final Design Changes
The key redesign for the adjustable dumbbell was replacing the single-component handle with a steel rod and rubber grip.
The previous approach of subtracting a single steel rod caused excessive material waste, as the shoulders are much taller than the rest of the dumbbell.
The following sections explore this material reduction's impact on performance and discuss the chosen rubber grip.
Grip Design and FEA
EPDM was selected for the grip due to its excellent resistance to weather, degradation, and chemicals like NaCl — critical for this design. With a Shore A Hardness of 70, it offers high wear resistance, ideal for frequent use. EPDM also adheres well to metals, enabling its assembly to the rod via insert-injection molding discussed later.
A concern was whether reducing the rod's metal thickness would compromise strength. FEA conducted in both SOLIDWORKS and ANSYS confirmed the maximum stress remains below the yield stress of all materials.
SOLIDWORKS was used to validate the dumbbell itself. Using Shared Topology within ANSYS allowed both the dumbbell and grip to be validated.
Nickel Electroplating
Nickel electroplating was selected for corrosion prevention due to its superior performance over chromium in chloride-rich environments, though it can still face similar challenges. Its uniform coating is especially effective in small areas and crevices, forming a protective Nickel Oxide layer through passivation. With only a minimal cost difference, its enhanced resistance made it the clear choice for the final design.
Final Design Fabrication
The dumbbell material was kept as 1018 steel, chosen for its strength, cost-effectiveness, and common use in dumbbells. Its machinability made it a practical choice. Since most metals are vulnerable in chloride environments, the coating became the primary defense, as the material itself could not be improved.
Additionally, the design incorporates an EPDM grip, applied via insert-injection molding. EPDM's adhesion to metal helps seal any crevices, further enhancing protection. Combined with the new nickel electroplating, these changes offer improved durability. The full fabrication process for the new design is outlined in the graphic below.
Conclusion
All design specifications were successfully met.
To meet condition 1, 2.85 cm diameter ends were selected, reducing the amount of material lathed, which saved time and cost. By using 1018 Steel for the same material type, the grooved pattern could still be fabricated, fulfilling condition 2. The FEA analysis confirmed that the 50-lb load on both ends could be sustained, satisfying condition 3. Lastly, the use of EPDM and insert molding reduced the crevice, while nickel electroplating enhanced corrosion resistance, meeting condition 4.
The original design required 336.74 cm³ of 1018 steel, while the new design uses only 123.6 cm³, resulting in a 63.3% reduction in steel usage and a significant decrease in material costs. When factoring in the cost of EPDM, the total material cost reduction is approximately 47%, excluding fabrication costs.
Fabrication costs are expected to be similar. While less material is cut during lathing, insert injection molding introduces an additional expense. The upfront mold cost is high, but long-term savings are significant. The cost difference between nickel and chromium electroplating is minimal, keeping overall fabrication costs comparable. Overall, the new design offers superior performance due to reduced material costs, enhanced environmental resilience, and sustained functionality.
References
All content here is derived from the formal report on this design optimization. Please feel free to reach out for resources or references.