The Die-Spring Evolution
|Spring manufacturers continue to implement more reliable and capable in-process SPC controls and advanced CNC coiling equipment to supply consistent-quality-level springs to metalformers, with zero defects.
Spring characteristics such as hole and rod fit, free length and squareness affect overall tooling performance. Now, as advances in spring materials, spring-manufacturing equipment and coatings allow stampers to realize dramatic quality improvements.
Key Parameters: Tensile Strength, Wire Shape
The tensile strength of the spring wire determines the spring’s energy-storage capacity and its relaxation. The ideal spring material has a high tensile strength and high elastic limit.
Initially, die springs were typically fabricated using music wire or hard-drawn round wire. Then, in the late 1980s, some manufacturers transitioned to chrome-vanadium steel wire and chrome-silicon wire, which delivered increased tensile strengths and elastic limits to increase energy-storage capacity and reduce spring relaxation. The next generation of enhanced spring materials—chrome-vanadium and chrome-silicon combinations—further improved mechanical properties. However, metalformers considering implementing these advanced-material springs should carefully consider the cost-benefit equation, since die springs often outlast the life of the tools.
Over the last 20 years, metalforming companies have transitioned from using die springs coiled from round wire to springs constructed with shaped wire—a rectangular or keystoned crosssection. Most die springs manufactured today feature a keystoned crosssection produced from pretempered/pretrapped wire or from pretrapped annealed wire, to maximize available material. These springs optimize energy-storage capacity, as more material can be allocated into the available space in the die.
Warding off Corrosion
General corrosion, galvanic corrosion, stress corrosion and corrosion fatigue threaten to reduce spring life and load-carrying capacity. To ward off corrosion and optimize spring life, manufacturers offer powder-coated or plated springs whose coatings typically meet or exceed 100-hr. salt-spray testing. These coated springs suffice in most applications.
In more challenging environments, to combat corrosion metalformers can opt for springs with sacrificial coatings —primarily water-based coatings that contain metal oxides and aluminum flakes. Zinc and aluminum platelets align in multiple layers, providing several advantages including barrier protection, passivation, positive galvanic action and self-repairing (sacrificial) qualities. These coatings, with the addition of advanced sealers, allow die springs to sustain as many as 1000 hr. of salt-spray resistance. The result is a more corrosion-resistant die spring, the reduced likelihood of a spring failure due to surface defects, and a lower coefficient of friction.
Standards for Design and Quality
Basic die-design parameters that dramatically affect spring performance and life include clearance between the spring, pocket and rod. The recommended clearances are based on how the diameter of the spring increases during compression, and are calculated using a formula based on initial spring pitch.
Die designers often find creative s to optimize the use of space in the die. For example, they can stack springs or nest them—both practices often can reduce cost compared to either increasing the size of the die or moving to nitrogen gas springs. To make it easier for designers to specify and select a die spring for a given application, spring manufacturers have adopted standard designs—originally to the NAAMS (North American Automotive Metric Standards) global standard that emanated in the mid-1990s from the automotive metalforming industry in an attempt to reduce the cost of stamping-die components, and then to European (ISO) and Asian (JIS) standards. The NAAMS line (defined by the Raymond spring standards and still incorporating the original Raymond colors of blue, red, gold and green) continues to lead the in the United States, followed by ISO and JIS.
All of these standards attempt to increase spring quality while making them worldwide commodities. Quality initiatives such as TS 16949, ISO 9000 and ISO 14000 demand that spring manufacturers continue to implement more reliable and capable in-process SPC controls and advanced CNC coiling equipment, to supply consistent-quality-level springs to metalformers, with zero defects.
The Case for Gas Springs
Growing demand from metalformers to develop complex, large tools to form increasingly complex parts requires springs capable of delivering consistent and repeatable forces. This trend has helped lead a transition in some cases to the use of nitrogen gas springs. As the ability to control spring force becomes more critical in these complex applications—typically to prevent wrinkles and tears—tool and die designers find that gas springs provide more consistent loading than do coil springs, but at increased cost. Die designers, therefore, should follow these guidelines when specifying die springs:
• Select the appropriate die springs early in the design process;
• Ensure that the springs are sufficiently preloaded;
• Protect the springs from adverse external elements;
• Provide proper spring guidance; and
• Maintain equipment regularly. MF
See also: Associated Spring-Raymond, Barnes Group
Related Enterprise Zones: Tool & Die
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