August 27, 2013 | Volume 09 Issue 32 |
Manufacturing Center
Product Spotlight
Modern Applications News
Metalworking Ideas For
Today's Job Shops
Tooling and Production
Strategies for large
metalworking plants
SPIROL disc springs (right) and the DEWALT high-performance industrial drill. [DEWALT product images used with permission by DEWALT Industrial Tool Co.]
DEWALT, a subsidiary of Stanley Black and Decker, is a brand with a reputation for providing high-quality professional power tools, contractor tools, and accessories. DEWALT's lead design engineer for drills was tasked with designing a high-power right-angle drill that was lightweight, compact, and portable. As is common with hand power tools, the goal with the design was to maximize performance while minimizing the size of the tool.
The design would incorporate a mechanical clutch built to limit maximum torque output to 70 ft-lb. Absent the clutch, with the huge gear reduction of 75:1, the drill would be capable of producing a lock rotor torque at the spindle of 175 ft-lb, transmitted directly to the operator of the tool. It was determined early on that the clutching thrust required in the space available could only be achieved by a disc spring design. Compared to conventional coil springs, disc springs offer a higher load capacity in less space. Additionally, longevity and reliability are high measures of quality for professional tools used on a daily basis. The life and performance warranty of the tool was reliant on the disc spring design.
Disc springs are conically shaped, washer-type components designed to be axially loaded. What makes disc springs unique is that based on the standardized calculations of DIN 2092, the deflection for a given load is predictable and the minimum life cycle can be determined. Disc springs can be statically loaded either continuously or intermittently, or dynamically subjected to continuous load cycling. They can be used singly or in multiples, stacked parallel, in series, or in a combination thereof.
The advantages of disc springs compared to other types of springs include the following:
Early on in development, DEWALT's engineers were working with a low-cost disc spring supplier. DEWALT had developed and theoretically calculated the initial disc spring design. The first prototype built failed, with the new design achieving less than half the targeted cycle life. Analysis indicated that premature fatigue failure of the disc spring was due to higher-than-expected stress, lesser quality material, and a less-than-ideal manufacturing process by the original supplier.
Location of the mechanical clutch assembly in the DEWALT unit.
Determined to find a solution, the lead engineer on the project contacted SPIROL to discuss the disc spring design. SPIROL's application engineering team evaluated the design objectives/constraints and quickly determined they could not be met by a standard disc spring. SPIROL essentially became an extension to DEWALT's team and focused specifically on the disc spring portion of the design. SPIROL recommended a custom disc spring design that had alternate dimensions, material, and secondary operations. For this demanding and high-performance application, these special features added significant value and ended up costing only pennies more compared to the product offered by the original supplier.
DEWALT tested SPIROL's recommended disc spring, and the tool far exceeded the cycle life requirement. By partnering together, DEWALT and SPIROL determined the best disc spring design for the new angle drill, minimizing costly development time for both companies.
Two disc springs stacked in series in the mechanical clutch assembly.
SPIROL's staff of engineers can review your application needs and work with your design team to recommend the best solution. One way to start the process is to go to "Disc Springs" in SPIROL's Optimal Application Engineering portal.
*Note: SPIROL considers stacking disc springs to be a challenge and encourages design engineers to consult SPIROL's special .pdf resource on "How to stack disc springs."
You can download the complete SPIROL disc springs catalog -- complete with extensive design guide -- by clicking here. For a .pdf of just the abbreviated guidelines at left, click here. Learn about stacking disc springs here.
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Disc spring design guidelines
Sizing and selection
Orientation
Fatigue life
Materials and finishes
Want more information? Go to www.spirol.com.
Source: SPIROL
Published August 2013