We hear from a lot of engineers that they learned a good deal about round wire design theory in school but not much about flat springs. Why is that? Who determined that flat springs aren’t interesting or cool enough?
Considering the multitude of industries and products that utilize these types of springs, we think they are not just “cool”, but that understanding them is a critical step in good machine design. In fact, Leonardo Di Vinci (he was kind of smart!) utilized flat steel springs in many of his inventions that were centuries ahead of their time – the self-propelled cart and car as well as the first iteration of a helicopter just to name a few.
For sure, if they were good enough for my buddy Leo (who’s home town and grave site I have actually visited), then they are good enough for the rest of us and it’s time to change this prejudice against flat springs!
During the next several blogs, we’ll present a summary of the various types of flat springs, their common applications and limitations, as well as design considerations. In this first article, we’ll focus on constant force (aka Conforce®) springs.
Types of Flat Springs
Why use a Constant Force Spring?
Constant force springs provide a nearly flat gradient as opposed to round wire extension and compression springs.
Counterbalancing: ie. Windows and shades, hospital tables, X-ray machines, industrial equipment, military tank seats, etc
Push or Pull: ie. Staple and nail guns, surgical staplers, drug delivery device, motor brushes, toasters, cup holders, fire dampers, retail displays, etc
Constant Force Design and Ordering Considerations
First, select the material considering the environment the spring will live in then determine life (cycle) and force required (consider hysteresis). You’ll then need to determine the space available, mounting method (spool, no spool), and end detail and attachment, including straight section on the end of the spring (“pick-up”). How many will you need and how quickly?? What are the critical characteristics and tolerances as well as packaging requirements? Will you issue a blanket order with releases or ship all at once?
Understanding Constant Force vs Constant Torque
Constant force springs provide a linear extending/retracting force whereas constant torque springs provide a rotational force (torque)
Limitations of Constant Force Springs
Cycle Life – the maximum number of cycles in our world is 100,000 measured as an extension and retraction of either the whole spring or a portion of it. Keep in mind that low design estimates will lead to early failure while high estimates make the spring larger and more expensive than necessary
Sample Design Chart (all Force values are estimates only):
Material Selection – the most common material is type 301 stainless steel. Texture-rolled high carbon steel can be used when a lower cost is required for high volumes; Inconel 625, and Elgiloy are utilized for the most demanding applications. Material thickness generally runs from .002” – .031” and tensile strength range from 270,000 – 310,000 psi. Finally, our standard finish is a deburred edge but you can request a “round” finish for a smoother edge.
Cost – 301 Stainless is the most–commonly specified material. As stated above, texture-rolled high carbon costs less in high volumes but Inconel and Elgiloy are significantly more expensive. Over-designing a spring will also require more material and, thus, a greater cost.
Force & Diameter Relationship
Spring force is generally constant as long as the diameter is constant. Longer springs increase in force slightly as they are extended (unless measures are taken to flatten the gradient). The normal tolerance for a constant force spring is +/- 10%
As the spring diameter is increased the spring force decreases and cycle life increases (lower stress). There is a direct relationship between the ID and the tolerance – if the ID must be held to a tighter tolerance then the load, tolerance must increase.
Testing and Inspection
Considering customer requirements, the following design aspects are typically tested and inspected: coil diameter, end detail (hole diameter; and location, pick up, etc); length; load or force; drum size and mounting; and “workmanship”.
Flat steel springs can be the forgotten “critical component” in engineering schools but they are not that difficult to understand if you work with the right supply partner. Don’t underestimate the importance of a good design and quality workmanship since a poor design and poor quality can have disastrous effects.
We hope you found this first article, focusing on constant force springs, valuable. Feel free to contact the leader in flat steel spring design and manufacturing – Vulcan Spring & Manufacturing – and look for future announcements detailing new, helpful blogs.
At Vulcan, we go “Beyond the Spring” and we’re here to help!
Want to know more about how we assist companies like yours? Check out our Design Guide or contact us. Our responsive and helpful team is standing by.Contact Us