There is a well-worn proverb that should hold special significance for engineers:
For want of a nail the shoe was lost.
For want of a shoe the horse was lost.
For want of a horse the rider was lost.
For want of a rider the message was lost.
For want of a message the battle was lost.
For want of a battle the kingdom was lost.
And all for the want of a horseshoe nail.
The basic idea is easy enough to grasp: seemingly unimportant omissions can have serious and far-reaching consequences. Of course, any engineer who’s worked on a complex assembly knows how crucial even the smallest component can be.
Take springs, for example.
Spring Basics
From an engineering perspective, springs are energy storage devices—mechanical parts that, when compressed, exert force on the surface compressing them. But to a layperson, a spring is essentially just a coil.
“That’s the standard, traditional spring—known as a round-wire coil or compression spring,” said a senior research and development engineer from Smalley’s engineering team. “It’s been around for many years and perfected by other companies, but at Smalley we developed a flat-wire wave spring, which is an adaptation of a spring that uses a bending formula rather than a traditional torsional formula.”
“The wave springs Smalley make have also been around for many years,” Smalley’s engineering team continued, “and the linear spring is an adaptation of a wave spring. It’s essentially the same thing—flat wire that is bent to put waves in it—but we don’t make it round. That gives you the ability to introduce loads along a straight line rather than a circle.”
In this sense, linear springs are essentially a series of leaf springs. “That’s right,” Smalley’s engineering team added, “The formula is slightly different than if you were to arrange a bunch of leaf springs side-by-side, but we’re talking about changing a constant, so it’s not that big a difference. Having said that, we do also manufacture miniature versions of leaf springs.”
Manufacturing Linear Springs
A unique aspect of linear springs is the process for manufacturing them, as Smalley’s engineers explained:
“When the material goes through our machine, the wave is put into it at the same time as it’s being cut to the proper length. It’s not a stamping process, where you’re forming the material in a mold. This allows us to control the wave height, which gives us better precision. We can program in the wave height, length and etc.”
Although one might worry that this process would affect the spring’s metallurgy, Smalley’s engineers state that this is not the case, and “if it does, it’s very minute—not enough to worry about. However, the process before the forming can change the material’s properties. At Smalley, we typically buy round wire and then roll it flat. That process will coldwork the material and increase its tensile properties, which gives us better springs.”
Choosing the Right Spring
Two key factors to consider when selecting a spring are free height and work height.
“There are at least three heights to take into consideration when selecting a spring. There’s the free height, which is the uncompressed height, when it’s sitting on a table doing nothing. That’s essentially how the spring has been formed from our machine,” said Smalley’s engineers.
“Then there’s the work height, which is how it’s being used within the application; the height you’re compressing the spring to. Sometimes there might be more than one work height, like if it’s cycling between two different positions. Then there’s solid height, which is the point where you can’t compress the spring any more—it’s essentially the thickness of the material being used.”
A factor to consider specifically when selecting a linear spring is the number of waves in the spring. How do you know the number of waves you need? Ultimately, it depends on the loads in your application with the number of waves being one variable that can be adjusted as necessary.
“There are a few variables that we can manipulate to change the spring rate and therefore change what the load at work height is going to be,” said Smalley’s engineering team. “So, we can change the number of waves, or the wire size. If we didn’t want to increase the spring rate, but we want to change the load, we might adjust the free height. If it’s taller and you have more deflection as you get down in your work height, you’re going to get more load. So, there’s many different options open to an engineer who’s trying to figure out how to design a part.”
Applications for Linear Springs
There’s a wide variety of linear springs to choose from in terms of size. In Smalley’s case, standard linear spring sizes vary from thicknesses of 0.012”” all the way to 0.062” and lengths varying from 1.5” to 12.0”. The sheer difference between the largest and smallest stock sizes indicates how varied the applications for linear springs can be.
“A few ideas that come to mind—and we need to think long and narrow in terms of applying loads—are cases where someone has a long seal, like rubber or neoprene, and instead of putting a bunch of pocketed coil springs, they can put in one linear spring,” said the engineering team.
“Another example might be a rotary vane pump,” Smalley’s engineers continued. “At high speeds, rotary vane pumps might not need a spring because of the centrifugal force, but at lower speeds you need something to keep a seal against the vanes and the outside contours of the pump. A linear spring can go along the length of that vane and push it up.”
“Lastly, any sort of detent preload,” the engineering team added. “Instead of pre-loading the end of a pin and pushing the pin axially, you can put a linear spring along the length of a pin and push it radially. That way it will fall into a contour and you’ll have a position lock. If it’s a positioning unit, you might want positive feedback when an operator’s turning a knob. This is a way of doing that.”
For Want of a Spring
When it comes to manufacturing, no component is too small to be worthy of due consideration. Even something as simple as a spring can make a huge impact on the success or failure of a product. For that reason, manufacturing professionals may want to keep in mind a variation on the old proverb which began this article:
For want of a spring the assembly was lost.
For want of an assembly the unit was lost.
For want of a unit the order was lost.
For want of an order the customer was lost.
For want of a customer the company was lost.
For want of a company the economy was lost.
And all for the want of a linear spring.
For more information on linear springs, visit the Smalley website.
Smalley Steel Ring has sponsored this post. All opinions are mine. --Ian Wright