Chances are you using dozens or even hundreds of springs every single day. Springs can be found in all kinds of machines and products today-suspensions and engines of your car, doorknob, and even your bed. Springs are everywhere, and it is impossible to imagine us achieving today's technological marvels without them. If you are an engineer trying to design a complex system or component, it is likely at some point, you will need to educate yourself about different types of springs and their applications.
So how exactly does an engineer go about finding the right spring to bring the latest marvel of human ingenuity to life? Let’s start with the basics.
What is a spring?
In their most basic form, springs are devices, usually metal, that store and release kinetic energy in a predictable and repeatable manner. Springs can come in any number of forms, shapes, materials, purposes, and strengths, but they always serve that same primary purpose of storing and releasing energy.
What types of springs are there?
To select the correct spring for an application, engineers must first figure out what kind of spring they need. Like most great inventions, Springs can come in limitless shapes and sizes. They also can be classified in nearly infinite ways. We will take a close look at three of the most common types;
A Compression spring stores energy when compressed and releases that energy when it expands. A coil suspension spring on your car is a familiar example. Compression springs are typically formed by twisting a metal rod or wire in a cylindrical shape with a pre-defined space between the coils to allow for compression. The compression spring is simple and highly predictable in the way that it performs. In general, when the coils are evenly spaced and sized, the compression spring will have a constant spring rate through its entire operating range. The spring rate is calculated using Hooke’s law, and it refers to the amount of force required to move the spring one inch. A spring with a five-pound spring rate will compress one inch under a weight of five pounds. Sometimes, a compression spring may be designed with different sized coils or abnormal shapes to create a single spring with variable spring rates. These springs can be found in conical shapes, hourglass shapes, or any number of different designs to achieve the desired performance.
An extension spring stores energy when it’s extended and releases that energy when contracting. Think of a trampoline spring. Like a compression spring, the extension spring is normally formed by wrapping a rod or wire in a coil. Unlike the compression spring, the extension spring is often manufactured with the coils in contact with each other when the spring is at rest. As the spring is extended by opposing forces, the energy required to extend the spring will be stored in the coil and will be released by returning the spring to its resting shape. Extension springs may also be designed as a constant-rate device or variable rate device based on the coils' shape and size.
Torsion springs act in a rotational direction, storing and releasing torque as they are twisted. A common torsion spring that everyone will recognize is the spring style found on a clipboard or a door hinge. Torsion springs of this type may also be formed as a coil, but in this case, the work is applied in a rotational direction with one end of the spring fixed and the other end rotating. The energy required to twist the spring is released when the spring returns to its original position, such as allowing the clipboard to snap shut on your papers. Torsion springs are not always designed as a coil. Another common torsion spring stores energy in a block of rubber or other elastic material as the working member is twisted inside that rubber, then returns to its original shape, releasing that energy.
There are many additional types of springs that we won’t go into detail about here. Flat spiral, power, and constant force springs are shaped like a spiral and store energy in the torque form when the coil is wound up. Leaf springs in an automobile use stacked arcs of steel pressing against each other. A simple bow and arrow use a single length of wood or fiberglass to store energy when bent. If you need to store energy and re release it in a predictable and repeatable manner, there’s some type of spring for the job.
Once the engineer has an idea of what type of spring will do the job, the next thing to look at is the application's properties.
The first property to consider is size. What length and width of spring will fit into the required space, and how far does the spring need to travel to serve its purpose? Keep in mind when designing that the diameter of a coil spring will increase as it’s compressed.
Spring set is the most extreme measure of travel for the spring before it becomes permanently deformed. Hooke's law works very well under normal conditions, but every spring has its limits. We’ve all seen that slinky with the single stretched out coil that never goes back to its original shape no matter how hard you try. It’s critical to balance the required force and travel of the design against the spring set for the spring you select.
Solid height is the minimum possible length for the coil spring. When all the coils contact each other, and the coil can’t get any shorter, that’s the solid height. When spring set is unknown or undefined, it’s common practice to avoid the solid height by at least 20%. So, if spring can compress 100 millimeters before reaching solid height, the engineer should avoid compressing it more than 80 millimeters.
Material selection, like any other component, is crucial in spring design. Most of the springs you see everyday are made from conventional materials such as carbon steel, stainless steel, and music wire. However, more exotic materials may be necessary if your application requires special corrosion resistance, vibration resistance, or other unique characteristics. Titanium, chromium, brass, silicon, vanadium, and even rubbers and plastics are common spring materials.
Infinite springs, infinite uses.
This article has only scratched the surface of design considerations when selecting springs for your application. Even a simple coil spring can come in various shapes; cylindrical, conical, hourglass-shaped, variable spaced coils, or any other configuration needed to serve the purpose.
Like any component, you need to define your spring's operational parameters and then work with spring manufacturers to find the right part for your application. The good news is there’s no shortage of great spring manufacturers out there. After all, humans have been building springs since before we even knew what a spring was!