Compression springs are mechanical devices that are specifically designed to be induced to axial compression loads. They can usually also be extended as well as rotated up to a point. In general, compression springs can store mechanical energy when a compression load is applied to them. They will return back to their original shape and size once the load is removed – undergoing an elastic deformation.
This unique ability to store potential energy combined with their relative simplicity and cheapness has made compression springs invaluable in a very wide range of applications. From mechanical keyboard buttons, mattresses and ballpoint pens, to firearms and car suspension shock absorbers. We have been using compression springs since the 15th century when the first compression springs were used in clockwork mechanisms.
Types of Compression Springs
Compression springs can have many different geometrical shapes. The most common of all is the coil or helical spring. This shape is preferred over others because it allows for seamless high compression and also expansion up to a point. It is also lighter since less material is used to serve the compression load absorption need. Finally the coil spring shape is what gives this type its relatively large spring constant (more about that later).
This category is further divided into subcategories, including:
Conical Coil Springs – offering an alternative and smoother resistance to compression. | |
Barrel Springs – offering non-linear resistance to compression loads while also providing further stability. The torsional springs found in pegs are also helical, not compression springs. | |
Volute Spring – consists of multiple layered coils that slide over each other when compressed. This type of spring is useful in applications where huge compression loads are applied to the element. Or when the spring needs to be quite small in size when compressed to its maximum point. Examples of volute spring application include tank suspensions and gardening secateurs. | |
Magazine Springs – mainly used in firearms that need compression absorption elements to fit in certain spaces that can’t always be round or cylindrical. The coils of these specialized springs can be rectangular or oval. While their ends usually have a formation that allows them to be held in place by pins. | |
Nested springs: These have one or more springs fitted inside a larger one. Nested springs enable the designer to get more loadbearing material into a limited space. Therefore, nested springs can support greater loads than a single spring could withstand. |
Materials for compression springs
Compression springs are generally made of spring steel, a category of steels that feature high yield strength. This allows them to deform to extreme points and still retain their original shape, size, and form. As a consequence these steels have a high margin to deform elastically when under stress. This is something that is taking place on a molecular level, so the composition of these steels can have a significant effect on their elasticity.
Generally, spring steels contain carbon and manganese, and can also contain nickel, chromium, molybdenum, tin, vanadium, copper, iron, tungsten, and aluminum. Spring steels are officially categorized by ASTM based on their yield strength and hardness, and so the different material compositions can be suitable for different applications. For example, ASTM A228 is used for piano strings, contains 0.7%-1% carbon and 0.2%-0.6% manganese, and has a maximum yield strength of 530 MPa and tensile strength of 400 MPa.
Properties of Compression Springs
In this section, I will focus on the open-coil helical springs since these are the most widely used compression springs. These springs have certain characteristics that can mean a lot about their performance. The outer diameter (D) is the diameter of the cylinder that is formed by the spring when viewed from top. The coil diameter which is the thickness of the spring wire (d) that is cylindrical as well. The free length (L) which is the total length of the spring when not under any compression, and the active (na) and total spirals (n) which are the number of coils that store and release the mechanical energy and the total coils respectively (at least two are dedicated to the ends/base of the spring). Another important morphological property is the direction of wind which can be either left or right.
The force exerted by a spring is proportional to its extension, this law was formulated and introduced by Robert Hooke back in 1676 and only a handful of years after the first springs started seeing application. Hooke introduced the world to the formula: “F = -kx” where F is the spring force, x is the extension distance, and k is the spring constant that is different for each spring – determined by the manufacturer through experimentation, or by the user with the formula: “k = Gd^{4}/[8^{3}Dna]”. As mentioned earlier, barrel and conical coils are nonlinear springs, so Hooke’s Law does not apply to them. Hooke’s Law does not apply to springs that have been deformed or taken beyond their general elastic limit.
Force of a Fully Compressed Spring
To figure out the force of a fully compressed spring, we may use the formula: F_{max} = Ed^{4}(L-nd)/[16(1+ν)(D-d)^{3}n]. E is the Young’s modulus, d is the wire diameter, L is the free length, n is the number of the active spirals/coils, ν is the Poisson’s ratio, and D is the outer diameter. Obviously, some of these are determined by the steel that is selected by the designer, while others are determined by the form, shape and size of the spring.
Design Considerations
When designing a compression spring, it is important to first decide what material you are going to use. Then find the shear modulus (G) and tensile strength (TS) from data tables. These two factors are important for determining the percentage of stress for example (100*σ/tensile strength) when calculating the load requirements, figuring out how much a spring is compressed when induced to a certain amount of load.
Another important consideration is the diameter of the spring as it compresses to its maximum point. Helical compression springs have the tendency to increase in diameter when compressed. So it is important to calculate this expansion with the formula “Expansion = {sq[(D-d)^{2}+(p^{2}-d^{2}/π^{2})+d]-D}”.
The index of the spring is important with designers trying to keep within the range of 4 to 10. It is calculated by “C = (D-d/d)” and provides a good idea of the proportion of the thickness of the wire in relation to the spring diameter. This will determine the overall strength of the spring (smaller is stronger but larger is more compressible).
Finally, the number of coils and active coils is determined by the type of the spring endings. So, if both sides must sit on a platform of basis the total coils must be two more than the active coils (one on each ending). Now, the coils per inch must be equal to 1/p where p is the selected pitch of the spring, but you can also work the other way around as well. So, knowing the free length in inches lets us calculate the number of coils as “na = L/p”.
Further reading on EngineeringClicks
- Corrosion Protection of Springs: Where the spring operates in a corrosive environment some form of surface protection is required.
- Factors affecting the Fatigue Performance of Helically Wound Springs: Reliability is of great importance to many spring users. Without the spring many applications will either cease to work or work less efficiently. Therefore much study has been made of the behaviour of springs under fluctuating loads.