Compression Spring Requirements — The Absolute Basics
Spring design is a diverse career due to the nearly limitless possibilities of helical spring requirements. One customer may want a spring made from .011″ music wire that requires force in ounces or grams, while another assembles a dump truck and needs a suspension design that supports tonnage. This means the designer needs to be savvy on not only forces and material, but the processes needed to produce this enormous range of possibilities.
Another skill needed is the ability to review a spring print and determine what makes sense and what doesn’t. It’s quite possible for blueprints and specifications to have incorrect or missing data. The needed design then becomes a puzzle and the engineer will need to fill in the missing data. That said, what is the very basic information that must be provided for a spring designer to analyze, verify and quote a design?
Wire Size, Shape and Size
This is the basic need of all springs—the material. However, just the size is not enough. Material is a critical need, based on environment. If a spring is not exposed to the elements or low/high temperature extremes, a good carbon steel such as oil tempered carbon steel will likely do the job. But, if corrosion resistance is needed, then a stainless steel will be needed. If the environment is even more intense, such as salt water, gases or acid, then an exotic material such as Inconel®, Monel® or Hastelloy® will be required.Most spring designs are made from round wire, but the wire shape can be anything required to produce a force at a needed height. Round wire is the best for distributing stress and handling repeated forces, but it has its limitations. Square and rectangular wire shapes are very common for die springs where high forces are needed without a lot of travel. Very rectangular material is often used when a spring is required to close to a very thin solid height. Regardless, these specifics are needed because wire is a product that uses a wide range of elements in its chemistry, which makes for extremes in price. Also, round wire has many stock sizes that are available for fast delivery, where special sizes require much longer lead times to the customer.
Compression springs, by their very nature, are ideal for fitting over a shaft or inside a cylindrical bore. If the spring is guided in either manner, the size of the shaft or bore should be shown on the print. Aside from just the basic requirement of size, there is another reason for knowing all the diameters. When a compression spring travels, the body diameter increases. If the spring is not guided, or just works over a shaft, this “growth” is not an issue. But, if the spring works in a bore, the bore size must be known to allow for the growth of the spring body during travel. If this requirement is not considered, the spring may come in contact with the bore wall, causing rubbing, wear, and possible failure of the application.
Free length is a basic geometric requirement. The free length of a compression spring is a direct result of the space between the coils. If the distance is measured from the center of a wire diameter to the center of the adjacent coil, this is called “pitch.” No matter which dimension is called out, they both determine how far the spring can travel and how much force will be produced from that travel, either to solid height or any height between the solid height and the free length.
Total Coils or Spring Rate
If the wire size and body diameter of a spring are known, the total coils are determined by the spring rate in pounds per inch. If the total coils are known, the spring rate can be calculated. Therefore, one determines the other. Spring rate is often very important, whereas the total coils needed to produce that rate is not critical. However, it is also common for spring blueprints to require a maximum solid height, which is solely dependent on the total coils. As long as the total coils do not breech the solid height requirement, all will be fine. If the maximum solid height is critical, the rate will have to be increased to allow for the removal of some of the coil/coils. Then, wire size or body diameter could possibly be adjusted to obtain the spring rate.
Grind or No Grind
Springs ends are usually ground, especially if the wire size is large. This allows a very square posture for the spring, both during deflection and at its free length. There are cases where an ungrounded spring will function well and the added cost of grinding isn’t needed. But, most designs need the spring to produce loads faithfully during its travel and grinding the ends flat will accomplish that.
There are many other specifics that can be required of compression springs, but the ones mentioned here are the basic needs required for a spring to be quoted and manufactured. If any of these requirements are missing, the engineer will need to contact the customer to be certain that no assumptions are made that could cause malfunction or failure in the application. ◆