Compression spring engineering is all around us, from miniscule springs in watch clasps, to our phone and computers we use on a day to day basis, to larger springs found in the oil and gas industry.
Compression springs are a type of mechanical device designed from a spring wire in a helical shape. These springs are used to release or store energy. They can also maintain or absorb shock between two surfaces.
Overview
A compression spring is a type of coil spring that works to resist an axially applied compressive force. Coil springs, or compression springs, hold a constant and may be convex, concave, tapered, conical or cylindrical in shape. Because compression springs are linear by design, they must hold a constant rate per inch in their entirety.
Compression coil springs come in all types including micro compression springs, small compression springs, heavy duty compression springs, and conical compression springs. Typically, these coil springs are wound from round wire into a helix shape. The finish, material, direction of helix, and compression spring ends allow compression springs to fit well within a wide variety of special needs.
Helix springs can be produced with very tight levels of tolerance, allowing the spring to fit precisely around a shaft or in a hole. The coil spring compression tester, or digital load tester, can be utilized to measure your metal spring’s specific load point with high accuracy. Because of these factors, the application possibilities are endless for these springs.
Compression Spring Engineering & Application
Compression springs have the versatility to accomplish a wide variety of applications that include twisting and pushing, thus enabling the user to achieve a plethora of results. Coil springs are one of the most energy efficient storage devices available, offering a high resistance to linear compressing forces (pushing).
A great example of a small compression spring engineering is a ballpoint pen. When the end of a ballpoint pen is clicked the tiny spring is compressed before returning to its original position. Other designs and uses include high temperature applications and vibration dampening. Springs that have been specially engineered for higher temperature applications can withstand extremely high temperatures.
Ends of Compression Springs
Generally, coil compression spring ends are square and closed. These ends may also be ground and closed, or even have ends that are open. Additionally, these springs can have hooks or either end, or both ends for fastening to your assembly.
Compression spring engineering may also permit the spring to remain in its position vertically due to close wounds for a certain number of coils. The square-ness of the ends will influence just how the produced axis force will be transferred to parts adjacent.
Another type of application includes the ability to thread a closed end spring into a threaded shaft. This is performed for fastening type purposes. Yet other end configurations can include reduced end diameters, such as barrel springs located on a bike seat. Compression springs may have dual and triple diameters for various assembly needs and situations.
An Engineer’s Guide to Selecting a Compression Spring
As mentioned previously, compression or coil springs are used throughout a variety of applications that require mechanical energy storage and compressive forces, such as push-button controls and pneumatic cylinders. The most commonly used compression spring is manufactured out of round metallic wire in a helix form. When selecting a helical compression spring, here are several important factors of compression spring engineering to keep in mind.
1. Diameter
The OD, or outside diameter, of a coil spring will expand when it is compressed. If your spring will be used in a bore or tube, consider this fact. Spring engineering manufacturers will also design tolerances applied to the outside diameter. This will add to the overall envelope size the assembly requires. Most suppliers will specify these factors.
2. Loading (or Travel) Requirements
Consider the travel or loading requirements on the spring. The constant, or rate, describes the relationship between a compressed spring force and unit length. This is generally measured in pounds per inch. Therefore, with the given load, the designer will be able to calculate the expected spring constant, or spring travel. The further the spring travels the more stress it has. At a point this stress will yield a material catalyzing a phenomena known as spring set.
When spring set occurs the spring will not expand to return to its original length. However, a spring may still be functional dependent on its use case in the assembly. Stress calculators and formulas are available in order to predict spring set. A solid rule of thumb to live by is to avoid solid height by a minimum of 20%.
3. Special or Standard Compression Ends
End types can be special or standard. Standard compression ends can be a combination of open or closed, ground and not ground. Ends that are open or closed will have an affect on the spring rate given the constant total of OD, wire size, and total coils. Although ground ends may require a greater manufacturing effort, combined with a closed end design this feature will reduce buckling tendencies and improve the squareness of loading force.
Some suppliers include ground and closed ends in their standard stock catalog designs while others do not. Before you make a purchasing decision make sure that you understand the difference. Other special examples include enlarged coils to snap into ring grooves, alignment pins offset lets, and reduced coil for screw mounting.
4. Choose Your Material
A variety of materials are available, from exotic alloys to carbon steel. The most common material used in a variety of applications is high carbon spring music wire. Another popular option, stainless steel 302, has less strength than music wire but holds a corrosion resistance. Beryllium copper and phosphor bronze are copper alloys good for electrical conductivity and corrosion resistance. Nickel allows are branded for various types of trademarks and chosen for low or high operating temperatures.
Takeaway
When your product design calls for springs, first identify the load conditions. When prototyping, you’ll want to begin with off the shelf springs. Use maximum load and maximum deflection as a starting point to filter through springs in supplier catalogues. Then, narrow down your choice using geometric constraints and spring rate. When you are ready for production, work closely with your spring manufacturer and supplier to fine tune your design to exact specifications.