Compression springs are used in many applications having linear compressive forces and needing mechanical energy storage, such as pneumatic cylinders and push-button controls. The most conventional compression spring type is made of a round metallic wire coiled into a helical form. Here are the most important factors to consider when selecting helical compression springs.
1. The outside diameter (OD) of a spring will expand under compression. Be sure to consider this if the spring will be used in a tube or a bore. Spring makers also have manufacturing tolerances on the outside diameter, which can add to the envelope size required of the assembly. Most spring suppliers will specify a work-in-hole diameter for a spring that factors their manufacturing tolerance and expected OD expansion. Look for this information to quickly select from stock spring catalogs, or use this information to better communicate your product needs when ordering custom-made springs.
2. Consider loading or travel requirements on the compression spring. The spring rate, or spring constant, is the relationship of the force to compress a spring by a unit of length, typically pounds per inch (lbs/in). So with a given load the product designer can calculate expected spring travel. The further you travel the more stress on the spring. At some point the stress can yield the wire material causing a phenomena referred to as spring set. After spring set the spring will not expand back to the original unloaded length. Still, this spring can be functional depending on the assembly. Stress formulas and calculators are available to predict spring set, or a starting rule of thumb is to avoid solid height by at least 20% (still remaining is 20% of total travel range from unloaded to solid).
3. Compression spring end types can be standard or special. Standard ends have a combination of:
1) plain open or closed, and 2) ground or not ground. Having open or closed ends will affect the spring rate given the same total coils, wire size, and OD. Ground ends require more manufacturing effort; however, combined with closed ends, this feature will improve squareness of the loading force and reduce spring buckling tendencies. Some manufacturers include closed and ground ends in standard catalog stock design, while some don’t. Be sure to know the difference. Special end examples include reduced coil for screw mounting, offset legs to work as alignment pins, and enlarged coils to snap into ring grooves.
4. Many spring materials are available from carbon steel to exotic alloys. Music wire is a high carbon spring steel and is the most widely used material. Stainless steel 302 has less strength than music wire, though it adds general corrosion resistance. Nickel alloys are branded in various trademarks and are chosen for extreme high or low operating temperatures, specific corrosive environments, and non-magnetic qualities. Phosphor bronze and beryllium copper are copper alloys for good corrosion resistance and electrical conductivity.
Filed Under: Motion Control Tips