Compression Springs Information
Compression springs are the most recognizable spring and are intended to oppose compaction in the direction of the axis. The spring is extended at rest; shortened and stored with energy when a load is applied, and is one most efficient energy storage devices available. Traditionally, they are wound and uniform in pitch and diameter, but these traits vary considerably today.
Dimensions, load and materials determine the deflection of a compression spring. The spring is fully extended when at rest. Upon compression, the spring will express stored torsional energy in a pushing force. The shape-restoring force is proportional to the deflected distance of the spring. Thicker-coil, thinner-diameter springs require heavier loads to begin compression. Spring material also determines the linear elasticity of the spring, with more rigid materials providing stiffer resistance and a higher return force.
Compressions springs are typically metal due to the stiffness of the material. Types of steel, alloys, and brass are all readily available, but custom applications may employ other materials. Compression springs differ greatly in their design, with different styles of pitch, end configurations, coil design, and diameters regularly occurring.
Helical Compression Springs
Straight compression springs have the same inner and outer diameter throughout the length of the spring. These are the most common style of compression spring.
Mini compression springs are a type of straight compression spring, except on a miniaturized scale. Common in medical devices, and chemical, marine and aerospace industries, typical free heights range in 1.27 to 26 mm.
Conical compression springs have a diameter that tapers towards one spring end; the spring radius decreases towards only one spring terminus. Conical springs offer a unique telescoping ability, with a very small height at maximum deflection.
Hourglass compression springs have a diameter that tapers towards the center of the spring length. The springs' ends have matching diameters.
Barrel compression springs have the largest measurement of their diameter in the middle of the spring. Both ends are tapered to a gradual smaller diameter.
Reduced-end compression springs have a consistent diameter across the middle springs, but taper only on last coil or two.
Die springs are a robust type of helical compression springs consisting of rectangular wire. For the same value deflection, die springs carry 30% more load. These springs are designed to carry very high compression loads in hostile environments. Made mainly for punch presses to provide consistent and reliable resistance, die springs also find use in other industries.
Non-Helical (Wave) Compression Springs
Some contemporary compression springs employ flattened, kinked coils which provide additional spring effect predetermined by the material. These compression springs, called wave washers or wave springs, are wavy metal washers designed to provide a compensating spring force or absorb shock when under load. When wave washers are stacked upon each other, these types of compression springs only occupy 30-50% of the compressed height of a similar helical spring.
Helical compression springs are wound in either a clockwise or counter-clockwise direction. Since the load is applied perpendicular to the axis, wind direction is typically unimportant when only one is used. For a spring-in-spring configuration, springs of opposite wind should be used.
In wave compression springs, wires are flattened, stacked lines of stainless steel. Most compression springs are made of rounded wire, but compression springs consisting of squared wire with very high load capacity exist in the form of die springs.
There are four typically types of spring ends, with each configuration affecting spring pitch, height, active coils, and mounting characteristics.
- Open-end springs have a consistent coil pitch when at free length.
- Closed-end springs have a reduced pitch at the end coil, with the last coil resting on the adjacent inner coil.
- Open, ground springs have no reduction in pitch, but the last coil is filed flat to help distribute load evenly.
- Closed, ground springs reduce in pitch upon the last coil wrap, and have its ends filed flat to help distribute load.
Compression Spring Specifications
- Outside diameter is measurement across the perpendicular face of the spring.
- Inner diameter is the measurement across the inner cavity of spring.
- Wire diameter is the width of the wire coiled to make the spring.
- Mean diameter is the outside diameter minus the wire diameter.
- Spring rate is the amount of force, in pounds, needed to extend the spring one inch. This is affected by wire diameter, material, and coil pitch.
- Maximum load/deflection is the maximum weight a spring can handle before it disfigures and its mechanical properties are erratic.
- Free length is the length of the spring with zero load.
- Solid height is the length of the spring with a maximum load, with each coil resting on the adjacent coil.
- Stress exists in compression springs in the form of torsion. Stress governs life expectancy, and the higher a spring's stress range, the lower the maximum stress must be to obtain comparable life.
- Pitch is the distance between a coil center to the adjoining coil center. In compression springs, this may vary considerably.
- Active coils are the number of coils that express deflection when under load. In an open, ungrounded spring it will be all of the coils; for a closed, grounded spring, this will be the coils that are not incorporated in the filed base.
Compression springs are usually mounted on rods of specific inner diameter or within an allocated space or guide. As compressed the spring will become slightly larger in both inner and outer diameter to a degree of about 10%. Grounded ends help distribute a load across the terminal coils evenly.
When a spring is compressed for the first time and the stress is high enough, the spring may not return to its full height. This is called "setting," and many manufacturers will specify their springs' setting measurement. It's also possible to make the spring slightly longer and for a manufacturer to provide that initial compress to provide a consistent height. This process is also called "scragging," and a machine demonstrates this process below.
While subject to fatigue like any spring, compression springs have long lives if employed correctly. They are subject to side deflection when not used with a guide, which could crumple the spring and severely damage machinery. While metal is resilient to high temperatures, many springs do have working temperatures to help enhance spring lifespan. Springs should be regularly inspected, and if the spring requires oil it should be regularly lubricated.
- Music wire is a common and relatively inexpensive high-carbon steel alloy used for spring manufacture. It is cold drawn and offers uniform tensile strength.
- Stainless steel exhibits relatively high strength with good corrosion resistance for specialty applications.
- High carbon steel (more than .3%) displays resistance to abrasion and impact.
- Hard drawn steel is inexpensive, but has a low working load.
- Specialty metals and alloys can be customized per application. This may include beryllium copper, beryllium nickel, niobium, tantalum, and titanium.
- Brass is inexpensive, easy to mold, and corrosion resistant, but may not be strong enough for the application.
- Composite springs often incorporate plastic or vinyl into the spring composition, supplementing the metal or steel construction.
- Other materials may be used for custom applications, based on the purpose requirements.
- Cleaned and oiled springs rely on oil t0 maintain a friction-free compression.
- Phosphated springs have additional corrosion resistance, lubrication, and the coating can serve as a base for painting or other further coating.
- Powder coating adds abrasion resistance and can also change the exterior color of the spring.
- Shot peening uses glass beads or bird shot blasted at high speed to dimple and strengthen the material surface.
Compression springs are exceptionally common. They are active in combustion engines, stamping presses, cellular phones, electronics and hand tools. Compression springs can also be used for vibration insulation such as in a suspension. One of the most overlooked employments of this type of spring is in ball point pens, while the working mechanism of a mattress box spring is widely recognized as a compression spring.
BS EN 13906-1 - Cylindrical helical springs made frim round wire and bar - calculation and design part 1: compression springs