Compression Springs Design, Calculate Compression Spring Rate
Compression Springs
Compression SpringsDefinition: A coil spring made of spring wire.
Overview: Compression springs are coil springs that resist a compressive force applied axially. Compression springs or coil springs have a spring constant and may be cylindrical springs, conical springs, tapered, concave or convex in shape. You can have large compression springs, jumbo compression springs, 3/32 compression springs, long compression springs, small compression springs, or even micro compression springs. Coil compression springs are wound in a helix usually out of round wire. The changing of compression spring ends, direction of the helix, material, and finish all allow a compression spring to meet a wide variety of special industrial needs. Coil springs can be manufactured to very tight tolerances, this allows the coil spring to precisely fit in a hole or around a shaft. A digital load tester, or coil spring compression tester can be used to accurately measure the specific load points in your metal spring. Compression springs are able to have two entirely different spring rates or a different spring constant associated with it's design. Compression springs can be made from non-magnetic spring wire like Phosphor Bronze or Beryllium Copper. Additionally, compression springs can be made from high carbon spring steels to stainless steel wire to name a few. The possibilities are almost endless because there are so many applications for metal springs.
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Compression spring applications:
Compression springs or coil springs can accomplish many types of applications like pushing or twisting, thus allowing you to achieve numerous results. Typical coil spring applications include force or load, which makes it shorter by pushing back against the load. The role of the compression spring is to return to it's original length. Compression springs offer resistance to linear compressing forces (push) and are in fact one of the most efficient energy storage devices available. A ballpoint pen is a excellent example of how small compression springs work. The small springs will compress when the pen is clicked and then the small springs will return to their original position. Other uses include vibration dampening and high temperature applications. Compression springs that are engineered for high temperature applications can reach up to 1,100 degrees Fahrenheit.
Ends: Compression spring ends are usually closed and square. These ends can also be closed and ground, or have open ends. Furthermore a helical spring or compression spring can have hooks on one end or both ends so as to fasten it to your particular assembly. You can also have compression springs bearing surface definition. If you are wondering about measuring bearing surface of compression springs then you can check the formulas at the bottom of the article. The ends of a helical spring can also be close wound for a certain number of coils on the ends permitting the spring to remain in a vertical position. The squareness influences how the axis force produced by the spring can be transferred to adjacent parts. Another application includes being able to thread a closed end coil spring onto a threaded shaft for fastening purposes. Other end configuration examples are reduced end diameters like a barrel spring on a bicycle seat. Springs can have dual diameters as well as triple diameters for achieving different assembly situations.
Spring Wire: The spring wire material choices available for coil springs today work well for corrosion resistance, electrical conductivity, non-magnetic, and high temperature applications. Basic compression springs are normally music wire springs, music wire is a high carbon spring steel. It is important to remember which spring wire you choose for your compression spring design application. Choosing the right spring wire for your spring will greatly enhance the life and repeatability of your spring, as well as give you many years of service life. To see the properties of common spring materials please visit the link from the home page of Planetspring.com
Key Parameters for compression spring calculation:
Dimensions: Outer Diameter, Inner diameter, Wire diameter, Free Length, number of coils, solid height and end configuration.
Outside diameter: The outer diameter of a spring.
Inner diameter: The Inner diameter of a spring.
Wire diameter: The outer diameter of round wire.
Free Length: The overall length of a spring in the unloaded position.
Solid Height: The length of a compression spring when all the coils are fully compressed and touching.
Spring Rate: (Stiffness) Is the spring rate of force in pounds per inch of compression. Examples: If the spring rate of a compression spring is 10 lbs. It will take you 10 lbs of force to move it 1 inch of distance. If you move it 2 inches of distance it will take you 20 lbs of force. The rate is linear.
End configuration:
Closed and Square: If the space between the coils is reduced at the ends to the point where the wire at the tip make contact with the next coil, the end is said to be closed and square. This is done so that the spring can stand on it's own. If there is no reduction in pitch at the end coils, the end is referred to as "open" and the spring will not stand up vertically on it's own .
Closed and Ground Ends: means an additional grinding operation may be applied to the closed end configuration. Grinding removes material from the spring's end coils to create a flat surface perpendicular to the spring axis. This may be done for a variety of reasons including a more even distribution of the spring force.
Open Ends Ground Square: are ends where there is no reduction in pitch at the end coils yet are ground square.
Technical Info For Compression Spring Design
1. How to measure compression spring:
How do I figure out how many active coils a compression spring has? With any type of custom springs, a portion of the end coils will probably be inactive. The number of coils not closed are the active coils. Example: If a spring has a total number of 10 coils and the first and last coils are closed then the spring has 8 active coils. Usually the first and last coil on a compression spring are closed. It's a good rule of thumb to count all the coils then subtract two coils to determine the amount of active coils. Sometimes in compression springs there is more than one coil on each end closed, so it's important to count all the closed coils to determine the number of active coils on your spring. The following equations give approximate active coil counts, assuming that the springs are compressed between parallel plates.
Use these equations to perfect your coil spring design
For closed ends (ground or unground): Na = Nt -2
For open ground ends: Na = Nt -1
For open unground ends : Na = Nt
In practice, the number of inactive coils varies slightly as a spring is compressed. If the spring output at two operating heights is known, the number of active coils over the operating height range can be calculated using the following equation for any end configuration.
Na =G d ( h 1 - h 2 )___
8 ( OD - d ) 3 ( P2 - P1 )
G = shear modulus of the spring wire
d = wire diameter
OD = spring outside diameter
h1, h2, = spring operating heights
P1, P2 = spring force at heights h1 and h2, respectively.
2. What is a safe design stress for a compression spring? This question doesn't have a single simple answer. The answer depends greatly on the certain factors such as the type of spring wire used ( i.e. music wire, stainless steel, chrome-silicon, etc.), material grade (i.e. commercial vs. valve quality, standard or high strength, etc.) and the service environment (i.e. static vs. cyclic, corrosive atmosphere, very high or low temperatures, etc.).
Helical springs that have infinite fatigue life under low deflection conditions may take a set if compressed to solid height . Another example of poor fatigue life when cycled in air would be a spring optimized for static life in salt water.
The compression spring design process usually begins with selecting the correct spring wire type that is appropriate for the application environment. Metal springs come in a variety of different metals such as stainless steel, music wire, Phosphor Bronze, etc. For static conditions in the coil spring design, the spring designer will take into consideration where the compression springs will be used and select the material best suited for the purpose, so as to assure stable compression spring force output over time. For cyclic conditions, not only does calculating spring rate have to be stable, but the compression spring must be able to survive the intended life without breaking. Finally, spring manufacturers limitations can also restrict design stress levels.
The best recommendation here is to understand what is wanted from the compression springs while they're in service. A spring design engineer can help develop the optimum compression spring design for the appropriate conditions. Ask yourself a few simple questions before calling a custom spring manufacturer so they may better assist you.
Will the compression spring operate under static or cyclic environments? If cyclic, what are the minimum and maximum operating loads, deflections (travel) or heights? What is the life you want from the compression spring? Where is the operating environment? What is the operating temperature? Will it be a small spring? Will it go into a hole or around a shaft and if so what is the size of each? These are the questions you need to ask yourself before calling compression spring suppliers.
3. Which spring wire gives the best corrosion resistance?
The actual operating environment of the spring wire plays a large role. Many coatings are available that can greatly reduce corrosion. These include powder coating and Phosphate with an oil dip or spray. Additionally there are many types of economical plating solutions available.
When the environment is such that a coated spring wire will not meet the requirements of an application, then stainless steel spring wire should be considered. Type 302 stainless steel is usually the first choice. This spring wire can yield very corrosion resistant springs for most applications. When the environment is a high temperature application then 17-7 PH stainless steel is recommended (650 F. max operating temp.) If you need to go higher then inconel x750 or inconel 718 can go up to (1,100 F) is a high-quality choice.
4. If I cut a spring in half, would the spring rate still be the same?
Cutting a spring in half greatly increases the rate or strength of the spring. Because you have decreased the numbers of active coils in the spring the spring rate increases. The spring rate is proportional to 1/Na, so reducing the number of active coils by half doubles the spring rate.
5. What does maximum safe deflection (travel) mean?
It means the maximum safe deflection from a free state that will not result in the spring taking a permanent set. Example: For a compression spring, the permanent set will result in reduced free length and force output. For an extension spring, the permanent set will reduce the force output by increasing the free length of the spring.
Compression Springs Theory
Compression spring design Formulas:
The following are just a few of the most basic formulas for spring design & calculating spring constant on compression spring design. Please bear in mind that effective coil spring design can only be accomplished by using a computer program capable of running hundreds of simultaneous calculations. If you need to calculate your compression spring please visit the compression spring calculator page from the home page of Planetspring.com
Compression spring design formulas
Formulas for compression spring constant (or rate)
K = Gd^4 / 8D^3na
G = E / 2(1+v)
D = Douter - d
Formula for pitch, rise angle, and solid height
Coilpitch = L free / na
theta = a tan (coilpitch / 3.14159D)
Lsolid = nt * d
Formulas to calculate the maximum force on a compression spring
Fmax = k (Lfree - Lsolid)
Tmax = (8WD / 3.14159d^3) * Fmax
W = (4C -1 / 4C - 4) + (0.615 / C)
C = D / d
Variables used in compression spring design formulas
Spring wire diameter = d
Spring Outside Diameter = Douter
Mean Diameter of Spring =D
Young's modulus of material = E
Max force at solid = Fmax
Shear modulus of material = G
Free Length = Lfree
Wire Length = Lwire
Solid Height = Lsolid
Maximum displacement = Ldef
Wahl correction factor = W
Spring Constant = k
Active Coils = na
Total Coils = nt
Density of material = p
Poisson ratio of material = v
Rise angle of spring coils = theta
Maximum shear stress = Tmax
Torsional Modulus of Elasticity for common spring wires.
6
Music Wire 11.5 x 10
6
Stainless Steel 10 x 10
6
Chrome Vanadium 11.5 x 10
6
Chrome Silicon 11.5 x 10
6
Phosphor Bronze 6.25 x 10
6
Brass 5.5 x 10
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