Coil Spring Manufacturing Process
Whether you have an existing spring application in need of re-engineering or a new one that requires designing this guide will help. The engineering team and Duer Carolina Coil has been manufacturing springs for well over 100 years and has refined the coil spring process for the industry.
The scope of this manual is limited to data which will be helpful to the design engineer in solving problems frequently encountered in helical springs. General information such as maximum stress limits, relative costs of materials, and the more often used formulas are presented for references.
For the sophisticated engineer desiring more technical design information we highly recommend Dr. A.M. Wahl’s “Mechanical Springs” published by the McGraw-Hill Company.
And for those interested in cost reduction, skip ahead and download your free guide of 21 ways to save money when it comes to coils.
Coil Spring Tolerances
Cold Wound Springs Manufacturing
The Spring Manufacturers Institute composed of over 300 major cold wound spring manufacturers has produced a handbook for “Standards and Design for Compression, Extension, Torsion, and Flat Springs.” This manual, available from your spring manufacturer, is highly recommended as a guide to practical tolerances and for its coverage of spring terminology, formulas and design information for cold-formed springs.
Hot Wound Springs Manufacturing
The accepted standard for manufacture and tolerances of how wound springs is ASTM-125, latest revision. This specification covers such points as tolerance on out side diameter, free height, maximum solid height, loaded height, and squareness, as well as definitions and inspection procedure.
The ASTM committee, composed of technical representatives of the major hot coil spring manufacturers, has cognizance over the updating of the specification to keep pace with technological advances.
Coil Spring Stress Correction
The conventional stress formulas for compression and extension springs shown in this guide give values in pure torsion. Most springs are used for light or moderate service where the life requirement will not exceed 10,000 cycles. It is generally accepted practice to ignore correction factors for such service.
For heavy duty service where life expectancy is 100,000 cycles or greater, total stress must be considered. The Wahl correction factor takes into account increased stresses caused by the curvature of the wire and shear. A corrected stress is obtained by multiplying the conventional stress by the appropriate Wahl factor.
In most elevated temperature tests where relaxation data are given for various temperatures and stresses, the stress figures include the Wahl correction factor.
Another stress correction factor, shown by H.C. Keysor, covers the increase in stress due to eccentric loading, being most pronounced in short springs. With only 2 active coils, this correction factor is 1.23, but with 4½ active coils, the factor drops to less than 1.10.
Compression and Extension Springs
W = compression rate, lbs/in.
S = uncorrected torsional stress, PSI
G = torsional modulus (see tables I, II, and III)
d = wire diameter or square size, in.
D = mean coil diameter, in.
P = load, pounds
N = number of active turns
b = long side of rectangular wire, in.
t = short side of rectangular wire, in.
K1 and K2 = constants (see above)
M = moment of torque, inch pounds
M/T = torque rate, or moment of cause 360° angular deflection, inch pounds per 360° deflection
b = width or axial dimension
S ₜ = tension stress, PSI
E = modulus in tension = 28,500 for hot wound carbon or low alloy steel springs or 30,500,000 for cold wound carbon or low alloy steel springs
t = thickness of radial dimension
Square Wire: Coiling distortion creates a trapezoidal shape, increasing the size along the inside diameter, and hence the solid height. The increased side dl can be estimated:
where d= side of square before coiling; C = D/d
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Rust and environmental pitting act as stress risers and may cause early failure in the best designed spring, carefully manufactured from otherwise sound material.
Where cost and other factors dictate against corrosion resistant materials, the spring surface, which is the most highly stressed area, must be protected. Unprotected shotpeened steel springs start rusting almost immediately. Protection starts with choosing the right coating.
Powder Coating, often the coating of choice, offers impact, abrasion, salt spray, humidity, and chemical resistance that meets the majority of customer requirements.
Epoxy powder coatings are designed for general purpose interior use and for applications where maximum chemical and solvent resistance is required.
TGIC-Polyester powder coatings are substituted for those applications requiring PVC coating. They offer excellent exterior durability and good resistance to most chemicals and solvents except alkalis and ketones.
At DC Coil, we have our own in-house powder coating capabilities, making TCIC-Polyester coating our preferred choice. It offers a lower cost and quicker turnaround times than other coatings.
Electroplating with zinc with .0002″ to .0006″ thickness is one of the more practical methods of coil protection.
Mechanical cleaning by blasting immediately prior to plating facilitates electroplating without lengthy acid pickling (which must be avoided).
Controlled stress relieving after plating reduces the effect of hydrogen embrittlement, a constant danger, particularly to oil tempered wire.
Electroplating cold wound springs made from oil tempered chrome vana dium or chrome silicon wire is not advisable. Electroless nickel plating is an effective substitute when cadmium is environmentally objectionable.
Dip painting to assure complete coverage with lacquer or enamel is another practical method, particularly for large springs. Waterbased paints are now available for all colors in both lacquer and enamel.
Baking can be used for faster drying time and better adhesion when using waterbased paints. This process usually involves individual handling. For this reason, springs small enough for barrel plating can be protected in this manner at a lower cost.
Oxide or phosphate coatings with supplemental oil or paint coatings are useful treatments, particularly for oil tempered chrome vanadium or chrome silicon wire springs. For limited protection against rusting on the shelf in inside storage, protective oils may be used.
The use of pre-coated wire, such as cadmium plated or galvanized wire, is practical for limited protection in small sizes for extension springs and compression springs with unground ends.
Hot dip galvanizing for finished springs is not normally practical. It has been successful only where large volume justifies the cost of very closely controlled processing.
Plastic coatings, such as polyvinyl chloride and nylon can be applied at a reasonable cost by the fluidized bed process for particularly corrosive applications.
Where the free length of a compression spring exceeds four times the mean diameter, lateral deflection or buckling becomes notice able during compression.
Piloting and guiding on the O.D. or I.D. helps correct this problem, although friction against a supporting member may be objectionable due to possible scoring and affected load reading.
For springs with squared and ground ends, one end on a plate and the other on a ball, objectionable buckling will occur when the coordinates are to the right of line 1.
Objectionable buckling will occur to the right of line 2 when springs with squared and ground ends are compressed between parallel plates, as on a spring testing machine.
When possible the spring should be designed so the end of the coils (tip ends) are 180° apart. This is accomplished by having the number of coils active and inactive always end on the half coil (i.e., 4.5 active and 6.5 total).
This allows for even distribution of the spring material preventing the spring from buckling sooner than shown on the graph above. This condition also helps prevent the spring from going out of square under load.
The natural frequency of springs should be considered where heavy duty service and rapid cycling are involved. If the natural frequency of the spring is too low, surge and coil clashing with augmented stresses will result.
It is best to design the spring with a natural frequency of at least 13 times the vibrating speed (cycles per minute). Samples for high-speed applications should be thoroughly tested to avoid premature failures in the field.
The formula for calculating natural frequency of a spring is:
n = cycles per minute of spring vibrating between its own ends
d = wire diameter, in.
D = mean coil diameter, in.
N = number of active coils
Heavy Duty Coil Springs
For the purpose of this guide, heavy duty springs are those with a service life requirement of over 100,000 cycles for cold wound springs and over 10,000 cycles for hot wound springs. For heavy duty service, optimum life will result from the application of the following recommendations:
- Provide adequate surface quality. Failure will be accelerated by surface defects such as laps and seams. For cold wound springs, music wire, carbon rocket wire, and valve quality carbon wire are the best choice. Oil tempered, valve quality alloy wire is more susceptible to surface defects and embrittlement from plating and is recommended only in sizes above the available carbon wire range. Although AISI-1095 has been the steel of choice for hot wound springs, with moderate stress levels it is no longer readily available. Because of improved rolling practices, the AlSI-5160 and 6150 have become viable substitutes for the AISI-1095 grade and with centerless grinding these grades display equivalent results.
- Specify shotpeening. This relatively low-cost operation induces beneficial stresses which will increase life as much as 500%. The intensity and coverage of .006 inches on an Almen C Strip and the coverage of 90%, as referenced in ASTM-A-125 S3, can be achieved with proper equipment. The more stringent intensity and coverage require outside processing and increase the cost.
- Magnaflux inspection for hot wound springs. Inspection on the finished spring is desirable, particularly for hot wound springs. Magnafluxing of material before coiling is not practical, nor will it detect forming or heat treat cracks. Specifying S-2 in ASTM-A-125 for hot wound springs will impose Magnaflux inspection. The acceptance level is established by the end use and can vary from seams no deeper than 3% of the bar diameter to no seams or indications at all.
- Specify surface protection against corrosion consistent with the operating environment. Corrosion pits are one of the most common causes of spring failure.
- Corrected Stress Values. For high cycle applications where the order of cycles is 108 the stress values corrected must be kept lower than 30,000 PSI for any consistency in the life of spring sets. Even with this, low-stress breakage does occur but in a limited predictable amount.
- Design to a conservative operating stress range. Recommended limits for shotpeened springs, assuming adequate surface preparation and protection are as follows.
Elevated Temperature Coil Springs
Technological advances in nuclear energy, high-speed aircraft, missiles, and steam turbines have increased requirements for springs to operate at high temperatures. The data shown here will enable the designer to estimate load loss or relaxation under load at various stress and temperature conditions for various materials.
Main factors to consider when designing elevated temperature springs:
- Space available and its effect on spring stress
- Operating temperature
- Frequency of exposure to temperature
- Allowable relaxation
- Corrosive environment
- Material cost
The torsional modulus of materials decreases as the temperature increases. For example, the torsional modulus of Inconel X-750 varies as follows:
Thus elevated temperature loads will be correspondingly lower than room temperature loads. Only room temperature loads should be specified for inspection purposes.
At DC Coil, we have a continuing test program for evaluating elevated temperature materials. Our efforts have been directed mainly to larger hot wound sizes where relatively little information is available from other sources.
Of the six materials suitable for hot wound springs which we have tested, only Inconel 718 and Inconel X- 750 showed good results at 700° F (see Table below).
Bar Size 5/8″ Diameter (Hot Wound) Initial Stress of 77,500 PSI (Corrected) 168-Hour Exposure at 700°F
With permission of the National-Standard Company, we show the results for their test of various cold wound spring materials in Table 4. Tests were made on springs of .080″ diameter wire. All stress figures are corrected.
Low Temperature Springs
As the temperature decreases, the tensile strength and modulus of carbon and low alloy spring stresses increase accompanied by a reduction of impact strength and increased notch sensitivity.
However, compression springs have the configuration to readily absorb impact energy, so with conservatively stressed designs, spring steels are suitable to temperatures as low as -320° F under ordinary conditions.
As the temperature approaches absolute zero or if high toughness is desired Type 302/ 304 stainless is the least expensive and safest choice of materials for cold wound springs in sizes up to approximately 1.125″ (28.575 mm) diameter.
For heavier sizes where hot wound springs are required, Inconel 718, Inconel X- 750, or A-286 are the best choices.
Table 1: Carbon and Low Alloys
Table 2: Corrosion Resisting Alloys
Table 3: High Alloys
Note: where a stress range is given, the lower value should be used for the larger sizes.
Table 4: Relaxation Tests
No Springs Heat Test