Coil Spring Design and Flat Spring Design Manufacturing and Assembly Considerations

 

Close Tolerances

Tolerances cost money! In your coil spring design or any type spring design, for that matter, the best technique is to apply tolerances only to those dimensions that are significant.

The tolerances listed in the charts on this page should serve only as a guide. If you want closer limits, they may be feasible, but they may also be costly. It is best to consult with a spring manufacturer before deciding on tighter tolerances.

Helical Spring Tolerance Data: Compression Spring, Extension Spring, Torsion Spring

Manufacturing variations for helical springs really depend on the coil spring design chosen. If tighter tolerances are required there are certain premium materials that can be used to facilitate this. Also, if the production requirements are high volume then it is more cost effective for the spring manufacturer to establish refined techniques that result in less piece-to-piece variation.

Note that very tight AQL (Acceptable Quality Limits) may require tolerance limits tighter than commercial in some dimensions. It is best to discuss tolerances and AQL for your coil spring design with the spring maker. The data shown on this page represent typical values for production runs.

The manufacturing techniques and distortion in heat treating present so many variables for flat springs that it is not practical for us to present any standard tolerance values.

We can present some values for thickness tolerance for spring steel strip.

You should only use commercial stamping tolerances as a guide. Many flat springs are hardened after forming and this usually results in some type of part distortion. Special processing, like fixturing, can aid in holding very tight tolerances.

Good coil spring design practice means making bends in highly stressed areas with as large a radius as possible. This cuts down on potential stress concentration areas when the part is in service.

Most wire parts are made of pretempered or prehardened wire. Flat springs can be made of either annealed or pretempered spring material. This choice is normally dictated by cost or by material formability. See the charts on this page.

Tooling

A flat spring may perform any number of functions. There are infinite possibilities for design variations. This is the reason that flat springs normally require special tooling. These tools can be complex or simple. It depends on the design of the part, the tolerances specified, and the quantity required.

Normally, temporary tooling is used for prototype or sample quantities.

Again, because of all these variables, it is best to check with the spring manufacturer to make sure the design is cost efficient and sensible to manufacture.

Conventional blanking may create a burr which can be removed by an inexpensive tumbling or similar operation. This may not be a good operation for intricate or delicate parts. In some cases it is possible to control the burr so it will be on the side where it will not interfere with the part application.

It is a good practice on flat spring design to provide liberal radii on corners and formed bends. It is also good to provide sufficient length in these bends to eliminate unnecessary secondary operations. Also, keep all holes well away from edges and bends.

Electroplating

In coil spring design one can say, in most cases, plating for these types of metal parts is for corrosion protection, not for decoration. Zinc or cadmium plating is used, even if gaps in the plating exist, since these protect steel sacrificially. The electroplating process must be carried out with great care to minimize hydrogen embrittlement problems that will cause premature fracture. A baking procedure after plating is essential in this case.

The designer needs to consider these points before finalizing his coil spring design:

1. Minimize sharp corners and similar stress concentration points.
2. Keep hardness as low as possible, preferably below Rc 48.
3. Keep operating stress down, in accordance with lowered hardness value.
4. Specify the plating thickness depending upon requirements.
5. Specify that parts be baked after plating.
6. If a large amount of residual stress from forming operations is present prior to plating and is essential to part functioning, the part CAN NOT be electroplated.

Mechanical plating provides an effective means of zinc or cadmium protection with minimum hydrogen embrittlement. It is particularly recommended where parts have high residual stress, have been hardened above Rc 48, and are used with high static loads.

Other Finishes

In addition to plating, protection can be provided by any coating suitable for protection of metals. This includes paint, phosphate, oil, grease, wax, enamel, and plastic materials. The chart on this page lists some common finishes used for springs. Remember that a non-sacrificial corrosion barrier like paint may leave the spring vulnerable to stress corrosion failure if the paint chips off.

An alternative that should be looked at in all cases is preplated wire or strip material before finalizing the coil spring design.

Packaging

It is common for these types of parts to be shipped in bulk. On arrival at the customer they are poured or scooped out of containers. The result can be a tangled mass that is difficult and costly to untangle. This can also result in distortion of flimsy parts and possibly surface damage to the material.

Special packing in these cases may be required. The other advantages, other than the obvious, for this are:

• Inventory control-parts can be packaged in specific quantities per package.
• Automatic assembly-parts can be supplied in packages designed to coordinate with automation operations.
• Easier selection and inspection.
• Better control of work standards in assembly operations.

Testing Procedures and Quality Levels

Some spring configurations are difficult to inspect for dimensions or test for performance characteristics. It may be necessary for the designer to specify in the coil spring design how the spring is to be inspected or tested as well as what is to be measured. This is especially true of torsion springs.

Motor springs and belleville spring washer test results are affected by hysteresis in the load/deflection curves. In this case it is beneficial to come to an agreement with your customers testing department on the exact testing methods to be used. This may require testers duplicated at the customer’s and part manufacturer’s plants. Such a tester may include a portion of the mechanism actually used in the function of the part.

Some specifications can create both manufacturing and inspection problems. These include:

• Do not specify a test load at a given deflection from free position. Specify the test load at a specific height or angular position. Note that test loads that are specified at a height near the free height or near the solid height are difficult to maintain.
• Helical compression springs that are preset at a high stress level may be dynamically unstable and recover or grow in length during shipment. In this case, it should be noted that these springs must be preset prior to testing.
• Hardness test should be made on a scale appropriate to stock thickness.

The choice of inspection level and method is just as important with respect to cost and reliability as the choice of tolerances. Tight AQL may increase part cost both for additional operations as well as inspection methods needed. If the tolerances must be close for proper functioning and nonconforming parts can be discarded at assembly, then loose AQL will minimize the cost of parts.

The designer should allow freedom on all but essential dimensions in his coil spring design. The spring maker normally controls the load variations by altering one dimension to compensate for variations in another dimension. For instance, he may decrease the free length slightly if the wire is running on the high side of the tolerance.

Surface integrity is of utmost importance to springs used in cyclic service. Surface integrity can be checked using magnetic particle inspection. Parts made of non-magnetic materials can be inspected by fluid-penetrant techniques.

It is now possible to install mechanisms directly in-line in the manufacturing processes to inspect for surface defects. Defective spots on the wire are identified and the springs coiled with the suspicious areas are automatically rejected.

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