Accepted tolerances in compression spring production have been cut in half over the last three years. This requirement, going along with the need for zero ppm production, triggered the development of an enhanced series of machines for the high-speed production of compression springs. The specifications of these types of compression spring coilers were introduced for the first time to the public during the November 2005 Japan Spring Machine Show (JSMS).
Initially a 4.5mm size machine, the FUL 45 was presented to the industry on this occasion. In 2007, development of the whole series of machines, starting at a wire range of ?0.4mm (FUL 25) and ending at a wire range of 7mm the (FUL 75), was completed.
 Figure 1: Five sections in the geometry of a compression spring.
Figure 2: Samples of a typical compression spring.
Figure 7: A typical compression spring coiler (FSE 83).
Figure 3: “Simple” compression springs no longer exist.
Figure 4: Die springs with different spring loads.
Figure 6: Three sections of wave spring geometry.
Figure 5: A wave spring is made from flat material.
Figure 8: Drive concept.
Figure 9: A matching machine working range for
every customer application.
Figure 10: Compression spring coiler development
stages over the last 80 years.
Figure 11: The focus in machine development.
Figure 12: The new generation of x5 universal spring coilers.
Figure 13: The large electronic compression spring coilers.
Figure 14: Machine for a wire working
range of up to 1.6 mm.
Figure 16: Different feed rollers.
Figure 15: Machine for a wire working
range of up to 3.0 mm.
Figure 18: Different coiling pin back plates.
Figure 17: FUL 75 heavy-duty ø7.0 mm valve
spring coiler with four pairs of infeed rollers.
Figure 20: Differences in the PTP setup in a
single conical spring.
Figure 19: The PTP coiling finger.
Figure 24: PTP coiling fingers on every universal
compression spring coiler.
Figure 22: Pitch generation with the help of the PTP coiling finger.
Figure 21: Higher initial tension closes the spring at both ends.
 
Figure 23: PTP coiling fingers in a setup for
left- and right-hand coiling on an FUL 45.
Figure 25: Left, a typical single-layer wave ring;
right, multi-layer wave rings called “wave springs.”
Figure 26: The new generation of wave spring coilers.
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Figure 27: Wave spring coiler schematics
with five-roller coiling area.
Figure 28: Coiling area with a servo-driven
wave spring mechanism.
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Moreover, during the last two years additional new ideas have been implemented in the above-mentioned machines to allow for an even better spring production process. This article will offer a closer look at new possibilities in setup and programming of high-tech compression springs. Additional servo axes allowing control of the forming process of the spring coil will be explained and discussed in detail.
Specifics in Compression Springs
From the view point of a machine tool builder for the production of compression springs, his job is done when a machine is built that is capable of producing the required springs according to the technical drawings.
Material specifications are only taken into account as far as they influence the spring forming process. Post-processing operations, such as shot peening, spring setting or heat treatment are not looked at in this article.
A review of the basics for a compression spring
The geometry of a compression spring is typically described in five sections.
Figure 1, below, presents a schematic of these five typical sections within a spring. Each section requires special attention.
At the beginning of the coiling process (a) special attention from the machine setter has to be given to the roundness or form of the spring. The diameter of the first half coil of a spring is determined by the setting of the coiling fingers of the previously produced last coil of the spring. The pre-tension of the first coil of the spring is also determined by the setting of the coiling fingers.
Next, (b) while the pitch tool is forcing the wire into the required position for the spring pitch, coil diameter reductions occur. These have to be compensated by the respective movement of the coiling fingers on the form axis. This will avoid formation of hour-glass shaped spring geometries.
This very important section of a compression spring is also the area where adjustments to the spring geometry can be carried out without violating the required tolerances and dimensions of the spring, in case the spring load, the spring characteristics or the resonance frequencies, are not according to its specifications.
The third section (c) of the spring, the so called “constant middle part” of a spring, can vary greatly in its number of coils. While the shortest springs do not have a constant-pitch middle part at all, springs used in a fly wheel clutch application, for example, might have more then 50 coils within this middle section.
What is said about the second section of a compression spring applies in a similar way to the fourth part (d), the “spring coil area with the decreasing pitch.” Nevertheless, the wire forming process, going from a no-pitch area to a high-pitch area within a spring is different from going to a high-pitch area to a no-pitch area. It requires different programming of the spring coiler and different tool positions.
The last section (e) of a compression spring determines how straight the spring stands once it is ground according to the specific grinding requirements (Schliffbild). Again, the closing of the last coils is determined by the pre-tension produced based on the coiling finger position.
The “typical” compression spring and its tolerances
All five sections of a compression spring are very visible in the photo shown in Figures 1 and 2, page 34. Output rates of up to nearly 1,000 springs/minute in the small wire range are possible nowadays for these standard springs.
Another “typical” modern compression spring is shown in Figure 3, left. While on first glance it seems to be a very simple spring, the tolerances given on the spring length (Lo within ±0,05mm), the spring outside diameter (De within ±0,05mm) and the chosen material, hint that only a precision machine tool will be able to manufacture such a product.
A different type of compression spring
A technical discussion of the latest compression springs should also consider springs made out of flat wire instead of round wire.
Of course, die springs made out of a nearly square-shaped cross-sectional wire are well known in the press tooling industry. Figure 4, above left, shows such springs. These springs are known for their high grade of stiffness in a small area.
However, if the ratio of material width to material height becomes significant (>3), such flat wire material can be used in wave springs. Figure 5, left, presents a photo of a typical wave spring.
Compression springs of this type can be divided in three sections (Figure 6, left). The first section (a), which is the first coil of the spring, is completely flat. In the second section (b), which typically consists of three to five coils, a certain number of “waves” are formed. This is the elastic area, where the spring can be deformed in case a compression force is applied. The third section (c) is once again the last coil, which provides a flat surface for the spring to stand perfectly upright (even without grinding).
Compression springs typically are described with geometry parameters such as:
Wire diameter.
Number of coils.
Spring outside diameter.
Spring length.
In contrast, a wave spring is
described by:
Wire cross-section (width, height).
Number of coils.
Number of waves/coil.
Wave height.
Spring length.
Thus, wave springs provide new opportunities to create a new spring force diagram in a compact area.
Machines for the Production of Compression Springs
Each of the above-mentioned compression spring types have special requirements for the production machine. The following discussion will focus on the various spring coilers available to produce these springs.
The “standard” electronic spring coiler for the basic compression spring
The requirements
Figure 7, page 35, displays a fully electronically driven compression spring coiler. Freely programmable features are needed to cope with small batch sizes and quick change-over times from one spring size to the next.
The HMI (Human Machine Interface) input menu in Figure 8, page 35, is based on the previously mentioned typical geometry parameters describing a compression spring.
These production machines feature servo drives for the infeed unit, the pitch tools, the cutting unit and the shape tools as a standard. Optional products also provide servo-driven mandrel positioning. Servo-driven height positioning of the mandrel box is also available for such modern machines.
Typically, standard machines only use the “straight cut” mode for the cutting of the spring. Limits are set when it comes to the single coiling finger process or sophisticated computer programs needed, for example, for machine networking.
The FSE- machine program.
A carefully sized machine program for a wire working range from 0.16mm (2,600N/mm2 max. tensile strength) up to 8.0mm (2,000N/mm2 max. tensile strength) is shown in Figure 9, page 36. All these machines fulfill the requirements of a typical spring and can be programmed according to the five spring sections initially described. The programming can be carried out in high resolution.
The “universal” electronic coiler, also for the “high-end” compression spring
The history of compression machine development is briefly summarized in Figure 10, page 36.
The requirements
Requirements for universal spring coiling machines include:
High-speed infeed units.
Multi-cutting systems.
Shape tools, freely programmable for left- and right hand coiling in one- or two-finger coiling systems.
Touch screen panels.
Modern computer interfaces, PC Windows-based setup for networking.
Optical camera systems with servo-controlled sorting devices.
A graphical user interface to cope with the tight tolerances of the next generation of “simple” springs.
Figure 11, page 36, summarizes the above-mentioned features of the Wafios FUL spring coiling series version x5.
The FUL machine program.
An overview of the latest compression spring coiler series titled FUL x5 is given in Figure 12, page 36. The large high-end spring coilers have a working range up to ?17mm (2,000N/mm²) in the XL version. These are listed in Figure 13, left.
During the design phase for all these machines, special focus was given to the infeed unit, the cutting unit and the “coiling pin back plates,” in particular.
A new generation of feed rolls is applied to the small FUL 25 and FUL 35 machines (Figures 14 and 15, middle). By narrowing and changing the take-up of the infeed rolls, the camera installation allows for better monitoring of short springs. Moreover, the rusting (friction oxidization) of the infeed rolls over time due to micro motioning on the feed roll seat is eliminated. Three screws pull the feed roll tightly against a large cam support (Figure 16, bottom). This setup allows for easy change even after a long time.
An optional four-pair infeed unit with large infeed rolls, electronically adjustable for a size 6/7 machine, is a state-of-the-art necessity to meet automotive requirements (Figure 17, page 39).
Special attention was also given to the coiling-pin back plate of the FUL x5 machines. Up to four servo motors with gears without any play ensure that the programmed tool position is being held correctly. Any play or deflection, as seen in belt-driven mechanisms, cannot occur anymore.
The mechanics holding the coiling fingers are best supported if they are mounted directly on the machine body. A wide, cast coiling-pin back plate mounted on play-free linear guides provides a universal and stiff platform that will not give way – even if the tightest index springs of the maximum wire size of super clean high-tensile wire are produced. Figure 18, below, shows a two-motor and a four-motor solution for the two-finger coiling system as it is implemented on the FUL 75 spring coiler.
New possibilities – the PTP coiling finger
The platform, as seen in Figure 18, provides a new possibility for controlling the guidance of the wire during the coiling process. A Pre-Tension Positioning (PTP) coiling finger allows for servo-driven adjustment of the coiling fingers within a production cycle (Figure 19, page 40).
As shown in Figure 19, a servo motor moves the tip of a coiling finger – or even better, a coiling finger insert – either toward the machine operator or against the machine front. The coiling finger is not twisted or turned to steer the wire in the desired direction, however.
An uplifting of the wire, guided on the complete width of the coiling finger, avoids unacceptable edge pressure, which causes damage to the sensitive wire surface. Furthermore, as experienced in extensive tests, the wear of the tip of the coiling finger is significantly less than when using a device that rotates the coiling finger to direct the wire.
Commonly known as the “pre-tension” adjustment for a spring, this was performed by mechanical means in a rotary fashion on the previous generation of compression spring coilers. Today, servo-driven adjustment for each of the five spring sections is now possible within each spring production cycle.
Figure 20, page 40, shows on the left the results for a single conical compression spring set up in the traditional way; the once chosen rotating position of the coiling finger is kept constant throughout the production process. While the first coil is closed due to the correct setup of the coiling finger (first section), it has an open end on the last coil. The right side of Figure 20 shows the results when the spring is produced with changing pre-tensioning position throughout the spring production cycle.
The same results can be recognized on a ground compression spring, as shown in Figure 21, page 41. The spring ends were still closed when the spring fell off the machine. After grinding the spring ends, the initial tension in the wire end pieces made them turn upward. This does not occur if the pre-tensioning is done with servo control in the right way in each section of the compression spring.
The possibilities of the PTP tool are displayed in Figure 22, page 41. Not using the pitch tool, it is possible to generate either high initial tension or even pitch without any geometry changes using the PTP coiling finger. A tension spring body is used to demonstrate these possibilities.
For ease of operation in case a lot of changes between left- and right-hand coiling setups take place, two of these fingers can be installed on the machine shown in Figure 23, page 41, an FUL 45. The PTP coiling finger is available for the whole FUL x5 machine range (Figure 24, page 41).
Concluding this chapter on the new possibilities in compression spring production, it becomes clear that this tool is extremely helpful to fulfill the ever tightening spring tolerances of the automotive industry. The positioning of the coiling fingers allows for efficient and precise geometry corrections within a spring production cycle.
The “specialty” spring coiler for a different kind of compression spring
Compression springs typically are made out of round or nearly square-shaped cross-sectional wire. However, wave springs used in typical compression spring applications are made from flat wire (strip material).
Spring details
One-layer or multi-layer wave rings are shown in Figure 25, page 41. Whether these springs have a flat beginning or end depends on the application. The number of waves per coil also vary based on the requirement. While small springs typically consist of coils with four waves, large rings of more than 10 waves are also known.
Another parameter not known to a typical compression spring is the wave height, which can vary greatly based on the desired spring characteristics. Being somewhat comparable to the pitch required in a round wire compression spring, the multiple waves within one coil and the friction in the contact area of the upper and lower coil do not allow for the use of standard calculation programs for compression springs. In fact, no widely accepted theory or model exists to calculate the possibilities of this type of spring.
The production process
Special spring coiling machines that guide the flat wire in an efficient way are needed to create wave springs. Therefore, a new generation of machine series, the SNA, has been introduced in different sizes to the market. Figure 26, page 42, displays photos of the SNA 22 and the SNA 33.
The working principle of these machines is shown in the schematics of Figure 27, page 42. Two pairs of infeed rolls feed the flat wire into the coiling area. The infeed unit is tilted 90° so that the feed rolls are in contact with the wire on the wide flat surface. This ensures a perfect grip and no slippage.
A wave ring unit is utilized to create the pitch for the wave spring to be produced (Figure 28, page 42). At the position of the upper coiling finger in this two-finger coiling system, a fork-like tool generates the pitch.
Synchronization of this servo-driven tool with the infeed unit is essential to ensure the correct position of the wave in each coil of the wave spring. Wave height can be programmed with the help of an Industrial Personal Computer (IPC) user front end.
The cutting process for flat wire differs greatly from the cutting process for round wire. A cutter-mandrel setup would deform the cross-section of the wire in the cutting area. Therefore, a press with the respective press tool is used to provide an accurate clean cut. Depending on the type of machine, press force between 50kN (SNA 22) and 100kN (SNA 33) is available on these machines.
The whole coiling process is set up in an upright vertical position. Similar to a standard spring coiling machine, the drop-out of the produced part is supported by gravity and controlled by a sensor.
Summary
Compression springs are the backbone of automotive technology. Although most people think of valve and suspension springs for vehicles, there are countless more applications – not easily noticeable – that require precise compression springs.
The rapid development in automotive technology, therefore, does not surpass the spring industry. Tighter tolerances, less available space and higher spring loads push spring development a step further.
A PTP coiling finger is just one tool in supporting the springmaker in producing precision springs at a zero ppm level.
Wave springs in dynamic applications are not yet implemented. Knowledge of the friction among the spring coils has not been investigated extensively. Nevertheless, material know-how and FEM programs will help provide all the necessary information for the springmaker to have a new generation of spring types on hand.
Dr.-Ing. Thomas Blum, who has a doctorate in engineering, was the director of engineering at Wafios Aktiengesellschaft when he wrote this article. He has since moved on to other professional pursuits. Readers may contact Wafios by phone at +49 7121-1460, fax at +49 7121-45218, e-mail at sales@wafios.de or Web site www.wafios.de. With headquarters in Reutlingen, Germany, Wafios also has manufacturing and service centers in the U.S., Canada and Brazil.
This paper was originally presented at the Japan Society of Spring Engineers (JSSE) International Symposium, Nov. 1-3, 2007, Nagoya, Japan.
