Description

Technical Field

[0001] The invention relates to a method of manufacturing a spring comprising a steel wire and a silane based coating.

The invention further relates to a spring comprising a steel wire provided with a silane based coating.

Background Art

[0002] Springs are well known and widely used. Protective coatings have been used to protect the steel wires from corrosion. Corrosion protection is for example important when springs are used in in extreme environments.

[0003] It is common practice to apply a protective coating on the spring. Such protective coatings are generally applied by spraying or dipping

techniques. In the manufacturing methods known in the art, the protective coating is applied on the coiled springs. This means that first the spring wire is coiled to form the spring and that subsequently the protective coating is applied on the coiled spring. Such manufacturing method has a number of drawbacks. The main drawbacks are that the manufacturing process is expensive and results in springs having a non-uniform coating. Therefore there is a need to provide an improved method of manufacturing coated springs.

Disclosure of Invention

[0004] It is an object of the present invention to provide a method of

manufacturing a spring comprising a steel wire and a silane based coating avoiding the drawbacks of the prior art.

It is another object of the present invention to provide a method of manufacturing a spring whereby a coated wire is coiled.

It is a further object of the present invention to provide a spring provided with a coating whereby the coating is concentric and has a low thickness, for example a thickness lower than 10 μηη.

It is still a further object of the present invention to provide a spring having good corrosion resistance.

[0005] According to a first aspect of the present invention a method of

manufacturing a spring coated with a coating layer is provided.

The method comprises the steps of

- providing a steel wire;

- coating said steel wire with a silane based coating in the range of 4 to 10 Mm, and preferably in the range of 5 to 8 μηη, to provide a coated steel wire;

- subjecting said coated steel wire to a mechanical deformation to obtain a coated and mechanical deformed steel wire;

- subjecting said coated and mechanical deformed steel wire to a stress relief treatment.

[0006] It can be preferred to add one or more curing or drying steps after the application of the coating. The curing is for example UV curing or IR curing, for example near infra-red curing.

[0007] For the purpose of this invention with "silane based coatings" is meant any coating comprising an organofunctional silane. In the present invention, term "silane based coatings" is meant the main component of coating is an organofunctional silane. In the other word, the coating may contain other components including trace components which are less than the organofunctional silane. In case of 100 wt parts silane based coating, the range of the quantity of an organofunctional silane is at least more than 51 wt parts, and preferably the quantity of an organofunctional silane is more than 70 wt parts and more preferably more than 90 wt parts. The organofunctional silanes are usually used as coupling agents between an inorganic, e.g. a metal or glass, and an organic phase, e.g. a polymer or resin, in order to enhance the adhesion between the two phases. In the present invention, the silane based coating applied on the steel wire has multiple functions and is much thicker than the silane based coating usually applied between an inorganic and an organic phase as a coupling agent. The silane based coating according to the present invention has a thickness in the range of 4 to 10 μηη; and preferably has a thickness ranging between 5 and 8 μηη. Herein, the thickness of silane in the range of 4 to 10 μηη refers to the average thickness of silane over the coated steel wire product. The coating is preferably homogeneously distributed and has uniform thickness over the coated surface of the steel wire.

However, the thickness may have some deviations, i.e. exceed 10 μηη and may lower than 4 μηη at certain locations of the coated steel wire. This quantity is distinguishing with the normal dose for applications of silane based polymer between an inorganic and an organic phase as coupling agents. Usually one or more monolayer of silane as coupling agent is applied to the surface of inorganic substrate. The quantity of the coated silane as a coupling agent is desired to provide adhesion between inorganic and organic phase. The silane based polymer of the present invention has multiple functions and its thickness is significantly thicker than the normal use of silane as coupling agent. The silane based coating of the present invention can adhere well to steel wires and provide corrosion resistance for the steel wires. Since it is available in several colors, it can provide aesthetics for the wires. Importantly, the silane based coating has sufficient flexibility and durability such that e.g. the coated steel wire can survive the spring making operation. The silane based coating also has sufficient heat resistance such that it can be survived e.g. in a stress relieving heat treatment after spring making. These advantages make it possible to coil, bend or mould a coated steel wire with silane based coating into a spring. Spring making from a coated steel wire has cost competitive advantages over a post coating operation where a steel wire without polymer coating is first coiled, bent or moulded into a spring and afterwards the spring is coated with polymers. Organofunctional silanes comprise in general an organic functional group Y, linked through a spacer R to one or more silicon atoms having functional groups X. [0008] Preferably, the organifunctional silane has the following formula

Y-R-SiXs

whereby

- X comprises a first functional group, each of these first functional

groups being independently selected from each other;

- R comprises a spacer;

- Y comprises a second functional group.

[0009] The first functional group X represents a silicon functional group, each of the silicon functional groups being independently selected from the group consisting of -OH, -R', -OR', -OC(=O)R' and the halogens such as -CI, -Br, -F, whereby -R' is an alkyl, preferably a C1 -C4 alkyl, most preferably -CH3

[0010] Preferaby, at least one of the silicon functional groups X comprises a

hydrolysable alkoxy group (-OR).

[001 1] Functional group Y preferably comprises at least one of the following

groups : an amine group (-NH2, -NHR', -NR'2), an unsaturated terminal double or triple carbon-carbon group, an acrylic, methacrylic acid group and its methyl or ethyl esters, -CN, -SH, an isocyanate group, a

thiocyanate group or a cyclic ether group as for example an epoxy group or an oxetane group.

Preferred functional groups Y comprise an amine group, a vinyl group, an epoxy group or an oxetane group.

In case the functional group Y comprises an epoxy group, this epoxy group forms preferably part of a glycidoxy group.

[0012] Spacer R preferably comprises an alkyl group or an alkoxy group. This alkyl or alkoxy group preferably has from 1 to 6 carbon atoms. Examples comprise methyl, ethyl, propyl, isopropyl, butyl, isobutyl, t-butyl, pentyl, hexyl, methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy, t-butoxy, pentyloxy and hexyloxy. [0013] The organic functional group Y may react with a polymer backbone. The at least one hydrolysable group OR' can react with water, forming reactive silanol groups (SiOH), and then bond to a substrate or self-condense to form siloxane crosslinks (SiOSi).

[0014] Examples of suitable commercially available organofunctional silanes

include

amino silanes such as 7-aminopropyltriethoxysilane,

glycidoxy alkyl alkoxy silanes such as (3-glycidoxy propyl)- trimethoxysilane, (2-glycidoxyethyl)-trimethoxysilane, (3-glycidoxy propyl)-triethoxysilane, (2-glycidoxyethyl)-triethoxysilane and 3- glycidoxypropyl methyl diethoxysilane.

epoxy cyclohexylalkylalkoxy silanes such as [β (3,4-epoxy cyclohexyl)ethyl] triethoxysilane;

silanes comprising an oxetane group such as 3-alkyl-3-[(trialkoxy silyl alkoxy)methyl] oxetane, for example 3-methyl-3-[(3-trimethoxy silyl propoxy)methyl] oxetane.

[0015] The coating can be applied by any technique known in the art, for example by dipping or by spraying. A preferred spraying technique comprises electrostatically assisted spraying.

In electrostatically assisted spraying a coating is applied by applying a static electricity charge to the droplets of a spray and an opposite charge to the part being sprayed, which then attracts the droplets directly to its surface. Electrostatically assisted spraying allows obtaining a

homogeneous coating.

[0016] Preferably, the coating composition further comprises a photoinitiator.

Preferred photoinitiators comprise cationic photoinitiators. [0017] According to the present invention, the steel wire coated with the silane based coating is subjected to a mechanical deformation.

Any type of mechanical deformation can be considered. Preferred deformation processes comprise bending or coiling. As coiling cold winding as well as hot winding can be considered.

[0018] By the mechanical deformation step such as the coiling step, stresses are created within the material. To relieve theses stresses and to allow the spring to maintain its characteristic resilience, the spring is subjected to a stress relief treatment. Therefore, the spring is preferably subjected to a heat treatment. The spring is thereby heated at a predetermined temperature during a predetermined time period.

The spring is for example heated to a temperature above 200 °C, above 250 °C, above 300°C or even above 350 °C and this during a time period of 10 minutes, 15 minutes, 30 minutes or even 45 minutes.

A typical heat treatment comprises heating to a temperature of 250 °C during 45 minutes, or heating to a temperature of 280 °C during 30 minutes.

[0019] Preferably, the spring is cooled after the stress relief treatment.

[0020] Possibly, the method comprises additional steps, for example finishing steps such as grinding and/or shot peening.

[0021] The steel wire may be made of any kind of steel suitable for coiling

springs, examples are steel with a high carbon content, stainless steel, chromium-vanadium alloyed steel, chromium-silicon alloyed steel and chromium-silicon-vanadium alloyed steel.

[0022] The wire used for the manufacturing of springs may have different cross- sections such as round, square, rectangular, oval, half oval, half round, trapezoidal, triangular cross-sections.

Compression springs are preferably made from round wire. In a preferred embodiment compression springs are made from wires having a diameter ranging between 0.1 and 10 mm.

Torsion or power springs are preferably made from flat wire.

This flat wire has preferably a width between 1 and 30 mm, for example between 5 and 25 mm. The thickness of the flat wire is preferably between 1 and 6 mm.

[0023] Preferably, the steel wire is provided with a zinc coating or a zinc alloy coating before the silane based coating is applied.

Zinc alloy coatings comprise for example brass coatings, zinc aluminium coatings or zinc aluminium magnesium coatings. A further suitable zinc alloy coating is an alloy comprising 2 to 10 % Al and 0.1 to 0.4 % of a rare earth element such as La and/or Ce.

[0024] An important advantage of the method of manufacturing springs according to the present invention is that it allows the mechanical deformation of the coated steel wires to make springs. This is a main advantage compared to coated springs known in the art. For the coated springs known in the art the springs are first coiled and the coiled springs are coated in a

subsequent step. Springs manufactured by such methods have the drawback that it is difficult to obtain a uniform coating. Furthermore the manufacturing process is expensive.

[0025] The method according to the present invention allows to manufacture

coated springs having a uniform and homogenous coating that is completely covering the surface of the wire. For a spring manufactured by the method according to the present invention the wire is completely coated. Even for springs having no gap or very small gaps between adjacent coils, the full surface of the wire is coated also between two adjacent coils.

[0026] The method according to the present invention allows to manufacture a coated spring wire having a high degree of concentricity. Furthermore the method according to the present invention avoids post- coating process steps.

[0027] A further advantage of a spring manufactured by the method of the

present invention is that the coated spring has a high corrosion resistance.

[0028] It is clear that a coating used to coat a spring according to the present invention needs to survive the spring-making process.

In order to do so the coating material should be flexible. Furthermore the coating should resist the temperatures reached during the coil making step and/or during the stress relief treatment. The coating should preferably resist a temperature of 200 °C, 250 °C, preferably 300°C or even 350 °C and this preferably during a time period of at least 10 minutes, for example 15 minutes, 30 minutes or 45 minutes.

[0029] According to a second aspect of the present invention a spring having a silane based coating is provided. The spring comprises a steel wire coated with a silane based coating in the range of 4 to 10 μηη, preferably in the range of 5 to 8 μηη. The silane base coating is applied on the steel before the coated steel wire is coiled to form the spring.

[0030] The coated wire according to the present invention can be used to

manufacture any type of springs. Springs are defined as elastic bodies used to store mechanical energy.

Springs can be classified in different types depending on how the load force is applied to the springs. Different types of springs are tension springs, compression springs and torsion springs:

Tension springs are designed to operate with a tension load. This means that a tension spring stretches as the load is applied to the spring.

Compression springs are designed to operate with a compression load. This means that a compression springs get shorter as the load is applied to the spring.

Torsion springs are designed to operate with a twisting force. The end of the spring rotates through an angle as the load is applied. [0031] Springs can also be classified depending on their shape. One can for example distinguish helical springs and flat springs (leaf springs).

Helical springs are made of wire coiled in the form of a helix.

Flat springs or leaf springs are made of a flat or conical shaped metal.

[0032] With the method according to the present invention it is possible

manufacture springs having a spring index lower than 10, for example lower than 9, lower than 8 or even lower than 7.

The spring index is defined as the ratio of the mean coil diameter (Dmean) to the wire diameter (d) : D _m ean/d.

The mean coil diameter Dmean is the average of the outer diameter of the coil and the inner diameter of the coil.

[0033] The method according to the present invention is also suitable to

manufacture closed springs, i.e. springs having no or very small gaps between adjacent coils. Such coils are difficult to manufacture in a post coating method as it is difficult or even impossible to apply the coating on the wire between two adjacent coils having no or very small gaps between adjacent coils.

Brief Description of Figures in the Drawings

[0034] The invention will now be described into more detail with reference to the accompanying drawings whereby

- FIGURE 1 shows a helical coiled coated spring according to the

present invention;

- FIGURE 2 shows the cross-section of a coated wire of a coated spring according to the present invention.

Mode(s) for Carrying Out the Invention

[0035] The present invention will be described with respect to particular

embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

[0036] Figure 1 shows the cross-section of a spring 100 according to the present invention. The spring 100 comprises a coated steel wire 102. Figure 2 shows the cross-section of the coated steel wire 102. The coated steel wire has a diameter d of 1.6 mm. The coated steel wire 102 comprises a core of high carbon steel wire 104. A zinc or zinc alloy coating 106 is applied on the steel wire 104. A silane based coating 108 is applied on the zinc or zinc alloy coating 106. The silane based coating 108 comprises for example (3-glycidoxy propyl)-trimethoxysilane. The silane based coating 108 is preferably applied by electrostatically assisted spraying. Possibly, the silane based coating 108 further comprises a photoinitiator such as a cationic photoinitiator. The thickness of the silane based coating 108 is in the range of 4 to 10 μηη and preferably in the range of 5 to 8 μηη.

[0037] The coated steel wire is subsequently subjected to a mechanical

deformation step, for example a coiling step. The spring is for example coiled to form a spring having a spring index equal to 10 or to a spring having a spring index equal to 8.

[0038] To relieve the stresses induced during the coiling, the coated and

mechanical deformed steel wire is subjected to a stress relief treatment. The steel wire is for example subjected to a heat treatment comprising heating to a temperature of 250 °C during 45 minutes or heating to a temperature of 280 °C during 30 minutes.

[0039] To evaluate the corrosion resistance of a coated spring according to the present invention the coated springs are subjected to a salt spray test according to ASTM B1 17. The time until the first with rust appears is determined.

Three different springs are compared in table 1. All the springs are made of a steel wire, coated with a zinc or zinc alloy coating and a silane based coating (3-glycidoxy propyl)-trimethoxysilane).

The first spring comprises a compression spring having a spring index equal to 10, the second spring comprises a compression spring having a spring index equal to 8 and the third spring comprises a tension spring having a spring index equal to 10.

Column 3 of Table 1 shows the corrosion resistance of the coated wire of the springs (expressed in hours till the first white rust appears). The corrosion resistance of the coiled springs is tested at different

temperatures. Therefore the coiled springs are subjected to room temperature, to a temperature of 250 °C during 30 minutes and to a temperature of 270 °C during 30 minutes. The results of the corrosion tests are given in Table 1.

Table 1

From the results of Table 1 one can conclude that good corrosion resistance is maintained even after subjecting the spring to a heat treatment.