METHOD FOR MAKING A SPRING CORE FOR A MATTRESS OR FOR SEATING PRODUCTS

Abstract

A method to manufacture a steel wire spring core for a mattress or for seating is described, which comprises the steps of providing a carrier comprising steel wire; repeatedly cold coiling a steel wire spring from steel wire taken from the carrier; and connecting a series of the coiled steel wire springs to each other. The steel wire has a diameter d between 0.8 and 4.5 mm; and has a drawn pearlitic microstructure. The steel wire comprises a steel alloy having a carbon content between 0.35 wt % and 0.85 wt %. The steel wire on the carrier has a ratio—expressed as a percentage—of the yield strength R.sub.po 2 (in MPa) over the tensile strength R.sub.m (in MPa) higher than 85%.

Claims

1. A method to manufacture a steel wire spring core for a mattress or for seating, comprising the steps of providing a carrier comprising steel wire; repeatedly cold coiling a steel wire spring from steel wire taken from the carrier, and connecting a series of the coiled steel wire springs to each other; wherein the steel wire has a diameter d between 0.5 and 4.5 mm; wherein the steel wire comprises a steel alloy, wherein the steel alloy has a carbon content between 0.35 wt % and 0.85 wt %; wherein the steel wire has a drawn pearlitic microstructure; wherein the steel wire on the carrier has a ratio—expressed as a percentage—of the yield strength R.sub.p0.2 (in MPa) over the tensile strength R.sub.m (in MPa) higher than 85%.

2. The method according to claim 1, wherein the steel alloy has a carbon content higher than 0.6 wt %, preferably higher than 0.7 wt %.

3. The method according to claim 1, wherein the carrier is a bobbin onto which the steel wire is wound.

4. The method according to claim 1, wherein the steel alloy comprises between 1.3 and 1.6 wt % Si; and between 0.6 and 0.9 wt % Cr.

5. The method according to claim 4, wherein the steel alloy consists of between 0.35 and 0.85 wt % C, between 1.3 and 1.6 wt % Si, between 0.6 and 0.9 wt % Cr, unavoidable impurities and the remainder being iron.

6. The method according to claim 1, wherein the steel alloy comprises between 0.02 and 0.06 wt % aluminum.

7. The method according to claim 1, wherein more than 120 steel wire springs are manufactured per minute.

8. The method according to claim 1, wherein the tensile strength R.sub.m (in MPa) of the steel wire is higher than the value obtained via the formula 2200−390.71*ln(d); wherein d is the diameter of the steel wire in mm.

9. The method according to claim 1, wherein the steel wire does not comprise a metallic coating layer.

10. The method according to claim 1, wherein the steel wire is provided with a metallic coating, preferably wherein the microstructure of the metallic coating comprises a globularized aluminum rich phase.

11. The method according to claim 1, wherein connecting a series of the coiled steel wire springs to each other is performed by inserting the coiled steel wire swings in compressed state in pockets made from a cloth, wherein a linear string of pocketed springs is obtained.

12. The method according to claim 11, wherein the pockets are formed from one single piece of cloth and wherein pockets are closed and linearly bonded to each other by means of welded bonds.

13. The method according to claim 1, wherein a two-dimensional matrix of coiled steel wire springs is provided, wherein the plane of the two-dimensional matrix is perpendicular to the longitudinal axes of the coiled steel wire springs; wherein the coiled steel wire springs are encased in pockets; wherein the pockets are formed by a first fabric ply on top of the coiled steel wire springs, by a second fabric ply below the coiled steel wire springs and by seams between the first fabric ply and the second fabric ply, wherein the seams surround the coiled steel wire springs.

14. The method according to claim 1, comprising the step of connecting the coiled springs to each other by lacing a steel wire through the coiled springs.

15. The method according to claim 1, wherein a multitude of steel wire springs are coiled without cutting the steel wire such that the steel wire runs continuously through the multitude of steel wire springs in the spring core.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

[0056] FIG. 1 illustrates the tensile stress-strain curve of a steel wire.

[0057] FIG. 2 shows a pocketed spring mattress core as can be made using the method of the invention.

[0058] FIG. 3 shows an example of a Bonnell spring.

[0059] FIG. 4 shows a Bonnell spring core for a mattress, as can be made using the method of the invention.

[0060] FIG. 5 shows an example of an LFK spring.

[0061] FIG. 6 shows an LFK spring core for a mattress, as can be made using the method of the invention.

[0062] FIG. 7 shows a continuous spring, as can be made using the method of the invention.

[0063] FIG. 8 shows another type of steel wire spring core wherein the steel wire spring are encased in fabric pockets.

MODE(S) FOR CARRYING OUT THE INVENTION

[0064] FIG. 1 provides information about the way the mechanical properties of the steel wires are described in this document. The mechanical properties are described and tested according to ISO 6892-1:2016 (which is entitled “Metallic materials—Tensile testing—Part 1: Method of test at room temperature”). FIG. 1 schematically illustrates a stress-strain curve of a steel wire in an uniaxial tensile test. In the X-axis, the strain is provided. The vertical (Y) axis provides the tensile stress (in MPa). The elongation at breakage is represented by A.sub.t. The tensile strength R.sub.m is the maximum stress. The yield strength R.sub.p0.2 is the stress when crossing the tensile curve with the line through 0.2% strain and parallel with the elastic modulus line.

[0065] FIG. 2 shows a pocketed spring mattress core as can be made using the method of the invention. FIG. 3 shows an example of a Bonnell spring. FIG. 4 shows a Bonnell spring core for a mattress, as can be made using the method of the invention. FIG. 5 shows an LFK spring. FIG. 6 shows an LFK spring core for a mattress, as can be made using the method of the invention. FIG. 7 shows a continuous spring as can be used to manufacture a mattress core using the method of the invention.

[0066] FIG. 8 shows another type of steel wire spring core wherein the steel wire springs are encased in fabric; and that can be made with a method according to the invention. The steel wire springs are positioned in a two dimensional matrix. On top and below the two dimensional array of steel wire springs a nonwoven fabric is provided. The pockets are formed by a first nonwoven fabric on top of the coiled steel wire springs, by a second nonwoven fabric below the coiled steel wire springs and by seams between the first nonwoven fabric and the second nonwoven fabric. The seams surround the coiled steel wire springs. The seams can be established by thermal welds (e.g. made by means of ultrasonic welding equipment) bonding the two nonwoven fabrics to each other.

[0067] A first series of experiments related to pocketed spring cores for mattresses. A 2 mm diameter steel wire was used, made out of a steel alloy consisting out of between 0.71 and 0.75 wt % carbon, between 0.6 and 0.9 wt % manganese, at maximum 0.03 wt % aluminum; unavoidable impurities, and the balance being iron. A 2 mm diameter steel wire has been drawn starting from a wire rod of 5.5 mm diameter. The tensile properties of the steel wire have been tested according to ISO 6892-1:2016: tensile strength R.sub.m=2018 MPa, R.sub.p0.2=1507 MPa (meaning that the R.sub.p0.2 is 75% of the tensile strength R.sub.m), and elongation at breakage=3%. A bobbin of steel wire has been treated in a furnace at 300° C. during 2 hours. After this heat treatment, the tensile properties have been tested again: tensile strength R.sub.m=2052 MPa, R.sub.p0.2=1823 MPa (meaning that the R.sub.p0.2 is 89% of the tensile strength R.sub.m), elongation at break 7%.

[0068] Helically coiled springs have been made according to the pocketed spring design with the steel wire that had been treated in the furnace as described in the previous paragraph. The spring height was 210 mm, the springs had diameter 80 mm and the springs had 7 coils. The springs have been tested according to Brazilian standard ABNT 15413-1:2013; entitled “Spring mattress and bases—part 1: Requirements and test methods”. This part of ABNT NBR 15413 establishes the requirements and test methods for spring mattresses and bases. The test method described in this standard involves compressing a single spring by hand to full compression during 10 seconds. After removing the load and allowing the spring to recover, a new compression cycle by hand to full compression is performed during 10 seconds. After removing the load and allowing the spring to recover, a new compression cycle by hand to full compression is performed during 60 seconds. After removal of the load, the loss of height of the spring compared to its initial height is measured and expressed as a percentage of the initial height of the spring. A maximum height loss of 8% is accepted according to this Brazilian standard. This test performed on the helically coiled springs with the steel wire that had been subjected to the thermal treatment showed no height loss.

[0069] The fatigue resistance of the spring cores made according to the method of the invention have been tested: the outcome is that the helically springs made with steel wire that has been subjected to the heat treatment are highly resistant to fatigue resistance.

[0070] A second series of tests related to steel wires for making Bonnell spring cores. A 2.2 mm diameter steel wire was made out of a steel alloy consisting out of between 0.55 and 0.59 wt % carbon and between 0.6 and 0.9 wt % manganese, at maximum 0.03 wt % aluminum; and unavoidable impurities, the balance being iron. The steel wire was drawn to 2.2 mm diameter starting from a wire rod of 5.5 mm diameter. The tensile properties of the steel wire have been tested according to ISO 6892-1:2016: tensile strength R.sub.m=1415 MPa, R.sub.p0.2=1050 MPa (meaning that the R.sub.p0.2 is 74.2% of the tensile strength R.sub.m), and elongation at breakage=3.25%. Bobbins of steel wire have been treated in a furnace at different temperatures during one hour. After this heat treatment, the tensile properties have been tested again, the results are given in table I. The first column of table I indicates the temperature at which the heat treatment in the furnace has been performed.

TABLE-US-00001 TABLE I Tensile test results of steel wire from heat treated bobbin Temperature R.sub.m R.sub.p0.2 R.sub.p0.2/R.sub.m Elongation (° C.) (MPa) (MPa) (%) (%) 200 1507 1427 94.7 1.5 220 1526 1453 95.2 2 240 1547 1465 94.7 2 260 1557 1449 93.1 3.5 280 1525 1420 93.2 2.5 300 1517 1427 94.0 5

[0071] The table hereunder illustrates how the invention may be applied to realize weight savings in steel wire spring cores for mattresses or for seating.

TABLE-US-00002 TABLE II Wire diameter (mm) d 2 1.8 Minimum tensile strength (Mpa) R.sub.m 1800 1800 Spring outer diameter (mm) D.sub.outer 65 65 Free length of spring (mm) L.sub.free 160 160 Solid height (mm) L.sub.solid 14 9 Max. Working length (mm) L.sub.n 35.9 31.7 Number of active coils N.sub.a 6 4 Spring index C 31.5 35.1 Wahl correction factor W 1.0 1.0 Coil pitch (mm) Pitsch 26.3 39.6 Rise angle (°) Θ 7.6 11.3 Spring rate (N/mm) R 0.106 0.103 Max. Load at solid height (N) F.sub.max 15.5 15.6 Max. Total shear stress (MPa) T.sub.max 324.4 448.2 Max. Working load (N) F.sub.max load 13.2 13.3 Max. Working shear stress (MPa) T.sub.max load 275.8 380.9 Max. Allowed shear stress (MPa) T.sub.zul 1008 1008 Safety margin on shear 3.11 2.25 Max. Displacement possible (mm) L.sub.def 146 151 Required length to make the spring L.sub.wire 1.6 1.2 (mm) Mass of the spring (kg) m.sub.spring 0.039 0.024

[0072] Following the results of Table II, 39% of weight savings are realized.