WO2017102478A1 - Flexible rack with steel cord embedded in polymer - Google Patents

Flexible rack with steel cord embedded in polymer Download PDF

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Publication number
WO2017102478A1
WO2017102478A1 PCT/EP2016/079989 EP2016079989W WO2017102478A1 WO 2017102478 A1 WO2017102478 A1 WO 2017102478A1 EP 2016079989 W EP2016079989 W EP 2016079989W WO 2017102478 A1 WO2017102478 A1 WO 2017102478A1
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WO
WIPO (PCT)
Prior art keywords
flexible rack
steel cord
diameter
steel
polymer
Prior art date
Application number
PCT/EP2016/079989
Other languages
French (fr)
Inventor
Katrien Bert
Raf CLAUWS
Filip De Coninck
Chris Dhulst
Original Assignee
Bekaert Advanced Cords Aalter Nv
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Bekaert Advanced Cords Aalter Nv filed Critical Bekaert Advanced Cords Aalter Nv
Priority to EP16808614.8A priority Critical patent/EP3390701A1/en
Publication of WO2017102478A1 publication Critical patent/WO2017102478A1/en

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Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16H—GEARING
    • F16H55/00—Elements with teeth or friction surfaces for conveying motion; Worms, pulleys or sheaves for gearing mechanisms
    • F16H55/02—Toothed members; Worms
    • F16H55/26—Racks

Definitions

  • the invention relates to a flexible rack for use in a drive system.
  • flexible rack is specifically designed to move parts back and forth such as for example a rapier in a textile loom, a door panel in an elevator, a window pane in a vehicle, a sunroof in a car or any other like applications.
  • One system for opening or closing sunroofs comprises cords that are held under tension by and guided over pulleys (see e.g. US3137491 ).
  • the system needs a lot of pulleys, takes quite some space (as the cords must go back and forth) and is not easy to mount and repair.
  • An alternative system is based on toothed belts that are held under tension (US
  • DE 1003517 is an early example of this wherein a metal wire, metal strand or metal cord is coated with a polymer jacket that is provided with teeth imprinted into the jacket.
  • the teeth can extend over about half of the perimeter of the cable or they can be provided as pockets, cavities, recesses or depressions wherein the teeth of the drive gear engage.
  • the central wire can be off centre or can be central.
  • the cross section of the flexible rack can be substantially round or can be of oval shape.
  • EP 0499135 describes a method to produce such a cable by embossing the teeth profile into the still soft jacket of the freshly extruded polymer jacket on a steel cord.
  • 5150631 suggests to use a steel band provided with openings where the pockets, recesses or cavities should occur. It is also suggested to have two steel cords at either side of the recesses.
  • DE 10 2012 007 104 suggests to use carbon, glas or aramid fibres instead of or in combination with a steel cord.
  • the suggested alternative must be easily mountable by preference as easily mountable as the flocked cable into the guiding parts of the drive system. It must also be wear resistant and survive at least ten thousands back and forth cycles of compression and tension. As for example a door pane with a window elevator system can get very hot in summer and very cold in winter the rack must remain functional between -40°C and +90°C.
  • the flexible rack is intended for use with a drive system with a drive gear.
  • the flexible rack comprises a steel cord embedded in a polymer jacket.
  • the polymer jacket is provided with regularly spaced recesses arranged along a straight line in the longitudinal direction of the flexible rack.
  • the recesses are for receiving the teeth of the aforementioned drive gear.
  • the recesses have closed side walls with tops (i.e. the hills that are between two recesses) that are strictly within the circumscribed circle of the flexible rack in a cross section.
  • the steel cord comprises steel filaments that are twisted together into a strand.
  • the steel cord may comprise strands that are twisted together into a rope.
  • a steel cord can be stranded out of two, three or more strands twisted together and forming a single steel cord.
  • a 'steel cord' therefore may refer to a strand or a rope.
  • the steel cord may also comprise other non-steel components like yarns or monofilaments of an organic nature.
  • the steel cord is embedded in a polymer jacket. Thereby it is understood that the steel cord is completely surrounded, encased, covered by the polymer and by preference the surface of the steel cord is insulated from its surrounding. Such insulation from the environment is important to prevent wear and tear of the steel cord but also to exclude corrosion.
  • the polymer must remain sufficiently stiff at high temperatures of for example 90°C or more and must not become brittle at temperatures down to -40°C or even lower;
  • the polymer must be wear resistant as the outer surface will glide in the guide elements of the drive system. Also the teeth of the drive gear engage into the recesses and push and pull against the walls thereof. The polymer must therefore have sufficient stiffness in order not to yield under the pressure of the gear teeth;
  • the polymer must be able to enter the steel cord and surround it completely in order to ensure a sufficient grip of the steel cord and to seal the steel cord from the outside.
  • the polymer must be sufficiently formable in order to form the jacket around the steel cord and to form the recesses into the jacket.
  • a suitable polymer must therefore have a flexural modulus according
  • ISO178 between 350 to 1 100 N/mm 2 .
  • a shore D hardness between 60 to 80 is desirable.
  • a tensile modulus at 100% elongation as per DIN53504 of at least 18 MPa is preferred. It is not excluded that the polymer is loaded by reinforcing materials such as glass, aramid or carbon fibres, carbon black or silica minerals in order to increase its toughness.
  • thermoplastic polymers such as
  • polyurethane elastomers polyurethane elastomers, polyester elastomers, polyamide elastomers, poly(etherimide)-polysiloxane, polyamide or polyester/silicone rubber or polyetheramide. Most preferred are polyester elastomers.
  • E is the modulus of the steel cord and A is the cross sectional metallic surface area i.e. the sum of the cross sectional areas of the individual filaments constituting the steel cord.
  • the total longitudinal stiffness of the flexible rack (EA) Rack is thus the sum of both longitudinal stiffnesses of steel cord and polymer jacket.
  • the 'elongation ratio' or ⁇ -ratio' of the longitudinal stiffness ER sc that can be attributed to the steel cord is then:
  • This ratio should be at least 90% or equal thereto. If this ratio is lower, the cord will elongate or compress too much and the flexible rack does not work properly. Due to the elongation/compression of the steel cord the regular recesses will - after prolonged use - not mesh properly with the gear teeth resulting in a total disruption of the flexible rack.
  • the ratio is preferably higher than or equal to 92%.
  • the ratio can be up to 100% but not equal thereto.
  • 100% no polymer jacket is present and in that case no recesses can be present in the longitudinal direction of the flexible rack.
  • the preferred steel cord - polymer jacket combinations have an E-ratio below 95% or even below 93%.
  • the ranges formed by combining any lower and upper limit are herewith also included.
  • the polymer jacket is applied.
  • the force - elongation curve is again determined in the linear region of the force elongation curve for example in the same load range used for the bare steel cord.
  • the total longitudinal stiffness of the flexible rack corresponds to the slope of the linear region and is expressed in newton.
  • the E-ratio can be calculated by dividing the longitudinal stiffness of the steel cord by that of the flexible rack.
  • the bending stiffness of the rack is considered. It is preferred that the steel cord accounts for at most 15% of the total bending stiffness of the flexible rack.
  • the bending stiffness (£7) sc of the steel cord is the proportionality constant between the curvature applied ( ⁇ / s) in (mm "1 ) (i.e. the change in tangent angle along the length of the bent flexible rack) and the moment (M sc ) (in Nmm) induced :
  • the bending stiffness (EI) SC is the product of the bending modulus E (usually the modulus of steel is taken for that) and the geometric inertial moment /.
  • a too high total stiffness will result in excessive friction with the guiding tubes at bends leading to a too high power need to move the flexible rack necessitating a larger and heavier drive motor. Also the mountability of the flexible rack is compromised. On the other hand some bending stiffness is desired in order to prevent the flexible rack from vibrating in the guide.
  • the total bending stiffness of the flexible rack is tuned by means of the contribution of the steel cord as the contribution of the polymer jacket is limited by the geometrical constraints of the rack guide. The inventors find that the contribution of the steel cord is best limited to below 15%, even better below 10% or even below 6%.
  • Bending stiffness is generally measured in a three point bending test. A sample of the flexible rack is supported at sliding support points separated by a distance. At the middle the sample is impressed by a roll
  • the total stiffness (El) Rack of the flexible rack can be determined.
  • the first deflection is at least up to four times the diameter of the steel cord.
  • the longitudinal stiffness of the steel cord is at least 190 kN but lower than 260 kN.
  • the contribution of the polymer jacket is then between 5 kN and 21 kN.
  • the resulting E-Ratios are all larger than 90%.
  • the bending stiffness of the steel cord is between 500 Nmm 2 and 1 400 Nmm 2 .
  • the contribution of the polymer jacket to the bending stiffness is between 8 000 Nmm 2 to 22 000 Nmm 2 .
  • the resulting B-Ratios are all between 2 and 15%.
  • the modulus of the steel cord is at least 180 000 N/mm 2 while the modulus at room temperature of the polymer is between 350 to 1 100 N/mm 2 .
  • the modulus of the steel cord must be taken between 10% to 50% of the breaking load of the steel cord.
  • the modulus of a steel wire is about 200 000 N/mm 2 which sets an upper limit to the possible attainable steel cord modulus.
  • the stiffness is partly determined by the modulus of the materials used. When using the above moduli the ratio of metallic area to polymeric area have to be between limits in order to meet the longitudinal stiffness requirement.
  • the cross section of the flexible rack is substantially circular. Therewith is meant that the difference between maximum and minimum calliper diameter (also called Feret diameter) remains smaller than 15% of the average of maximum and minimum diameter. This Out-of-roundness ratio' may also be smaller than 10% or even smaller than 5%, but is always larger than or equal to 0 %.
  • the calliper diameter is the diameter measured between the two parallel jaws of a calliper. Care should be taken to have close to zero measurement pressure. In that respect optical devices for measuring Feret diameters are more preferred.
  • the cross section is not necessarily convex as recess are provided in the flexible rack. Also additional grooves or flats can be provided in order to guide the flexible rack in the guide channels.
  • the cross section can be inscribed within a circle of minimum radius
  • the minimum circumscribed circle It is the circle with the smallest diameter that completely comprises the
  • the diameter 'D' of this 'minimum circumscribed circle' is between 3 and 8 mm, or between 4 to 6 mm for example 4.8 to 5.2 mm.
  • the position of the centre of the steel cord and the centre of the minimum circumscribed circle can also be easily determined on a cross section of the flexible rack.
  • the center of the steel cord is situated in or at least very close to the plane that mirrors the cross section of the flexible rack i.e. the plane that cuts the recesses in the middle and goes through the centre of the minimum circumscribed circle.
  • the maximum distance of the centre of the steel cord to this mirror plane is by preference less than 5% or less than 3% or even less than 2%. If this distance is too large, the flexible rack will tend to curve in a direction perpendicular to the mirror plane which is a less preferred bending direction.
  • the distance of the centre of the steel cord to the plane through the centre of the minimum circumscribed circle that is perpendicular to the mirror plane - the 'perpendicular plane' - should be less than 15% of 'D' or less than 10% even less than 7% of 'D'. If this distance becomes too large the flexible rack will have a too outspoken preferred bending direction and will be difficult to mount without the flexible rack tending to turn.
  • the centre of the steel cord is by preference situated in an ellipse centered at the centre of the minimum circumscribed circle and oriented along the mirror plane.
  • the ellipse has a half major axis of 0.15*D and a half minor axis of 0.05*D.
  • the distance between the perpendicular plane and the centre of the steel cord is at least 1 % or even more than 3% of the diameter 'D' of the minimum circumscribed circle while the distance between the mirror plane and the centre of the steel cord is kept minimal.
  • circumscribed circle 'D' or preferable between 2 and 8%. Note: the 'bottom of the recess' is not to be regarded relative to the orientation of gravity but in radial direction towards the centre of the flexible rack.
  • a plain carbon steel composition typically contains at least 0.65 % of carbon, a manganese content ranging from 0.30 % to 0.70%, a silicon content ranging from 0.15% to 0.30%, a maximum sulphur content of 0.03%, a maximum phosphorus content of 0.30%, all percentages being
  • a low carbon steel typically has a composition of between 0.04 wt % and 0.20 wt % of carbon while the other named elements are in similar ranges as that for high carbon steel except that copper may be present up to 0.18 wt%.
  • Preferred stainless steels contain a minimum of 12% by weight of Cr and a substantial amount of nickel. More preferred stainless steel compositions are austenitic stainless steels as these can easily be drawn to fine diameters. The more preferred compositions are those known in the art as AISI 302 (particularly the 'Heading Quality' HQ), AISI 301 , AISI 304, AISI 314 and AISI 316.
  • the tensile strength is less important than the longitudinal or bending stiffness a minimal tensile strength of at least 1770 N/mm 2 is expected from the filaments.
  • a higher tensile strength can be obtained by using higher carbon levels and/or by strain hardening the wire by wire drawing. In this way tensile strengths in excess of 2000 N/mm 2 , or even 2400 N/mm 2 or more than 2700 N/mm 2 can easily be obtained.
  • the modulus of the steel itself is always between 190 000 to 200 000 N/mm 2 .
  • the steel filaments are covered with a corrosion inhibiting
  • a coating such as a zinc coating or a zinc-aluminium coating (Bezinal® of Bekaert). This can be combined with an adhesion enhancing coating that is selected to adhere the polymer used in the polymer jacket to the steel cord.
  • the diameter of the filaments 'd' plays a key role in the longitudinal or bending stiffness of the steel cord.
  • the longitudinal stiffness scales with the metallic surface area of the steel cord i.e. with 'd 2 ' while the bending stiffness scales with 'd 4 ' making the choice of filament diameters a sensitive matter.
  • the diameter of the filaments will be between 0.15 to 0.50 mm but more preferably it will be between 0.20 to 0.30 mm for example between 0.22 and 0.28 mm. Filaments of different diameters can be combined in a single steel cord.
  • Both the longitudinal and bending stiffness scale proportionally with the number of steel filaments.
  • the number of filaments is preferably between 7 and 48; for example from 12 to 30 filaments. Too many filaments necessitate the use of fine filaments adversely affecting the bending stiffness (not enough stiffness) and longitudinal stiffness (not enough metallic surface in the circumscribed circle of the steel cord). Too few filaments will result in thick filaments increasing the bending stiffness to a too high level and impeding the fatigue life of the flexible rack.
  • the configuration of how the filaments are positioned in the steel cord - generally denominated by 'construction' - has a large influence on the behaviour of the flexible rack.
  • Steel cords exist in the form of strands or ropes.
  • a strand subsequent layers of steel filaments are helically wound around a core.
  • Those filaments show a first-order helical deformation.
  • the core can be a single wire that is substantially straight which represents a zero-order helical deformation.
  • the core can itself be a strand of two to five filaments twisted together without a filament in the centre. Those two to five filaments will also show a first-order helical deformation.
  • the different layers of the steel strand may have equal or different lay lengths and directions.
  • the steel cord is called a 'layered' cord.
  • layered cords are for example 1 +6+12 i.e. one core wire surrounded by 6 first layer filaments of a first lay length and direction, surrounded by 12 second layer filaments of a second lay length and direction.
  • Other examples are
  • lay direction between adjacent layers maybe equal or opposite.
  • the cord is less prone to torsion under tension or compression.
  • the different layers may have all the same lay length and
  • filament diameters may be carefully chosen and arranged such that the outer filaments are tangent to one circumscribed circle.
  • Such configurations are known as Warrington constructions (for example 1 +6
  • Constructions having one lay length and direction with all filaments having the same diameter are called compact constructions. Their outer perimeter can be circumscribed with a polygon. Typical examples are (12CC, 19CC, 27CC).
  • a steel cord can also be composed of several outer strands that are
  • the outer strands themselves will obtain a first-order helical deformation that will add to the degree of deformation of the filaments.
  • the outer strands will show filaments that have a second-order helical deformation or a first-order helical deformation (if the core of the strand is a single wire).
  • the core strand may show zero-order or first order helical deformed wires.
  • the degree of helical deformation can be progressed by making a rope of a rope. But in practise second-order helical deformation is the highest degree of deformation given like for example in a 7x7 type of steel cord.
  • deformations are best suited for reinforcing the flexible rack. If second order or higher order filaments are present, the longitudinal stiffness becomes insufficient. Especially when the cord is being compressed the filaments lack compressive resistance and give in too early.
  • the steel cord only comprises steel filaments with first-order helical deformation. If zero-order helical filaments - i.e. straight filaments - are present they will tend to wick out and move in the direction of tension under repeated compression and tension cycles. This is because under tension the core filament is pinched and held by the surrounding filaments while it is tensioned and thus stretched. Contrary: under compression the core filament is released while it is under compression. This pushes the core filament minutely forward. But after several cycles the minute pushes result in a core filament migrating out of the flexible rack. This is a highly undesirable situation that can result in the core filament getting entangled into the drive mechanism.
  • the anchoring force can be increased by chemical means e.g. by an adhesive coating on the individual filaments or on the steel cord as a whole.
  • the anchoring force can be increased by improving the mechanically anchoring of the polymer jacket to the steel cord. This can for example be achieved by providing gaps in the outer layer of the steel cord. Gaps can be created by eliminating a filament from an otherwise saturated outer layer.
  • a saturated outer layer is an outer layer wherein no further filament of the same diameter as the other filaments of that layer can be added without pushing the other filaments out of the layer.
  • An unsatured layer is a saturated layer wherefrom one or more filaments have been removed. Typically one or two filaments are removed.
  • a 'wrap' is a single thread that is helically twisted around a core steel cord i.e. the wrap has first order helical deformation.
  • the wrap and core steel cord together form the steel cord of the claims.
  • the wrap has a very short lay length (for example between 2 and 10 mm, for example 2.5 to 5 mm).
  • the lay direction is preferably opposite to that of the outer layer of the core steel cord.
  • the wrap may have the same lay length as the distance between consecutive recesses.
  • the wrap may have a lay length that is a multiple of the distance between consecutive recesses.
  • there is no need to have a correspondence between the periodicity of the recesses and the lay length of the wrap as long as the wrap wire is not too thick in that it would interfere with the bottom of the recesses.
  • wraps instead of a single wrap there may be two or even more wraps. In case of two wraps they can be in the same lay direction with the same lay length for example separated one half of a lay length from each other.
  • the two wraps can be in opposite directions.
  • the wrap As the wrap is oriented oblique to the axis of the core steel cord it acts as a screw that is encased in the polymer jacket thereby greatly increasing the anchoring force.
  • the wrap filament can be a fine steel wire e.g. a wire of 0.15 mm or thinner.
  • the wrap may be an organic monofilament or a yarn made of poly-aramide fibres, poly(p-phenylene-2,6-benzobisoxazole) fibres, polyurethane fibres, carbon fibres, polyolefin fibres, polyamide fibres, polyester fibres , polycarbonate fibres, polyacetal fibres, polysulfone fibres, polyether ketone fibres , polyimide fibres, polyether imide fibres or mixtures thereof.
  • a wrap has a further advantage in that - when the flexible rack is
  • the wrap holds the steel filaments of the steel cord together and prevents them from opening.
  • a wrap therefore increases the compression resistance of the steel cord and therefore of the complete flexible rack.
  • the flexible rack is intended for use with a drive system.
  • a drive system Such a drive
  • the system comprises at least a gear that is driven by a motive force.
  • the motive force can be generated for example by hand crank or by an electrical motor.
  • the drive gear has a drive gear diameter that is equal to the diameter of the circumscribed circle tangent to the teeth. The relation between said drive gear diameter and the dimensions of the flexible rack are important as the ratios have a large influence on the lifetime of the flexible rack.
  • the largest diameter of the steel filaments of the steel cord is less than 4% of the diameter of the drive gear.
  • the maximum diameter of the filaments is important for the fatigue life of the flexible rack. If the bending imposed by the drive gear is too severe for example when the diameter of the drive gear is smaller than 25 times the diameter of the thickest filament, the bending stresses induced in the filaments will reduce the useful life of the flexible rack. If filaments break they can wick out of the steel cord, puncture the polymer jacket and come out of the flexible rack leading to a failure of the drive system. It is therefore preferred that the maximum diameter of all filaments is smaller than 4%, or even better smaller than 2% for example less than 1 % of the drive gear diameter.
  • the diameter of the minimum circumscribed circle 'D' is less than 15% of the diameter of the drive gear. Even more preferred is if it is less than 13% for example less than 10%.
  • the radially outer regions of the polymer jacket are stretched in an amount equal to the ratio of minimum circumscribed circle 'D' over drive gear diameter. While polymers can take an appreciable amount of stretching they do this in a predominantly plastic way. However, under repeated bending cracks may propagate from the stretched outer regions of the polymer jacket resulting in a tearing of the polymer jacket. In order to prevent this cracking the outer elongation must be limited.
  • FIGURE 1 shows the flexible rack according the invention in perspective view
  • FIGURE 2 shows a cross section of the flexible rack according the
  • FIGURE 3 shows a load-elongation diagram of a bare steel cord and of the flexible rack for determination of the longitudinal stiffness
  • FIGURE 4 shows a force-deflection diagram of a bare steel cord
  • FIGURE 5 shows a force-deflection diagram of a the flexible rack to
  • FIGURE 6 shows a cross section of a real sample.
  • FIGURE 1 shows a perspective view of the flexible rack 100 according the invention.
  • the flexible rack comprises a steel cord 102 embedded in a polymer jacket 104.
  • the polymer jacket is provided with regular recesses 106 in the longitudinal direction of the flexible rack.
  • the bottom of the recess 108 is there where the recesses come closest to the steel cord 102.
  • Between two subsequent recesses a crest or top 1 10 is present.
  • the recesses match the gear tooth shape of the drive gear of the drive system.
  • FIGURE 2 shows the cross section of another preferred embodiment 200 of the flexible rack.
  • the steel cord 202 is embedded in the polymer jacket 204.
  • Recesses 206 start from bottom 208, closest to the steel cord 202 up to the top 210.
  • a groove 214 is made along the length of the flexible rack 200. Such a groove can serve to guide the flexible rack and prevent it from rotation. Likewise flats 215, 215' can be introduced for the same purpose.
  • F1 and F2 indicated calliper diameters of which F2 is the maximum calliper diameter.
  • the minimum circumscribed circle is indicated with C1 and has a diameter indicated with'D'.
  • the centre of that circle is situated at M1 .
  • the core is covered with 9 filaments of diameter 0.23 mm with a lay length of 12.5 mm in S direction.
  • the steel cord is wrapped with a 0.15 mm steel wire 216 at lay 3.5 mm in Z direction.
  • the lay corresponds to the distance between recesses.
  • the centre of the steel cord is situated at M2 and the steel cord has a minimum circumscribed circle C2.
  • the eccentricity of the two centres is indicated with ' ⁇ '.
  • the distance between the bottom of the recess and the steel cord is indicated with ' ⁇ '.
  • a steel cord of construction (12)+15x0.25 was made from hot dip galvanised wires.
  • the construction is made of 12 wires with diameter 0.25 arranged in a compact cord configuration of lay 10 mm in S around which 15 filaments of the same diameter are twisted at lay 21 mm in Z.
  • the polymer jacket is made of IROGRAN D 74 P 4778 available from Huntsman. This is a thermoplastic polyether-polyurethane for injection moulding and extrusion applications. It has a Shore D of 70. The polymer is extruded around the steel cord until a diameter of 5 mm is reached with a round cross section. After the extrusion the cord is allowed to cool down.
  • Figure 6 shows a cross section of the resulting flexible rack. It has a
  • the steel cord has a centre M 2 and diameter 1 .57 mm.
  • the out-of-centre distance between a plane through the centre of minimum circumscribing circle perpendicular to the mirror plane of the flexible rack to the centre of the steel cord is 0.22 mm or 4.4% of D.
  • the perpendicular distance between the mirror plane and the centre of the steel cord remained below 2 ⁇ .
  • the polymer thickness between the bottom of the tooth and the steel cord is 0.20 mm or 4% of D.
  • constructions were produced such as 7x7 with a diameter of 1 .60 mm, 0.55+6x0.53, 0.34+6x0.32+12x0.295 and 3+9+15x0.25 but using the same polymer jacket.
  • the zero deflection is set at the point when a force on the indenter is first sensed. Deflection is continued until an elongation of 1 % in the outer fibre of the steel cord is obtained. This elongation is equal to the ratio of the diameter of the steel cord to twice the radius of curvature at the deepest indentation point. For a basis of 50 times the diameter of the cord, this is achieved with a total displacement of about 4.2 times the diameter of the steel cord. This is the first upward curve indicated with 402 in Figure 4. When the maximum deflection is reached, the movement of the indenter is reversed until no force is sensed (the point 405). The curve in the downward direction 404 is parallel to the upward curve 402 but is displaced parallel to it.
  • L is the distance between the fulcrums in mm (65 mm in this case).
  • AF is the force difference in newton in the linear region while AX
  • the cord was provided with a polymer jacket and indented to form recesses. Again a stiffness measurement was performed wherein the cord was deflected perpendicular to the plane formed by the indentations and the axis of the steel cord. A typical trace is shown in Figure 5. The same conditions as for the bare cord applied (distance between fulcrums, degree of bending applied etc .). Note the difference in force scale compared to the bare cord. Again a first upward trace 502 is recorded in order to 'set' the cord. At turning point 503 the deflection direction is reversed (following trace 504) until zero force is detected at 505.
  • the cord After direction reversal the cord is deflected half of the previous deflection (second upward trace 506) till turning point 507 after which the movement of the indenter direction is again reversed tracing the line 508.
  • a linear fit to the second upward trace gives a measure for the stiffness that is calculated in the exact same way as for the bare cord. Again three measurements on different pieces of the flexible rack are made. The resulting values are 21634 Nmm 2 , 19505 Nmm 2 , and 19958 Nmm 2 yielding an average of 20366 Nmm 2 .
  • the contribution of the bending stiffness of the steel cord to the total bending stiffness of the flexible rack is therefore 1042/20366 or 5.1 % which is lower than the maximum value 15%.
  • the absolute contribution of the polymer jacket is 19324 Nmm 2 .
  • the procedure is repeated at least 12 times and the average of the results is taken in order to reduce the variability in the test.
  • Some of the flexible racks were tested in a drive system simulating the bending in a first plane and one bend in a plane perpendicular to the first plane.
  • the flexible rack was guided through a tubular guide channel and driven back and forth by a drive motor.
  • the drive gear had a diameter of 70 mm but in principle the flexible rack can be used with drive gears having a diameter of 33 mm or higher.
  • the tests showed that at least a flexible rack according the claims can be used to transfer enough force in tension and compression to move a window pane. Endurance tests showed that the flexible rack according the claims could survive the required number of drive cycles.

Abstract

A flexible rack for use in a drive system such as a window elevator system, sunroof drive system, sun shade or cover actuators in a vehicle comprises a steel cord embedded in a polymer jacket. In the polymer recesses are provided at regular distances that engage with the teeth of the drive gear. In order to act as a backbone to the flexible rack, the steel cord must account for at least 90% of the total longitudinal stiffness of the flexible rack otherwise it succumbs to compressive stresses and/or stretches too much under tensile loads. In contradistinction therewith the bending stiffness contribution of the steel cord must remain below 15% of the total bending stiffness of the flexible rack so that the flexible rack can follow the curvature of the guiding channel the flexible rack is running in. By preference the steel cord is free of second and higher order helical deformation. Particularly preferred is that the steel cord is situated slightly 'off- centre' as therewith a preferred bending direction is induced.

Description

Title: Flexible rack with steel cord embedded in polymer
Description
Technical Field
[0001 ] The invention relates to a flexible rack for use in a drive system. The
flexible rack is specifically designed to move parts back and forth such as for example a rapier in a textile loom, a door panel in an elevator, a window pane in a vehicle, a sunroof in a car or any other like applications.
Background Art
[0002] For the purpose of this application, focus will be given to flexible rack for drive systems as used in vehicles for example to move up and down side window panels or to move back and forth sunroofs, in glass or textile, interior sun shades or exterior covers. However, it will be clear that the inventive principles can be extended to any other application wherein a back and forth movement over a length of decimetres to metres is desired.
[0003] One system for opening or closing sunroofs comprises cords that are held under tension by and guided over pulleys (see e.g. US3137491 ). The system needs a lot of pulleys, takes quite some space (as the cords must go back and forth) and is not easy to mount and repair. An alternative system is based on toothed belts that are held under tension (US
6786540).
[0004] Other drive systems are based on transmission cables such as already described in US1983962 (1934). Such cable comprises a core - usually assembled metallic wires - with a thick metal wire twisted and hammered around the core. The pitch of the helix leaves sufficient space between turns in order to allow a drive pinion to grip into the helix and drive the cable back and forth. As such systems work both ways - in tension and compression - no reversing pulleys are needed thereby saving space and cable length.
[0005] Of course the cable must run in a tubular casing or through sleeves to guide the cable and to prevent buckling when coming under compression. This guiding does not constrain the bending in a single plane and many systems require that the wire moves out of the flat plane in a track with bends in different directions. [0006] While such systems work satisfactory they do generate noise:
• There is the engagement noise generated by the drive pinion teeth gripping and leaving the helix. As the round helix wire does not allow for a gentle exit of the pinion tooth this generates noise;
• There is also the scraping noise as the helix wire scrapes the side of the guide elements;
• When the drive cable is not held in the sleeves or cable guides, it can vibrate - for example due to the running engine - and generate a rattling noise
[0007] The current solution to prevent the generation of noise is to provide a
brush like flocking in between the helical winding (see e.g. DE3513093). The flocking serves to centrally hold the wire in the guide and at the same time dampens the vibration. Although this reduces the noise, the solution is not entirely satisfactory in that the flocking wears and generates dust. After prolonged use the noise increases. An alternative suggested solution was to coat the complete wire with a plastic material (DE7136899U).
[0008] Already at an early stage it has been contemplated to make a flexible rack that comprises a flexible but strong core on which a plastic jacket is deposited that is provided with teething that engages with a drive gear.
[0009] DE 1003517 is an early example of this wherein a metal wire, metal strand or metal cord is coated with a polymer jacket that is provided with teeth imprinted into the jacket. The teeth can extend over about half of the perimeter of the cable or they can be provided as pockets, cavities, recesses or depressions wherein the teeth of the drive gear engage. The central wire can be off centre or can be central. The cross section of the flexible rack can be substantially round or can be of oval shape.
[0010] EP 0499135 describes a method to produce such a cable by embossing the teeth profile into the still soft jacket of the freshly extruded polymer jacket on a steel cord.
[001 1 ] As an alternative to the metal wire, metal strand or metal cord US
5150631 suggests to use a steel band provided with openings where the pockets, recesses or cavities should occur. It is also suggested to have two steel cords at either side of the recesses. [0012] Alternatively DE 10 2012 007 104 suggests to use carbon, glas or aramid fibres instead of or in combination with a steel cord.
[0013] Also the tooth profiling has been subject of many alterations as
exemplified in DE 10 2012 105 372 and DE 10 2012 105 375.
[0014] Hence, although there has been a desire already from the beginning of the nineties of the previous century to come up with an alternative for the ubiquitous flocked drive cable with its associated disadvantage of noise generation, none of the suggested alternatives seem to have made it to the real world. The desire remains to realise an improved alternative.
Disclosure of Invention
[0015] The inventors set themselves the task to provide an alternative to the
flocked cable by improving the drive cable with the polymer jacket. If such an alternative flexible rack can be found it will solve long standing issues such as noise problems. Furthermore, the suggested alternative must be easily mountable by preference as easily mountable as the flocked cable into the guiding parts of the drive system. It must also be wear resistant and survive at least ten thousands back and forth cycles of compression and tension. As for example a door pane with a window elevator system can get very hot in summer and very cold in winter the rack must remain functional between -40°C and +90°C.
[0016] Quickly after the first tests the inventors realised that the prior art
mentioned gives a gross simplification of reality. One of the problems that immediately occurred is that due to the crimp of the polymer after extrusion the central steel cord comes under compression. As the steel cord is compressed, due to the helical arrangement of the filaments, the filaments tend to twist in the closing direction of the steel cord. As a result, the imprinted recesses do not form a line, but spiral around the surface of the cable. Clearly such a cable is not mountable. Furthermore, initial tests showed that - due to the alteration of compression and tension - individual steel wires wick themselves out of the cord. The inventors found a solution to overcome these problems that will now be disclosed. [0017] According a first aspect of the invention a flexible rack for use in drive system is disclosed. The flexible rack is intended for use with a drive system with a drive gear. The flexible rack comprises a steel cord embedded in a polymer jacket. The polymer jacket is provided with regularly spaced recesses arranged along a straight line in the longitudinal direction of the flexible rack. The recesses are for receiving the teeth of the aforementioned drive gear. By preference the recesses have closed side walls with tops (i.e. the hills that are between two recesses) that are strictly within the circumscribed circle of the flexible rack in a cross section.
[0018] Special about the flexible rack is that the longitudinal stiffness of the steel cord accounts for at least 90% of the total longitudinal stiffness of the flexible rack.
[0019] One steel cord is present approximately centrally positioned in the flexible rack. The steel cord comprises steel filaments that are twisted together into a strand. Alternatively the steel cord may comprise strands that are twisted together into a rope. For example a steel cord can be stranded out of two, three or more strands twisted together and forming a single steel cord. For the purpose of this application a 'steel cord' therefore may refer to a strand or a rope. Next to the steel filament the steel cord may also comprise other non-steel components like yarns or monofilaments of an organic nature.
[0020] The steel cord is embedded in a polymer jacket. Thereby it is understood that the steel cord is completely surrounded, encased, covered by the polymer and by preference the surface of the steel cord is insulated from its surrounding. Such insulation from the environment is important to prevent wear and tear of the steel cord but also to exclude corrosion.
[0021 ] In the selection of the polymer the following properties must be balanced:
• The polymer must remain sufficiently stiff at high temperatures of for example 90°C or more and must not become brittle at temperatures down to -40°C or even lower;
• The polymer must be wear resistant as the outer surface will glide in the guide elements of the drive system. Also the teeth of the drive gear engage into the recesses and push and pull against the walls thereof. The polymer must therefore have sufficient stiffness in order not to yield under the pressure of the gear teeth;
• The polymer must be able to enter the steel cord and surround it completely in order to ensure a sufficient grip of the steel cord and to seal the steel cord from the outside.
• The polymer must be sufficiently formable in order to form the jacket around the steel cord and to form the recesses into the jacket.
[0022] It must be realised that the larger part of the forces induced by the gear teeth are ultimately transferred to the steel cord, hence these forces must pass through the polymer.
[0023] A suitable polymer must therefore have a flexural modulus according
ISO178 between 350 to 1 100 N/mm2. A shore D hardness between 60 to 80 is desirable. A tensile modulus at 100% elongation as per DIN53504 of at least 18 MPa is preferred. It is not excluded that the polymer is loaded by reinforcing materials such as glass, aramid or carbon fibres, carbon black or silica minerals in order to increase its toughness.
[0024] Possible polymers are therefore thermoplastic polymers such as
polyurethane elastomers, polyester elastomers, polyamide elastomers, poly(etherimide)-polysiloxane, polyamide or polyester/silicone rubber or polyetheramide. Most preferred are polyester elastomers.
[0025] With the longitudinal stiffness (EA)SC (in N) of the steel cord is meant the Hooke's constant of the steel cord in a load (Fsc) vs elongation (AL/L) diagram:
Fsc = {EA)SC x (AL/L)
E is the modulus of the steel cord and A is the cross sectional metallic surface area i.e. the sum of the cross sectional areas of the individual filaments constituting the steel cord.
[0026] The presence of a polymer jacket will further add to the longitudinal
stiffness of the flexible rack. Although the modulus of the polymer is significantly lower than that of the steel cord there is a substantial area of polymer present in the cross section of the flexible rack, leading to an increase in the longitudinal stiffness of the flexible rack. The total longitudinal stiffness of the flexible rack (EA)Rack is thus the sum of both longitudinal stiffnesses of steel cord and polymer jacket. The 'elongation ratio' or Έ-ratio' of the longitudinal stiffness ERscthat can be attributed to the steel cord is then:
ER = EA)sc
SC (EA)Rack
This ratio should be at least 90% or equal thereto. If this ratio is lower, the cord will elongate or compress too much and the flexible rack does not work properly. Due to the elongation/compression of the steel cord the regular recesses will - after prolonged use - not mesh properly with the gear teeth resulting in a total disruption of the flexible rack. The ratio is preferably higher than or equal to 92%.
[0027] Additionally, when the longitudinal stiffness of the steel cord is less than 90% of the total longitudinal stiffness, due to the inevitable shrinkage of the polymer after extrusion, the steel cord will be too easily compressed. When compressed the extruded steel cord will tend to torque leading to a line of recesses that is not straight. If the longitudinal stiffness of the steel cord is higher than 90% of the total, the steel cord can resist the
compression by the shrinkage of the polymer.
[0028] In principle the ratio can be up to 100% but not equal thereto. In the case of 100% no polymer jacket is present and in that case no recesses can be present in the longitudinal direction of the flexible rack. The preferred steel cord - polymer jacket combinations have an E-ratio below 95% or even below 93%. The ranges formed by combining any lower and upper limit are herewith also included.
[0029] The fraction of the steel cord that contributes to the longitudinal stiffness can be determined as follow:
• First the stiffness of a bare steel cord is determined. This is done in the linear region of the force - elongation curve for example between about 10% to about 50% of the breaking load of the steel cord. The longitudinal stiffness of the steel cord corresponds to the slope of the linear region and is expressed in newton.
• Then the polymer jacket is applied. After application of the polymer jacket, the force - elongation curve is again determined in the linear region of the force elongation curve for example in the same load range used for the bare steel cord. The total longitudinal stiffness of the flexible rack corresponds to the slope of the linear region and is expressed in newton.
• The E-ratio can be calculated by dividing the longitudinal stiffness of the steel cord by that of the flexible rack.
These measurements are to be performed at room temperature (20 to 22°C)
[0030] In a further preferred embodiment of the flexible rack the bending stiffness of the rack is considered. It is preferred that the steel cord accounts for at most 15% of the total bending stiffness of the flexible rack.
[0031 ] Similar to the case of the longitudinal stiffness both the steel cord and the polymer jacket contribute to the bending stiffness. The bending stiffness (£7)sc of the steel cord is the proportionality constant between the curvature applied (Δθ/ s) in (mm"1) (i.e. the change in tangent angle along the length of the bent flexible rack) and the moment (Msc) (in Nmm) induced :
Figure imgf000008_0001
In principle the bending stiffness (EI)SC is the product of the bending modulus E (usually the modulus of steel is taken for that) and the geometric inertial moment /.
[0032] As in the case of longitudinal stiffness one can separate the contribution of the steel cord to the total bending stiffness by means of a bending ratio or B-Ratio of the bending stiffness BRSC that can be defined as: [0033] However, in contrast with the longitudinal case, the influence of the polymer jacket is much larger as it forms a substantially tubular sheet around the steel cord. And as for a round wire of diameter d the inertial moment / scales with the diameter of the wire to the fourth power d4 it follows that the polymer sheath will have a larger influence on the bending moment than the steel cord even though the modulus of the former is much lower.
[0034] A too high total stiffness will result in excessive friction with the guiding tubes at bends leading to a too high power need to move the flexible rack necessitating a larger and heavier drive motor. Also the mountability of the flexible rack is compromised. On the other hand some bending stiffness is desired in order to prevent the flexible rack from vibrating in the guide.
[0035] The total bending stiffness of the flexible rack is tuned by means of the contribution of the steel cord as the contribution of the polymer jacket is limited by the geometrical constraints of the rack guide. The inventors find that the contribution of the steel cord is best limited to below 15%, even better below 10% or even below 6%.
[0036] It is best that the contribution of the steel cord to the total bending stiffness is larger than 2% for example larger than 3%. A too low B-ratio results in a too flexible steel cord. As the meshing of the drive gear in the recesses of the flexible rack exerts a moment on the polymer tops, this will induce local bending in the steel cord. If the steel cord is not stiff enough it will give in and the meshing will not be as optimal as it should be.
[0037] Bending stiffness is generally measured in a three point bending test. A sample of the flexible rack is supported at sliding support points separated by a distance. At the middle the sample is impressed by a roll
perpendicular to the sample. By measuring the force on the roll as a function of the deflection, the total stiffness (El)Rack of the flexible rack can be determined.
[0038] Like in the case of longitudinal stiffness the contribution of the steel cord must be separated from the total stiffness. This can be done by performing a three point bending test on the bare steel cord under the same
conditions as on the flexible rack.
[0039] During measurement the following must be considered: • When making the measurement on the flexible rack care must be taken that the deflection is perpendicular to the axial plane
comprising the recesses.
• The stiffness (EI)SC or (EI)Racfe is determined in the linear part of the force-deflection graph and after a first load cycle has been applied.
• The first deflection is at least up to four times the diameter of the steel cord.
• Measurements are performed at room temperature.
The ratio of (£7)Sc to (EI)Rack then follows.
[0040] In an even more particular embodiment of the invention, the longitudinal stiffness of the steel cord is at least 190 kN but lower than 260 kN.
Typically the contribution of the polymer jacket is then between 5 kN and 21 kN. The resulting E-Ratios are all larger than 90%.
[0041 ] The bending stiffness of the steel cord is between 500 Nmm2 and 1 400 Nmm2. Typically the contribution of the polymer jacket to the bending stiffness is between 8 000 Nmm2 to 22 000 Nmm2. The resulting B-Ratios are all between 2 and 15%.
[0042] In a particular embodiment of the invention, the modulus of the steel cord is at least 180 000 N/mm2 while the modulus at room temperature of the polymer is between 350 to 1 100 N/mm2. The modulus of the steel cord must be taken between 10% to 50% of the breaking load of the steel cord. The modulus of a steel wire is about 200 000 N/mm2 which sets an upper limit to the possible attainable steel cord modulus. As mentioned: the stiffness is partly determined by the modulus of the materials used. When using the above moduli the ratio of metallic area to polymeric area have to be between limits in order to meet the longitudinal stiffness requirement.
[0043] The cross section of the flexible rack is substantially circular. Therewith is meant that the difference between maximum and minimum calliper diameter (also called Feret diameter) remains smaller than 15% of the average of maximum and minimum diameter. This Out-of-roundness ratio' may also be smaller than 10% or even smaller than 5%, but is always larger than or equal to 0 %. The calliper diameter is the diameter measured between the two parallel jaws of a calliper. Care should be taken to have close to zero measurement pressure. In that respect optical devices for measuring Feret diameters are more preferred. The cross section is not necessarily convex as recess are provided in the flexible rack. Also additional grooves or flats can be provided in order to guide the flexible rack in the guide channels.
[0044] The cross section can be inscribed within a circle of minimum radius
enclosing the cross section: the minimum circumscribed circle. It is the circle with the smallest diameter that completely comprises the
perpendicular cross section of the flexible rack. This minimum
circumscribed circle can be determined by means of digital image analysis software packages. The diameter 'D' of this 'minimum circumscribed circle' is between 3 and 8 mm, or between 4 to 6 mm for example 4.8 to 5.2 mm.
[0045] The position of the centre of the steel cord and the centre of the minimum circumscribed circle can also be easily determined on a cross section of the flexible rack. By preference the center of the steel cord is situated in or at least very close to the plane that mirrors the cross section of the flexible rack i.e. the plane that cuts the recesses in the middle and goes through the centre of the minimum circumscribed circle. The maximum distance of the centre of the steel cord to this mirror plane is by preference less than 5% or less than 3% or even less than 2%. If this distance is too large, the flexible rack will tend to curve in a direction perpendicular to the mirror plane which is a less preferred bending direction.
[0046] The distance of the centre of the steel cord to the plane through the centre of the minimum circumscribed circle that is perpendicular to the mirror plane - the 'perpendicular plane' - should be less than 15% of 'D' or less than 10% even less than 7% of 'D'. If this distance becomes too large the flexible rack will have a too outspoken preferred bending direction and will be difficult to mount without the flexible rack tending to turn.
[0047] In summary: the centre of the steel cord is by preference situated in an ellipse centered at the centre of the minimum circumscribed circle and oriented along the mirror plane. The ellipse has a half major axis of 0.15*D and a half minor axis of 0.05*D.
[0048] On the other hand some eccentricity of the steel cord is desired to prevent rattling of the rack in the guide when not in use. So by preference the distance between the perpendicular plane and the centre of the steel cord is at least 1 % or even more than 3% of the diameter 'D' of the minimum circumscribed circle while the distance between the mirror plane and the centre of the steel cord is kept minimal.
[0049] Another feature of the invention that turned out to be important is the
presence of a thin polymer fleece or layer at the bottom of the recesses and the steel cord. If no such polymer layer is present the teeth of the gear appear to quickly damage the recesses in the polymer rack. Also the presence of the polymer at the bottom of the recess protects the steel cord against corrosion. The distance between the bottom of the hollow and the steel cord must be at least 2% of the diameter of the minimum
circumscribed circle 'D' or preferable between 2 and 8%. Note: the 'bottom of the recess' is not to be regarded relative to the orientation of gravity but in radial direction towards the centre of the flexible rack.
[0050] The steel filaments that make up the steel cord are made out of plain
carbon steel or stainless steel or even out of low carbon steel. A plain carbon steel composition typically contains at least 0.65 % of carbon, a manganese content ranging from 0.30 % to 0.70%, a silicon content ranging from 0.15% to 0.30%, a maximum sulphur content of 0.03%, a maximum phosphorus content of 0.30%, all percentages being
percentages by weight. There are only traces of copper, nickel,
molydenum and / or chromium.
[0051 ] A low carbon steel typically has a composition of between 0.04 wt % and 0.20 wt % of carbon while the other named elements are in similar ranges as that for high carbon steel except that copper may be present up to 0.18 wt%.
[0052] Preferred stainless steels contain a minimum of 12% by weight of Cr and a substantial amount of nickel. More preferred stainless steel compositions are austenitic stainless steels as these can easily be drawn to fine diameters. The more preferred compositions are those known in the art as AISI 302 (particularly the 'Heading Quality' HQ), AISI 301 , AISI 304, AISI 314 and AISI 316.
[0053] Although in the inventive flexible rack in general the tensile strength is less important than the longitudinal or bending stiffness a minimal tensile strength of at least 1770 N/mm2 is expected from the filaments. A higher tensile strength can be obtained by using higher carbon levels and/or by strain hardening the wire by wire drawing. In this way tensile strengths in excess of 2000 N/mm2, or even 2400 N/mm2 or more than 2700 N/mm2 can easily be obtained. The modulus of the steel itself is always between 190 000 to 200 000 N/mm2.
[0054] Preferably the steel filaments are covered with a corrosion inhibiting
coating such as a zinc coating or a zinc-aluminium coating (Bezinal® of Bekaert). This can be combined with an adhesion enhancing coating that is selected to adhere the polymer used in the polymer jacket to the steel cord.
[0055] The diameter of the filaments 'd' plays a key role in the longitudinal or bending stiffness of the steel cord. The longitudinal stiffness scales with the metallic surface area of the steel cord i.e. with 'd2' while the bending stiffness scales with 'd4' making the choice of filament diameters a sensitive matter. Typically the diameter of the filaments will be between 0.15 to 0.50 mm but more preferably it will be between 0.20 to 0.30 mm for example between 0.22 and 0.28 mm. Filaments of different diameters can be combined in a single steel cord.
[0056] Both the longitudinal and bending stiffness scale proportionally with the number of steel filaments. The number of filaments is preferably between 7 and 48; for example from 12 to 30 filaments. Too many filaments necessitate the use of fine filaments adversely affecting the bending stiffness (not enough stiffness) and longitudinal stiffness (not enough metallic surface in the circumscribed circle of the steel cord). Too few filaments will result in thick filaments increasing the bending stiffness to a too high level and impeding the fatigue life of the flexible rack.
[0057] The configuration of how the filaments are positioned in the steel cord - generally denominated by 'construction' - has a large influence on the behaviour of the flexible rack. Steel cords exist in the form of strands or ropes. In a strand subsequent layers of steel filaments are helically wound around a core. Those filaments show a first-order helical deformation. The core can be a single wire that is substantially straight which represents a zero-order helical deformation. Or the core can itself be a strand of two to five filaments twisted together without a filament in the centre. Those two to five filaments will also show a first-order helical deformation. The different layers of the steel strand may have equal or different lay lengths and directions.
[0058] When the different layers differ in at least one of the lay length and lay direction the steel cord is called a 'layered' cord. Typically layered cords are for example 1 +6+12 i.e. one core wire surrounded by 6 first layer filaments of a first lay length and direction, surrounded by 12 second layer filaments of a second lay length and direction. Other examples are
3+9+15, 4+10+16. The lay direction between adjacent layers maybe equal or opposite. When the lay direction between adjacent layers are opposite, the cord is less prone to torsion under tension or compression.
[0059] Alternatively the different layers may have all the same lay length and
direction. In that case filament diameters may be carefully chosen and arranged such that the outer filaments are tangent to one circumscribed circle. Such configurations are known as Warrington constructions (for example 1 +6|6|6 indicating a single core filament around which 18 filaments of three different diameters are twisted with the same lay length and direction), Seale constructions (for example 1 +8|8 indicating a core wire surrounded by 8 first layer filaments wherein 8 second layer filaments fall into the recesses of the first layer) or Warrington-Seale constructions (such as 1 +6|6|6|12 i.e. a Warrington construction covered with 12 filaments of equal diameter fitting into the recesses of the outer filaments).
[0060] Constructions having one lay length and direction with all filaments having the same diameter are called compact constructions. Their outer perimeter can be circumscribed with a polygon. Typical examples are (12CC, 19CC, 27CC).
[0061 ] A steel cord can also be composed of several outer strands that are
twisted around a core strand. The outer strands themselves will obtain a first-order helical deformation that will add to the degree of deformation of the filaments. Hence the outer strands will show filaments that have a second-order helical deformation or a first-order helical deformation (if the core of the strand is a single wire). The core strand may show zero-order or first order helical deformed wires. The degree of helical deformation can be progressed by making a rope of a rope. But in practise second-order helical deformation is the highest degree of deformation given like for example in a 7x7 type of steel cord.
[0062] Steel cords that are free of second-order or higher order helical
deformations are best suited for reinforcing the flexible rack. If second order or higher order filaments are present, the longitudinal stiffness becomes insufficient. Especially when the cord is being compressed the filaments lack compressive resistance and give in too early.
[0063] Even more preferred is that the steel cord only comprises steel filaments with first-order helical deformation. If zero-order helical filaments - i.e. straight filaments - are present they will tend to wick out and move in the direction of tension under repeated compression and tension cycles. This is because under tension the core filament is pinched and held by the surrounding filaments while it is tensioned and thus stretched. Contrary: under compression the core filament is released while it is under compression. This pushes the core filament minutely forward. But after several cycles the minute pushes result in a core filament migrating out of the flexible rack. This is a highly undesirable situation that can result in the core filament getting entangled into the drive mechanism.
[0064] In a flexible rack the force exerted by the drive gear is conveyed by the meshing recesses to the polymer jacket. The polymer jacket is too weak to absorb these forces that subsequently have to be transmitted to the steel cord. The steel cord forms the 'backbone' of the flexible rack and carries at least 90% of the tensile or compressive load. It is therefore imperative that the forces on the polymer jacket are adequately transferred to the steel cord. Therefore the steel cord must anchor well into the polymer jacket. The inventors have found that an anchoring force, in newton, of at least 50*D, D being the diameter of the minimum circumscribed circle in millimetre, is needed to pull off 25 mm of the polymer jacket from the steel cord at room temperature. If the anchoring force falls below this limit there is a genuine risk that the polymer jacket loosens from the steel cord during use. Better is if the anchoring force is larger than 70*D or even larger than 100xD. [0065] The anchoring force can be increased by chemical means e.g. by an adhesive coating on the individual filaments or on the steel cord as a whole. Alternatively the anchoring force can be increased by improving the mechanically anchoring of the polymer jacket to the steel cord. This can for example be achieved by providing gaps in the outer layer of the steel cord. Gaps can be created by eliminating a filament from an otherwise saturated outer layer. A saturated outer layer is an outer layer wherein no further filament of the same diameter as the other filaments of that layer can be added without pushing the other filaments out of the layer. An unsatured layer is a saturated layer wherefrom one or more filaments have been removed. Typically one or two filaments are removed.
[0066] Another alternative solution to increase the anchoring force is to use a wrap or wraps. A 'wrap' is a single thread that is helically twisted around a core steel cord i.e. the wrap has first order helical deformation. The wrap and core steel cord together form the steel cord of the claims. Typically the wrap has a very short lay length (for example between 2 and 10 mm, for example 2.5 to 5 mm). The lay direction is preferably opposite to that of the outer layer of the core steel cord. In one embodiment the wrap may have the same lay length as the distance between consecutive recesses. In another embodiment the wrap may have a lay length that is a multiple of the distance between consecutive recesses. Of course there is no need to have a correspondence between the periodicity of the recesses and the lay length of the wrap as long as the wrap wire is not too thick in that it would interfere with the bottom of the recesses.
[0067] Instead of a single wrap there may be two or even more wraps. In case of two wraps they can be in the same lay direction with the same lay length for example separated one half of a lay length from each other.
Alternatively the two wraps can be in opposite directions.
[0068] As the wrap is oriented oblique to the axis of the core steel cord it acts as a screw that is encased in the polymer jacket thereby greatly increasing the anchoring force. The wrap filament can be a fine steel wire e.g. a wire of 0.15 mm or thinner. Or the wrap may be an organic monofilament or a yarn made of poly-aramide fibres, poly(p-phenylene-2,6-benzobisoxazole) fibres, polyurethane fibres, carbon fibres, polyolefin fibres, polyamide fibres, polyester fibres , polycarbonate fibres, polyacetal fibres, polysulfone fibres, polyether ketone fibres , polyimide fibres, polyether imide fibres or mixtures thereof.
[0069] A wrap has a further advantage in that - when the flexible rack is
subjected to compression - the wrap holds the steel filaments of the steel cord together and prevents them from opening. A wrap therefore increases the compression resistance of the steel cord and therefore of the complete flexible rack.
[0070] The flexible rack is intended for use with a drive system. Such a drive
system comprises at least a gear that is driven by a motive force. The motive force can be generated for example by hand crank or by an electrical motor. The drive gear has a drive gear diameter that is equal to the diameter of the circumscribed circle tangent to the teeth. The relation between said drive gear diameter and the dimensions of the flexible rack are important as the ratios have a large influence on the lifetime of the flexible rack.
[0071 ] In a particularly preferred embodiment of the flexible rack and drive gear combination, the largest diameter of the steel filaments of the steel cord is less than 4% of the diameter of the drive gear. The maximum diameter of the filaments is important for the fatigue life of the flexible rack. If the bending imposed by the drive gear is too severe for example when the diameter of the drive gear is smaller than 25 times the diameter of the thickest filament, the bending stresses induced in the filaments will reduce the useful life of the flexible rack. If filaments break they can wick out of the steel cord, puncture the polymer jacket and come out of the flexible rack leading to a failure of the drive system. It is therefore preferred that the maximum diameter of all filaments is smaller than 4%, or even better smaller than 2% for example less than 1 % of the drive gear diameter.
[0072] In another particularly preferred embodiment of the flexible rack and drive gear combination, the diameter of the minimum circumscribed circle 'D' is less than 15% of the diameter of the drive gear. Even more preferred is if it is less than 13% for example less than 10%. When the flexible rack follows the drive gear the radially outer regions of the polymer jacket are stretched in an amount equal to the ratio of minimum circumscribed circle 'D' over drive gear diameter. While polymers can take an appreciable amount of stretching they do this in a predominantly plastic way. However, under repeated bending cracks may propagate from the stretched outer regions of the polymer jacket resulting in a tearing of the polymer jacket. In order to prevent this cracking the outer elongation must be limited.
Brief Description of Figures in the Drawings
[0073] FIGURE 1 shows the flexible rack according the invention in perspective view;
[0074] FIGURE 2 shows a cross section of the flexible rack according the
invention;
[0075] FIGURE 3 shows a load-elongation diagram of a bare steel cord and of the flexible rack for determination of the longitudinal stiffness;
[0076] FIGURE 4 shows a force-deflection diagram of a bare steel cord to
determine the bending stiffness of the steel cord;
[0077] FIGURE 5 shows a force-deflection diagram of a the flexible rack to
determine the bending stiffness of the flexible rack;
[0078] FIGURE 6 shows a cross section of a real sample.
[0079] Corresponding elements over different figures have equal units and tens numbers.
Mode(s) for Carrying Out the Invention
[0080] FIGURE 1 shows a perspective view of the flexible rack 100 according the invention. The flexible rack comprises a steel cord 102 embedded in a polymer jacket 104. The polymer jacket is provided with regular recesses 106 in the longitudinal direction of the flexible rack. The bottom of the recess 108 is there where the recesses come closest to the steel cord 102. Between two subsequent recesses a crest or top 1 10 is present. The recesses match the gear tooth shape of the drive gear of the drive system.
[0081 ] FIGURE 2 shows the cross section of another preferred embodiment 200 of the flexible rack. The steel cord 202 is embedded in the polymer jacket 204. Recesses 206 start from bottom 208, closest to the steel cord 202 up to the top 210. A groove 214 is made along the length of the flexible rack 200. Such a groove can serve to guide the flexible rack and prevent it from rotation. Likewise flats 215, 215' can be introduced for the same purpose.
[0082] F1 and F2 indicated calliper diameters of which F2 is the maximum calliper diameter. The minimum circumscribed circle is indicated with C1 and has a diameter indicated with'D'. The centre of that circle is situated at M1 . These geometrical features have been determined with the digital image analysis software package Olympus Analysis'. The steel cord consists of 12 filaments 203 and one wrap 216 that are arranged in a 3+9+1 configuration that is
• 3 filaments of diameter 0.25 mm are twisted together in a first operation with a lay length of 6.3 mm in S direction thus forming a core;
• The core is covered with 9 filaments of diameter 0.23 mm with a lay length of 12.5 mm in S direction.
• The steel cord is wrapped with a 0.15 mm steel wire 216 at lay 3.5 mm in Z direction. The lay corresponds to the distance between recesses.
The centre of the steel cord is situated at M2 and the steel cord has a minimum circumscribed circle C2. The eccentricity of the two centres is indicated with 'Δ'. The distance between the bottom of the recess and the steel cord is indicated with '∑'.
[0083] Another representative embodiment of the invention was produced as
follows:
• A steel cord of construction (12)+15x0.25 was made from hot dip galvanised wires. The construction is made of 12 wires with diameter 0.25 arranged in a compact cord configuration of lay 10 mm in S around which 15 filaments of the same diameter are twisted at lay 21 mm in Z.
• Around this steel cord a polymer jacket is extruded. The polymer jacket is made of IROGRAN D 74 P 4778 available from Huntsman. This is a thermoplastic polyether-polyurethane for injection moulding and extrusion applications. It has a Shore D of 70. The polymer is extruded around the steel cord until a diameter of 5 mm is reached with a round cross section. After the extrusion the cord is allowed to cool down.
• After a brief reheating, up to the softening temperature of the polymer (200°C) recesses are imprinted in the longitudinal direction of the coated steel cord. The recesses have a distance of 3.5 mm from one another so as to engage with the teeth of a drive gear with same tooth spacing. The resulting product is a flexible rack.
[0084] Figure 6 shows a cross section of the resulting flexible rack. It has a
smallest circumscribed circle indicated with Ci with a diameter 'D' of 5.0 mm and centre Mi . The steel cord has a centre M2 and diameter 1 .57 mm. The out-of-centre distance between a plane through the centre of minimum circumscribing circle perpendicular to the mirror plane of the flexible rack to the centre of the steel cord is 0.22 mm or 4.4% of D. The perpendicular distance between the mirror plane and the centre of the steel cord remained below 2 μιτι. The polymer thickness between the bottom of the tooth and the steel cord is 0.20 mm or 4% of D.
[0085] According the same procedure flexible racks with different steel cord
constructions were produced such as 7x7 with a diameter of 1 .60 mm, 0.55+6x0.53, 0.34+6x0.32+12x0.295 and 3+9+15x0.25 but using the same polymer jacket.
[0086] It will now be described how the elongation stiffness can be determined.
All load elongation tests are performed at room temperature with the use of an extensometer in order to achieve sufficient precision.
[0087] First the bare steel cord is subjected to a load elongation test. A typical result - notably for the construction (12)+15x0.25 - is represented with the line 302 in FIGURE 3. At the start of the loading, some setting of the cord occurs but this does not affect the longitudinal stiffness of the cord. The slope of the fitted line 303 in the region between 10% to 50% of the breaking load of the steel cord gives the elongation stiffness of the bare cord (EA)SC that is in this case 235386 N.
[0088] After extrusion and impressing of the recesses the resulting flexible rack is again submitted to a load-elongation test. This results in the curve 304 in FIGURE 3. Note that the setting of the cord has disappeared. A linear curve fitting 305 yields the total stiffness (EA)Rack of the flexible rack as being 254561 N. The contribution to the stiffness by the polymer jacket is thus 19175 N. The E-ratio of both is then 235386/254561 or 92.5%. This means that the steel cord carries that amount of the total force exerted in elongation.
[0089] The cord 7x7x0.15 failed to meet the 90% requirement and showed a too high elongation during use.
[0090] In furtherance the bending stiffness of the bare steel cord and the flexible rack has been determined. To this end a specimen of the steel cord is supported horizontally between two frictionless fulcrums 50 times the diameter of the cord (1 .29 mm for this steel cord) apart. The wire is deflected at the middle with a roll indenter. The force exerted on and the displacement of the indenter are recorded as shown in the curve of Figure 4.
[0091 ] The zero deflection is set at the point when a force on the indenter is first sensed. Deflection is continued until an elongation of 1 % in the outer fibre of the steel cord is obtained. This elongation is equal to the ratio of the diameter of the steel cord to twice the radius of curvature at the deepest indentation point. For a basis of 50 times the diameter of the cord, this is achieved with a total displacement of about 4.2 times the diameter of the steel cord. This is the first upward curve indicated with 402 in Figure 4. When the maximum deflection is reached, the movement of the indenter is reversed until no force is sensed (the point 405). The curve in the downward direction 404 is parallel to the upward curve 402 but is displaced parallel to it. This is due to the internal friction in the cord. The measurement is resumed (second up curve 406) till an elongation of 0.5% is reached (the point 407). Again the direction is reversed and a second downward curve 408 is traced. When now fitting a line 410 to the linear region of the second upward curve a measure for the bending stiffness of the steel cord is obtained. This can be derived out of classical bending theory to be:
_ L3 - AF Herein L is the distance between the fulcrums in mm (65 mm in this case). AF is the force difference in newton in the linear region while AX
represents the corresponding deflection in mm over that same region. The factor 48 results from bending theory at small deflections (which is applicable here). The measurement is repeated thrice and averaged to reduce measurement error. For this cord a bare stiffness of 1042 Nmm2 was obtained.
[0092] Subsequently the cord was provided with a polymer jacket and indented to form recesses. Again a stiffness measurement was performed wherein the cord was deflected perpendicular to the plane formed by the indentations and the axis of the steel cord. A typical trace is shown in Figure 5. The same conditions as for the bare cord applied (distance between fulcrums, degree of bending applied etc .). Note the difference in force scale compared to the bare cord. Again a first upward trace 502 is recorded in order to 'set' the cord. At turning point 503 the deflection direction is reversed (following trace 504) until zero force is detected at 505. After direction reversal the cord is deflected half of the previous deflection (second upward trace 506) till turning point 507 after which the movement of the indenter direction is again reversed tracing the line 508. A linear fit to the second upward trace gives a measure for the stiffness that is calculated in the exact same way as for the bare cord. Again three measurements on different pieces of the flexible rack are made. The resulting values are 21634 Nmm2, 19505 Nmm2, and 19958 Nmm2 yielding an average of 20366 Nmm2. The contribution of the bending stiffness of the steel cord to the total bending stiffness of the flexible rack is therefore 1042/20366 or 5.1 % which is lower than the maximum value 15%. The absolute contribution of the polymer jacket is 19324 Nmm2.
[0093] The bending ratios of the steel cords 0.55+6x0.53, 0.34+6x0.32+12x0.295 covered with the same polymer jacket turned out to be too high i.e. the steel cords were too bending stiff in order to result in an acceptable flexible rack. This is mainly due because the filaments are too thick. As the stiffness of the flexible rack then becomes too high, the friction forces between flexible rack and the guides in which the rack moves becomes too high. Only the cord 3+9+15x0.25 also passed the bending
requirements.
[0094] The anchoring force of the steel cord in the polymer jackets was tested as follows:
• A flexible rack was cut straight at one end;
• A circular incision was made through the polymer jacket down to the steel cord at 25 mm from the end;
• The piece of the 25 mm long polymer jacket was pulled in axial direction from the polymer rack while the force was monitored;
• The maximum force was noted.
The procedure is repeated at least 12 times and the average of the results is taken in order to reduce the variability in the test.
[0095] For the flexible rack with steel cord construction (12)+15x0.25 a pull off force of 350 N was needed to pull 25 mm of the polymer jacket from the steel cord. The minimum required is 50x5 i.e. 250 N.
[0096] Some of the flexible racks were tested in a drive system simulating the bending in a first plane and one bend in a plane perpendicular to the first plane. The flexible rack was guided through a tubular guide channel and driven back and forth by a drive motor. The drive gear had a diameter of 70 mm but in principle the flexible rack can be used with drive gears having a diameter of 33 mm or higher. The tests showed that at least a flexible rack according the claims can be used to transfer enough force in tension and compression to move a window pane. Endurance tests showed that the flexible rack according the claims could survive the required number of drive cycles.

Claims

Claims
1 . A flexible rack for use in a drive system, said drive system comprising a drive gear, said flexible rack comprising a steel cord embedded in a polymer jacket, said polymer jacket having regular recesses in the longitudinal direction of said flexible rack, said recesses for receiving the teeth of said drive gear, said steel cord comprising steel filaments, and wherein in cross section said flexible rack is circumscribed with a minimum circumscribed circle, said circle having a diameter 'D'
characterised in that
the longitudinal stiffness of said steel cord accounts for at least 90% of the total longitudinal stiffness of said flexible rack.
2. The flexible rack according to claim 1 wherein the bending stiffness of said
steel cord accounts for at most 15 % of the total bending stiffness of said flexible rack.
3. The flexible rack according to any one of claims 1 to 2 wherein the longitudinal stiffness of said steel cord is between 190 kN and 260 kN and wherein the contribution of the polymer jacket to the longitudinal stiffness is between 5 kN and 21 kN.
4. The flexible rack according to any one of claims 1 to 3 wherein the bending stiffness of said steel cord is between 500 and 1 400 Nmm2 and wherein the contribution of the polymer jacket to the bending stiffness is between 8 000 Nmm2 and 22 000 Nmm2.
5. The flexible rack according to any one of claims 1 to 4 wherein the modulus of said steel cord is at least 180 000 N/mm2, while the modulus at room
temperature of said polymer is between 350 and 1 100 N/mm2.
6. The flexible rack according to any one of claims 1 to 5 wherein the distance between the plane through the centre of said minimum circumscribed circle and that is perpendicular to the mirror plane of said cross section and the centre of said steel cord is within 15 % of the diameter 'D' of said minimum circumscribed circle .
7. The flexible rack according to claim 6 wherein said distance is at least 1 % of the diameter 'D' of said minimum circumscribed circle.
8. The flexible rack according to any one of claims 1 to 7 wherein the polymer between the bottom of said recesses and said steel cord is at least 2 % of the diameter 'D' of said minimum circumscribed circle.
9. The flexible rack according to any one of claims 1 to 8 wherein said steel filaments are free of second or higher order helical deformation.
10. The flexible rack according to claim 9 wherein said steel filaments have first- order helical deformation.
1 1 . The flexible rack according to any one of claims 9 or 10 wherein said filaments are arranged in concentric layers, said layers having a lay direction that is opposite or equal between adjacent layers.
12. The flexible rack according to claim 1 1 wherein the outer of said concentric layers is an unsaturated layer.
13. The flexible rack according to any one of claims 1 to 12 wherein said steel cord comprises one or more wrap filaments twisted around said steel cord.
14. The flexible rack according to any one of claims 1 to 13 wherein said steel filaments have a diameter between 0.15 to 0.50 mm.
15. The flexible rack according to any one of claims 1 to 14 wherein a force of at least 50*D in newton is needed to axially pull said polymer jacket from said steel cord over a length of 25 mm.
16. The flexible rack according to any one of claims 1 to 15 for use with a drive gear having a drive gear diameter, wherein the diameter 'D' of said minimum circumscribed circle is less than 15% of the diameter of said drive gear.
PCT/EP2016/079989 2015-12-18 2016-12-07 Flexible rack with steel cord embedded in polymer WO2017102478A1 (en)

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Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2092629A (en) * 1981-02-06 1982-08-18 Bekaert Sa Nv Improvements in fatigue resistant cables
JPH01145463A (en) * 1987-12-01 1989-06-07 Yokowo Mfg Co Ltd Drive cord with rack
EP0499135A1 (en) * 1991-02-15 1992-08-19 GATE S.p.A. A toothed, flexible transmission cable, a method for its manufacture, and a transmission mechanism including the cable
WO2007020156A1 (en) * 2005-08-19 2007-02-22 Nv Bekaert Sa A polymer impregnated steel cord
WO2011045215A1 (en) * 2009-10-14 2011-04-21 Inventio Ag Elevator system and suspension for such a system
US20120277047A1 (en) * 2011-04-28 2012-11-01 Hi-Lex Corporation Toothed cable

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2092629A (en) * 1981-02-06 1982-08-18 Bekaert Sa Nv Improvements in fatigue resistant cables
JPH01145463A (en) * 1987-12-01 1989-06-07 Yokowo Mfg Co Ltd Drive cord with rack
EP0499135A1 (en) * 1991-02-15 1992-08-19 GATE S.p.A. A toothed, flexible transmission cable, a method for its manufacture, and a transmission mechanism including the cable
WO2007020156A1 (en) * 2005-08-19 2007-02-22 Nv Bekaert Sa A polymer impregnated steel cord
WO2011045215A1 (en) * 2009-10-14 2011-04-21 Inventio Ag Elevator system and suspension for such a system
US20120277047A1 (en) * 2011-04-28 2012-11-01 Hi-Lex Corporation Toothed cable

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