Elevator tension member with a hard thermoplastic polyurethane elastomer jacket

Abstract

An elevator tension member is presented that has one or more steel cords as strength members that are encased in a jacket of thermoplastic polyurethane elastomer. The thermoplastic polyurethane elastomer is selected on the basis of its thermal properties in that the glass transition temperature of the hard phase (Tg HS) is above 90° C. In preferential embodiments that thermoplastic polyurethane elastomer has a crystallisation temperature (Tg) that is at least 20° C. higher than Tg HS. In other preferential embodiments the sum of Tg HS and Tc is higher than 200° C. Such thermoplastic polyurethane elastomers exhibit an unexpected increase in useful lifetime when compared to conventionally used polyurethanes. Moreover the invention provides a simple method to select an appropriate thermoplastic polyurethane elastomer.

Claims

1. An elevator tension member comprising one or more steel cords and a jacket encasing said steel cords, wherein said jacket comprises a thermoplastic polyurethane elastomer, said thermoplastic polyurethane elastomer having a hard crystalline phase and a soft phase, wherein the glass transition temperature of said hard crystalline phase is higher than 90° C., wherein said thermoplastic polyurethane elastomer further has a crystallisation temperature, said crystallisation temperature being measured during cooling from the melt and wherein the sum of said glass transition temperature of said hard crystalline phase and said crystallisation temperature is higher than 200° C.

2. The elevator tension member according to claim 1, wherein the diameter of each of said steel cords is lower than or equal to 8 mm and larger than or equal to 1 mm.

3. The elevator tension member according to claim 1, wherein the Shore D hardness of said thermoplastic polyurethane elastomer is between 40 and 90.

4. The elevator tension member according to claim 3, wherein the Shore D hardness of said thermoplastic polyurethane elastomer is between 45 and 60.

5. The elevator tension member according to claim 1, wherein the bending stiffness of the elevator tension member is at least five times the total bending stiffness of the one or more steel cord.

6. The elevator tension member according to claim 1, wherein the thickness of said jacket is at least 8% of the maximum diameter of said one or more steel cords said thickness being the minimum of distances between any of said one or more steel cords and the outer surface of said tension member.

7. The elevator tension member according to claim 1, wherein said thermoplastic polyurethane elastomer occupies at least 80% of the available area inside the circumscribed circle of any one of said one or more steel cords in a perpendicular cross section.

8. The elevator tension member according to claim 1, wherein one steel cord is encased in said jacket, said tension member having a substantially circular cross section and wherein the thickness of said jacket is thinner than 20% of the diameter of the tension member, said thickness being the minimum of distances between said one steel cord and the outer surface of said tension member.

9. The elevator tension member according to claim 8, wherein the bending stiffness of the bare one steel cord is between 8 and 17 kNmm.sup.2.

10. An elevator tension member comprising one or more steel cords and a jacket encasing said steel cords, wherein said jacket comprises a thermoplastic polyurethane elastomer, said thermoplastic polyurethane elastomer having a hard crystalline phase and a soft phase, wherein the glass transition temperature of said hard crystalline phase is higher than 90° C., wherein said thermoplastic polyurethane elastomer further has a crystallisation temperature that is at least 20° C. higher than glass transition temperature of said hard crystalline phase, said crystallisation temperature being measured during cooling from the melt.

11. The elevator tension member according to claim 10, wherein said crystallisation temperature is less than 80° higher than the glass transition temperature of said hard crystalline phase.

12. An elevator tension member comprising one or more steel cords and a jacket encasing said steel cords, wherein said jacket comprises a thermoplastic polyurethane elastomer, said thermoplastic polyurethane elastomer having a hard crystalline phase and a soft phase, wherein the glass transition temperature of said hard crystalline phase is higher than 90° C., wherein said one or more steel cords is treated with an adhesion primer to improve the adhesion between said one or more steel cords and said jacket such that the shear stress needed to pull a 10 mm long embedded steel cord out of said jacket is higher than 4 N/mm.sup.2.

13. A method of selecting a thermoplastic polyurethane elastomer for use as a jacket encasing one or more steel cords in an elevator tension member, said method comprising the steps of: obtaining a thermoplastic polyurethane elastomer in a differential scanning analysis performed on said thermoplastic polyurethane elastomer: i. determining the highest glass transition temperature during heating of said polyurethane, said glass transition temperature corresponding to the glass transition temperature of the hard segments in said polyurethane; ii. determining the crystallisation temperature of said polyurethane during the cooling of the melt; selecting the thermoplastic polyurethane elastomer for use as a jacket for encasing one or more steel cords in an elevator tension member if and only if: i. said glass transition temperature of the hard segments is higher than 90° C. and; ii. the sum of said glass transition temperature and said crystallisation temperature is larger than 200° C.

14. A method to produce an elevator tension member comprising the steps of: providing one or more steel cords arranged in a single plane; selecting a thermoplastic polyurethane elastomer according the method of claim 13; extruding the selected thermoplastic polyurethane elastomer around said one or more steel cords; thereby obtaining an elevator tension member.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

(1) FIG. 1 shows an elevator tension member according the invention with one single steel cord: an elevator rope.

(2) FIG. 2 shows an elevator tension member according the invention with eight steel cords: an elevator belt.

(3) FIGS. 3a and 3b show schematic Differential Scanning Calorimetry (DSC) curves indicating the thermal features of a TPE.

(4) FIG. 4 shows a test system to evaluate the fatigue life of an elevator tension member.

(5) FIG. 5 shows the relation between the number of fatigue cycles obtained on various TPEs in relation to the sum of T.sub.g HS+T.sub.c.

MODE(S) FOR CARRYING OUT THE INVENTION

(6) FIG. 1a shows an elevator tension member that is in this case an elevator rope. The rope consists of a steel cord 106 that is surrounded by a polymer jacket 110. The steel cord is of the generic type 7×7+19W that in more detailed form is:
{[(0.34+6×0.31)+6×(0.25+6×0.25)]+7×(0.34+6×0.31|6×0.33/6×0.25)}

(7) The numbers indicate the diameters of the filaments in millimetre. The brackets indicate one operation wherein steel filaments are assembled into strands and strands into cord. The core of the steel cord 104 is of the 7×7 type, that has a king strand (0.34+6×0.31) surrounded with 6 strands of make (0.25+6×0.25). Around the 7×7 core 7 strands of the Warrington type are twisted, wherein all filaments are twisted in one single operation. The lay direction between different layers are alternating and have a magnitude between 5 to 12 times the diameter of the strand or cord. The steel cord can be circumscribed with a circle 102 and has a calliper diameter ‘D’ which is in this case 5.0 mm.

(8) The tension member has a jacket 110 that is extruded around the steel cord 106. The jacket has a substantial circular cross section with a total diameter ‘D.sub.tot’ of 6.5 mm. The thickness—indicated with ‘t’—is therefore about 0.75 mm which corresponds to the minimal of distances between the steel cord 106 and the outer surface of the tension member. The polymer fills to a large degree—in this case 85%—the available area inside the circumscribed circle 102.

(9) FIG. 2 shows an alternative elevator tension member 200 wherein 8 steel cords 202 are arranged in a side by side relationship in a single plane. Neighbouring steel cords have opposite lay directions. The cords have a 7×7 configuration with formula
[(0.21+6×0.19)+6×(0.19+6×0.175)]
The cords are encased, embedded, surrounded in a polymer jacket 210 consisting of a TPE.

(10) The inventors evaluated a large number of commercially available TPE's as obtainable from known suppliers such as Bayer, BASF, Teknor-Apex, Lubrizol, etc. . . . . The same one steel cord as depicted in FIG. 1 was extruded with all these TPEs.

(11) The thermal properties of the TPEs were determined in a DCS measurement. FIGS. 3a and 3b describe such a trace of TPE 5 (see further): 3a upon second heating, 3b during first cooling. In the abscissa the temperature is represented (in ° C.) while in ordinate the heatflow (in mW/g) is represented. The relevant glass transition temperatures of soft segments (T.sub.g SS), hard segments (T.sub.g HS) and melting (T.sub.m) temperatures are determined on second heating, after erasing the thermal history of the sample and after all water is evaporated. The skilled lab technician knows how to determine these transition temperatures. Upon cooling (FIG. 3b) an exothermic peak is noticed when the sample starts the crystalize at the crystallisation temperature T.sub.c. The measurement of these properties is simple and takes less than one hour.

(12) The extruded samples of elevator ropes were tested for fatigue life in a test system such as depicted in FIG. 4. In the test system 400, the elevator tension member 401 is tensioned by two weights 416, 418 to 12% of the breaking load of the elevator rope. The test system 400 comprises one traction sheave 414 driven by an electrical motor and one additional deflection sheave 412. Both sheaves 412, 414 have round grooves with a groove radius slightly larger than the diameter of the tested load bearing assemblies 401. During fatigue testing the motor drives the load bearing assembly 401 back and forth over both traction sheave 412 and the deflection sheave 414. The test system is a good representation of a real life elevator. The diameter ‘D.sub.sheave’ of both traction sheave 412 and deflection sheave 414 is 16.1 times the total diameter ‘D.sub.tot’ of the elevator tension member. In the test the ratio ‘D.sub.sheave/D.sub.tot’ is much lower than the conventionally used ratio 40.

(13) The ratio D/D.sub.tot was intentionally chosen low to test the elevator tension member in extreme conditions. The test is continued till the jacket of the elevator tension member cracks or shears off. For a single cord this can take between 50 000 to 2 000 000 bends. As one bend takes about one second the duration of one test is between ½ and 24 days. There is therefore a large benefit if one can reduce the selection of the TPE by performing a simple DCS test. Based on this test, the number of candidate TPEs can already be largely reduced before having to resort to elaborated fatigue testing of the elevator tension member in its entirety.

(14) In Table 1 an overview of the samples tested is shown: Column (1) identifies the TPE type, the second column (2) is the glass transition temperature of the hard segments (T.sub.g HS (° C.)), column (3) is the melting temperature of the TPE, column (4) is the crystallisation temperature T.sub.c (° C.), column (5) is the difference of the crystallisation temperature and the glass transition temperature of the hard segments (T.sub.c−T.sub.g HS (° C.)), followed by the sum of both ((T.sub.c+T.sub.g HS (° C.)), column (6)). Column (7) lists the Shore D hardness values. Column (8) lists the number of bends (per 1000 bends or kBends) attained with each cord. The last column (9) is the measured bending stiffness on the elevator tension member (in Nmm.sup.2).

(15) TPE 1 to 7 and TPE 12 all have a hard segment glass transition temperature above 90° C. (indicated bold). Of those TPE 1, 3, 4, and 7 have a crystallisation temperature that is at least 20° C. above the hard segments glass transition temperature (indicated bold underlined).

(16) From another perspective, the TPEs 1, 2, 3, 4, 7 and 12 have the sum of the hard segment glass transition and the crystallisation temperature above 200° C. (indicated bold double underlined).

(17) Table 1 proves the assertion of the inventors that in order to obtain more than 490 000 bends in the test system a TPE with a hard segment glass transition temperature that is larger than 90° is needed. Even longer fatigue life can be obtained when the crystallisation temperature is at least 20° C. higher than the hard segment glass transition temperature. There appears also a trend that the fatigue life increases with the sum of the hard segment glass transition temperature and the crystallisation temperature. This is graphically represented in FIG. 5. There the number of bending cycles (kBends) attained is plotted as a function of the sum of the hard segment glass transition (T.sub.g HS) temperature and the crystallisation temperature (T.sub.c). The vertical dashed line indicate the 200° C. limit, while the horizontal dashed line indicates the 490 000 bends limit line.

(18) Next to that the bending stiffness of the elevator tension member was determined. To this end a specimen of the elevator tension member is supported horizontally between two frictionless fulcrums 50 times the diameter of the steel cord (5.00 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. Out of classical bending theory the bending stiffness can be derived from:

(19) ${\left(EI\right)}_{SC}=\frac{L^3.Math.\Delta F}{48.Math.\Delta X}$

(20) Wherein L is the distance between the support fulcrums, ΔF, ΔX indicated the change in force and the change in displacement in the upper linear region of the curve.

(21) For the bare steel cord i.e. the steel cord used prior to extrusion the bending stiffness measured was 14 000 Nmm.sup.2. The elevator ropes that attain the best fatigue results have a bending stiffness that is at least 5 times the bending stiffness of the bare cord.

(22) It is remarkable that fatigue results increase so drastically in function of the TPEs used, while the steel cord remains exactly the same. The selection of TPEs is by the invention much easier and only relies on a simple DCS measurement.

(23) TABLE-US-00001 (1) (2) (3) (4) (5) (6) (7) (8) (9) Nr Tg HS (° C.) Tm (° C.) Tc (° C.) Tc-Tg (° C.) Tc +Tg HS (° C.) Shore D kBends El (Nmm.sup.2) TPE 1   90 179 111 21 201 46  686  80513 TPE 2   99 155 110 11 208  491 TPE 3  105 186 173 69 278 52 1749 101807 TPE 4  108 180 163 55 271 42  904  88021 TPE 5  100 179  98 −2 198  75  31892 TPE 6  104 162  92 −12  195 40  470  94712 TPE 7   92 180 153 60 245  763  85795 TPE 8   63 173 104 42 167  250 TPE 9   60 129  88 28 148  50  48034 TPE 10  50 118  87 37 137  118  46346 TPE 11  60 175 103 43 163 42  208  63969 TPE 12 100 180 107  7 207 50 1201 TPE 13  25 160 114 89 138 54  108