WO2024012949A1

WO2024012949A1 - Drive sheave for an elevator

Info

Publication number: WO2024012949A1
Application number: PCT/EP2023/068507
Authority: WO
Inventors: Raf CLAUWS
Original assignee: Bekaert Advanced Cords Aalter Nv
Priority date: 2022-07-12
Filing date: 2023-07-05
Publication date: 2024-01-18

Abstract

A drive sheave of an elevator for use with a coated steel cord comprises a metal body and separators. The metal body has shallow circumferential grooves with a groove depth. The grooves are separated from one another with separators that extend out of the metal body much more than the groove depth. The separators are made of or are provided with a low friction coating. The friction coefficient between the separators and the coating of the coated steel cord is much lower than the friction coefficient between the metal body and the coating of the steel cord. The metal body can be assemble out of discs each having a single groove. The friction force of the coated steel cord can further be influenced by making the groove width less than the diameter of the coated steel cord. An elevator comprising coated steel cords running over the sheave is also claimed.

Description

Title: Drive sheave for an elevator

Description

Technical Field

[0001] The invention relates to a drive sheave for an elevator. Such a drive sheave transfers force and torque between a drive train and a tension member. The drive sheave is suitable for use with polymer coated elevator rope.

Background Art

[0002] From the turn of the century elevator builders have been striving to eliminate the rooftop machine room above the elevator shaft. One crucial element that facilitated this was the introduction of tensile members comprising high tensile, fine filament steel cords in replacement of the thick wire, low tensile steel ropes. While in the old technology elevator ropes are 8 mm or thicker, in the new technology steel cords have a diameter of less than or equal to 5 mm, in case a single steel cord is used, or even diameters lower than 2 mm when multiple steel cords are encased parallel in the polymer jacket of a belt.

[0003] In the following ‘steel ropes’ will refer to the older technology with steel wire assemblies having an overall rope diameter of more than 8 mm with steel wires thicker than 0.30 mm with a tensile stress below 2 000 N/mm², while the term ‘steel cords’ will be reserved for steel wire assemblies made from fine wires thinner than 0.30 mm with a tensile strength in excess of 2000 N/mm². The tensile stress of a steel filament - expressed in newton per square millimetre - is the force needed to rupture the filament - called ‘breaking load’ (in newton) - divided by its metallic surface in perpendicular cross section (in mm²).

[0004] Indeed, such fine, high tensile filaments allow to reduce the diameter of the tensile member on which cabinet and counterweight are suspended. This in turn allows the use of a smaller diameter drive sheave that moves the tension member. A smaller drive sheave can be driven by a direct drive motor, thereby eliminating the bulky gear box needed when a larger sheave had to be used. All this makes it possible to design a smaller drive system that can be mounted either at the top or bottom of the shaft. No longer a machine room above the shaft on the roof top is needed.

[0005] However, the use of fine, high tensile steel filaments brings particular problems that are not present in old technology steel ropes. The finer filaments generate higher transversal stresses, called Hertzian stresses, between the filaments due to their smaller diameter when being bent under load. On top of that, high tensile steel filaments are less ductile and more susceptible to breaking under transversal forces.

[0006] This problem is generally solved by encasing the steel cord or steel cords in a jacket or mantle of a polymer. In particular thermoplastic polyurethane is preferred because it has good wear resistance properties and, as it is a thermoplastic material, does not need a thermal treatment to cure it. In what follows a single steel cord that is encased in a polymer jacket will be called a ‘coated steel cord’. Multiple steel cords arranged in parallel that are held together and encased in a polymer jacket will be called a ‘coated elevator belt’. A ‘tension member’ refers to either a ‘coated steel cord’ or a ‘coated elevator belt’.

[0007] Introducing a polymer casing brings an additional difficulty with it in that the coefficient of friction between the polymer and the drive sheave behaves differently from the well-known steel rope/cast steel drive sheave. Not only the materials are different (polymer vs. steel) but also the geometrical configuration: large contact surface in case of a coated elevator belt, smaller contact surface in case of coated steel cord. In general the friction between polymer and steel sheave is higher in comparison with known steel rope/steel sheave combinations.

[0008] Moreover, the use of thin steel filaments results in a steel cord that cannot resist torque as good as the conventional steel ropes as steel cords have a much lower ‘torsional rigidity’ compared to steel ropes. The torsional rigidity is the product of the shear modulus G (a material constant, in MPa) with the torsion constant ‘J’ that is - for filaments with circular cross sections of diameter ‘d’ - equal to ird⁴/32, the second order polar moment of a round filament. Hence, the fine steel filaments in a steel cord will resist much less the torque exerted on it than a steel rope made of thick steel wires, not even when many wires are combined. The coated steel cord is therefore easily twisted around its axis compared to conventional steel ropes. One twist of a cord around its axis is called ‘a torsion’. The torsion can be applied when a torque twists the steel cord or it can be an elastic torsion, that is a torsion that releases when the torque on the cord is removed. Usually torsions - either applied or released - are counted over a unit length of the cord.

[0009] The combination of an increased coefficient of friction between polymer and sheave surface and the much lower torsional rigidity of the coated steel cord leads to the build-up of torsions in a coated steel cord during use. Indeed, when the coated steel cord entering the drive sheave is not in the plane of the drive sheave the coated steel cord will roll in the sheave, when the sheave turns, due to the increased friction between sheave and polymer jacket and the reduced torsional rigidity. The plane of the drive sheave is the plane perpendicular to the axis of the sheave and through the middle of the sheave. Unfortunately, there are few elevators that are completely free of misalignment of the tension members.

[0010] This repeated up- and un-torquing on every flight makes the filaments in the steel cord move relative to one another which is a source of fretting, abrasion of filaments one against the other leading to an earlier failure of the coated steel cord due to ‘torsion fatigue’.

[0011 ] As the total load on the rope during acceleration differs when going upward (the acceleration of the sheave adds to the gravity acceleration) compared to when going downward (the acceleration of the sheave subtracts from gravity acceleration) there may be a tendency that in the coated steel cord torsions build up, accumulate or torsions decumulate, are taken out of the cord between the sheave and the anchoring point of the coated steel cord. In general, when these induced torsions are in the ‘closed’ direction of the steel cord, the inventors have found that this is less harmful then when those torsions are in the ‘open’ direction of the steel cord. The ‘closed’ direction of a steel cord is that direction that shortens the lay length of the outer strands in the steel cord. The ‘open’ direction of a steel cords is that turning direction that increases the lay length of the outer stands in the steel cord.

[0012] A further complication is that coated steel cords in elevators are used with a smaller drive sheave diameter, sometimes even below the commonly accepted D > 40*d where ‘D’ is the smallest diameter of the drive sheave and ‘d’ the diameter of the steel cord. This implies that in one travel of cabin, the coated steel cord hits the side of the groove much more than in the prior art elevators using large diameter steel wire ropes. So also this aggravates the torsion accumulation or torsion decrease problem.

[0013] To overcome this problem of ‘torsion fatigue’ it has been suggested in US 2004/0256180 A1 to coat the sides of the groove wherein a coated cord runs with a friction reducing coating that has a lower coefficient of friction with the coated cord than the bottom of the groove.

[0014] However, it turns out to be very difficult to do this in a consistent and lasting way. Therefore the inventors suggest the following solution that will be described in detail hereinafter.

Disclosure of Invention

[0015] It is a prime object of the invention to provide a drive sheave for an elevator that prevents torsion fatigue in coated steel cords. It is a further object of the invention to provide an elevator arrangement that diminishes the torsion fatigue problem.

[0016] According a first aspect of the invention, a drive sheave for an elevator is provided. The drive sheave serves to transmit the torque of the drive motor of the elevator to the linearly moving coated steel cord that lifts or lowers the cabin and the counterweight. The drive sheave comprises a metal body. In the metal body circumferential grooves for receiving the coated steel cord are present.

[0017] The grooves have the shape of an arc of a circle or an arc of an ellipse with its longer axis parallel to the drive sheave axis. The grooves have a bottom, a valley at the radial smallest distance and circumferential ridges at the largest radial distance. The depth of the grooves - the ‘groove depth’ - is the radial distance from the bottom of the groove to the ridge and is equal to the difference between the largest radial distance and the smallest radial distance on the metal body. Note that the cross-section of the groove is always concave, never convex.

[0018] The drive sheave further comprises separators that circumferentially separate the grooves from one another. The separators are not part of the metal body. The separators radially protrude above the circumferential ridge of the groove with a separator height. The separator height is larger than the groove depth. The number of separators is equal to the number of grooves or the number of grooves plus one.

[0019] The surface of the separators, at least the surface of the separators that may contact the coating of the coated steel cord, has a first coefficient of friction between the separator surface and the coating of the coated steel cord. The grooves in the metal body and the coating of the coated steel cord slide over one another with a second coefficient of friction. The first coefficient of friction is lower than the second coefficient of friction. With ‘coefficient of friction’ is meant the static coefficient of friction which is a dimensionless constant.

[0020] The invention eliminates the difficulty to apply friction reducing coating of the prior art: the cross-over from high to low friction surface contacting the coating of the coated steel cord is clear and well defined. There is no gradual transition in friction when the coated steel cord climbs up the sides of the groove. Further a large difference between first friction coefficient on the separator and the second friction coefficient is possible. By the invention the build up of torsions in the coated steel cord in a misaligned installation is reduced and can even be prevented.

[0021 ] Preferably the separator height is larger than twice the groove depth, or even three times the groove depth, or more. The separator height is limited by the diameter of the coated steel cord that runs in it. There is little benefit in making the separator height larger than twice the diameter of the coated steel cord.

[0022] The metal of the metal body is steel, cast iron, or non-ferrous metals and alloys such as copper, brass, bronze or other copper comprising alloys.

Steel grades such as high tensile steel grades S690QL, S355JR, S960QL, Q355A, A514 Grade F, A514 Grade P, A517 Grade F according EN 10025-6 are preferred steels for this. Typically the metal body is milled out of a steel cylinder. Alternatively the metal body can be milled from carbon steel cylinders with steel grades: S235JR, Q235A, A283 Grade C, A36, St37-2, A537 Grade 70, Q345, SS400, SM400A according EN 10025. Also stainless steels such as X5CrNi18.9, Werkstoffnummer 1 .4301 , X5CrNiMo18.10, Werkstoffnummer 1 .4401 , X2CrNiMo18.10, Werkstoffnummer 1 .4404, 42CrMo4-V Werkstoffnummer 1 .7225 can be used. The surface of the groove can be treated in order to increase the coefficient of friction by etching, sanding, buffing, laser texturing or similar operations.

[0023] The groove width is defined as the axial distance between the facing edges of the circumferential ridges of the groove in the metal body. The separator spacing is the widest axial distance between two facing surfaces of a pair of separators. Preferentially the ratio of groove width to separator width is between 0.4 and less than 1 .0, preferably between 0.45 and 0.95 even more preferred between 0.50 and 0.95 such as between 0.55 and 0.90. The ratio allows to fine tune the force brought over from the drive sheave to the coated steel cord: if this ratio is lower, less force can be transmitted, while when the ratio is 1 .0 the transmitted force is maximal. This is also important for tuning the deceleration experienced by the passengers in the case of an emergency stop: of the deceleration is too high, passengers may get injured at an emergency stope. If the deceleration is too low, the cabin may hit the bottom of the elevator shaft.

[0024] The shape of the groove is preferentially an arc of an ellipse having a minor axis in the radial direction of the drive sheave and a major axis in the axial direction of the drive sheave. The ratio of the major axis to the minor axis is between 1 and 1 .5. Obviously a ratio of one corresponds to a circular groove. More preferred is if the ratio of major to minor axis is between 1 .05 and 1.1. Using a groove shape that is an arc of an ellipse takes into account the flattening of the coating of the coated steel cord. As the coating is generally softer, it will give in and flatten.

[0025] The separators are annular bodies that are mounted between the grooves. The annular bodies comprise a first and second flank directed towards the groove. In one preferred embodiment the annular bodies are made of metal and the first and second flank of the annular bodies are coated with a friction reducing coating. Alternatively the first and second flank of the annular body have received a friction reducing treatment.

[0026] A friction reducing coating is for example an amorphous carbon coating (Diamond Like Coating, DLC), a polytetrafluoroethylene (PTFE, Teflon (R)) coating, an ethylene tetrafluoroethylene coating (ETFE, Tefzel(R)), a ceramic coating.

[0027] In one other preferred embodiment the annular bodies are made of polymer material, preferably hard polymer material. The polymer can be one out of the group comprising polyamide (Nylon), poly oxymethylene, (POM), polytetrafluoroethylene (PTFE, Teflon (R)), ethylene tetrafluoroethylene (ETFE, Tefzel(R)), polypropylene (PP), polyethylene (PE), polyvinylchloride (PVC), polyimide (PI), polyetherimide (PEM), polyetheretherketone (PEEK), ultra high molecular weight polyethylene (LIHMPE). Using a polymer is generally considered cheaper than having to use metal annular bodies that further have to be coated with an antifriction coating.

[0028] In a preferred embodiment the polymer is completed with a friction reducing agent or slip additive such selected from the group comprising polyglycols, natural or synthetic waxes, silicone oil based, erucamide based, siloxane compounds such polydimethylsiloxane, soaps such as metal soaps, or fine silica (0.5 pm to 5 pm). This embodiment has the advantage of having a very low friction coefficient when in contact with the coating of the coated steel cord.

[0029] One preferred way to realise the metal body is to machine it out of a cylinder of metal. As by regulations it is required that an elevator should at least have two tension members the minimum number of grooves is two. Consequently, the drive sheave will count from two to ten grooves depending on the load requirements of the elevator.

[0030] Typically such drive sheaves have a diameter of between 70 to 210 mm, such as between 800 and 120 mm. The use of a coated steel cord made of high tensile strength filaments (above 2 500 N/mm²) of fine diameter (less than 0.30 mm) allows to diminish the diameter of the steel cord to between 4 and 8 mm. Using the rule of thumb that a sheave should at least be 40 times the diameter of the cord (40*d), this results in drive sheave diameters that are between 160 and 320 mm. However, for this kind of coated steel cords, the 40*d rule is no longer kept and generally drive sheave diameters of between 80 to 130 mm are currently in use. Such a small diameter of sheave allows the use of direct drive motors thereby eliminating the need to use a gear box between motor and drive sheave.

[0031] Possibly the surface of the groove is machined, buffed, grounded, sanded or shot blasted to give it a desired degree of roughness. An increased roughness will raise the coefficient of friction between metal body and coated steel cord. A desired degree of roughness Ra is between 0.1 and 2 micrometer, for example between 0.5 and 4 micrometer (pm), for example from 0.5 to 2 pm, or from 1 to 3 pm, or from 2 to 4 pm.

[0032] In one preferred embodiment, the separators are held in slots between the grooves. Possibly the slots lock, hold, engage the separators to the metal body.

[0033] In one preferred embodiment, the metal body is build from, assembled out of units or discs wherein one groove is machined. In between these single grooved discs the separators are mounted. At either end of the drive sheave an end piece is provided: one for connecting to the axis of the direct drive motor, and one as an end cap for holding the last separator away from the motor.

[0034] In a further refinement of the embodiment, the single grooved disks and separators are held together by bolting, axial and circumferential interlocking or combinations thereof. With ‘axial and circumferential interlocking’ is meant that facing parts of the single grooved discs have protrusions and recesses that axially fit to one another and that prevent that the grooved discs would shift circumferentially from one another. The parts are ‘form-fitting’ (‘Formschlussig’ in German). Possibly the single discs may be welded to one another, but this is less preferred as it does no longer allow for separating the discs without damaging them. [0035] In a further embodiment, the separators can rotate relative to the single grooved discs. For example by a friction bearing, possibly assisted with a lubricant such as mineral oil, silicone oil, graphite, molybdenum sulphate are similar substances.

[0036] According a second aspect of the invention an elevator is claimed, more particularly a traction-type elevator. An elevator generally comprises a car riding on its tracks and a counterweight riding on its own tracks. The counterweight and the cabin are mechanically connected by means of two , three or more separate tension members. With ‘mechanically connected’ is meant that if the cabin moves then the counterweight also moves.

[0037] The counterweight and the tracks are connected in a 1-on-1 , 2-on-1 , 3-on- 1 , or 4-on-1 roping or reeving by introducing diverting pulleys that carry load but do not transmit torque to the tension member. Only the drive sheave transmits torque to the tension members that is received from the drive motor, preferably a direct drive motor. In all cases there is one drive sheave over which the tension members, in casu coated steel cords, run in the grooves of the drive sheave. The drive sheave is according the described previous embodiments.

[0038] Preferred is if the coated steel rope has a round cross section. That is the steel cord is extruded with a polymer jacket and the polymer jacket is substantially round. The fine diameter and high tensile steel filaments in the steel cord in combination with the polymer jacket allow to reduce the diameter of the drive sheave ‘D’ - taken at the smallest i.e. at the bottom of the groove - below the commonly accepted 40 x d, wherein ‘d’ is the diameter of the steel cord without coating. The diameter of the steel cord ‘d’ is the diameter of the smallest circle circumscribing the steel cord in a perpendicular cross section of the coated steel cord. Currently the applicant is running tests with sheave diameters of 20xd. So a drive sheave diameter of below 25 x d appears very well possible.

[0039] In absolute terms smaller drive sheave diameters ‘D’ allows for faster running motors that on their turn allow a better controlled ramp up and slow down of the elevator cabinet. This improves the passenger experience in the elevator. However, this also aggravates the problem of torsions building up or been taken out of the coated steel cord: as the diameter of the sheave diminishes the number of times the coated steel cord hits the side of the groove increases. Compared to the prior art with steel wire ropes this number of sidewall contacts is much larger because:

- first the overall diameter of drive sheave and coated steel cord diminishes and

- second an even lower D/d ratio is being applied, further decreasing the sheave diameter, while

- the distance travelled by the cabin does not decrease accordingly. By the invention this problem of torsions building up or being taken out is greatly reduced, as the low friction separator surfaces prevent the induction of torque to the coated steel cord.

[0040] By changing the groove width of the groove in the metal body and/or adapting the roughness of the metal body in the groove, the absolute friction force between coated steel cord and drive sheave can be finetuned such that the friction force does not become too high, leading to a risk of cabin lift without the counterweight moving or too low in which case the cabin would not lift. Also in case of a downward emergency stop, the coated steel cord should not stick as this would lead to a large deceleration that would be unpleasant to the passengers. A ratio of groove width to coated steel cord diameter ‘d0’ of between 0.4 and 1.0 is preferred. ‘d0’ is the diameter of the coated steel cord inclusive the polymer coating. Even better is if the groove width is less than dO, that is the ratio is smaller than 1 .0, for example less than 0.95 or even 0.90.

[0041 ] Additionally or alternatively one can choose the radius of the groove at the bottom of the groove somewhat larger than the semi-diameter of the coated steel cord e.g. the radius of the groove can be 1 to 1 .5 times the semi-diameter (that is the radius) of the coated steel cord. With ‘at the bottom of the groove’ is meant that the radius of curvature is to be determined locally, in the groove, where the coated steel cord contacts the drive sheave. More preferred is if the radius of the groove is between 1.05 to 1.1 times the radius of the coated steel cord. Best is if the radius of the groove is between 1 .06 to 1 .1 . [0042] A radius of curvature at the bottom of the groove that is higher than the semi-diameter of the coated elevator cord accommodates for the flattening of the coating, the polymer jacket, when the coated elevator rope is under tension during use. However, when the radius of curvature of the groove becomes too large compared to the semi-diameter of the coated steel cord, the coated steel cord starts to wander, to go left or right in the groove which is not desired.

Brief Description of Figures in the Drawings

[0043] Figure 1 shows a known drive sheave as is used for coated elevator ropes.

[0044] Figure 2 shows a first embodiment of the invention and indicates the various geometrical parameters involved.

[0045] Figure 3 shows a second embodiment of the invention.

[0046] Figure 4 shows a third embodiment of the invention.

[0047] Figure 5 shows a schematic of an elevator showing the drive sheave [0048] In the reference numbers the unit and tens digit refers to equal items across figures, the hundred digit indicates the number of the figure.

Mode(s) for Carrying Out the Invention

[0049] Figure 1 shows a drive sheave 100 as it is known in the art. The drive sheave is for use with a coated steel cord in an elevator. The drive sheave is made out of one metal piece 110 of hot rolled 42CrMo4-V 1 .7225 steel. There are three grooves 102, 102’, 102” with a radius of curvature of 3.5 mm at the bottom of the groove. The total depth of the groove, taken from the top of the ridge is 5.5 mm. The spacing of the grooves is 9.5 mm. The groove is polished to a roughness ‘Ra’ of 1 .6 pm. A slot 104 is provided for locking the drive sheave to the axis of the drive motor. Holes 106 are provided for receiving bolts to ensure proper attachment to drive motor.

[0050] Figure 2 shows a first embodiment of the invention. Also here the drive sheave consists of a metal body 210 wherein different grooves 202, 202’, 202”,.. are provided. However, the groove depth ‘5’ is relatively shallow compared to the prior art groove. The groove depth is to be taken from the radial bottom of the groove to the circumferential ridge of the groove in the metal body. The radius of curvature at the bottom of the groove Ri is taken about 1 .05 times the semi-diameter of the coated steel cord ‘d0’.

Characteristic of the drive sheave 200 is that the grooves 202, 202’, 202”, 202”’,... are further provided with separators 212, 212’, 212”,... in between adjacent grooves. The separators 212, 212’, 212”,.. protrude from the metal body with a separator height ‘A’ from the top of the metal body ridge. Note that separator height ‘A’ is five times larger than the groove depth ‘5’. Furthermore the groove width, indicated with ‘w’, is smaller than the separator width, indicated with ‘W. An end cap 216 is provided to support the end separator. The drive sheave diameter is indicated with ‘D’.

[0051] In this embodiment the separators are made of polyamide, Nylon 6/6. By itself Nylon 6/6 has already a very low coefficient of friction with many materials. The coefficient of friction can further be reduced by introducing slip additives. The separators 212, 212’, 212”,.. are held in circumferential slots 214 that are machined in the metal body 210. The separators are made by a subtractive manufacturing method:

- First the metal body according the appropriate dimensions given is prepared. Also the slots 214 are machined in the metal body. The slots 214 have a dovetail shaped cross section.

- In a cylindrical container, the metal body is mounted and fixed coaxially to the cylindrical container. The container has a diameter inclusive twice the height of the separators.

- The space between container and metal body is drawn vacuum.

- Molten Nylon 6/6 is drawn and pressed into the vacuum cavity. The nylon is left to solidify.

- The cylindrical container is opened and the intermediate elevator sheeve is further allowed to cool and harden.

- The nylon sleeve is machined such that only the separators remain: The rest of the material is chiseled away.

[0052] At the end, away from the drive motor the drive sheave is provided with an end cap 216, holding the last separator by means of screws (not shown). In this manner the separators are well held in between the grooves by the circumferential slots 214 by the dovetail connection. [0053] In a second embodiment depicted in Figure 3, the metal body of the drive sheave 300 is build up from a series of single grooved discs 311 , 31 T, 311 ”, 311 ’” each having a groove. The single grooved discs 311 , 31 T, 311”, 31 T” have a protruding square shaped part 320 that inserts into a recess of the same shape and depth. By means of bolts 322, the single grooved discs are firmly bolted to the end block 324 that connects to the motor sheave (not shown). In between the single grooved discs separators 312, 312’, 312”, 312”’ are mounted. These are bronze discs covered with a diamond like coating such as CeraTough™ applied by IBC Coating technologies. Such diamond like coating is though and shows a very low friction coefficient. An end cap 326 ensures that the last separator is well held to the last single groove disc 31 T”. The drive sheave diameter ‘D’ is indicated.

[0054] In a third embodiment 400 depicted in Figure 4, the ratio between groove width ‘w’ and the separator spacing ‘W is greatly reduced. The coated steel cord 430 contacts the single grooved disc 411 only over a narrow region. Most of the coated elevator cord is contacted by the separator discs 412, 412’. As a result the friction between coated steel cord and drive sheave is reduced. These separator discs 412, 412’ are in this case machined annular bodies out of steel that are coated with a Teflon(R) coating. Care must be exercised that the transition from separator to single grooved disc is very smooth and no edge is present that could damage the coating.

[0055] Figure 5 depicts an elevator 550 with a car 552 riding on its tracks 554. The weight of the car is balanced by a counterweight 556 riding on its tracks 558. The car 552 is mechanically connected to the counterweight 556 through three coated steel cords 560 running parallel. The coated steel cords 560 have a substantially round cross section. The coated steel cords 560 are connected on one end to the ceiling of the elevator shaft above the counterweight 556 with a clamp 568’, run down the shaft towards the a diverting pulley 566” on the counterweight 556, continue upward to the drive sheave 500. The drive sheave 500 is driven by a motor 562. Continuing after the drive sheave the coated steel cords are led under car 552 by the diverting pulleys 566’, 566 and finally the coated steel cords are connected at the top of the shaft with clamp 568. This type of tension member path is generally known as 2 on 1 reeving. The elevator is driven by a drive sheave 500 as described hereinabove. [0056] In a particular embodiment, the groove radius as measured locally at the bottom of the groove is 1 .5 times the semi-diameter of the coated steel cord. The semi-diameter or radius is half of the diameter of the coated steel cord i.e. dO/2.

[0057] Due to the decreased diameter of the drive sheave 500 the motor 562 can be kept dimensionally small thereby making it possible that the car 552 can rise up to the top of the shaft 564. No machine room is necessary.

Claims

1 . A drive sheave for an elevator with coated steel cord, said drive sheave comprising a metal body, said metal body comprising grooves for receiving the coated steel cord of the elevator, the groove depth of said grooves being the radial distance from the bottom of said groove to the circumferential ridge of said groove, said drive sheave further comprising separators, circumferentially separating said grooves, characterised in that said separators protrude over the circumferential ridge of said groove with a separator height that is equal or larger than said groove depth, said separators having a surface with a first coefficient of friction between said separator surface and the coated steel cord, said grooves having a surface with a second coefficient of friction between said groove surface and the coated steel cord, the first coefficient of friction being lower than the second coefficient of friction.

2. The drive sheave according claim 1 wherein the groove width is the axial distance between the facing edges of the circumferential ridges of the groove in the metal body and the separator spacing is the widest axial distance between two facing surfaces of a pair of separators, and wherein the ratio of groove width to separator spacing is between 0.40 and less than 1 .0.

3. The drive sheave according to claim 1 or 2 wherein said separators are annular bodies comprising a first and second flank.

4. The drive sheave according to claim 3, wherein said annular bodies are made of metal, said first and/or second flank of said annular bodies being coated with a friction reducing coating or having received a friction reducing treatment.

5. The drive sheave according to claim 4, wherein said friction reducing coating is one out of the group comprising an amorphous carbon coating (DLC), a polytetrafluoroethylene (PTFE, Teflon (R)) coating, an ethylene tetrafluoroethylene coating (ETFE, Tefzel(R)), a ceramic coating.

6. The drive sheave according to claim 3 wherein said annular bodies are made of polymer, said polymer being one out of the group comprising polyamide (Nylon), polytetrafluoroethylene (PTFE, Teflon (R)), ethylene tetrafluoroethylene (ETFE, Tefzel(R)), polypropylene (PP), polyethylene (PE), polyvinylchloride (PVC), polyimide (PI), polyetherimide (PEM), polyetheretherketone (PEEK), ultra high molecular weight polyethylene (UHMPE).

7. The drive sheave according to claim 6 wherein to said polymer a friction reducing agent is added, said friction reducing agent being selected from the group comprising polyglycols, natural or synthetic waxes, siloxane compounds such as polydimethylsiloxane, soaps such as metal soaps.

8. The drive sheave according to any one of claims 1 to 7 wherein said metal body is made of steel, cast iron, or non-ferrous metals and alloys such as copper, brass, bronze or other copper comprising alloys.

9. The drive sheave according to any one of the claims 1 to 8 wherein said separators are held in slots between said grooves.

10. The drive sheave according to any one of the claims 1 to 9 wherein said metal body is build from separate, single grooved discs where, in between, said separators are mounted,

11 . The drive sheave according to claim 10, wherein said single grooved discs and separators are held together by bolting, welding, axial and circumferential interlocking or combinations thereof.

12. The drive sheave according to claim 10 or 11 wherein the separators can rotate relative to said single grooved discs.

13. An elevator comprising a car riding on its tracks, a counterweight riding on its tracks, said car being connected to said counterweight by two or more coated steel cords, said coated steel cords being driven by a drive motor, said coated steel cord having a substantially round cross section, characterized in that said coated steel cord is contacted by a drive sheave according to any of the claims 1 to 12 for driving said coated steel cord.

14. The elevator according to claim 13, wherein said coated steel cord has a semidiameter, said circular groove has groove radius at the bottom of the groove, the radius of said groove is between 1 to 1 .5 times the semi-diameter of said coated steel cord.

15. The elevator according to claim 13 or 14 wherein the coated steel cord has a diameter inclusive the polymer coating and wherein the ratio of the groove width to the coated steel cord diameter is between 0.4 and 1.0,