The present invention relates to suspension and traction systems with a pulley and the load bearing elements or suspension and traction elements thereof. It particularly relates to a system provided with at least one pulley and a suspension and traction element with at least one shape memory alloy (SMA) wire.

Suspension and traction systems, such as elevators, are provided with a car, a counterweight and one or more pulleys and they are operated by traction machines or units, usually an electric motor, connected to the traction pulley. The motor is arranged in a machine room above the shaft through which the car travels. There is currently a trend in designing and developing elevators to optimize use of the shaft and dispense with the machine room. Components such as motors are small enough to allow enormous space savings. Pulleys, however, cannot be reduced as much as desired because they have the problem that the smaller they are, the greater the wear the bearing elements sustain as the pass through said pulleys. The stresses sustained due to traction and contact drastically reduce the fatigue lifetime of ropes or belts. Patent application EP2511219 seeks to prevent wear between adjacent strands of the rope by preventing localized contacts and separating them with resin. However, the invention does not solve the problem of the increases in stresses due to the bending component when the pulley diameter is reduced.

To prevent degradation of the suspension and traction element as it passes through smaller sized pulleys, the present invention is based in recognizing that said degradation is considerably reduced when a martensite transformation takes place in bearing elements provided with at least one shape memory alloy wire. Said transformation enables an increase in flexibility with large recoverable deformations and without a large increase in stress. The present invention therefore provides a suspension and traction system provided with at least one suspension and traction element and at least one pulley wherein pulley diameter, Ø_pulley, is less than or equal to 130 mm, the suspension and traction element comprises a load bearing section with at least one shape memory alloy wire chosen from the shape memory alloys the characteristic Af temperature of which is below room temperature and the value of the Ø_pulley/Ø_smawire quotient is between 50 and 2000.

Characteristic Af temperatures below room temperature assure that the invention will work.

The added value of shape memory alloys is based on the unique property that such materials have, i.e., superelasticity. This property originates in the special characteristics of martensite transformation occurring in these materials when they are subjected to a certain level of stress. This peculiar behavior makes these materials extremely flexible, with considerable recoverable deformations of up to 8%, and without a large increase in stress, which is unattainable for any other metal alloy. This peculiarity allows small bending radii without damage (without permanent deformations) in the rope, which in turn allows pulleys having a smaller diameter. Furthermore, stresses between wires are homogenized, reducing contact pressures, which would improve local contacts and reduce the breaking of individual wires.

A superelastic deformation cycle begins with the application of a load. The material, in an initial austenitic state, elastically deforms until reaching a critical stress level (σM s) such that the austenitic structure becomes thermodynamically unstable and its transformation into detwinned martensite is induced. This direct conversion or transformation is macroscopically characterized by a great homogenous deformation in a very small increase in stress due to reorientation of the martensite variants in the load application direction. However, at the microscopic level, deformation is not homogenous, but rather the interface between austenite/martensite phases advances in the form of a transformation front from nucleation thereof until the entire structure is in the detwinned martensite phase (σM f).

As a result of the selection in the pulley diameter and wires proposed by the invention together with the use of an SMA material, martensitic transformation and therefore the superelasticity characteristics occur in the rope without needing any special activation signal, at normal use temperatures for an elevator. Degradation of the suspension and traction element with smaller sized pulleys is thereby prevented.

Other additional advantages and particular implementations are defined in the dependent claims.

For the purpose of aiding to better understand the features of the invention according to a preferred practical embodiment thereof, the following description refers to a set of drawings in which the following has been depicted with an illustrative character:

• Figure 1 illustrates the evolution of longitudinal stress of a wire when it is bent around a pulley.
• Figure 2 is a comparison between a conventional steel wire and a steel wire comprising an SMA material.
• Figure 3 shows the operating conditions of an SMA wire as it passes through a pulley according to the dimensionless pulley diameter to wire diameter ratio.

The system of the invention is provided with at least one suspension and traction element (rope, belt...) and at least one pulley the diameter of which is less than or equal to 130 mm. The load bearing section of the element comprises at least one shape memory alloy wire. The following, among others, are included among the materials suitable for this purpose:

• o NiTi (49-57 at% Ni)
• o NiTiCu (8-20 at% Cu)
• o NiTiCr (< 1 at% Cr)
• o NiTiCo (< 4 at% Co)
• o NiAl (36-38 at% Al)
• o NiTiPd (0-40 at% Ni)
• o NiTiNb (0-40 at% Ni)
• o CuZn (38.5-51.5 at% Zn)
• o CuZn-X (X=Si, Sn, Al, Ga)
• o CuAlNi (28-29 at% Al, 3-4.5 at% Ni)
• o CuAlNi (16-18 at% Al, 9-13 at% Mn)
• o CuAlBe (0.5-8 at% Be, 22-25 at% Al)

All these materials undergo martensitic transformation at normal working temperatures for an elevator, since the characteristic temperature of these shape memory alloys (i.e., the Af temperature at which transformation occurs) is below room temperature.

• Figure 1 shows the longitudinal stress that a wire can sustain when it is bent around a pulley. Before traveling around the pulley (straight rope), the wire withstands a stress (σ1), but said stress increases due to the sum of the bending component once the rope is on the pulley (σ2). After leaving the pulley, it recovers its original stress (σ1). While passing around the pulley, there is a localized increase as it enters the pulley (σ3), and an also localized decrease as it leaves the pulley (σ4).
• Figure 2a shows a diagram of the suspension system of an elevator, whereas Figures 2b and 2c show the comparison of what happens with the rope as it passes around the pulley, in terms of stress/contact force and fatigue/wear, when the rope is conventional (steel) and when it is an SMA rope. In areas in which the rope is straight, the wires of the rope withstand similar stress regardless of if it is a steel rope or an SMA rope, Figure 2a. However, as it passes around the pulley, deformation increases, and while stress in a conventional rope shoots up due to the linear behavior of steel, in an SMA rope deformation occurs without an increase in stress, which means that contact forces are lower. This has an effect on the lifetime of the rope, Figure 2c, because the lower contact force the lower the fatigue and wear in wires.
• Figure 3 shows the operating conditions of an SMA wire as it passes around a pulley according to the dimensionless pulley diameter to wire diameter ratio. If this ratio is above 2000, transformation in the wire does not occur, so use of these alloys is not of interest for the described objective in the invention. When the ratio is below 50, damage occurs in the wire, which can give rise to a dramatic reduction in the lifetime of the rope. Therefore, the range of interest suitable for using SMA materials for being able to use pulleys having a smaller diameter without causing damage in the rope is between 50 and 2000.

The invention can be applied to both ropes and belts. In the case of a rope, the total section (bearing section plus sheath) has a shape the width/thickness ratio of which is substantially equal to 1. The sheath is preferably, though not necessarily, a polymer sheath.