In elevators, one or more ropes are used as the means by which the elevator car is suspended. Each rope end needs to be fixed to a fixing base, which is typically either the load to be lifted or a stationary structure, depending on the type of suspension chosen for the elevator. The rope ends can be fixed directly to the load, such as the car or counterweight, which is the case when these are to be suspended with 1:1 ratio. Alternatively, the rope ends can be fixed to a stationary structure of the building, which is the case when the car and counterweight are to be suspended with 2:1 ratio, for instance.
Ropes of an elevator typically include one or several load bearing members that are elongated in the longitudinal direction of the rope and each of them forms a continuous structure that continues unbroken throughout the length of the rope. The load bearing members are the members of the rope which are configured to bear together the load exerted on the rope in its longitudinal direction. The load suspended by the rope causes tension on the load bearing member in the longitudinal direction of the rope, which tension can be transmitted by the load bearing member in question all the way from one end of the rope to the other end of the rope. Ropes may further comprise non-bearing components, such as a coating, which cannot transmit tension in the above described way. The coating can be utilized for one or more purposes. For instance, the coating can be used to provide rope with a surface via which the rope can effectively engage frictionally with a drive wheel. The coating can also be used to provide the load bearing members of the rope with protection and/or for positioning these relative to each other.
In prior art, elevator ropes have been fixed to the fixing base with a rope terminal arrangement. Such a rope terminal arrangement has been contemplated, where the rope end is compressed in a gap delimited by two compression members. Thereby, it is subjected to compression in its transverse direction and tensile loading in its longitudinal direction.
Reliability of this kind of configuration relies largely on the grip produced by the compression between the rope surface and the compression member. The rope end should be firmly gripped such that it can't slide out of the compression gap, because this would mean that the suspension of the particular rope would be lost. This kind of rope terminal arrangement has the drawback that a reliable grip is difficult to provide simply. This is the case particularly, when the surface of the rope end is made of material sensitive to deformation under stress, such as elastic polymer materials, like polyurethane, for instance. The surface material is subjected to continuous compression and shear stress, which may cause increasing deformation over time (creep). In long term, the creep phenomenon can lead to rupture of the surface material, slipping and in the worst case unexpected loss of suspension of the particular rope fixed by the rope terminal arrangement.
The rupture lifetime of a coated rope termination, in particular, is difficult to determine on the basis of laboratory tests. In normal operating conditions, the rupture lifetime can be on the order of years, whereas testing can be done up to a few months for practical reasons. Test results should be extrapolated to cover the entire product lifetime, but this is difficult due to the complexity of the creep phenomenon. Because the rupture lifetime is difficult to predict, the long-term safety of the rope termination need to be guaranteed by alternative or additional measures, as a sudden loss of suspension could occur without prior warnings.