Lift Report

The rope elongation module describes the elongation behaviour of steel wire ropes in the longitudinal direction under the influence of mechanical stress. The rope elongation module is determined from the rope stress-elongation curve based on static tension testing. These rope stress-elongation curves are distinctly non-linear. The rope elongationmodule is consequently not a constant and is dependent on a range of rope andstress parameters. These parameters exerting a significant influence on the rope elongation module include the rope construction, rope core, tension level during loading and unloading cycles and the utilization status of the ropes. A distinction is made in tangent and secant elongation modules, whereby a difference is drawn according to the tangential rope elongation modules

ET,up(σZ) - as the tangent with any optional stress at the load curve and

ET,down(σZ, σupper) - as the tangent with any optional stress at the unloading curve taking into account the stress reversal σupper from the load to the unloading curve

and according to the secant rope elongation modules

ES(0, σupper)-  as the secant between the lower stress σlower =0 of an optional stress level σupper and

ES(σlower, σupper) - as the secant between two optional stress levels σlower and σupper subject to stress reversal at σupper.

An illustration of the different definitions of the rope elongation module is taken from FeyJah1990, Fig. 1.

Of significance for operation in the elevator construction sector is the secant elongation module ES(σlower, σupper). The rope elongation module ES(0, σupper) is subordinate, but permits an estimation of the elongation during the short mounting phase. In FeyJah1990, the rope elongation modules from wide-ranging tensile tests on 6- and 8-strand steel wire ropes with different cores and in different serviceability conditions have been illustrated. The rope elongation between the stress applications σlower and σupper is

With σlower=0, the elongation is

The constants A, B and C are stored in Fey2000 for 6- and 8-stranded ropes with up to 3 wire lengths in a new and prestressed condition separately according to loading and unloading. Rope stresselongation curves are regularly determined up to fixed stress reversal points. The rope elongation module of importance for elevator construction between two stress applications with stress reversal at σupper is calculated subject to the condition of equal elongation levels for loading and unloading Eup(σupper) = Edown(σupper) at the stress reversal point. FeyJah1990 determined that an unloading curve can occur at any optional point of the loading curve through elongation of the unloading curve through the reversal and the zero point. The constant Bdown is determined by iteration and transferred to Bdown upper. The rope elongation module in this intermediate unloading stage is

or

Due to the complexity of the method for determining the rope elongation module between two stress applications after a stress reversal, the rope elongation modules have been calculated and listed for selected stress pairs, Fey2000.

For current ropes and/or ropes deviating from the list in Fey2000, the rope stresselongation curves have to be measured and the elongation modules determined separately. These ropes also include 9-strand rope constructions such as the Drako 300T and 300TX and/or ropes with compacted strands. During these tensile tests, the rope tensile stress is continuously increased up until around 10 % of the rope minimum breaking strength. The elongation is measured and recorded. After releasing the tensile stress on the rope to the starting value, 10 cycles are executed with a load of up to 59 % of the rope minimum breaking strength, then the stress is released. Subsequently load is applied again to 10 % of the rope minimum breaking strength and released with stepped measurements of rope elongations. Fig. 2 illustrates the rope stress-elongation curves for a Drako 300T rope when new and after threshold stress application. It is evident that the curves are steeper after loading, i.e. the rope elongation module rises. In addition, the load and unloading curve converge at the starting value, i.e. the remaining initial elongation is no longer increased after a short period with these ropes. Consequently, in conditions typically occurring in elevators for an elevator installation with a vertical rise of H=400 m, elongation of substantially less than 2 mm results per additional load unit of q=100 kg.

At defined stress levels σlower and σupper, adjusted tensile tests can be carried out with suitable intermediate unloading cycles. For already available measurements and optional stress levels σlower and σupper, to determine the rope elongation module between two stress applications with stress reversal, a simplified method can be used as used in Vog1996 to describe spring rates and damping parameters for rubber springs under pressure with a distinctly non-linear curve. This method can be transposed here to the rope stresselongation curves. During testing, socalled original load and unloading curves are determined. For smaller stress levels deviating from the maximum stresses,the current unloading curve is determinedby horizontal and vertical shift of the original unloading curve. This requires the current unloading curve to run through the reversal and zero point of the original load curve.

In times of increasing raw material shortages and dwindling energy resources, the watchword energy efficiency is on everyone’s lips. The possibilities for increasing energy efficiency in the field of elevator technology are many and varied, and are not restricted to electrical and electronic modules. Thin ropes, small sheaves with small output torque levels, consistent lightweight technology where possible, improvements to shaft efficiency through optimized installation, reduced friction between moving and stationary components of an elevator installation are only a few examples of the contribution which can be made by “mechanical” means towards improved energy efficiency. The interface to the rope elongation module lies here in reducing translated rope masses.

Omission of a rope causes the stress in the remaining ropes to be increased. The operating point moves upwards on the rope stress-elongation curve into the steeper area. The rope elongation module rises. The rope spring rate reduces under otherwise identical conditions. Dispensing with a suspension rope is possible while using the same rope construction, for instance by increasing the nominal wire strength as with the Drako 300TX (nominal wire strength R=1960 N/mm²). With increasing vertical rise, the rope mass increases. With the vertical rise typical in high-rise applications of around 400 m, dispensing with an upper rope means a mass reduction of around 10- 15 %. Added to this is the possibility where applicable of making savings with the compensating ropes. In summary, this results in lower initial costs, lower operating costs and lower spring rates. The number of ropes can be reduced when using the Drako 300TX. The tension in the ropes increases, as does pressure between the rope and groove. This must be expected to compromise the service life of ropes. This is the case for moderate vertical rises, in which the elimination of rope mass is of subordinate importance. In typical high-rise applications where vertical rises of up to 400 metres are not uncommon, the mass reduction is significant. The rope stress is reduced, while the rope elongation module remains almost unaffected. Due to the lower rope mass and the influence of the nominal wire strength, comparative calculations looking at the Drako 300T (R=1570 N/mm²) with n-ropes and the Drako 300TX (R=1960 N/mm²) with (n-1) ropes show a comparable increase in service life, or in the case of a high-rise installation with a vertical rise of 400 m, to an increase of around 10 %.

It is customary for more demanding lift installations to be fitted with tried and tested steel wire ropes with independently stranded rope core. These ropes demonstrate a long service life coupled with extremely good running characteristics, and due to the regular lay of the bearing elements are tolerant to mounting imperfections. Due to drawbacks such as greater mounting complexity, ropes with parallel stranded steel core have so far been reserved to special applications: the outer strands laid in one direction and the strands in the core tend to twist already under their own intrinsic load. However, ropes with double parallel core offer scope for developments in the field of elevator technology. An initial rough approximation for rope stress ratios indicates that the elevator car mass to added load ratio F/Q = 1, and rope safety between an empty and fully loaded elevator car (with 50 % payload compensation without consideration of other masses) is 12 ≤ Sf ≤ 24. Using the results from Fey2000, it is possible to determinethat the rope elongation module of new ropes with parallel laid core is around 10% greater than for independently stranded rope cores. For used ropes, this difference converges to less than 5 %, however. Against the backdrop of elongation, vibrations, energy efficiency of readjustment etc., the possibilities offered by the double parallel construction should be utilized. This is supported by the increasing mounting quality and extended knowledge of the characteristics of parallel laid rope cores, in particular in highrise projects.

The Drako 250T, a proven steel wire rope with independently stranded rope core, is now being joined by the Drako 250TPC, Fig. 3. The dimensions of the outer strands and rope core in this 8-strand rope are ideally coordinated. To further increase service life and to improve quiet running, the strands are compacted. The rope elongation module under typical rope force conditions reaches that of rope elongation modules of the magnitude discussed. Further development also with other constructions and technologies is currently under way.

To increase the service life and further tolerance to mounting imperfection, already available rope constructions can be fitted with sheathed steel cores, Fig. 4.

Against the background of conditions typically prevailing in lifts, the rope elongation module is discussed and findings from measured rope stress-elongation curves outlined. Against the backdrop of energy efficiency, rope elongation, costs and rope service life, rope constructions are presented and compared.

Fey2002 Feyrer, K.: Drahtseile. Bemessung, Betrieb, Sicherheit. (Wire ropes. Tension, Endurance, Reliability) SpringerVerlag 2000

FeyJah1990 Feyrer, K.; Jahne, K.: Seilelastizitätsmodul von Rundlitzenseilen (Rope elasticity module of round stranded ropes) DRAHT 41 (1990) p.498- 504

DIN 18 800 Steel structures; Design and construction, 03.1981

VDI 2358 Wire ropes for mechanical handling equipment, 10.1984

ISO 4344 Steel wire ropes for lifts – Minimum requirements, 02.2004

Vog1996 Vogel, W.: Zur Dimensionierung von Hydraulikpuffern für Treibscheibenaufzüge (On dimensioning of hydraulic buffers for traction sheave elevators). Dissertation Universität Stuttgart 1996

VogScheu 2008 Vogel, W.; Scheunemann, W.: Extrusion von Stahleinlagen (Extrusion of steel cores). Internal Report Technical Competence Center TCC Pfeifer Drako 2008