Lift Report

As regards elevator ropes, two classes emerged in the past: designs based on fiber cores such as 8 x 19S + NFC(Fig. 1). This design represents a good choice for many elevators. In fact, it is by far the most frequently used rope design, all around the world. Increased demands on ropes resulting from greater ascent heights, higher speeds, and enhanced demands in regard to ride comfort have prompted the development of a further class of ropes, those with an independent wire rope core (IWRC). Increased loadbearing capacity, at the same diameter, make it possible to reduce rope diameter in comparison to ropes with a fiber core. IWRC ropes (Fig. 2) with a diameter of 22 mm are used in some high-rise lifts, for example. If the same number of NFC (natural fiber core) ropes was used, they would have to be 26 mm in diameter.

    

Particularly at great ascent heights, these IWRC ropes can improve ride comfort considerably due to their comparatively lower stretch. A large number of deflections along the course of the rope’s travel can also be readily managed by using these ropes. A further advantage deriving from the greater carrying capacity is lower rope diameter when compared with NFC ropes. This property makes possible less expensive drives – thanks to more favorable torque demands.

A new factor in rope design results from requests to reduce space needs, as was mentioned at the outset of this article. The EN 81-1 elevator standard currently in force specifies 8 mm as the smallest permissible rope diameter. In conjunction with the minimum requirement of D/d = 40, the result is that the smallest drive sheave will have a diameter of 320 mm. The situation in lifts without a machine room can be considerably improved with a further (significant) reduction in drive sheave diameter. Lifts are already being built today incorporating sheaves 240 mm in diameter and the trend is toward even smaller units. The slender ropes required for such systems represent a further class of ropes for manufacturers, a class that will have to be engineered according to its own rules. The obvious solution is to use steel-core ropes for these small diameters. The following section will discuss some of the aspects to be taken into consideration here.

The effects of the smaller rope diameters are to be depicted in the coming paragraphs with regard to the number of ropes to be needed and the demands that increased nominal wire strengths place on the material used in the drive sheave.

A model elevator (Fig. 3) for a payload of 630 kg and 20 m ascent height is to serve as the basis for calculation.

The following parameters show the supplementary data, chosen arbitrarily, that are used for calculation purposes:

Reeving:                             1:1

Car weight:                        1250 kg

Counterweight:                  1565 kg

Rope diameter:                  8 mm

Nominal strength:              1570 N/mm²

Drive sheave:                    320 mm,

Groove angle:                   80°

D/d:                                 40

The table of results (Table 1) shows the number of ropes required for an assumed groove with an 80° undercut as well as the resultant pressure between the ropes and the drive sheave, corresponding to the calculations specified in TRA 003, which permits pressure of 9.0 N/mm² as the upper limit for this groove shape.

If, while maintaining all the other parameters, rope diameter is reduced to 6.0 mm, then the number of ropes required will rise as per table 2, this resulting from the proportional reduction in minimum breaking force for these thinner ropes.

Although the pressure remains unchanged, at less than 9.0 N/mm² (Table 3), realization of such an elevator is made questionable by the large number of ropes required, i.e. 13. One potential solution to this problem would be to increase rated rope strength from 1570 N/mm² to 1960 N/mm²; here it is necessary to note, however, that this strength level is not provided for in the DIN EN 12 385-5 standard. This change reduces the number of ropes to 10 and experience shows that it would possible to assemble such an array; the number should, however, be no higher than this. Regrettably, this solution causes the pressure between the rope and the drive sheave to rise to 10.4 N/mm² so that we would have to expect at the least a significant reduction in rope service life and/or increased wear at the drive sheave.

One option available to reduce the pressure between the rope and drive sheave in this model lift would be to adopt 2:1 reeving (Fig. 4/Table 4). This step makes it possible to reduce the number of ropes to 7, in spite of the slightly increased rope safety requirements for 1570 N/mm² nominal strength; at the same time pressure could be kept down to a value below 8 N/mm².

Figure 5 depicts the service life calculation scheme developed by Prof. Feyrer at the Institute of Mechanical Handling and Logistics, University of Stuttgart. Greater bending performance is achieved (blue curves) at constant load and increasing D/d ratio. This effect can be achieved in much the same way with declining loads (orange curves).

The effect of the trend toward reducing drive sheave diameter, as mentioned at the outset, can easily be read from this figure. A reasonable service life for the ropes can be achieved in the case of further reduction of the D/d ratio only with a simultaneous – and in some instances significant – increase in rope safety. With identical minimum breaking force Fmin for the ropes, this measure will result a higher number of ropes. One alternate solution would be to use ropes with a higher minimum breaking force value. What will ultimately be used are ropes with an IWRC and greater rated strength for the selected wires. The problem associated with the pressure between the ropes and the drive sheave, depicted in the model lift introduced above, should not be underestimated. When conducting internal performance testing, sheaves with a hardness of 50 HRC are currently being used to avoid making an impression in the sheaves

Summary

The use of suspension ropes of less than 80 mm in diameter will increase in numerous elevator installations. Elevator product series have already been developed here and in some cases have already been in use for several years. These special- design ropes, when compared with the conventional lift components included in the norm, require greater effort in design approval and acceptance of the actual lift. Test certificates issued by notified bodies are employed are used to support makers, certifying the suitability of the special-design ropes for use as elevator suspension ropes. The fact of a separate expert evaluation for these ropes makes it permissible both to go below a D/d ratio of 40 and also to go below the 8 mm minimum diameter for ropes  specified in the standard. Over and above that, the rated strength of the ropes can exceed the 1770 N/² mentioned in EN 81-1 and EN 12 385-5 since safe utilization of the ropes in a lift will have to be demonstrated by way of the prescribed testing. A carefully conducted expert assessment describes the peripheral conditions needed for the use of the ropes in regard of the properties mandated by EN 81. The unquestioned emphasis is, of course, on the elevator’s safety and the satisfaction of all the required properties such as, for instance, the maintenance of the minimum number of trips as per Annex N.

The user of these special-design ropes may assume that safe system operation is assured by the assessment and certification of the ropes. It is obvious that when installation teams are dealing with ropes of a diameter that they otherwise only encounter in conjunction with speed governors will have to exercise greater care in handling and installation than is the case with conventional suspension ropes.

Moreover, the tolerances for the drive and deflection sheaves have to be kept quite narrow in regard of the diameter and uniformity of the profiles one to another. Due to the reduced circumference in relation to a “standard” elevator, these sheaves will have to execute a significantly greater number of revolutions in order to achieve the same ascent height. Deviations in the circumference will unavoidably result in irregular wear in the ropes; this will lead to increased wear at the ropes and drive sheave and oscillations being induced in the ropes.