Wire Ropes Under Bending and Tensile Stresses - Wire Ropes: Tension, Endurance, Reliability - page 272

Table 3.6 General requirements for the rope drives in passenger lifting installations

Installation Min. safety factor m Min. diameter ratio D/d Max. pressure

9 N/mm 2a Elevator 12 40

2 N/mm 2b Mine hoisting 8 80

Ropeway 3.8-4.5 80 -

a Specific rope pressure, k
b Global rope pressure, p

The minimum safety factor is a general requirement for the rope drives in
passenger lifting installations, Table 3.6 . The safety related part of this general
requirement is to prevent wire rope breakage by extreme forces which occur only
rarely. The minimum rope force F min has to be several times stronger than the
nominal rope tensile force S.

3.3.2 Lifting Installations for Passengers

There are technical regulations governing the requirements, for important pas-
senger installations using rope drives and these include not exceeding a given
tensile force and not falling short of a given diameter ratio D/d of sheave and rope
diameters. These technical regulations are also used for installations needing
comparable safety requirements. The general design requirements for lifting
installations for passengers listed in Table 3.6 also represent to a great extent the
various current technical regulations.
The so-called rope safety factor in Table 3.6 varies considerably depending on
the installation being considered although, of course, in all cases the passengers
have to be provided with the same degree of safety. The required rope safety
factors guarantee that extreme rope forces—which occur only rarely—in any case
will be surely surpassed by the rope breaking force. However, the predominant
criterion that defines the required rope safety factors is a reasonable rope,
endurance allowing sufficient time for distinct indications of an approaching rope
discard to be produced and detected safely. Therefore, in combination with the
required diameter ratio of sheave and rope and the different kinds of sheave
grooves, an elevator with the rope safety factor m = 12 is not safer than a ropeway
with the smaller factor m = 4 and the higher diameter ratio D/d.
The different requirements for these two parameters: rope safety factor m and
diameter ratio D/d for elevator, mine hoisting and rope way have practical reasons.
In elevators, the diameter of sheaves and traction sheaves need to be small because
of the space available and also to keep the price for the gear down. On the other
hand, the rope safety factor for ropeways has to be small so that the suspension
bow can be kept low. In between, the wire ropes for mine hoistings which are
worked intensively should not be too heavy.