Civil Engineering Reference
In-Depth Information
3.3.1.4 Extreme Force
To prevent wire rope breakage by extreme forces which occur only rarely, tech-
nical , regulations normally require the minimum breaking force F min to be several
times stronger than the nominal rope tensile force S
F min v S :
The so-called rope safety factor m takes the increase of the tensile force by
possible impact forces, acceleration forces and overloading into consideration (by
means of impact factor f imp ) as well as the weakening of the wire rope breaking
force over time due to fatigue, wear or corrosion (by means of residual breaking
factor f R ). The rope safety factor m is a combination of the impact factor f imp and
the residual breaking factor f R
v ¼ f imp
f R
:
The decreasing rope breaking force over time is characterised by the residual
breaking factor f R = F R /F min , whereby F R is the residual breaking force of the
rope. The weakening of the wire rope breaking force under the effect of rope
drives, first investigated by Woernle ( 1929 ), will be rated as sufficient with the
residual rope breaking force F R = 2/3F min or with the residual breaking factor
f R = 2/3.
The impact factor f imp varies widely. A minimum impact factor can be set at
f imp = 1.67. Then the minimum rope safety factor will be
v min ¼ 2 : 5 :
Due to the impact force, however, in some cases the safety factor has to be
much greater. For example, in a high speed elevator operating with safety gear or a
buffer, the impact force can be more even than five times the normal rope tensile
force; that means the impact factor is f imp [ 5, Feyrer ( 1977 ), Vogel ( 1996 ),
Nußbaumer ( 2011 ).
As far as extreme forces are concerned, the rope terminations only have to be
considered.
3.3.1.5 Optimal Rope Diameter
The optimal diameter for a wire rope is the diameter which enables the wire rope
to reach an optimum for the number of bending cycles. If the wire rope diameter is
smaller or larger, the endurance of the wire rope is reduced. For economic reasons,
the diameter of the wire rope should not be larger than the optimal rope diameter.
The disadvantage of using a rope diameter larger than the optimal one is that the
rope endurance is reduced and costs increased.
Search WWH ::




Custom Search