|
The rope strength design factor is the ratio of
the rated strength of the rope to its operating stress. If a particular
rope has a rated strength of 100,000 lbs. and is working under an
operating stress of 20,000 lbs., it has a rope strength design factor
of 5. It is operating at one fifth or 20% of its rated strength. Many
codes refer to this factor as the "Safety Factor" which
is a misleading term, since this ratio obviously does not include
the many facets of an operation which must be considered in determining
safety. Wire rope is an expendable item - a replacement part of a
machine or installation. For economic and other reasons, some installations
require rope to operate at high stress (low rope strength design factors).
On some installations where high risk is involved, high rope strength
design factors must be maintained. However, operating and safety codes
exist for most applications and these codes give specific factors
for usage. When a machine is working and large dynamic loadings (shock
loadings) are imparted to the rope, the rope strength design factor
will be reduced which could result in overstressing of the rope. Reduced
rope strength design factors frequently result in reduced service
life of wire rope.
O.S.H.A. (A.N.S.I.) Removal Criteria 5. A.N.S.I. Safety Codes, Standards
and Requirements - rope must be removed from service when diameter
loss or wire breakage occurs as follows:
|
A.N.S.I.
|
Equipment
|
Number broken wires in running ropes
|
Number broken wires in standing ropes
|
|
No.
|
|
in one rope lay
|
in one strand
|
in one rope lay
|
at end connection
|
|
B30.2
|
Overhead & Gantry
Cranes |
12
|
4
|
Not Specified
|
Not Specified
|
|
B30.4
|
Portal, Tower & Pillar
Cranes |
6
|
3
|
3
|
2
|
|
B30.5
|
Crawler, Locomotive &
Truck Cranes |
6
|
3
|
3
|
2
|
|
B30.6
|
Derricks |
6
|
3
|
3
|
2
|
|
B30.7
|
Base Mounted Drum Hoists |
6
|
3
|
3
|
2
|
|
B30.8
|
Floating Cranes &
Derricks |
6
|
3
|
3
|
2
|
|
A10.4
|
Personnel Hoists |
6*
|
3
|
2*
|
2
|
|
A10.5
|
Material Hoists |
6*
|
Not Specified
|
Not Specified
|
Not Specified
|
*Also remove
for 1 valley break. OSHA requires monthly record keeping of
wire rope condition.
Note: Current industry recommendations and OSHA standards are based
upon the use of steel sheaves. The manufacturer of plastic or
synthetic sheaves or liners should be consulted for their recommendation
on the safe application of their product, and possible revision in
rope inspection criteria when used with their product. |
