Fiber rope Safety Factors
Sended from: Bahman Mombeini
bmn151@gmail.com
Fiber rope, like wire rope, must have a factor of safety to account for Loadings over and above the weight being hoisted and for reductions in capacity due to:
The reduced capacity of the rope below its rated strength due to ordinary usage, wear, broken fibers, broken yarns, age, variations in size and quality.
Extra loads imposed by acceleration and inertia (starting, stopping, swinging and jerking of the load).
Increases in line pull due to friction of the rope passing over sheaves.
─ Inaccuracies in the weight of the load.
─ Reduced strength due to bending over sheaves.
─ Reduced strength due to drying out, mildew and rot.
Severe strength reductions caused by knots in the rope.
Weakened yarns due to ground-in dirt and abrasives.
This list is not complete. It illustrates why the factor of safety is required and why the safe working loads must never be exceeded. The factor of safety does not give you extra usable capacity.
The factor of safety for all fiber ropes is 5. For ropes used to hoist or support personnel, it is 10.
Safe working loads (SWL) are calculated as follows:
SWL = Breaking Strength of Rope
Factor of Safety
Or: SWL = Breaking Strength of Rope
5
Example: A rope rated at 1500 lbs breaking strength has a safe working load of 300 lbs.
1500 lbs = 300 lbs
5
Safe Working Loads
Because no rigger can be expected to remember the safe working loads of ropes, "rules of thumb" are used to estimate the loads.
The following rules of thumb work well for new ropes when load tables are not available.
Manila Rope
Change the rope diameter into eighths of an inch
Square the enumerator and multiply by 20
Example:
a) 1/2 inch manila rope = 4/8 inch diameter
SWL = 4 X 4 X 20 = 320 lbs
b) 5/8 inch manila rope
SWL = 5 X 5 X 20 = 500 lbs
c) 1 inch manila rope = 8/8 inch diameter
SWL = 8 X 8 X 20 = 1280 lbs
Nylon Rope
Change the rope diameter into eighths of an inch
Square the enumerator and multiply by 60
Example:
a) 1/2 inch Nylon rope = 4/8 inch diameter
SWL = 4 X 4 X 60 = 960 lbs
Polypropylene Rope
Change the rope diameter into eighths of an inch
Square the enumerator and multiply by 40
Example:
a) 1/2 inch Polypropylene rope = 4/8 inch diameter
SWL = 4 X 4 X 40 = 640 lbs
Polyester Rope
Change the rope diameter into eighths of an inch
Square the enumerator and multiply by 60
Example:
a) 1/2 inch Polyester rope = 4/8 inch diameter
SWL = 4 X 4 X 60 = 960 lbs
Polyethylene Rope
Change the rope diameter into eighths of an inch
Square the enumerator and multiply by 35
Example:
a) 1 inch Polyethylene rope = 8/8 inch diameter
SWL = 8 X 8 X 35 = 2240 lbs
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