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Safety Factors

Fibre 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 fibres, 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 fibre 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

منبع : گروه ایمنی SKLF

http://kickinteractive.com/dc/units/1_1/safety.html