Breaking Strain and Safe Working Loads

Breaking Strain

• The breaking strain is the amount of force or weight that a rope can withstand before breaking. It is typically measured in pounds or kilograms and is usually printed on the rope packaging or provided by the manufacturer.
• The actual breaking strain of the rope may be reduced as it experiences wear and tear.  Rope in use should be regularly inspected to ensure it’s still capable of performing it’s task safely.
• Breaking strain of rope can be reduced by the following:
  • Abrasion – fraying of fibers and strands
  • Sunlight damage
  • Melting
  • Introduction of knots or splices (rope that contains a bolen for example can have its breaking strain reduced by up to 50%)
• Breaking strain of cable can be reduced by the following:
  • Abrasion – fraying of wires and strands
  • Crushing, flattening
  • Pinching
  • Bird caging
  • Strand pulls
  • Rust
  • Kinks, knots
  • Use of clips or splices
• Breaking strain of chain can be reduced by the following:
  • Abrasion and surface wear
  • Thinning in between links
  • Rust
  • Link damage
• In general, the breaking strain of both rope and cable can be impaired when tension is applied to a sharp bend.
• The use of chafing gear (material secured around the line in order to prevent rubbing damage) and thimbles (a teardrop shape groove that a rope or cable can make use of that protects the inside of an eye splice) can help to reduce damage

Safe Working Loads

• Safe working loads (SWL’s) should also be found on the rope or cable packaging and is typically 1/5 of the manufacturer’s listed breaking strain
• Because breaking strain can’t be accurately measured in the case of working rope or cable:
  • All general work should make use of SWL’s as a maximum load for the rope/cable
  • Rope and cable should be taken out of service if excessively damaged
• Note that lifting gear is held to a higher standard than other equipment.  Lifting gear used in cranes/derricks needs to be certified and should be inspected regularly prior to use.  If there the ship makes use of certified lifting equipment, this equipment must be kept separated from any uncertified working equipment.