Properties of Rope

Published On:Jan 16,2018

Properties of Rope
Rope Dimensions: In order to discuss properties of Rope, we should define the rope size that can be considered as an approximate starting point. A diameter of approximately 4mm is usually taken as the lower limit, which differentiates rope from small cords, and the largest ropes now made approach 300mm. The Range is thus from the first few divisions of an ordinary ruler to its whole width. However, the diameter is an uncertain measure of rope size, because of the irregular boundaries of ropes and the variable spaces between fibres, yarns and strands. Rope size is best expressed in terms of its linear density, i.e. mass per unit length.
Strength and Weight: Strength is the first measure that comes to mind when we consider the properties of Ropes. Fibre rope engineering normally normalizes strength in terms of weight, so that strength & weight should be considered together. There is an important mechanical quantity, known as breaking load, commonly expressed in Kilogram- force, kiloNewtons(102kgf), tones(1000kgf) or pounds force. The properties are best expressed in terms of linear density, but can also be expressed in Pascals, pounds force per square inch or other conventional stress units which are normalized by area. Nylon and polyester ropes in three-strand laid, eight-strand plait and various braids are produced for demanding service with strengths about 15% higher than the ordinary ropes.
  • Minimum Strength: The breaking load values are for minimum strengths. This is defined in some standards as two standard deviations below the mean of a statistically significant no. of break-test results.
  • Termination Strength: In use, rope is only as strong as the means used to hold or anchor it at its ends; this is called the “termination”. The disturbance of rope structure to make a splice or the interface with the fitting inevitably weakens a rope at the termination.
  • Strength-to-weight ratio: This is a measure of rope tenacity & provides a quick view of weight relative to strength for comparison between various products. Wire rope has a lower strength-to-weight ratio by a factor of nearly two or more when compared with any synthetic fibre rope. For rope made of very high tenacity fibres, the factor can be over eight.
  • Submerged Weight: Using polyester parallel strand construction & wire rope is very nearly twice as strong, but its submerged weight is 20 times as great. Therefore, on a strength-to-weight basis, the polyester is 10 times better. This is significant for applications such as deepwater mooring where the submerged weight of wire rope becomes very difficult to manage.
  • Service Conditions: As some ropes may degrade faster than others, the safety margin might have to be increased for a very high strength line, as compared to one of lower strength which may have a much longer life at a larger diameter. Although everyone tends to look at strength first, the selection must view all aspects of the application.