Rope is a compact and flexible structure that is used in numerous applications, from recreational marine to mining, safety, and rescue. This blog post goes over certain types of ropes, their mechanical properties and applications of use in sailing.
Types of Rope
Rope is one of those materials that goes opposite to the saying “one-size-fits-all”. Ropes are produced from a variety of materials and have a wide range of characteristics and capabilities. Some rope applications require ropes that are lightweight or stretchable, while for others strength, durability, and lasting under extreme environmental conditions are needed from the rope material used. To meet such specific demands, rope manufacturers engineer products based upon the application of use and test them to verify properties determined from the proposed criteria.
Ropes are often categorized set up on their application, construction, fiber, and splice. We will not go over the details of rope material properties on this post. Yet, briefly, styles of ropes made from a variety of materials can be summarized into the categories below:
Braided rope – cylindrically produced rope made by intertwining, maypole fashion, several to many strands according to a definite pattern with adjacent strands normally containing yarns of the opposite twist.
Fiber rope – rope produced primarily from textile fibers. The linear density (in kg/100 m or lbs/100 ft) and strength of fiber ropes are the two main criteria for final product evaluation.
Plaited rope – rope made from eight strands arranged in four pairs intertwined in a maypole fashion. One strand is placed adjacent to the second in each pair and each strain in each pair is twisted in one direction and the alternate pair is twisted in the opposite direction, with the four pairs of strands
Twisted rope – rope made from three or more strands that are twisted together in opposite directions.
Application Note: Marine Rope Specifications & Use
Sailing around the world is a risky business. One has to choose the correct rope type and material for safety when crossing the ocean for weeks or, at times, months long in a sailing boat. The right rope is not always the strongest or the most rigid, and often, a wide range of sizes and strengths are necessary. In addition to properties such as water and UV resistance, the common specifications that determine a circumnavigator’s rope selection criteria are outlined below.
Flexibility and Strength
Sheets in sailing are ropes that are flexible and stretchable and they are used to adjust and trim the sails. They act as shock absorbers by preventing the sails from ripping during momentary heavy forces; wind gusts or changes in wind speed when the boat is going up and down 16 ft. waves. A sheet has to be stretchable to absorb these above mentioned wind shocks, but it also needs to be strong enough not to be torn during heavy loading. Most oceangoing sailors are experienced seamen and they look for the proper ropes with the necessary flexibility and strength that best fit their boat size, sail surface area, and the expected weather conditions.
In a sailing boat there are also applications of rope that require rigidity. The rope length should not change during loading. The halyards pulling the sails up to the mast is a typical example. If a halyard stretches with stronger winds, the sails will get baggy and will catch more wind than usual. This is a problem when more wind speed in a particular direction is not needed or expected.
Rigidity increases with the increase in the diameter of ropes and larger diameter ropes often mean higher rope weight. In a sailing boat with a 50-65 feet tall mast, the weight of the rope on the mast plays an important role in the heeling of the boat during strong wind conditions. This brings another factor for rope selection: weight.
Manila rope consisting of hemp fibers used to be the most common rope material type used in marine applications. Lately, with new generations of synthetic fibers there are many different options to choose from and to rig a boat. Even steel wires running from the top of the mast down to the four sides of the boat can be replaced by special no stretch, high strength, and low weight ropes. The advantages are numerous and some ocean sailors and most ocean racers prefer this new generation of ropes instead of the usual stainless steel cables.
Mechanical Properties of Rope
In addition to the rope properties highlighted above for sailing applications, you may have heard of ropes marketed for other uses as low stretch, high endurance, high resistance to wear and cuts, flexible, torque-free, and more. These properties are determined after identification and verification of needs, diligent engineering, and mechanical testing.
Mechanical properties of rope that need verification and testing include:
- Breaking force (kN or lbf)
- Elongation (m or in)
- Fatigue lifetime
- Fatigue strength (MPa or ksi)
- Linear density (kg/m or lbs/ft)
- Rope diameter (mm or in)
- Torque strength (Nm or lbf-in)
Breaking force and elongation are major properties of rope to determine the serviceability and suitability. In addition, ropes may appear the same size in diameter, but their linear densities may be different enough to affect their breaking force. Thus it is important to take linear density information into consideration when designing or selecting rope samples.
Fatigue resistance is a common distinguisher for wire ropes. Higher fatigue strength means the rope is able to withstand forces under a certain amount of time.
New Rope vs. Used Rope
A rope’s strength decreases over time. Rope testing may involve samples from new ropes and used ropes. When used rope samples are tested, test reports often include information on the application of use as well as how long the rope was, or is designed to be, used for. Depending on the variations of new and used rope mechanical properties, manufacturers may provide warranty information or include a recommended life span on the product specifications.
Rope Testing Equipment
Reliable determination of rope properties needs top quality, precise and versatile mechanical testing systems. To measure the breaking force and elongation, tension testing must be performed using a tensile testing machine that can maintain a certain rate of tensile pull. The rope specimen must be pulled in one continuous direction without interruption, thus it is important to select a testing system with adequate stroke and ample vertical test space.
Picture below shows ADMET eXpert 2600 table top Universal Testing System with a pancake style load cell and capstan rope grips. These frames have up to 54 in (1,370 mm) vertical test space to accommodate longer length rope specimens for accurate tensile test data.
Clamping rope samples can be a challenge. Small diameter ropes including threads and fine wires may easily slip from common tensile grip configurations such as manual or pneumatic vise grips. On the other hand, large diameter ropes require robust designs with clamping housing that can sufficient space for the specimen diameter.
ADMET offers thread grips that come with guide horns and centering grooves for small diameter fibers, wire ropes, sutures, and more. For larger ropes, we carry capstan rope grips featuring roller drums to avoid specimen slippage. Manual rope and thread grips are available with T-screws to tighten the specimen between the jaws. We also offer the same configurations with pneumatic and hydraulic clamping mechanisms.
Principles of Operation: Capstan Rope Grips
- Select the applicable rope grip based on the maximum force required for the testing as well as the specimen diameter and length (rope and thread grips have (1) maximum specimen diameter and (2) minimum specimen length requirements).
- To install the rope specimen in capstan rope grips, wrap the rope around the roller drum once or multiple times.
- Tighten the clamping jaws.
- Start the test.
Rope tensile testing for breaking force and elongation can be hazardous. It is recommended to utilize a safety enclosure when testing rope specimens to protect operators and people near the testing area from rope samples that can snap back and whiplash out of the test machine area.
Standard Test Methods
ASTM A931 – Standard Test Method for Tension Testing of Wire Ropes and Strand
ASTM D885 – Standard Test Methods for Tire Cords, Tire Cord Fabrics, and Industrial Filament Yarns Made from Manufactured Organic-Base Fibers
ASTM D2256 – Standard Test Method for Tensile Properties of Yarns by the Single-Strand Method
ASTM D2970 – Standard Test Methods for Testing Tire Cords, Tire Cord Fabrics, and Industrial Yarns Made From Glass Filaments
ASTM F3410- Standard Test Method for Testing Fiber Rope Used in Water Sports
Rope is a compact and flexible structure that is used in a variety of applications. Rope manufacturers provide specifications and comparative data that are based on the rope material properties. When used in sailing boats, strength, flexibility, rigidity, resistance to UV, water, and more need to be taken into account to select the right rope for a sole application.
ADMET is a leading global manufacturer of universal testing systems, fixtures and accessories. Manufacturers and product developers choose ADMET testing machines to test large diameter and small diameter ropes. Click here to fill out our Sales Inquiry form and a Sales Engineer will be in touch to discuss how we can help with your testing.