The Effect of Knot Type on the Breaking Strength of Rope

The Experementer

Researched by Brittney S.
2004-05




PURPOSE

The purpose of this experiment was to determine which knot or hitch, among four pairs with similar uses, had a higher breaking strength.

I became interested in this project because I enjoy rock climbing and it would be nice to know what knots are best to use and which are safest.

The information gained from this experiment could prove or disprove known information about knots, hitches, and rope breaking strength it could also be useful information regarding industry and recreation such as; fishing, timber and rock climbing.



HYPOTHESIS


My first hypothesis was that the figure of eight knot would be stronger than the over hand knot.

My second hypothesis was that the sheet bend would be stronger than the square knot.

My third hypothesis was that the bowline would be stronger than the fisher mans eye

My fourth hypothesis was that the timber hitch would be stronger than the clove hitch.

I based my hypothesis on Coast Guard data on the web called “Knot Strength”, at http://131.203.57.1/knots/knot_str.htm. It showed a table of breaking strengths in rope with various knots. It can be found in the appendix.



 EXPERIMENT DESIGN

The constants in this study were:
  • Type of rope used
  • Diameter of rope used
  • Rope testing device, all components
  • Testing done at room temperature
  • Speed at which rope gained tension
  • The procedure in which way rope was attached to winch
  • Indicator calculates in kilograms
The manipulated variable was eight different knots and hitches used and stretched to their breaking point.

The responding variable was the force needed to break the knots.

To measure the responding variable I made a device that increased rope tension and transferred the force to the load cell. The load cell then sent information to the indicator. The indicator displayed in kilograms from 0 to breaking point.



 
MATERIALS

QUANTITY ITEM DESCRIPTION
96 inch 11/4 inch x1/4 inch flat bar steel
49 inch 3 inch x3 inch x1/4 inch
10 inch 1 3/8 inch Sch. 40 pipe
1 3/8 inch x 4 1/2 inch carriage bolt with nut
1 1/2 inch x 9 inch carriage bolt with nut
4 5/16 inch x 1 inch grade 8 bolts with nut
1 1500 lb oat winch
1 500 lb s-type load cell
1 indicator 1000 lbs.
450 feet 1/4 sisal rope
2 cans of primer paint
2 cans of Caterpillar yellow paint
1 1/2 inch x 3/4 inch fine thread bolt
4 2x4x8 number 2 lumber
3 1x6x6 cedar fencing
4 2 inch hinges
2 caster wheels
4 table leg supports
2 barrel bolts
3 lbs. Deck screws
1 10 inch metal fiber cutting blade
1 10 inch compound miter saw
1 wire-feed MIG Welder
2 lbs. .030 MIG Welding wire
1 6 inch handles
1 5/16 inch drill bit
1 3/8 inch drill bit
1 1/2 inch drill bit
1 4 inch electric grinder



PROCEDURES

1. Build testing device
a. Get a 1.2 cm. solid, long piece of metal.
b. Put on safety glasses.
c. Put on leather gloves.
d. Measure 15 cm three times in the metal and mark where the 15 cm mark is.
e. Measure up the mark at the 15 cm to the saw blade.
f. Turn on the saw and cut the metal in the three spots you marked it.
g. Turn off the saw.
h. Weld two of the pieces of metal together in the center on the long side.
i. Put on your welding helmet so you don’t hurt your eyes.
j. Put on a leather, welding chest and arm coat.
k. Put on leather gloves.
l. Weld the third piece in the center where you had already welded the other two.
m. Weld the third piece on the long side.
n. Weld it sitting up on its side.
o. On the metal tube cut a 1.27 cm wide cut in it.
p. Cut it in a straight line.
q. Smooth out the edges and the sharp pieces of metal where you had cut the straight 1.27 cm wide cut.
r. Fit the half, inch wide cut to be just wide enough so that the metal piece you made in step eight has a little extra space to slide.
2. Cut rope
a. Cut the rope at 152 cm
b. Cut 5, 152 cm ropes for each knot
3. Tie knots
a. Tie the different knots in the middle of the rope
b. For the hitches (Bowline and Timber hitch) tie them on the bar at the end of the device that is being used.
4. Conduct trials.
a. Take the rope
b. Put the rope in the hole in the winch that has an indent in it that is facing the bar at the end of the device and tie a knot in it so it doesn’t slip out
c. Turn the winch forward so there is two loops all the way around it then put the rope in the middle of the two and go around it only one time
d. Tie the knot in the middle or the hitch at the end then tie it off on the solid metal triangle holding the bar
e. Turn the scale to measure kilograms
f. Turn the winch slowly while carefully reading the breaking strength on the indicator until the first strand breaks
g. Record the data
h. Repeat 4f with the second and third strands
i. Repeat 4a-4e with all the other knot types five times each
5. Record all the data
6. Average all trials for each knot type
a. Add all the data for one knot and the divide it by five because that is how many recordings there should be
b. Repeat 5a with each knot




  RESULTS

The original purpose of this experiment was to determine which knot or hitch, among four pairs with similar uses, had a higher breaking strength.

The results of the experiment were that the rope did break sooner with knot than it would without the knot in it. With some knots it did not break so soon because it was a stronger knot. I also found that the rope always breaks at the critical bend in the knot.

See my table and graph




 CONCLUSION

My original hypothesis was the figure of eight knot would be stronger than the over hand knot.

This hypothesis should be accepted because the figure of eight knot was stronger than the overhand knot.

My second hypothesis was the sheet bend would be stronger than the square knot.

This hypothesis should be accepted because the sheet bend was stronger than the overhand knot.

My third hypothesis was the bowline would be stronger than the fisherman’s eye.

This hypothesis should be rejected because the fisherman’s eye was stronger than the bowline

My fourth hypothesis was the timber hitch would be stronger than the clove hitch.

This hypothesis should be accepted because the timber hitch was stronger than the clove hitch.

Because of the results of this experiment, I wonder if testing the knots at extremely different temp would change the results.

If I were to conduct this project again I would do many more trials on breaking the knot. I would also start out with a softer and wider winch tube so that I could of attached the hitches with less hassle. I also wonder what would happen if I tested different ropes instead of different knots. Sisal is not ordinarily used in Mt. Climbing so I should have used a more tipical rope.
 

RESEARCH REPORT

Introduction
  Knot strength refers to how a knot weakens the breaking strength of the line. The twists, loops, and turns of a knot place unequal stress on the line that reduces the lines breaking strength by about 50% (Jacobson 1999). Lines of different materials, weaves, and diameters will have different characteristics and strengths.

Rope
Sisal rope is made from a hard natural fiber. It has excellent resistance to sunlight, little stretch, and good knot holding ability. It’s widely used in navigation, petroleum, exploration, mining, fishing, lumber, architecture, communication, and civil use.

Knots
The overhand knot and the figure of eight knot are used as a stopper knot to keep a line from running through a hole or a block. The square knot is used to tie two lines of similar size together . The sheet bend is used to tie two lines together that are the same or different sizes. The bowline and fisherman’s eye are used to tie a line around a post or person’s waist. The timber hitch and the clove hitch are used to tie a line around an object.

History
  People as hunters, farmers, craft workers, traders, and soldiers have used ropes for many purposes throughout history. Ropes are depict in Spanish cave paintings dating back 20,000 years. Knots have been used at least since the beginning of history. Primitive Egyptian and Greek societies used knots extensively.

Human Needs
  Knots are used in rope to tie objects together and many other things Humans need knots and ropes. Search and rescue people need ropes to help get large animals, children, and adults out of mud, water, deep holes, and many other awful accidents. Tugboats need rope to drag logs and drift wood across the water to land. Many other jobs need rope too.

Summary
  Knots’ are an important thing to human being’s. Knots help humans in many ways. The timber hitch was the strongest knot I tested. The timber hitch is used for tying objects to other objects. The timber hitch ties around a pole or other solid round or squared object.
 
BIBLIOGRAPHY


Chisholm, Geoffrey. “The Breaking Point of a Bowline.” Knot Performance. December 1, 2004. http://www.allaboutknots.com.

Chisholm, Richard. “The Study of Knot Performance.” Knot Performance. December 1, 2004. http://www.allaboutknots.com.

Hunter, Rob. "Knots, Hitches, and Splices." World Book Encyclopedia.” 1999.

“Knot Strengths.” http:// 131.230.57.1/knots/knot_str.htm.

Lowry, Mike. Telephone interview. November 15, 2004



ACKNOWLEDGEMENTS


I would like to thank the following people for helping make my project possible:
  • My dad for helping make the device and finding information.
  • My mom for taking the pictures.
  • My grandma for finding information.
  • Mr. Newkirk for helping me with the procedures.
  • Mrs. Helms for helping me find information on the computer.


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