University of Extrication: Tie-Down Webbing Destructive Testing
Topic: Tie-down webbing destructive testing
Objective: The rescue team will better understand proper application and use of 2-inch-wide tie-down webbing and ratchet units.
Task: The rescue team shall review their vehicle rescue ratchet strap units and ensure that the team is utilizing the tie-down webbing within the working load limit of the tool.
This University of Extrication column presents several important points that vehicle rescue personnel must understand in order to ensure that the team is safely and effectively utilizing 2-inch-wide tie-down ratchet strap units.
The breaking strength
There is an important difference between two critical webbing tie-down terms—Breaking Strength and Working Load Limit (WLL). The Breaking Strength rating is the maximum load beyond which either the webbing or a load-bearing component of the ratchet tie-down unit will fail. WLL describes the maximum load the complete ratchet assembly should ever be subjected to during routine and regular use.
The webbing industry maintains a 3:1 safety ratio between the Breaking Strength and the WLL of a tie-down. This is not the same as a 3:1 mechanical advantage gained in a rope system for example, through the use of pulleys. The 3:1 safety ratio means that loads on these tie-downs should not exceed one-third of the tool’s Breaking Strength. If you have ratchet strap tool with a Breaking Strength of 10,000 pounds, don’t think you have a WLL of 10,000 pounds. Working with the 3:1 ratio, a tool that has a Breaking Strength of 10,000 pounds would have a WLL of 3,330. It is likely that some component on the unit will fail if the load ever approached or reached the 10,000-pound Breaking Strength. Don’t be confused!
Destructive testing
To evaluate just what happens when we exceed the WLL and go to the Breaking Strength of a standard ratchet strap unit, destructive testing was conducted under a laboratory setting. The testing followed very specific step-by-step testing guidance from the Web Sling & Tie Down Association. Brand new tie-down ratchet units were placed in the test machine with everything set up according to specs—sufficient wraps on the mandrel, twin hooks connected to the machine properly, backside of the hooks down, pulling speed established, etc.
The ratchet tie-down was pulled in a straight line loading it to its Breaking Strength, at which point a load-bearing component failed. It must be repeated that these tests were conducted with brand new, factory-fresh ratchets and webbing under controlled laboratory conditions. These tests do not necessarily represent what would occur if used or aged tie-down equipment, showing signs of normal wear and tear, were tested under real-world conditions.
When the pull on the ratchet and webbing was below the 3,330-pound WLL, everything was fine with nothing unusual noted. The ratchet tool remained functional, validating why ratchet tie-downs should only be used within their WLL and not pushed beyond that point.
Catastrophic failure
As the load on the ratchet and webbing exceeded the WLL and started approaching the unit’s Breaking Strength, extensive stretching of the polyester webbing was evident. In fact, just prior to failure, the entire ratchet webbing assembly stretched and elongated more than 17 inches from its original length.
When the Breaking Strength was reached, there was an instantaneous and catastrophic failure. In every case, it was not the twin steel hooks that failed, nor was it the metal frame of the ratchet assembly or the gear-like sprockets of the ratchet that failed either. In every test, there was webbing failure near the end where the twin hook was attached to the webbing. The webbing at the point of failure pulled apart as the 2-inch-wide webbing frayed and separated almost explosively. The red marker lines failed right along with the webbing itself, but they did not “pop out” as you may have been told they would. These lines simply indicate the tensile strength of the webbing per inch of width, nothing more.
Due to this excessive Breaking Strength load, the portion of the 2-inch-wide webbing wrapped around the mandrel of the ratchet also showed significant physical deformity with stretching and crushing damage evident. After the test, with all webbing removed from the ratchet unit, even the ratchet’s steel mandrel showed physical damage where it had actually started to collapse under this extreme load.
Interestingly, the large bolt that connects a short length of webbing and one twin hook to the frame of the ratchet unit did not fail in any test. This connection bolt was not a weak link as might be expected, just the webbing itself was.
Rescue personnel must ensure that 2-inch-wide ratchet tie-downs are only used within their WLL otherwise tool damage and catastrophic failure can result.
Tie-Down Webbing Terms to Know
- Breaking Strength: This rating is the maximum load beyond which either the webbing or a load-bearing component of the ratchet tie-down unit will fail.
- Working Load Limit (WLL): This describes the maximum load the complete ratchet assembly should ever be subjected to during routine and regular use.
- Safety Ratio: The 3:1 difference between Breaking Strength and WLL of a ratchet tie down.
Ron Moore
RON MOORE, who is a Firehouse contributing editor, recently retired as a division chief with the McKinney, TX, Fire Department and now serves with Prosper, TX, Fire Rescue. He self-published the Vehicle Rescue 1-2-3 training manual and serves as the forum moderator for the extrication section of Firehouse.com . Moore can be contacted directly at [email protected].
