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Author Topic: Answering that age old superstition about ratchet straps  (Read 1697 times)

brian maddox

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Re: Answering that age old superstition about ratchet straps
« Reply #20 on: May 10, 2021, 03:30:28 pm »

I think the test was valid for what it was represented to be - a determination of failure strength in a singular test situation.  The load cell was attached between the piston ram and the test table anchor, not at the head of the ram.

That all of the straps failed before the ratchet mechanism surprised me, in a way.  Having encountered many failed ratchets the forces needed to cause those failures must be significantly greater than I'd guessed.

Well the torsional force on the ratchet assembly is directly proportional to the amount of wraps of strap around it [you know, leverage and all that jazz]. It looks like the tester used good ratchet strap technique and kept the number of wraps to a minimum. It would interesting to see a similar test with different numbers of wraps on the ratchet.
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Riley Casey

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Re: Answering that age old superstition about ratchet straps
« Reply #21 on: May 10, 2021, 07:19:45 pm »

Failed ratchets found in the trailer du jour at a 1 am load out would I suspect have meet with a fair amount of abuse from forces unrelated to their ratchet function at least in my experience.

.... Having encountered many failed ratchets the forces needed to cause those failures must be significantly greater than I'd guessed...

Russell Ault

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Re: Answering that age old superstition about ratchet straps
« Reply #22 on: May 12, 2021, 01:09:59 am »

{...} That all of the straps failed before the ratchet mechanism surprised me, in a way.  Having encountered many failed ratchets the forces needed to cause those failures must be significantly greater than I'd guessed.

My guess is that the ratchet is stronger than the strap under static loading, but the relative elasticity of the strap can handle momentary shocks much better.

(I'm reminded of the video they show in some of the fall prevention classes up here explaining the importance of shock absorbers and comparing fabric and wire rope lanyards. First, they attach a 100kg weight to 2m nylon lanyard hanging from a force metre, then they drop the weight from the lanyard's anchor point. The lanyard arrests the "fall", but the maximum force measured suggests that, with no shock absorber, a similar fall might cause internal injuries. Next, they replace the nylon lanyard with a wire rope lanyard and conduct the demonstration again. The wire rope lanyard just snaps.)

-Russ
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Jonathan Johnson

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Re: Answering that age old superstition about ratchet straps
« Reply #23 on: May 12, 2021, 01:58:55 am »

(I'm reminded of the video they show in some of the fall prevention classes up here explaining the importance of shock absorbers and comparing fabric and wire rope lanyards. First, they attach a 100kg weight to 2m nylon lanyard hanging from a force metre, then they drop the weight from the lanyard's anchor point. The lanyard arrests the "fall", but the maximum force measured suggests that, with no shock absorber, a similar fall might cause internal injuries. Next, they replace the nylon lanyard with a wire rope lanyard and conduct the demonstration again. The wire rope lanyard just snaps.)

Reminds me of a demonstration by my high school physics teacher. He had a brick hanging from the ceiling with a piece of thick twine, and a piece of thin twine hanging below the brick.

For the first part of the demonstration, he asked the class "if I pull on the thin twine, which string will break?" Naturally, the class chose "the thin twine." So he pulled slowly, and the thick twine broke.

He set up the demonstration again, the same way, and asked the class "now which string will break?" Of course, the class chose the thick twine. So he quickly and forcefully yanked the thin twine, and the thin twine broke.

The demonstration was to show that the strength of the thin twine plus the weight (mass under influence of gravity) of the brick exceeded the breaking strength of the thick twine, but the mass (under the influence of inertia) of the brick plus the strength of the thick twine exceeded the breaking strength of the thin twine.

Dynamic loading can be a cruel master.
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Re: Answering that age old superstition about ratchet straps
« Reply #23 on: May 12, 2021, 01:58:55 am »


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