Tuesday, September 23, 2014

Violence against cables

You recall the impotent bottle-jack debacle of course.  Well I went to Biltema (a magical place) and after long stingy consideration decided to get two 6 tonne jacks this time.  The next size up was 4, but the thought of them being too weak and having to get even more jacks convinced me that overkill was the right choice.

Got some stash
Took the wimpy old jacks off and hid them with their shame in the cupboard.  These beefy fellows will surely get the job done.

Beefcake!
Hooked up the temporarily spared victim and got to cranking.  Some tense high-tension creaking filled moments later and BANG!  We have breakage.  :-)

Feel the pain!
It's all a bit inconclusive though.  We didn't get a nice break in the middle like I wanted.  You can see that the splices are mostly intact, but it's not obvious that they didn't contribute to the break.  I had some more test pieces that I had made whilst practicing more, but even after breaking two more, I still had no clear answer :-(

I have been pondering various ways of actually measuring the tension in the cable, and even tried some, but none of them really worked.  I thought I could measure the cable stretch before it broke, and compare it with some standard stretching measurements for this size and kind of cable.  Problem is that the stretch is so little, only a couple of mm at best that it really is impossible for accurately measure, especially while you're anticipating a violent breakage of the cable.

A force gauge that can measure things like this is gonna be way out of my price range, so if I am to accurately measure it, there's gonna be more time wasting!

I found a video on youtube of a dude that had added a pressure gauge to a bottle jack.  By knowing the area of the piston and the pressure of the oil you can calculate the force required.  I might try that, but it requires some rather non-trivial modifications.

Whoops, there goes a shackle!

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