High-Pressure Accelerator
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  1. #1
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    Default High-Pressure Accelerator

    Sharing another piece of motion picture special effects equipment. The trade name is a “ratchet”, but basically a high pressure air motivated accelerator. I’m not positive, but I believe the overall design was probably taken directly from the previous generation aircraft carrier catapult. The main difference is that we use compressed nitrogen instead of steam.

    I wish I could say this was my design, it isn’t, I did have a hand in fabrication.

    Basically it’s a rope compound:



    Think block and tackle, working in reverse. Instead of pulling the two blocks together with a rope, we’re pulling the blocks apart and taking up the rope.

    The traveler is attached to a hydraulic ram:



    Finally there’s a high pressure accumulation and firing system. Nothing special on this one, literally just open the ball valve and things start happening... QUICKLY!



    The ball valve on the right is both a safety and a dump. It stays open until you’re ready to fill and prevents accidentally activating the system if the firing valve is open. In the picture above I gave the system a bit of pressure to keep the ram closed so I could stretch my rope out.

    Before my time guys used wire rope in machines like this, but the business graduated to synthetic before I got into it. Today I’m using 3/4” Technora 12-strand. The working load is significantly higher than 3/4” cable while weighing far less. 3/4” cable or wire rope breaks at 25.6 tons: 51,200 lbs. 3/4” Tech breaks at 32.5 tons: 65,000 lbs.




    In addition to weighing less than comparable cable, synthetic rope doesn’t have nearly as much energy in the event it does break. Tech-line, when it breaks, almost just “lays down”, unlike cable which tends to carry a lot of energy... enough to cut anyone unlucky enough to get caught by it, in half. You can also tailor the synthetic rope to the circumstances. Here the abrasion resistant characteristics of Tech line were why I chose it... in other situations I might choose something like Amsteel or Dyneema. Both Amsteel and Dyneema are lighter, Amsteel almost floats, but neither have the abrasion resistance that tech-line has. Running either Amsteel or Dyneema around a sheave typically welds the outer fibers together and the rope is unusable after only a couple pulls.

    Today’s work is gonna happen very fast. I’m pulling a Lincoln Town Car into a phone pole. The ratchet compound is a 6:1. The cylinder strokes 8’ so I’m pulling about 50’ total. The producers don’t want me to break the pole, but more important is that the accident appears to be a fatal one. I’m thinking 1800 psi, with both accumulators filled, ought to do the trick. I relieved the crap out of the car. There’s nothing at all between the bumper cover and the crank pulley. Ideally the machine stops pulling a two or three feet before impact, so I’m not actually ever pulling against the pole.

    Math period, isn’t my strongest suit and math on dynamic stuff like this is even more difficult. I’m guessing I’ll go from zero to 35-40 mph right before impact... if all goes according to plan.

    I’ll update later tonight with aftermath photos.




    Be safe





    Jeremy

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    After hours waiting for the shooting crew to put a camera on this thing... while doing maths I’m terrible at over and over again... I decided f*ck it, full send. I had four full 2500 psi bottles from a six-pack and one 6000 psi bottle. I saw 2400-2450 on the accumulator gauge before all my tanks were just equalizing. The pole moved... a lot! But it didn’t break. The car... little bondo, little paint, some of it’ll buff out:




    Not sure why the photos are blurry, but the rest of these people are trying to make a movie and I had to get out of the way.







    Be safe




    Jeremy

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    That's pretty neat. Thanks for sharing. That cord is amazing at strength vs weight and safety value.

    Sent from my SM-G960U using Tapatalk

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    Biggest problem is the rope manufacturers know how neat it is as well. The spool I showed cost almost $4k. Just under $10/ft. Worth it to the producers on the safety factor alone, but too often it gets cut and fidded for a specific application and always seems to be just too short for the next one...




    Be safe



    Jeremy

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    I've snapped some synthetic rope. It literally just dropped to the ground without even making a snap.

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    Default High-Pressure Accelerator

    Did you overload the rope? On the rare occasions when I’ve seen synthetic ropes break the failure has always been due to a combination of excessive snatch load in concert with less-than-ideal termination practices.

    I bought $30k order of Amsteel blue several years ago, a large enough order to warrant the Samson ropes rep to come out and “certify” me and the crew in the proper termination techniques. My takeaway, aside from the xeroxed certificate with my name on it, was that the manufacturers ratings are based on minimums. A proper eye in the end of the rope is made by running a minimum of three fid-lengths of rope-tail back into the hollow core of the “live” end of the rope. The tail needs to taper to prevent a sudden decrease in diameter. If the tail gets woven before running into the core you need a minimum of three chevrons of rope between the weaves. The smoother and more gradual you can make the transition the better off you are and the stronger your rope becomes. Do your best to pre-stretch it and things only get better. Every time I’ve seen these type ropes break it’s been because the taper, if the tail even did so, was too severe and the sudden reduction in diameter actually lowers the overall strength. In my world I assume we will always break the rules by “shocking” the rope, so it’s always drastically oversized to compensate.





    Be safe



    Jeremy

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    We were winching a pretty solid load and didn't realize the rope slipped off the pulley and bound in the snatch block. So we had a pinched line and were putting double the load we thought we were on it.

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    I watched your video bringing down that press. Good job, firstly, but like me you’re in a situation where no matter how safe you work and how far you think ahead there’s a good chance of unforeseen dynamic forces coming into play. You’re obviously doing everything you can to avoid those dynamic changes, while most of the time I’m chasing them to the ragged edge. I shudder when producers show me reference videos like the one you posted... even worse when they want to dupe something like an unintentional crane or building collapse.

    Too often the words I want to say are along the lines of: “so you want me to actively duplicate a SERIES of every last thing going terribly wrong... and you want to do it safely... you want to see a test AND you want to shoot it in two weeks?!?”

    Bite the tongue and say: “yes sir” if you want a call the next time. Especially when you have absolutely no idea how to even begin.





    Be safe





    Jeremy.

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    To give an idea of the force these machines can generate:


    These are two of the buffer plates from the traveler end of the accelerator. They hold a stack of rubber “donuts” and keep the piston from hammering the back end of the cylinder. The smaller one is 1/4” the larger, 1/2” thick.

    You can see where the clevis nut has made an impression:


    Maybe ten pulls on each to get this domed. Much more deformation and it starts to get harder to pull the clevis off.




    Be safe





    Jeremy

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    Tonight’s setup:





    Not my choice on any of it, I’m a hired gun tonight. Before I pull the trigger there will be a 12k Pettibone holding the truck back, but the risk versus reward of keeping the ratchet on the truck versus putting it on the ground with bull pricks and the Pettibone holding it. I’ll know in a couple hours...

    Plan is 2999 psi in the accumulator, just shy of the 3k psi rating on the tanks. All the other fittings are 6k. Experience tells me the [email protected] 2500 psi tanks ought to get me 1600 psi in the accumulator and the [email protected] 6k tanks should make the balance.

    3k psi gets me 60k psi on the rod side of the cylinder. I’m passing over 5 sheaves, so I’ll lose some pull to friction. 8-10% each time the rope touches a wheel is probably a fairly close estimate. Still gets me better than 35k of pulling force. I’m yanking a 12k bus, on its side, on pavement.





    We’ll see what happens




    Be safe






    Jeremy

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    The machine on the truck wasn’t an issue tonight. Too large of rope running over too small of sheaves was. The rope jumped the sheave twice. Second time it totally fragged the wheel:



    Somehow the rope not only jumped the wheel it bound up so hard it got behind the dust cover of the bearing. I dug those fibers out of the race after prying off the dust cover.

    Going from 6:1 to 5:1 meant the production for their shot. Third times a charm... By the end I was more worried about the rope breaking than anything else:


    The 12-strand rope was probably more like Technora 9 1/4-strand by the third pull. Thankfully it held and has earned itself an early retirement.

    I’ve got job-security for at least a week going through the ratchet, replacing every sheave and making sure there’s no other damage I missed. Then, on to the next one.





    Be safe





    Jeremy


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